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NASA Engineering and Safety Center

Technical Assessment Report

Volume II

Pilot Breathing Assessment

November 19, 2020
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Table of Contents

Technical Assessment Report

Appendix 1: PBA Study Design.............................................................................................................3

Appendix 2: Fundamentals of Pilot Breathing...................................................................................33

Appendix 3: VigilOX Sensors..............................................................................................................41

Appendix 4: Pilot Physiology...............................................................................................................48

Appendix 5: Development of JPL Mask .............................................................................................63

Appendix 6: Standardization of Test Flights......................................................................................81

Appendix 7: F-35 Pilot Interviews and Ground Test Data ...............................................................88

Appendix 8: Pilot Breathing Assessment (PBA) Considerations on NESC’s F/A-18 PE Report 

(2017) and Other Issues................................................................................................217

Appendix 9: Results of Pilot Questionnaires and Interviews..........................................................224

Appendix 10: PBA Machine Learning................................................................................................244
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Appendix 1: PBA Study Design

• This appendix contains the following examples referenced in Technical Section 1. 

1. A flight test report provided as an example of reports submitted by Pilot 

Breathing Assessment (PBA) pilots following flight. 

2. A metadata report provided as an example of those submitted by the attending life 

support specialist for a PBA flight. 

3. PBA Flight cards as executed for Profiles A-F, H (in flight), and Profile G (on the 

ground). These cards were reviewed during Crew Brief Review (pre-flight), 

followed and annotated by pilot during flight, and reviewed in Crew De-brief 

Review. 

Flight Cards for PBA Scripted Profiles

- Profile A: High Altitude

- Profile B: AeroBatics

- Profile C: Control

- Profile D: Down low

- Profile E: Elimination of Cabin Pressure

- Profile F: Functional Check Flight

- Profile G: Ground only

- Profile H: Health Check – Standardized Flight Test Profile 
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1. Typical example of flight test report submitted by the PBA pilot following a flight. 
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2. A metadata report provided as an example of those submitted by the attending life support 

specialist for a PBA flight. 

099-8-846-021920-E0-XX-XX Brief: 11:00

Step: 12:15

NAVY/USAF 13:00

Landing: 14:30

Pre-Flight 1:30

N/A

ISB003 ESB Serial # ESB001 Madge Tech Ser#

SW Ver. SW Ver. Cockpit Q59978

Sync Time: Sync Time: Pilot Q59998

Moisture Trap Yes

CRU73/05858 Madge Tech Ser#

ISB Serial # ESB Serial # ESB004 Pilot N/A

SW Ver. SW Ver.

Sync Time: Sync Time:

Moisture Trap N/A

ISB003

Data Name: Data Name: 

Rear Seat

ISB Serial # ESB Serial # 

Data Name: Data Name:

FVC1 FVC2 FVC3 VC RAD-97

11:32:15 11:33:22 11:34:09 11:33:27 11:25:49

12:22:36 12:23:11 12:24:06 12:24:11 12:18:36

14:09:56 14:10:33 14:11:11 14:10:38 14:06:02

14:50:51 14:51:33 14:52:24 14:52:29 14:46:35

FVC1 FVC2 FVC3 VC RAD-97

11:36:25 11:37:08 11:37:57 11:38:02 N/A

12:20:36 12:21:25 12:22:13 12:20:41 N/A

14:08:40 14:10:30 14:10:58 14:11:46 N/A

14:52:33 14:53:18 14:54:06 14:52:38 N/A

1.3 1.4

VigiLox Rear Seat

Installed Regulator Type/Ser#:

O2 Connector/Regulator Used:

1.3 1.4

6:10:42 6:12:40

ESB_ESB001-2020-02-19-12-27

6:14:29 6:16:04

Post-Flight 

O2 Connector/Regulator Used:

ISB Serial # ESB Serial # ESB001

Front or Single Seat

VigiLox Life Support Pre/Post Flight Report

Life Support Tech:

VigiLox Front or Single Seat

Flight Number: 

Phil/Mark

At Aircraft After

At Debrief

Duration:

Flight Config.: Takeoff:

CRU103/19820

N/A

At Mission Brief

At Aircraft Before

At Aircraft After

At Debrief

At Mission Brief

ISB Serial # 

ISB001

Installed Regulator Type/Ser#:

ISB_ISB003-2020-02-19-12-26

 ISB001 ESB004

ISB_ISB001-2020-02-19-12-27 ESB_ESB004-2020-02-19-12-27

At Aircraft Before

Spirometery Fwd/Single Seat

Spirometery Rear Seat
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3. Flight cards as executed for Profiles A-F, H (in flight), and Profile G (on the ground). 

Profile A: High Altitude
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Profile B: AeroBatics
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Profile C: Control
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Profile D: Down low
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Profile E: Elimination of Cabin Pressure
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Profile F: Functional Check Flight
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Profile G: Ground only
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Profile H: Health Check – Standardized Flight Test Profile 
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Appendix 2: Fundamentals of Pilot Breathing

• This appendix contains links to the two animations discussed in PBA Volume I, 

Technical Section 2

o Normal breathing animation

o G-Breathing animation

Also the following: 

The Fundamentals of How Regulators Operate in Breathing Systems

This appendix offers some fundamental descriptions of how regulators operate as a part of an 

integrated pilot breathing system. This appendix has three sections:

Section 1: Twenty statements about regulators in breathing systems

Section 2: Twenty paragraphs – expanding these statements with further explanation

Section 3: One scenario that considers a pilot breathing system upset, and the available 

information about that upset event

Section 1: Twenty statements about regulators in breathing systems:

1. Regulators act as pneumatic signal amplifiers. Regulators receive a relatively small 

pressure/flow pneumatic force input from pilot inhalation and deliver a larger 

pressure/flow response in the form of suppled air delivered to the mask.

2. Inhalation, which initiates and continuously controls regulator function, is highly 

variable. A pilot’s peak inhalation for any given breath can range from <1 mmHg to 

>30 mmHg. Rates of change of an individual’s inhalation pressure can range from 

<2 mmHg/sec to >200 mmHg/sec. The volume of a single breath can range from <0.5l to 

>5.0l.

3. Regulators need to provide air whether the call for air is gentle or intense, slow or 

sudden, small or large.

4. Breathing systems are compliant – some of the pressure/flow energy from inhalation is 

directed towards flexing breathing hoses and other compliant elements of the breathing 

system.

5. If the compliance of mechanical components like breathing hoses varies, regulator 

response varies. Compliance can vary if mask fit changes, or if hose length or hose shape 

changes. A regulator will deliver more air more quickly when it is installed in a breathing 

system with a short, stiff hose. A regulator will deliver less air with a greater delay when 

a longer hose or a softer hose is used.

6. Supply pressure effects regulator performance. Low pressure supply can delay delivery 

and limit driving force through restrictions in the flow path. High pressure supply can 

trigger overshoot and flow control instabilities. It is harder to smoothly match the timing 

and sequence of breathing when there is more energy to manage. Variable supply 

pressure is especially disruptive.

7. Regulators need to compensate for changes in cabin pressure and turbulence issues 

associated with airflow patterns in the cabin. This is accomplished through a system of 

diaphragms and through-holes. The through-holes have a specific size and are designed 
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for a specific pressure and a specific pressure differential. If the holes are too small, the 

response is too slow. If the holes are too large, internal pressure is not maintained and the 

regulator “droops” and the gas delivery is delayed.

8. Regulators need to compensate for changes in gas density. At high altitudes where gas 

density is lower, it takes a smaller pressure to deliver a set volume of breathing air. It is 

easier to breathe through a regulator at high altitudes, but the pilot receives fewer moles 

of breathing gas. An oxygen (O2) schedule adjusts O2 concentration to account for 

altitude/density effects. 

9. Some regulators play an active and intentional role in controlling the O2 percentage in 

pilot breathing air – some regulators play in unintentional role in controlling the O2

percentage in pilot breathing air.

10. Breathing is dynamic. During inhalation, the absolute pressure, the delta pressure, the rate 

of pressure change, the corresponding flow, and the rate of change of flow are always 

changing. The hard thing about regulators is they need to do more than just deliver flow –

they need to deliver the right amount of flow at the right time. Regulators need to match 

the sequence of pilot breathing.

11. A standard flow bench test of a regulator does not measure regulator dynamic response, 

nor does it measure the ability to match breathing sequence (which is the hard part of the 

regulator job). A standard flow bench test of a regulator involves fixed flows and 

constant conditions.

12. The most sophisticated standard test of a regulator involves a breathing simulator with a 

sinusoidal waveform. A sinusoidal waveform represents the smoothest rate of change 

possible. Sinusoidal waveform tests are non-conservative tests of breathing system 

dynamic response.

13. Pilots report difficulties breathing during and immediately after speaking, because the 

demand for air immediately after speaking is sudden and has a different sequence than 

the regulators are tuned for.

14. The hardest job of a regulator is matching breathing sequence; the regulator must match 

the timing of inhalation initiation, velocity increases, and velocity decreases. 

15. The hardest conditions for matching breathing sequence involve: 

• variable supply pressure

• variable cabin pressure

• large and variable amounts of system compliance

• talking and other sudden and variable demands for air

• long breathing hoses

• mask valves that are sticky or unseated

16. There is no Military Standard (MIL-STD) test that measures the ability of a regulator in a 

pilot breathing system to match breathing sequence. There is no MIL-STD requirement 

related to breathing sequence.

17. Breathing hysteresis and breathing phase shift are standard and quantitative 

measurements of system breathing sequence match (or breathing sequence insults).
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Recommended requirements for breathing system performance are less than 0.7 lps for 

breathing hysteresis and less than 30 degrees for breathing phase shift.

18. Regulators can insult breathing sequence both ways; sometimes they deliver not enough 

air early in the inhalation, and too much air late in the inhalation – sometimes they 

deliver too much air early in inhalation, and not enough air late in inhalation.

19. Regulators sometimes suffer a flow control problem where they deliver a small amount 

of flow at a high velocity.

20. Regulators sometimes suffer a flow control problem where they interrupt speaking, by 

delivering a sudden surge of air while the pilot is trying to speak and is exhaling. 

Section 2: Twenty paragraphs about regulators in breathing systems:

1. Regulators act as pneumatic signal amplifiers. Regulators receive a relatively small 

pressure/flow pneumatic force input from pilot inhalation and deliver a larger 

pressure/flow response in the form of suppled air delivered to the mask. Regulators 

receive a small pressure signal – that pressure signal is limited by the pressure force and 

volume of pilot breathing. This pressure fluctuation triggers regulator function – it serves 

as the control signal for the regulator. The job of a regulator is to receive air at high 

pressure and deliver the appropriate amount to the pilot at the correct time. The 

magnitude of the pressure forces involved in delivering air are substantially greater than 

the magnitude of the pressure forces involved in pilot demand. Some people find it 

helpful to think of a regulator as a pneumatic amplifier – especially when comparing 

different types of regulators. Some regulators quickly convert a small amount of signal 

into a large and sudden pneumatic delivery – these regulators can be thought to have 

“high gain.” These “high gain” regulators may have fewer flow delays, but they are less 

stable and more prone to overshoot.

2. Inhalation, which initiates and continuously controls regulator function, is highly 

variable. A pilot’s peak inhalation for any given breath can range from <1 mmHg to 

>30 mmHg. Rates of change of an individual’s inhalation pressure can range from 

<2 mmHg/sec to >200 mmHg/sec. The volume of a single breath can range from <0.5l to 

>5.0l. Regulators receive a pressure signal from pilot breathing that controls function–

that pressure signal will vary dramatically, because breathing is highly variable. Normal 

resting breathing in open air generally involves pressure changes on the order of 1 to 

3 mmHg. The greatest amount of pressure lungs can apply (under static, no-flow 

conditions) can be >30 mmHg. Rates of pressure increase and decrease can change by a 

factor of 100. The volume of a single breath can change by a factor of 10. Regulators 

receive pressure signals that are highly variable in magnitude, duration, and rate of 

change.

3. Regulators need to provide air whether the call for air is gentle or intense, slow or 

sudden, small or large. Pneumatic signal amplification would be comparatively easy if 

the pilot’s breathing demand were consistent. A regulator could be tuned to match a 

specific breathing profile – but tuning to match the needs of deep regular breaths would 

fail if the pilot was requesting slow gentle breathing, with a low flow and a small rate of 

change.
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4. Breathing systems are compliant – some of the pressure/flow energy from inhalation is 

directed towards flexing breathing hoses and other compliant elements of the breathing 

system. The job of a regulator would be substantially easier if the pilot breathing system 

was rigid and perfectly sealed. If the pilot breathing system was sealed and rigid, there 

would be a larger pressure signal caused by inhalation and the inhalation pressure signal 

would be more repeatable. Actual pilot breathing systems are compliant – hoses flex, and 

gas leaks around the face seal of the mask. When a pilot takes an inhale breath, some of 

the inhalation pressure energy goes to drop the pressure of the line – but some of the 

inhalation pressure energy goes to flexing the breathing hose and causing in-leakage 

around the mask. When compliance changes from flight to flight, regulator response will 

change from flight to flight – even if the mechanical elements of the regulator are 

performing identically.

5. If the compliance of mechanical components like breathing hoses varies, regulator 

response varies. Compliance can vary if mask fit changes, or if hose length or hose shape 

changes. A regulator will deliver more air more quickly when it is installed in a 

breathing system with a short, stiff hose. A regulator will deliver less air with a greater 

delay when a longer hose or a softer hose is used. Two-seat planes, where the length of 

the hose for the front seat and the length of hose for the back seat are different will have 

different breathing responses – even if the mechanical elements of the regulator are 

performing identically. Generally, the pilot with the longer hose will have the more 

difficult time breathing, because the volume of gas is greater, the pressure signal will be 

smaller, the amount of possible compliance is larger and more variable, and it takes 

longer for the air to reach the pilot.

6. Supply pressure effects regulator performance. Low pressure supply can delay delivery 

and limit driving force through restrictions in the flow path. High pressure supply can 

trigger overshoot and flow control instabilities. It is harder to smoothly match the timing 

and sequence of breathing when there is more energy to manage. Variable supply 

pressure is especially disruptive. MIL-STD tests focus on the ability for a regulator to 

flow freely and deliver a sufficient supply of air to the pilot – even when the supply 

pressure is low. This is important, but it is not sufficient. Regulators must deliver a 

consistent amount of flow for a given demand – regardless of supply pressure. It is hard 

to deliver a consistent amount of flow – with consistent timing and sequence – if supply 

pressure varies dramatically. Pneumatic flow control systems are prone to overshoot with 

high supply pressure and suffer delays at low supply pressure.

7. Regulators need to compensate for changes in cabin pressure and turbulence issues 

associated with airflow patterns in the cabin. This is accomplished through a system of 

diaphragms and thru-holes. The thru-holes have a specific size and are designed for a 

specific pressure and a specific pressure differential. If the holes are too small, the 

response is too slow. If the holes are too large, internal pressure is not maintained and 

the regulator “droops” and the gas delivery is delayed. Regulators are designed to leak 

slightly, with internal high-pressure gas leaking through the regulator to the outside. This 

slight leakage ensures reliable performance in a cabin environment with increasing 

pressure (descent) or decreasing pressure (ascent). This intentional leakage must be 

manufactured to meet a specific set of conditions – with a set supply pressure, a set cabin 

pressure, and a set rate of cabin pressure change. Different conditions result in different 
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performance. Systems designed to respond to faster rates of change of cabin pressure are 

generally less capable to meet demands for fast increases in inhalation. Systems designed 

for fast response to changes in inhalation are generally less capable to correct for sudden 

changes in cabin pressure.

8. Regulators need to compensate for changes in gas density. At high altitudes where gas 

density is lower, it takes a smaller pressure to deliver a set volume of breathing air. It is 

easier to breathe through a regulator at high altitudes, but the pilot receives fewer moles 

of breathing gas. An O2 schedule adjusts O2 concentration to account for altitude/density 

effects. At high altitudes, cabin pressure is low, the pressure in the breathing gas line is 

low, and small changes in pressure can maintain high volumetric flow rates. Regulator 

function is easier at high altitudes – small changes in pressure can meet flow 

requirements – but the reduced density of gas means that a given volume of air contains 

fewer moles of O2. An altitude/ O2 percentage schedule accounts for this and requires that 

pilot breathing gas contains a higher percentage of O2 at higher altitudes.

9. Some regulators play an active and intentional role in controlling the O2 percentage in 

pilot breathing air; some regulators play an unintentional role in controlling the O2

percentage in pilot breathing air. Demand regulators receive 100% of pilot breathing gas 

from a high-pressure supply. If there is an altitude/O2 percentage schedule, the gas supply 

system has to adjust the concentration of O2. One example is an On-Board Oxygen 

Generating System (OBOGS) that operates with a different cycle time at high altitudes to 

provide greater O2 percentage. The demand regulator does not intentionally mix gases 

and adjust O2 concentration. Diluter-demand regulators mix high-pressure supply gas 

with cabin air. Diluter-demand regulators intentionally adjust O2 concentration. For a 

given altitude – the ratio of high-pressure gas to cabin gas should be fixed regardless of 

breathing profile. In practice, this this ratio is not fixed. When a pilot takes a quick deep 

breath, flow rate from the high-pressure gas is choked and more cabin air is introduced.

The pilot taking a quick deep breath from a diluter-demand regulator generally gets a full 

breath, and it is generally easy to breathe, but the O2 concentration may be lower than 

specified. The pilot taking a quick deep breath from a demand regulator can suffer a 

choke in the breathing gas supply. The O2 concentration will match the schedule, but the 

air will come late, it will be difficult to breathe, and the quantity of gas may be reduced.

10. Breathing is dynamic. During inhalation, the absolute pressure, the delta pressure, the 

rate of pressure change, the corresponding flow, and the rate of change of flow are 

always changing. The hard thing about regulators is they need to do more than just 

deliver flow – they need to deliver the right amount of flow at the right time. Regulators 

need to match the sequence of pilot breathing. Inhalation is not simple. The amount of air 

requested at any given instant changes, and the rate of change is variable. Delivering air 

to the pilot early disrupts breathing. Delivering air to the pilot late disrupts breathing. In 

the early stages of inhalation, increasing the delivery rate too quickly disrupts breathing.

In the early stages of inhalation, increasing the delivery rate too slowly disrupts 

breathing. In the late stages of inhalation, decreasing the delivery rate too quickly 

disrupts breathing. In the late stages of inhalation, decreasing the delivery rate too slowly 

disrupts breathing.

11. A standard flow bench test of a regulator does not measure regulator dynamic response, 

nor does it measure the ability to match breathing sequence (which is the hard part of the 
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regulator job). A standard flow bench test of a regulator involves fixed flows and 

constant conditions. A standard flow bench sets and fixes supply pressure. A standard 

flow bench sets and fixes cabin pressure (usually at lab ambient pressure). A standard 

flow bench sets and fixes demand pressure – and measures the corresponding flow for 

these fixed pressure conditions. A standard flow bench measures steady state flow. A 

regulator in a standard flow bench has internal components that are generally not moving.

The difficult job of a regulator is matching breathing sequence and making dynamic 

changes that match the dynamic changes of pilot breathing. A standard flow bench test 

does not test the difficult part of regulator performance. 

12. The most sophisticated standard test of a regulator involves a breathing simulator with a 

sinusoidal waveform. A sinusoidal waveform represents the smoothest rate of change 

possible. Sinusoidal waveform tests are non-conservative tests of breathing system 

dynamic response. Testing a regulator with a breathing simulator is a dynamic test of 

regulator function. Breathing simulator tests are tests of the difficult part of regulator 

performance. Pilot reports of difficulty breathing indicate that difficulties are most likely 

when changes in the breathing sequence are the most abrupt; such as taking a fast breath 

at the end of talking, or taking a large and sudden breath during an anti-g straining 

maneuver. The standard breathing simulator uses a sinusoidal waveform. The sinusoidal 

waveform is the smoothest and most uniform dynamic pattern possible. Sinusoidal 

breathing simulators are the most non-conservative dynamic tests possible.

13. Pilots report difficulties breathing during and immediately after speaking, because the 

demand for air immediately after speaking is sudden, and has a different sequence than 

the regulators are tuned for. Meeting a sudden change in breathing sequence is the 

hardest thing for a regulator to do. Sudden changes cause changes in system compliance.

Sudden changes cause fast dynamic changes in the regulator.

14. The hardest job of a regulator is matching breathing sequence; the regulator must match 

the timing of inhalation initiation, velocity increases, and velocity decreases. There is an 

inherent lag in a regulated pilot breathing system: first the pilot needs to initiate the 

inhalation, then the pressure of the mask/line control volume needs to drop, then the 

regulator needs to receive the signal, then the regulator needs to mechanically actuate and 

respond to the signal, then the gas needs to flow through the regulator and through the 

mask/line control volume. Matching the timing and matching the sequence and increasing 

flow rate when the pilot desires increasing flow rate and decreasing the flow rate when 

the pilot desires decreased flow rate – the dynamic response is the hard thing.

15. The hardest conditions for matching breathing sequence involve: 

• variable supply pressure

• variable cabin pressure

• large and variable amounts of system compliance

• talking and other sudden and variable demands for air

• long breathing hoses

• mask valves that are sticky or unseated

Meeting the dynamic response and matching the timing of the pilot’s breathing sequence 

is always the hard part of the job. Sometimes, aspects of the pilot breathing system can 
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make this especially hard. It is harder to match the timing of the breathing sequence when 

the supply pressure is changing, because response rates of the internal components in the 

regulator change when supply pressure changes. Cabin pressure changes complicate 

system response. Compliance in the breathing system causes delays. Changes in 

compliance results in inconsistent timing. Sudden demands for air limit the allowable 

time for system response. Long breathing hoses delay the pressure signal and delay the 

delivery of gas. Mask components, especially inhalation and exhalation valves, can delay 

system response if they are sticky.

16. There is no MIL-STD test that measures the ability of a regulator in a pilot breathing 

system to match breathing sequence. There is no MIL-STD requirement related to 

breathing sequence. Timing, and matching the breathing sequence is the difficult and 

important job – but there is no standard way to test timing and breathing sequence, and 

there are no specifications related to timing and breathing sequence.

17. Breathing hysteresis and breathing phase shift are standard and quantitative 

measurements of system breathing sequence match (or breathing sequence insults).

Recommended requirements for breathing system performance are less than 0.7 lps for 

breathing hysteresis and less than 30 degrees for breathing phase shift. Quantitative 

measures of the ability of a breathing system to match the timing and breathing sequence 

of a pilot have been developed. Breathing hysteresis and breathing phase shift 

quantitatively and systematically measure timing and sequence – this is not a subjective 

assessment. Breathing hysteresis and breathing phase shift have been evaluated as part of 

the Pilot Breathing Assessment – there is a sufficient database to set provisional 

standards.

18. Regulators can insult breathing sequence both ways; sometimes they deliver not enough 

air early in the inhalation, and too much air late in the inhalation – sometimes they 

deliver too much air early in inhalation, and not enough air late in inhalation.

Measurements of breathing hysteresis and breathing phase shift show that regulators in 

breathing systems can err in both directions, sometimes supplying too early and 

sometimes too late. System lag is expected, delivering too early is likely caused by 

system interactions where the end of the previous breath effects system performance for 

the following breath.

19. Regulators sometimes suffer a flow control problem where they deliver a small amount of 

flow at a high velocity. Regulators are constantly adjusting to find a solution to the 

breathing supply problem. Sometimes, the regulators shift between overshoot condition 

and overshoot condition – resulting in a system that provides a small amount of high 

velocity gas.

20. Regulators sometimes suffer a flow control problem where they interrupt speaking, by 

delivering a sudden surge of air while the pilot is trying to speak and is exhaling. Pilots 

report getting their speech cut off by a regulator that pushes a large amount of air down 

their throat – making speech impossible. This occurs because a pilot will take a quick 

inhale breath at the end of a sentence – this quick breath triggers a large surge of gas from 

the regulator – which makes speaking impossible.
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Section 3: Summary Scenario:

When considering the difficult job of matching breathing system supply sequence to breathing 

system demand sequence, the following scenario makes sense:

• Pilot senses problems flying, declares a PE

• Medical assessment indicates signs of hypoxia or “hypoxia-like” symptoms

• Regulator is removed from the pilot breathing system, and tested as a single component 

• Regulator is tested under fixed flow conditions

• Breathing hysteresis / breathing phase shift are not measured

• No anomalies are detected

• Event is declared a UPE

The pilot suffered a PE because the pilot breathing system inflicted breathing sequence insults on 

the pilot, which compromised breathing, choked the supply air to the pilot, and triggered 

hypoxia. The cause could have been a stuck exhalation valve, a breathing hose that was too long 

for the system tuning, cabin pressure surges, supply pressure variability, or a breathing demand 

profile that was outside the tuning range for the regulator. These problems will never be 

identified by a post event bench test of the regulator as an isolated component (under fixed flow 

conditions). These problems can be identified by measuring pilot breathing in the jet, and 

measuring breathing hysteresis and breathing phase shift.

PilotBreathingNORM_anim.mp4 PilotBreathingGstrain_anim.mp4
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Appendix 3: VigilOX Sensors

Department of Defense (DoD) Technology Readiness Levels (TRL)

The Department of Defense (DoD) Technology Readiness Levels (TRLs) are defined in the table below:

Level Definition DoD DAG Description

1

Basic principles observed 

and reported

Lowest level of technology readiness. Scientific research begins to be translated into applied 

research and development. Examples might include paper studies of a technology’s basic 

properties.

2

Technology concept and/or 

application formulated.

Invention begins. Once basic principles are observed, practical applications can be invented. 

Applications are speculative and there may be no proof or detailed analysis to support the 

assumptions. Examples are limited to analytic studies.

3

Analytical and experimental 

critical function and/or 

characteristic proof of 

concept.

Active research and development are initiated. This includes analytical studies and laboratory 

studies to physically validate analytical predictions of separate elements of the technology. 

Examples include components that are not yet integrated or representative.

4

Component and/or 

breadboard validation in 

laboratory environment.

Basic technological components are integrated to establish that they will work together. This 

is relatively “low fidelity” compared to the eventual system. Examples include integration of 

“ad hoc” hardware in the laboratory.

5

Component and/or 

breadboard validation in 

relevant environment.

Fidelity of breadboard technology increases significantly. The basic technological components 

are integrated with reasonably realistic supporting elements so it can be tested in a simulated 

environment.

6

System/subsystem model or 

prototype demonstration in 

a relevant environment.

Representative model or prototype system, which is well beyond that of TRL 5, is tested in a 

relevant environment. Represents a major step up in a technology’s demonstrated readiness.

7

System prototype 

demonstration in an 

operational environment.

Prototype near, or at, planned operational system. Represents a major step up from TRL 6, 

requiring demonstration of an actual system prototype in an operational environment such as 

an aircraft, vehicle, or space.

8

Actual system completed 

and qualified through test 

and demonstration.

Technology has been proven to work in its final form and under expected conditions. In 

almost all cases, this TRL represents the end of true system development. Examples include 

developmental test and evaluation of the system in its intended weapon system to determine if 

it meets design specifications.

9

Actual system proven 

through successful mission 

operations.

Actual application of the technology in its final form and under mission conditions, such as 

those encountered in operational test and evaluation. Examples include using the system under 

operational mission conditions.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 42 of 260

Inhalation and Exhalation Sensor Block Evolution

Source Material: PowerPoint

Title: VigilOxTM Evolution

Date: 12/04/2018

Presenter: Lucas Mesmer, Cobham Senior Design Engineer

File Name: 2018_11_30_VigilOx_Evolution.ppt

There are two components of the VigilOx. There is the Inhalation Sensor Block (ISB) and the 

Exhalation Sensor Block (ESB). These two systems collect data independently. Within each 

system there are physical components (hardware) and digital components (software). The 

information below provides the version numbers and known modifications for the hardware and 

software of the ISB and ESB across the various builds between the initial release of the ISB in 

August 2017, ESB in October 2017 to December 2018.

Date Name Block

Softwar

e 

Version

Software Changes

Hardwar

e Version

Hardware Changes

Aug 

2017

Initial 

Release ISB V0.24

Initial Release (United States Air Force

School of Aerospace Medicine 

(USAFSAM))

DEV

Initial Release (USAFSAM) 

Oct 

2017

Update ISB V0.26

•Revised Oxygen (O2) sensor 

implementation to utilize Nano-Fiber

•Added O2 humidity compensation 

capability

DEV

•O2 sensing material changed from 

Ruthenium to Nano-Fiber

Initial 

Release ESB V0.12 Initial Release (USAFSAM) DEV Initial Release (USAFSAM) 

Jul 

2018

Cardinal 

Update

ISB V0.34

•Data Filtering - Improved data filtering 

and detection of “bit-collisions”

•O2 Sensor - Limited maximum reported 

partial pressure to current absolute gas 

pressure

•Humidity Sensor - Revised sensor heater 

control, Bounded by 0 and 100%

•Cabin Pressure Sensor - Implemented 

temperature compensation

•Real-Time Clock - Improved long-term 

stability (when used in conjunction with 

AMPSS TOOL Rev D or later)

•Micro-SD Card - Improved robustness 

of usability checks

•Status LED - New warning/message 

scheme

•Password Protocol - Added password 

protection on certain software constants 

and calibration tables

DEVA

•Circuit Board - Replaced various 

components to improve sensor bus 

voltage stability and long-term O2

sensor stability

ESB V0.27

•CO2 Sensor - Tare function available for 

calibration and improved heater control

•Flow Sensor - Limited maximum flow to 

400 slpm (-)

•Mask Pressure Adapter and Tube 

Retainers

•Exhalation Tube Adapter Ring

•Battery Divider - Chamfer 

increased to ease installation and 

removal of batteries

•ON/OFF Button
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 43 of 260

Date Name Block

Softwar

e 

Version

Software Changes

Hardwar

e Version

Hardware Changes

•Humidity and Gas Temperature 

Sensor Board - Porous sock 

installed

Oct 

2018

Secondary 

Update

ISB V1.01

•O2 Sensor - Calibration tables and partial 

pressure of O2 calculation updated to 

accommodate new O2 sensing material

A

•Envelope - Increased slightly to 

accommodate O2 sensor and 

hardware changes

•Circuit Board - Updated 

components and layout to 

accommodate new O2 sensor and 

provide further improvements to 

long-term O2 sensor stability

•O2 Sensor Redesign:

o Transmissive (same as ESB)

o Next-Gen Sensing Material

▪Humidity insensitive:

▪Altitude insensitive – no change

▪Temperature sensitive – no 

change

▪Little to no photo-bleaching or 

aging

▪Response time similar to 

previous sensors

▪Increased resolution at low O2

concentrations (<190 mmHg 

Partial Pressure of Oxygen 

(ppO2))

▪Decreased resolution at high O2

concentrations (>190 mmHg 

ppO2)

▪Recommended usage time 

between calibrations not yet 

determined

•Flow Straightener Retention

•Power Button - Button and cover 

redesigned to improve tactile feel

•Battery Compartment - Resized to 

better fit battery

ESB V1.01

•Mask Pressure Sensor - Will tare 

simultaneously with flow sensor

•Bootloader - Minimizes occurrence of 

solid red light on startup

A

None
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 44 of 260

VigilOX Change Logs and Known Issue Revisions from Cobham

Source Material: Excel Sheet

Title: VigilOx Evolution

Date: 12/11/2018

Presenter: Lucas Mesmer and Zoe Rabinowitz, Cobham Engineering

File Name: 2019_04_22_VigilOx_Evolution.xlsx

ISB Hardware Change Log (2018-12-11)

Aug-17 Oct-17 Jul-18 Oct-18

REV ISB (DEV) No Change ISB (DEVA) ISB (A) Hardware

- Initial 

Release

- Circuit board components 

changed to improve: battery 

voltage stability and detection, 

long-term O2 sensor stability

- Retention ring added to flow straighteners 

to prevent flow straighteners from 

inadvertently being pushed into tube

- Initial 

Release

- Implemented Next-Gen O2 sensor and 

applicable photo-filters; changed from 

reflective to transmissive sensing, 

shortened laminar flow plate, external 

protective cap added

- Initial 

Release

- Circuit board components updated to 

accommodate next-gen O2 sensor and 

further improve O2 sensor long-term 

stability

- Initial 

Release

- Orifice resized such that overall pressure 

loss, when compared to previous hardware 

versions, is maintained

- Initial 

Release

- Envelope increased to accommodate NextGen O2 sensor

- Initial 

Release

- Improved ON/OFF button's tactile feel

- Initial 

Release

- Battery compartment resized to allow less 

battery movement

- Initial 

Release

- Circuit board cover hardware change from 

flat head to pan head screws

- Initial 

Release

- Manufacturing defect: RTC negative 

terminal makes tenuous contact with 

battery and may need to be sent to 

manufacturer for repair

- Initial 

Release

- Manufacturing defect: Clocked pin press 

fit may not be to print causing loose or 

missing pins. Continually check integrity and 

send to manufacturer for repair if necessary
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 45 of 260

ESB Hardware Change Log (2018-12-11)

Aug-17 Oct-17 Jul-18 Oct-18

REV ESB (DEV) ESB (DEVA) ESB (-) ESB (A) Hardware

- Initial 

Release

- Circuit board 

components 

changed to improve:

-battery voltage 

stability and 

detection

- long-term O2

sensor stability

- Mask pressure adapter 

reconfigured to remove pig-tail; 

female and male connections 

were reversed

- Adapter body changed from 

polygonal to rectangular profile

- Implemented Next-Gen O2 sensor and 

applicable photo-filters

- Initial 

Release

- Mask pressure tube retainers 

changed from elastic rings to 

molded retainers to securely grip 

tube

- Circuit board components updated to 

accommodate next-gen O2 sensor and 

further improve O2 sensor long-term 

stability

- Initial 

Release

- Exhalation tube adapter ring 

retention set-screws 

reconfigured to prevent damage 

to exhalation valve threads

- Set screws replaced with 

button-head screws

- Envelope increased to accommodate 

next-gen O2 sensor

- Initial 

Release

- Battery divider chamfer 

increased to ease installation and 

removal of batteries

- Initial 

Release

- Improved ON/OFF button's 

tactile feel

- Initial 

Release

- Porous sock installed over 

humidity and gas temperature 

sensor board to repel water 

droplets

- Initial 

Release

- Manufacturing defect: RTC negative 

terminal makes tenuous contact with 

battery and may need to be sent to 

manufacturer for repair
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 46 of 260

ISB Software Change Log (2018-12-11)

Aug-17 Oct-17 Jul-18 Oct-18

Version Number ISB V0.24 ISB V0.26 ISB V0.34 ISB V1.01

Sensor

Real Time 

Clock

- Initial 

Release

- AMPSS TOOL Version D and later 

allows for minor adjustments of 

clock to bound drift by ~+/- 2.6 

seconds per 24 hours

Flow - Initial 

Release

Gas (Line) 

Pressure

- Initial 

Release

Gas (Line) 

Temperature

- Initial 

Release

Humidity - Initial 

Release

- Revised humidity sensor heater 

control to prevent heater overshoot

- Bounded sensor by 0 and 100% RH

O2

- Initial 

Release

- Revised O2 sensor 

implementation to utilize 

Nano-Fiber

- Added O2 humidity 

compensation capability

- Limited maximum reported partial 

pressure to current absolute gas 

pressure measurement

- Calibration tables and 

partial pressure of O2

calculation updated to 

accommodate Next-Gen 

O2 sensing material

Cabin 

Pressure

- Initial 

Release

- Implemented temperature 

compensation

Cabin 

Temperature

- Initial 

Release

Acceleration - Initial 

Release

Data Filtering - Initial 

Release

- Improved data filtering to detect 

and eliminate "bit-collisions"

- Filter settings are configurable for 

each sensor

Status LED - Initial 

Release

- Implemented new 

warning/message scheme

Miscellaneous - Initial 

Release

- Improved robustness of Micro-SD 

Card usability checks

- Implemented password protocol to 

protect certain software constants 

and calibration tables
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 47 of 260

ESB Software Change Log (2018-12-11)

Oct-17 Jul-18 Oct-18

Version Number ESB V0.12 ESB V0.26, V0.27, V0.28 ESB V1.01

Sensor

Real Time 

Clock - Initial Release

- AMPSS TOOL Version D and later 

allows for minor adjustments of clock 

to bound drift by ~+/- 2.6 seconds per 

24 hours

Flow - Initial Release

- V0.26 and later: limits maximum 

reported flow to 400 slpm

- V0.27 and later: References correct 

high pressure sensor for flow 

measurements above ~200 slpm

Gas (Line) 

Pressure - Initial Release

Gas (Line) 

Temperature - Initial Release

Humidity - Initial Release

- Revised humidity sensor heater 

control to prevent heater overshoot

- Bounded sensor by 0 and 100% RH

O2 - Initial Release

- Limited maximum reported partial 

pressure to current absolute gas 

pressure measurement

- Calibration tables and 

partial pressure of O2

calculation updated to 

accommodate Next-Gen 

O2 sensing material

Cabin 

Pressure - Initial Release - Implemented temperature compensation

Cabin 

Temperature - Initial Release

Acceleration - Initial Release

Carbon 

Dioxide - Initial Release

- Factory tare function available for 

calibration

- Heater control adjusted to minimize 

overshoot of temperature control

Mask 

Pressure - Initial Release - V0.28 and later: tares simultaneously with flow sensor

Data Filtering - Initial Release

- Improved data filtering to detect 

and eliminate "bit-collisions"

- Filter settings are configurable for 

each sensor

Status LED - Initial Release - Implemented new warning/message 

scheme

Miscellaneous - Initial Release

- Improved robustness of Micro-SD 

Card usability checks

- Implemented password protocol to 

protect certain software constants 

and calibration tables

- Bootloader updated to 

minimize occurrence of 

solid red light on startup
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 48 of 260

Appendix 4: Pilot Physiology

Standards, Hyperoxia and Hypoxia

Standards have been developed steadily to provide adequate breathing gasses to combat the 

effects of altitude on the human aircrew members. World War II saw the rapid expansion of 

knowledge in the development of countermeasures and standards to prevent the medical 

consequences of altitude. At the outset of the war, aircraft were neither pressurized nor heated. 

The aircraft had to fly as high as possible to avoid ground fire. Flying at 25,000 to 30,000 ft, the 

crews suffered hypoxia from the lack of oxygen (O2) and decompression sickness from the low 

pressure. Hypobaric chambers were developed to study the effects and develop these standards.

Today’s high-performance aircraft are pressurized during flight in order to provide acceptable 

pressure and thermal environments. Hypoxia during flight in these aircraft is prevented by 

delivering appropriate concentrations and pressure of O2 in relation to the cabin altitude. The 

delivery of 100% O2 as a routine in the flight environment has advantages and disadvantages. 

The United States Air Force (USAF) uses breathing systems that provide a mixture of O2 and air 

until 10,000 ft and then 100% O2 is delivered. Hypoxia is a critical hazard that follows a failure 

of the life support systems to provide adequate O2 relative to the altitude and dynamic flight 

environments of fighter aircraft. Modern 4th and 5th generation fighter aircraft fly higher than 

previous generations of aircraft. Above 40,000 to 43,000 ft, loss of cabin pressure requires the 

immediate provision of breathing gas with 100% O2 and an appropriate pressure to supply the 

alveolar O2 concentration to prevent the immediate onset of hypoxia. Numerous studies have 

investigated the relationship between the concentration of O2 supplied in the inspired gas and the 

cabin altitude. Also, work has been done on prevention of hypoxia and pressure breathing 

employing partial-pressure suits with oro-nasal masks. The results were encompassed in the Air 

Standardization Coordination Committee (Air Standards 61/101/6A and 61/101.1C) and the 

NATO Military Agency for Standardization (STANANG 3863).

Fighter aircraft in service today and for the future utilize the maximum differential pressure of 

5.0 Lb in2for aircraft altitudes above 23,000 ft. Numerous physiological factors influence the 

specifications of the gas composition for the aviator. Liquid O2 systems utilize Nitrogen as the 

diluent gas as it is readily available for mixing prior to delivery to the pilot. Molecular sieve 

concentrators a breathable mixture that contains argon as well as O2 and nitrogen. The maximum 

concentration of argon is 5 to 6%. Studies done by Cooke et al. showed that these lower 

concentrations of argon had no discernable physiological effect and it was safe as a diluent gas.

The standard concentration of O2 at a minimum to avoid hypoxia should be equal to or greater 

than the partial pressure of oxygen (ppO2) in the alveolar gas at sea level. This equates to 

103 mmHg. Another factor to consider is to prevent impairment of the aircrew if a sudden loss of 

cabin pressure occurs at high altitude. If a rapid decompression above 30,000 ft occurs, the rapid 

development of hypoxia will ensue and incapacitate the pilot until such time as adequate O2 to 

the alveolus is provided and the cells can recover function.

Breathing high concentrations of O2 in the operational environment of highly maneuverable 

aircraft significant disadvantages. It produces acceleration induced atelectasis in addition to 

absorption atelectasis and delayed otic barotrauma. The Royal Air Force (RAF) in the 1950s 

used pressure demand non-dilutional regulators that delivered 100% O2. Reports revealed that 

this produced symptoms of dry coughing accompanied by a sense of difficulty breathing. These 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 49 of 260

occurred during and after flights, especially those exposed to +Gz accelerations with G-trousers. 

Also notable were the episodes of substernal chest pain and tightness in the chest. (A similar set 

of symptoms occurred with the pressurized high concentrations of O2 used from startup to post 

landing in the F-22.) The coughing was usually provoked by attempts to take deep breaths wither 

in flight or upon standing and exiting the cockpit post flight. The coughing and shortness of 

breath lasted from a couple minutes to episodes lasting 10 to 30 minutes. Several studies 

conducted after this showed that 80 = 85% of pilots had the symptoms with inhaled 100% O2 and 

exposure to +Gz loads above 3 to 4 G. Chest radiographs revealed marked collapse of the basal 

segments of the lungs. The atelectasis remained until the individual took deep breaths or 

coughed. The chest x-ray returned to normal usually within 10 to 30 minutes, but occasionally 

these findings remained for over 24 hours. Centrifuge studies confirmed that absorption 

atelectasis by breathing 100% O2 combined with acceleration atelectasis with a +Gz load greater 

than 3 to 4Gs is significantly worsened by the inflation of G-suits. This is due to the rapid 

absorption of gas in non-ventilated alveoli in the base of the lungs. The nitrogen is replaced with 

O2 from breathing high concentrations of O2. Nitrogen is not absorbed into the circulatory 

system as acts as a splint to prevent alveolar collapse. The +Gz acceleration increases the weight 

of the upper lung segments resulting in the compression of the lower segments. This results in 

closing the small and intermediate airway passages. This is accentuated by the inflation of the 

abdominal bladder of the G suit.

If air is breathed prior to exposures to high Gz loading the nitrogen in the non-ventilated alveoli 

maintains the patency of the alveoli while under acceleration loads. Nitrogen has a far lower 

solubility in blood, and it acts as a break on the total absorption of gas from the alveoli. If the 

inspired gas is 100% O2, this displaces the nitrogen in the alveoli. Since the remaining gas in the 

alveoli is principally O2, the blood picks up this O2 rapidly and thus causes the alveoli to 

collapse. The surface forces cause the alveoli to remain collapsed until they are reopened by deep 

inspiration or coughing. This is referred as absorption atelectasis. In animal studies, Rahn and 

Dale showed that the rate of absorption from non-ventilated alveoli is increased by 60% when 

100% O2 is inhaled rather than air. RAF and USAF flight and lab studies established clearly that 

significant acceleration atelectasis does not occur with inhaling > 40% nitrogen. Thus, the 

combination of absorption and acceleration atelectasis has an extremely significant impact in the 

high G aircraft environment. With the introduction of molecular sieve concentrators, there is an 

increase in the amount of argon produced in the inhaled gas mixtures. Haswell et al. studied the 

effect of argon in the gas mixtures and showed that it was just as effective as nitrogen alone in 

prevention of acceleration atelectasis. Ernsting et al. in animal studies showed that up to 

25,000 ft, 40% nitrogen was required to prevent acceleration atelectasis. Flight studies also 

confirmed that 40% nitrogen was required to inhibit acceleration atelectasis up to a pressurealtitude of 20,000 ft. Since 1960, the RAF has required that O2 concentration will not exceed 

60% at cabin altitudes less than 20,000 ft. The specifications for the molecular sieve concentrator 

for the RAF Harrier GR Mk 5 aircraft specified the minimum nitrogen concentration as 40% or 

greater below 16,000 to 25,000 ft. The concentrator of the AV-8B derivative of the Harrier 

delivered 94% O2 and this resulted in prominent acceleration atelectasis with moderate Gz loads.

Venous blood continues to flow through the atelectatic segments and produces a right to left 

shunt. A right to left shunt is usually a cardiac abnormality in which O2-poor blood gets from the 

right half of the heart into the left side and thus into the systemic circulatory system. In this case 

it is the lung segment collapse that causes the O2-poor blood to return to the heart simulating a 

heart defect. The consequence of this is dependent on the magnitude of the acceleration 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 50 of 260

atelectasis. Moderate exposures of 4 Gz with 100% O2 can result in a right-to left shunt of 20 to 

25% unsaturated blood in the systemic output of the heart. (Green et al.). This would not have a 

significant effect with 100% O2 at low altitudes but would produce a severe condition and 

significant hypoxia if the alveolar PO2 fell below 103 mmHg. The above reasons were the 

driving force behind limiting sea level O2 to < 60% in inspired gas for the RAF and USAF. The 

United States Navy (USN) has employed 100% O2 in combat aircraft. Ostensibly, this is to 

provide protection against toxic fumes and against drowning upon entry into the water via a 

crippled aircraft or parachute. To date no studies have shown a benefit of this strategy in 

comparison to the deleterious effects of acceleration atelectasis.

No long-term deleterious effects have been previously found if acceleration atelectasis develops 

repeatedly in flight. The re-inflation with deep breathing or coughing is effective in reversing the 

atelectasis once breathing air at sea-level pressures post flight. The chest discomfort and the 

coughing make this condition unacceptable. If coughing occurs during dynamic flight it can over 

pressurize lung segments causing alveolar damage. Also, if the inhaled gas drops significantly, 

this can worsen the right to left shunt (hypoxia) as a significant portion of the lung can be 

utilized to compensate for the drop in O2 partial pressure.

Hyperoxia (Excessive O2)

A principle concern and area of special mention to be emphasized is hyperoxia in the aviation 

environment. It has been continually found that there are widespread misconceptions on the use 

of high concentrations in the aviation environment. (Note the Hyperoxia section is heavily 

referenced due to the distinct sections of medical evidence). The result of using high 

concentrations of inhaled oxygen (high FiO2) is DAA – Denitrogenation Absorption Atelectasis. 

As noted in the previous section, nitrogen in the alveoli will be washed out and replaced with O2

with high concentrations of O2 are respired. O2 is extremely soluble and in blood and attaches 

rapidly to hemoglobin and thus diffuses rapidly in the pulmonary circulation. Not enough gas 

remains in the alveoli to maintain patency and the alveolus collapses. (Lumb Ab, ed Nunn’s 

Applied Respiratory Physiology, 6th edition. Philadelphia PA, Butterworth-Heinemann/Elsevier, 

2005; Déry, R, Pelletier J, Jacques A, Clavert M, Houde J, Aveolar Collapse induced by 

denitrogenation. Can Anaesth Soc J. 1965, 12(6): 531-557). A contributing mechanism occurs if 

the inspired VA/Q ratio of a lung unit is reduced, a point is reached where the rate at which 

inspired gas entering the alveolus is exactly balanced by gas uptake from the alveolus into the 

blood. This point is known as the critical flow rate (VA/Q) ratio. If the inspired VA/Q ratio is 

less than this, the lung unit will collapse. This is likely when FiO2 is high and the gas uptake is 

large. Anesthesia literature has shown that when an FiO2 of 100% is used after a VCM (Vital 

Capacity Maneuver), atelectasis recurs within 5 min. Recurrence of atelectasis within five min 

after a vital capacity maneuver at FiO2 = 100% or immediately after removal of positive-end 

expiratory pressure (PEEP) at FiO2 = 40% suggests an instability in the alveoli that have been 

collapsed. It is possible that atelectasis, once formed, impedes surfactant function so that such a 

region is prone to collapse again after having been reopened. On the other hand, when 40% O2 is 

used, atelectasis will not recur for at least 40 min. In order to avoid atelectasis formation, lower 

O2 concentration has been used during induction of general anesthesia. With 100% O2, shunt 

increased from 0.3% to 6.5%, with atelectasis formation corresponding to an area of 8.0 cm2. 

With 30% oxygen, shunt increased to only 2.1%, with minimal atelectasis (0.2 cm2). Without 

any pre-oxygenation, no atelectasis was seen directly after induction. In contrast if the FiO2 was 

increased to 100% before intubation, atelectasis appeared. Moreover, increasing FiO2 at the end 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 51 of 260

of surgery to 1.0 before extubation increased atelectasis formation, which persisted in the 

postoperative period. (L. Magnusson and D. R. Spahn. New concepts of atelectasis during 

general anesthesia. British Journal of Anaesthesia 91 (1): 61±72 (2003). David C. Warltier, 

M.D., Ph.D., Editor. Pulmonary Atelectasis. Anesthesiology 2005; 102:838–54. Lena E. 

Andersson et al. Effect O2of Carbon Dioxide Pneumoperitoneum on Development of Atelectasis 

during Anesthesia, Examined by Spiral Computed Tomography. Anesthesiology 2005; 102:293–

9. Atelectasis during Anesthesia: Pathophysiology and Treatment. Rev Bras Anestesiol 2008; 58: 

1: 73-83. Raquel S Santos, Pedro L Silva, Paolo Pelosi, Patricia RM Rocco. Recruitment 

maneuvers in acute respiratory distress syndrome: The safe way is the best way. World J Crit 

Care Med 2015 November 4; 4(4): 278-286. Hedenstierna G1, Rothen HU. Atelectasis formation 

during anesthesia: causes and measures to prevent it. J Clin Monit Comput. 2000; 16(5-6):329-

35)

1) Since the partial pressures of gasses decrease in ascent to altitude, the 

denitrogenation occurs faster. The effects on vital capacity were demonstrated in the 

Royal Australian Air Force (RAAF) document #D18123622 submitted to the root cause 

corrective action (RCCA) Aerospace Medicine and Physiology Team. Vital capacity was 

reduced by a further 15% after flight, to an average of 28% below baseline (but as much 

as 35% in some cases). In aviation it is known that hyperoxia has resulted in complaints 

of sough, dyspnea, and chest pain in aviator’s flying at altitudes between 14,000 and 

20,000 ft for 5 hours or longer. (Gradwell DP. Prevention of hypoxia. In: Ernsting’s 

Aviation Medicine, edited by Rainford DJ, Gradwell DP. London: Hodder Arnold, 2006, 

p. 57–71, C. Dussault, E Gontier, C. Verret, M. Soret, A. Boussuges, G. Hedenstierna, X 

S. Montmerle-Borgdorff, Hyperoxia and hypergravity are independent risk factors of 

atelectasis in healthy sitting humans: a pulmonary ultrasound and SPECT/CT study, J 

Appl Physiol, 2016, 121: 66–77). Dussault et al. revealed that when breathing 100% O2, 

high-grade atelectasis was present by computerized tomography (CT) and was manifested 

by cough and chest pain. After inhaling only 44.5% O2, only a small-grade atelectasis 

was visualized and not manifested by frank symptoms. Dassault also found that 

acceleration and absorption atelectasis are independent of one another i

2) There are numerous deleterious effects of hyperoxia on the cardiovascular system. 

a. Smit et al. in a multivariate analysis of 85 studies, found that arterial hyperoxia 

induced various amounts of vasoconstriction peripherally. The magnitude of the 

constriction was proportional to the level of inhaled O2 and prominent in vessels 

~15 to 25 μm in diameter. Pronounced constriction was seen in muscle 

vasculature, while constriction was seen in the skin and intestines was not as 

prominent.( Smit B, Smulders Y, Eringa e, Oudemans-van Straaten H, Girbes A, 

Wever K, Hooijmans C, Spoelstra A, Effects of hyperoxia on vascular tone in 

animal models: systematic review and meta-analysis, Critical Care2018, 22:189, 

pp1-16). Thompson et al. found that One Hour of Hyperoxia with isocapnea 

increased systemic vascular resistance index (SVRI), reduced Heart Rate, Cardiac 

Index, and stroke index (SI). The effects on SVRI and Cardiac Index persisted for 

up to 1 hour after normoxic inhalation. Numerous other studies have shown 

similar effects. (Asmussen E and Nielsen M. The cardiac output in rest and work 

at low and high O2 pressures. Acta Physiol Scand 35: 73–83, 1955. Daly WJ and 

Bondurant S. Effects of O2 breathing on the heart rate, blood pressure and cardiac 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 52 of 260

index of normal men — resting, with reactive hyperemia, and after atropine. J 

Clin Invest 41: 126–132, 1962. Haque WA, Boehmer J, Clemson BS, 

Leuenberger UA, Silber DH, and Sinoway LI. Hemodynamic effects of 

supplemental O2 administration in congestive heart failure. J Am Coll Cardiol 27: 

353–357, 1996. Whitehorn WV, Edelmann A, and Hitchcock FA. The 

cardiovascular responses to the breathing of 100% O2 at normal barometric 

pressure, Am J Physiol 146: 61–65, 1946). Two other studies have shown these 

cardiovascular effects and that persistent increased peripheral vascular resistance 

remained for 30 to 40 minutes after being placed on normal 21% O2. (Eggers 

GWN, Paley HW, Leonard JJ, and Warren JV. Hemodynamic responses to O2

breathing in man. J Appl Physiol 17: 75–79. 1962. Waring WS, Thomson AJ, 

Adwani SH, Rosseel AJ, Potter JF, WebbDJ, and Maxwell SRJ. Cardiovascular 

effects of acute O2 administration in healthy adults. J Cardiovasc Pharmacol 42: 

245–250, 2003.) Waring et al. pointed out that the arterial and venous saturations 

were 85% and 86% respectively in their studies.

There are peripheral circulation disturbances also notes with high FiO2 inhalation.

Orbegozo et al. found that normobaric hyperoxia significantly altered microcirulation in 

healthy individuals. They found that this decreased the proportion of perfused vessels 

(PPV) from 92% to 66%, perfused vessel density (PVD) from 11.0 to 7.3 vessels/mm, 

perfused small vessel density (PSVD) from 9.0 to 5.8 vessels/mm and microvascular flow 

index (MFI) from 2.8 to 2.0, and increased PPV heterogeneity from 7.5% to 30.4%. 

Muscle oxygen consumption (VO2) decreased from 8.5 to 7.9%/s. Thirty minutes after 

return to air, PPV, PVD, PSVD and MFI remained partially altered. Normobaric 

hyperoxia alters the microcirculation in healthy subjects, decreasing capillary perfusion 

and VO2. (Diego Orbegozo Corté, FlorinPua, Katia Donadello Fabio SilvioTaccone

LeonardoGottinbJacquesCreteuraJean-Louis Vincenta Daniel De Backer, Normobaric 

hyperoxia alters the microcirculation in healthy volunteers, Microvascular Research, Vol 

98, March 2015, Pages 23-28)

b. Seals et al. demonstrated that breathing 100% O2 can decrease sympathetic 

activity (Seals DR, Johnson DG, Fregosi RF. Hyperoxia lowers sympathetic 

activity at rest but not during exercise in humans. Am J Physiol. 

1991b;260:R873–878), Specifically they showed that in healthy humans, 

hyperoxia lowered the efferent sympathetic nerve activity to skeletal muscle 

under resting conditions but it did not have an obvious modulatory effect on the 

nonactive muscle sympathetic nerve adjustments to rhythmic exercise. 

c. Hyperoxia has been found to decrease cardiac output and raises systemic vascular 

resistance (Whalen RE, Saltzman HA, Holloway DH, Jr, McIntosh HD, Sieker 

HO, Brown IW., Jr Cardiovascular and blood gas responses to hyperbaric 

oxygenation. Am J Cardiol. 1965; 15:638–646. Mak S, Azevedo ER, Liu PP, 

Newton GE. Effect of hyperoxia on left ventricular function and filling pressures 

in patients with and without congestive heart failure. Chest. 2001;120:467–473, 

Mak S, Egri Z, Tanna G, Colman R, Newton GE. Vitamin C prevents hyperoxiamediated vasoconstriction and impairment of endothelium-dependent 

vasodilation. Am J Physiol Heart Circ Physiol. 2002; 282:H2414–H2421. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 53 of 260

Reinhart K, Bloos F, Konig F, Bredle D, Hannemann L. Reversible decrease of 

O2 consumption by hyperoxia. Chest. 1991; 99:690–694).

3) Breathing high concentrations of O2 can lead to impaired O2 exchange (Aboab J, Jonson 

B, Kouatchet A, Taille S, Niklason L, Brochard L. Effect of inspired O2 fraction on 

alveolar derecruitment in acute respiratory distress syndrome. Intensive Care Med. 2006; 

32:1979–1986, Dantzker DR, Wagner PD, West JB. Instability of lung units with low 

VA/Q ratios during O2 breathing. J Appl Physiol. 1975; 38:886–895).

4) Hyperoxia has resulted in cerebral blood flow decreases and metabolic rates. As early as 

1947 there were indication that cerebral blood flow was inhibited by high concentrations 

of O2. Kety et al. showed that FiO2 inhaled in concentrations of 85 to 100% was 

associated with a reduction in cerebral blood flow of 13%. They showed that a FiO2 of 

10% O2 produced an increase of 35% in Cerebral Blood Flow (CBF). Research since that 

time has revealed that inhaling high concentrations of O2 results in decreases in cerebral 

metabolic rates (Xu F, Liu, P, Pascual JM, Xiao G, Lu H, Effect of hypoxia and 

hyperoxia on cerebral blood flow, blood oxygenation, and oxidative metabolism, Jou 

Cerb Blood Fl and Met, Oct 2012, p 1909-1998), decreased CBF (Watson NA, Beards

SC, Altaf N, Kassner A, Jackson A. The effect of hyperoxia on cerebral blood flow: a 

study in healthy volunteers using magnetic resonance phase-contrast angiography, Eur J 

Anaesthesiol, 2000, vol. 17 (pg. 152-9)). Breathing high concentrations of O2 can lead to 

impaired O2 exchange (Aboab J, Jonson B, Kouatchet A, Taille S, Niklason L, Brochard 

L. Effect of inspired O2 fraction on alveolar derecruitment in acute respiratory distress 

syndrome. Intensive Care Med. 2006; 32:1979–1986, Dantzker DR, Wagner PD, West 

JB. Instability of lung units with low VA/Q ratios during O2 breathing. J Appl Physiol. 

1975; 38:886–895). Xu demonstrated that hyperoxia decreased cerebral metabolic rate of 

oxygen (CMRO2) by 10.3±1.5% (P < 0.001) with using 50% inhaled O2 and 16.9±2.7% 

(P < 0.001) for FiO2 of 98%. Hyperoxic-induced vasoconstriction is variable and has 

been shown to decrease cerebral blood flow from 9 to 31% (Watson et al.) and up to 40% 

by Decker et al. (Decker, ASMA presentation, Effects of Hyperoxia on MRI and EEG, 

Aerospace Medicine Association Meeting, Dallas Texas. June 7, 2018. Article Pending.) 

Certain areas of the brain may be more susceptible to hyperoxia than other areas. 

Exercise increases CBF but it was found that with hyperoxia, the middle cerebral artery 

did not increase in flow, whereas the posterior cerebral artery did with exercise. (Smith 

KJ, Ainslie PN, Regulation of Cerebral Blood Flow and Metabolism during Exercise, 

Exper Phys, 1 Nov 2017, Vol 102, Iss 11, pp 1356-1371, Smith, K.J., Wong, L.E., Eves, 

N.D., Koelwyn, G.J., Smirl, J.D., Willie, C.K., & Ainslie, P.N. (2012). Regional cerebral 

blood flow distribution during exercise: Influence of O2. Respir Physiol, Neurobiol, 

184(1), pp.97-105). The reduced CBF restricts the excessive amount of O2 going to the 

brain, as the metabolic processes should be supported by the excess O2. Xu et al. (Feng 

Xu, Peiying Liu, Juan M Pascual, Guanghua Xiao, and Hanzhang Lu, Effect of hypoxia 

and hyperoxia on cerebral blood flow, blood oxygenation, and oxidative metabolism. J 

Cereb Blood Flow Metab. 2012 Oct; 32(10): 1909–1918) pointed out that when exposed 

to 98% oxygen the cerebral metabolism decreased by about 17%. Due to the 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 54 of 260

vasoconstriction and decreased cerebral metabolism, a vulnerable state is induced in the 

brain. Bulte et al. reported (Bulte DP, Chiarelli PA, Wise RG, Jezzard P. Cerebral 

perfusion response to hyperoxia. J Cereb Blood Flow Metab. 2007; 27:69–75) observing 

regional perfusion of grey matter even at moderate levels of hyperoxia; however, 

perfusion changes at all O2 levels were relatively mild. Nishimura used stepwise 

increases in FiO2 to 40%, 70%, and 100%. The measured End-tidal carbon dioxide (CO2) 

decreased significantly at 70% and 100% O2. Steady-state cewrevral blood flow velocity 

(CBFV) decreased significantly at FiO2 at or above 40%, while mean arterial blood 

pressure (MBP) was unchanged. Associated with these changes, cerebral vascular 

resistance index increased at 70% and 100% O2. This indicated that even during mild 

hyperoxia, reduction in steady-state CBFV occurs. (Nishimura N, Iwasaki K, Ogawa Y, 

Shibata S. Oxygen administration, cerebral blood flow velocity, and dynamic cerebral 

autoregulation. Aviat Space Environ Med. 2007; 78:1121–1127).

5) Changes in neural activity are not delineated by indirect measures using functional 

magnetic resonance imaging (fMRI), Positron Emission Tomography, or functional 

optical imaging. These studies delineate the vascular effects hyperoxia, but not the neural 

activity. The assumption has always been that the excess O2, despite vasoconstriction and 

down regulation of cerebral metabolism, does not alter neuronal function. Xu et al.

measured the cerebral metabolic rate of oxygen (CMRO2) and observed decreases in 

CMRO2 as previously mentioned. Electroencephalography (EEG) studies have shown 

significant EEG disturbances. Croal et al. used magnetoencephalography (MEG) and 

observed a moderate reduction (3 to 5%) in occipital lobe signal power. They showed 

focal decreases in oscillatory power across the alpha, beta and low gamma bands. 

Interestingly enough was the fact that the T1 imaging in this study did not show isocapnic 

hyperoxia had a significant effect on carotid blood flow. (Croal PL, Hall EL, Driver ID, 

Brookes MJ, Gowland PA, Francis ST (2015), The effect of isocapnic hyperoxia on 

neurophysiology as measured with MRI and MEG. Neuroimage 105: 323–331) Shen 

et al. demonstrated in healthy subjects that resting EEGs had significant alterations. The 

testing was done with resting state and task evoked states and compared to Sham 

controls. They showed hyperoxia suppressed α (8 to 13 Hz) and β (14 to 35 Hz) band 

power (by 15.6 ± 2.3% and 14.1 ± 3.1%, respectively), but did not change the δ (1 to 3

Hz), θ (4 to 7 Hz), and γ (36 to 75 Hz) bands. Thus, the statements of hyperoxia has a 

pronounced effect on brain neural activity. (Sheng M, Liu P, Mao D, Ge Y, Lu H, The 

impact of hyperoxia on brain activity: A resting-state and task-evoked 

electroencephalography (EEG) study, PLoSONE 12 (5): e0176610: 

https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0176610&type=pri

ntable). Decker et al. also reported the α and β decreases in correlate with the fMRI 

decreases in CBF. Wesly and associates studied the effects of short term (15 minute) 

exposures of 100% O2. (Wesley Vuong, Sayeed A.D. Kizuk, Joanna MacLean, Clayton T 

Dickson, Kyle Mathewson, Electrophysiological correlates of hyperoxia during restingstate EEG in awake human subjects, https://doi.org/10.1101/355941). The results showed 

again an increased blood-O2 saturation levels, decreased heart rate, and a slight, nonsignificant, decrease in breathing rate. There were significant neuronal activity changes 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 55 of 260

including decreases in low-alpha (7 to10 Hz), high-alpha (10 to 14 Hz), beta (14 to 30 

Hz), and gamma (30 to 50 Hz) frequency ranges during eyes-opened hyperoxic

conditions. Paradoxical changes in eyes-closed hyperoxia, increased in the delta (0.5 to

3.5 Hz) and theta (3.5 to 7 Hz) frequency bands were apparent together with decreases in 

the beta range. The decreased alpha in eyes opened is often associated with increased 

attentional processing, but the changes in delta and theta indicated an induction of a sleep 

state. These results suggested a state-specific and perhaps opposing influence of shortterm hyperoxia.

6) The Retinal Artery has been shown to have distinct impact with Hyperoxia. WangsaWirawan et al. showed significant constriction of retinal vessels and a reduction in retinal 

blood flow with inhalation of 100% O2. This is in order to maintain a normoxic retinal 

profile of the retinal neurofiber layers. (Norbert D. Wangsa-Wirawan, PhD; Robert A. 

Linsenmeier, PhD Retinal Oxygen Fundamental and Clinical Aspects, Arch Ophthalmol; 

2003, 121 (4) pp54-557). This can be concerning as high concentrations of O2 in 

combination with high levels of illumination can damage photoreceptors. (Ruffolo JJ

JrHam WTMueller HAMillen JE Photochemical lesions in the primate retina under 

conditions of elevated blood O2 .Invest Ophthalmol Vis Sci 1984; 25893-898, Jaffe

GJIrvine RWood ISSeveringhaus JWPino GRHaugen C Retinal phototoxicity from the 

operating microscope Ophthalmology; 1988; 951130-1141). Another significant change 

was noted that during hyperoxia there was a large increase in choroidal PO2. This 

increase is a consequence of the lack of metabolic regulation of choroidal blood flow 

Huan et al. also demonstrated that with 80% inhaled O2 there was a significant reduction 

in retinal perfused vessel density. The reduction was greatest in the macular area than in 

the peripapillary area. They also demonstrated no real change in flow at the foveal 

avascular zone. (Huan Xu; Guohua Deng; Chunhui Jiang; Xiangmei Kong; Jian Yu; 

Xinghuai Sun, Microcirculatory Responses to Hyperoxia in Macular and Peripapillary 

Regions Investigative Ophthalmology & Visual Science August 2016, Vol.57, 4464-

4468.) It was once theorized that hyperoxia may play a part in treating retinal conditions. 

A fundamental misconception countering that premise is that the O2 supply to the tissue 

relies on O2 saturation. This is an incorrect premise as in reality, O2 moves into tissue by 

simple diffusion. This is driven by gradients of PO2, not saturation. The PO2 gradient 

from the choroid is much steeper during hyperoxia, and thus results in a greater portion of 

the retina can be supplied by the choroid during hyperoxia than during normoxia. This 

oxygenation occurs despite the retinal reduction in blood flow. Another concern lies in 

the fact that hyperoxia can be damaging to photoreceptors if combined with high levels of 

illumination. (Ruffolo JJ JrHam WTMueller HAMillen JE Photochemical lesions in the 

primate retina under conditions of elevated blood O2. Invest Ophthalmol Vis Sci. 1984; 

25893- 898 .Jaffe GJIrvine RWood ISSeveringhaus JWPino GRHaugen C Retinal 

phototoxicity from the operating microscope. Ophthalmology. 1988; 951130- 1141). 

7) Recent clinical concerns have been raised about the use of normobaric or hyperbaric O2.

Analysis of emergency department and Intensive Care Unit (ICU) patients has shown that 

outcomes are significant worse in patient with elevated levels of O2. Several papers point 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 56 of 260

to the fact that Emergency Department hyperoxia is an independent predictor of hospital 

mortality. Most of the multivariate studies focus on patients with hyperoxia in the first 24

to 48 hours of admission. Animal studies have shown that the deleterious effects of 

hyperoxia are both time and dose dependent. These animal models have shown that only 

a few hours of hyperoxia provoke pathologic changes in pulmonary mechanics and 

deleterious inflammatory changes. Stolmeijer et al. conducted a clinical literature review 

of hyperoxia in acutely ill patients. The outcomes of neurological and functional states as 

well as mortality were examined. Only one study showed a transient improvement in 

traumatic brain injury, other studies revealed higher mortality rates after cardiac arrests, 

cerebrovascular accidents (CVAs), and traumatic brain injuries. (R. Stolmeijer , H. R. 

Bouma, J. G. Zijlstra, A. M. Drost-de Klerck, J. C. ter Maaten, and J. J. M. Ligtenberg, A 

Systematic Review of the Effects of Hyperoxia in Acutely Ill Patients: Should We Aim 

for Less? BioMed Research International Volume 2018, Article ID 7841295, 9 pages

8) Breathing 100% O2 is well known to cause the development of delayed otic barotrauma 

or barotitis. This can occur in moderate ascent to and descent from altitude. The usual 

course is that the aircrew member will awaken the next day after the flight or altitude 

chamber run with ear pain and moderate deafness. Physical examination reveals the 

tympanic membrane (TM) (ear drum) is drawn inwardly towards the middle ear and this 

usually contains fluid. The deafness and discomfort can be relieved by introducing air 

into the middle ear. This is done by Frenzel’s maneuver or Valsalva maneuver. The 

mechanisms for barotrauma from excessive O2 are similar to the G induced atelectasis. 

With the use of 100% O2 the nitrogen is washed out of the middle ear and replaced with 

O2. The capillaries rapidly absorb the O2 and this reduces the pressure in the middle ear. 

The TM is pulled into the middle ear cavity resulting in pain and deafness. If numerous 

ascents to altitude have been performed (even to 5,000 ft) on 100% O2 without 

reintroduction of nitrogen, a painful retraction of the TM occurs. Also, if a rapid change 

in altitude or frequent changes in altitude occur whether with or without 100% O2, and 

the middle ear is not allowed to compensate for the changes, then a painful barotitis 

occurs. This condition is amplified with breathing 100% O2. In this case both breathing 

> 60% O2.

Standards for Aircraft

In the development of standards, the prevention of absorption and accentuated acceleration 

atelectasis and traumatic barotitis were crucial in highly dynamically maneuvering aircraft. The 

standards were established for the RAF and USAF from the 1950s to the 1990s to keep the 

concentration of O2 should not exceed 60%. There are limits to the altitude range that this can be 

applied. As altitude increases the concentration of O2 must be increased to prevent hypoxia, 

which is of critical importance. There are factors that must be taken into consideration to adjust 

the maximum amount of O2 in relation to the cabin altitude. The first factor to consider is the 

pressurization schedule of the aircraft. The aircrew should only be exposed to cabin altitudes 

greater than 20,000 to 25,000 ft in the case of a decompression of the cabin. These are rare 

events, but the risk increases in combat operations. The primary goal is preventing 

Decompression Sickness. The next factor to consider is the effect of high levels of G forces. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 57 of 260

Many combat aircraft cannot maintain +Gz accelerations greater than about 3 to 4 G at altitudes 

above 36,000 ft. This limits the amount of acceleration atelectasis that occurs at these altitudes. 

This should equate to cabin altitudes > 15,000 ft. The next factor to consider is that it is 

technically difficult to match the narrow physiological limits if the O2 concentration is not 

allowed to exceed 60% at altitudes greater than 15,000 ft. The minimum concentration of O2 at a 

cabin altitude of 18,000 ft is 49%. (Figure 1). Above 30,000- to 33,000-ft cabin altitude, 100% 

O2 is required to maintain alveolar O2 concentration greater than 103 mmHg. Above 40,000 ft,

positive pressure for breathing (additional pressure provided to the mask with the O2) is required 

to maintain the alveolar O2 partial pressure.

Figure 1

From: Raising the Operational Ceiling, Proceedings of a Workshop held at Armstrong 

Laboratory, Brooks AFB, 13-15 June 1995

Relationship between concentration of O2 in the inspired gas and the pressure altitude within the 

cabin required to:

1) Maintain an alveolar PO2 of 103 mmHg

2) Produce an alveolar PO2 of 30 mmHg on an instantaneous decompression from the cabin 

altitude denoted on the X axis to the cabin altitude/absolute intrapulmonary pressure 

denoted by the broken horizontal curves [final cabin altitudes or 30, 35, 37.5 and 40

thousand ft and final intrapulmonary absolute pressures of 141 mmHg (40,000 ft Curve) 

125, and 110 mmHg] This is the minimum concentration of O2 to prevent hypoxia on a 

subsequent rapid decompression from the altitude of flight. An alveolar PO2 of < 30 mmHg, 

even for a few seconds will result in unconsciousness.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 58 of 260

3) Ensure with a cabin pressure differential of 5 Lb. in2, that an instantaneous decompression 

form the cabin altitude on the X axis the alveolar PO2 immediately after the decompression 

will be 30 mmHg when

a. using a pressure breathing system which provides a breathing pressure of 30 mmHg 

at 50,000 ft and

b. using a pressure breathing system which provides a breathing pressure of 70 mmHg 

at 60,000 ft

Combining all the relationship to the cabin altitude, the scheduling of the cabin, the G forces 

involved, and the technical difficulties lead to the limitation should not exceed 60% O2 from 

ground level to 15,000-ft cabin altitude. This limit should not exceed 75% O2 from 15,000 to 

20,000 Ft cabin altitude. These requirements were incorporated into Air Standards 61/101/6A 

and STANAG 3865. (Figure 2).

Figure 2. Relationships between minimum and maximum concentrations of O2 in inspired gas 

and the cabin altitude with the cabin pressurized for a typical agile combat aircraft with a ceiling 

of 50,000 ft.

From: Raising the Operational Ceiling, Proceedings of a Workshop held at Armstrong 

Laboratory, Brooks AFB, 13-15 June 1995.

As an example of not addressing physiological standards, for the F-22, the normal standards in 

design and development were not followed. The USAF found that through early 2012, there had 

been an increasing number of hypoxia-like incidents in the F-22 Raptor. This was surmised as 

being related to the On-Board Oxygen Generating System (OBOGS) or its installation. There 

were numerous concerns raised by the USAF Safety Accident Boards and the NASA 

Investigation into the F-22 investigation of the incidents. These findings were summarized in the 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 59 of 260

USAF Scientific Advisory Board Report on Aircraft Oxygen Generation, February 1, 2012. 

dsp.dla.mil 31.

1) An applicable multi-national standardization document from the Air and Space 

Interoperability Council (ASIC; formerly Air Standardization Coordinating Committee)—

currently ASIC Advisory Publication 4060, ‘The Minimum Quality Criteria for On-Board 

Generated Oxygen’—was called out as advisory guidance for the F-22. 

2) The USAF substantially diminished its application of systems engineering and reduced its 

acquisition core competencies (e.g., systems engineering, human systems integration (HIS), 

aviation physiology, cost estimation, contracting, and program and configuration management). 

3) Lost capabilities and expertise to perform the critical function of HSI led to atrophy of 

policies/standards and research and development expertise with respect to the integrity of the life 

support system. 

4) Three life support system-critical subsystems (OBOGS, Back-up Oxygen System [BOS], and 

Emergency Oxygen Subsystem [EOS]) were not classified as “safety-critical items” and were 

integrated or eliminated without sufficient analysis. 

5) Modeling, simulation, and integrated hardware-in-the-loop testing to support the development 

of the F-22 life support system and the thermal management system were insufficient to provide 

an ‘end-to-end’ assessment of the range of conditions likely to be experienced by the F-22.

6) The OBOGS was developed as a “fly-to-warn/fail” system with no requirement for initial or 

periodic end-to-end certification of the breathing air or periodic maintenance and inspection of 

key components.” This led to the development and implementation of “ ...a comprehensive 

Aviation Breathing Air Standard to be used in developing, certifying, fielding, and maintaining 

all aircraft oxygen breathing systems.”

(http://www.dsp.dla.mil/Portals/26/Documents/Publications/Journal/160301-DSPJ-06.pdf) 

These were incorporated into the Military Standard (MIL-STD) 3050 document. “For fighter and 

trainer aircraft the O2 concentration delivered by the breathing system using OBOGS shall be 

above the O2 warning threshold at steady-state breathing gas flows from 7 to 60 

liters/minute/crew member Ambient Temperature and Pressure Dry (ATPD) from Sea Level to a 

cabin altitude of 7,999 feet; and 2) 7 to 80 liters/minute/crew member (ATPD) from a cabin 

altitude of 8,000 feet to the aircraft maximum ceiling. (NOTE: If the breathing system using 

OBOGS will supply O2 to a demist system, for example as part of a chemical defense ensemble, 

the steady-state flow should be increased accordingly.) The system shall be capable of achieving 

peak inspiratory and expiratory flows of up to 4.3 liters/sec ATPD (258 L/min ATPD)” 

and“...the concentration of oxygen in the inspired gas shall not exceed 60% at cabin altitudes 

between 0 and 15,000 ft or 75% at a cabin altitude of 20,000 feet, except momentary 

excursions...” This again reinforces the findings that have been made historically by aviation 

physiology studies. The reports did address some of the physiological issues, but there were 

many open items not addressed. This study’s findings accelerate closing the knowledge gaps on 

many questions. 

The above background makes it imperative that this be reflected in any standards going forward. 

The Aircrew Breathing study has delineated breathing system dysfunctions that compound the 

effects of aviation in the normal and dynamic regimes. It would seem that in aircraft programs 

problems that affect the performance of the human through the human-machine interface have 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 60 of 260

either not been identified early in the design phases, and/or have not been acted upon once 

discovered. There is no doubt that the twin pressures of budget and schedule exert a tremendous 

force in the programmatic management of any large-scale system. And based on the fundamental 

human research performed by the military and others over the years gone by, it is tempting to 

think that all of the human performance requirements for a given system are known, thereby 

rendering further funding for human testing and design change unnecessary. Yet when 

assumptions on human performance are made, designers may not realize where these 

assumptions are not completely valid. That is, if the assumptions underlying requirements are 

incorrect and/or are based on an incomplete understanding of human physiology, then such 

assumptions can be very costly in the long run. The continuing belief that Hyperoxic 

concentrations of O2 seem to validate these misunderstandings. There must be requirements 

established to adequately meet the actual demands of aircrew. Furthermore, subsequent changes 

to the aircraft that can affect the human within this system; however, there has to be significant 

effort to re-assess the human design requirements or to determine if those requirements actually 

meet the current demands of the aircrew operating the system. In short, there must be a HSI

process. These are findings that have been identified in the F/A-18 and F-22 NESC Life Support 

Systems reviews.

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FRC and occlusion pressure in human subjects. Rap. Physioi., 33,241. 

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by breathing oxygen and increased gravitational force. PH.D. Thesis, London University. 

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NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 62 of 260

Manco, J. C, and Hyatt, R. E. (1975). Relationships of air trapping to increased lung recoil 

pressure induced by chest cage restriction. Am. Rev. Rapir. Dis., 111,21. 

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pulmonary volume-pressure hysteresis. /. Appl. Physiol, 10,191. 

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volume, closing volume and gas exchange. /. Appl. Physiol., 52,1453. 

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(1967). Radiologically undetectable pulmonary collapse in the supine position. Lancet, 2, 

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expiratory flow in normal subjects. /. Appl. Physiol., 32, 325. 

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lung function. /. Appl. Physiol., 39, 707. 

Wagner, P. D., Laravuso, R. B., Uhl, R. R., and West, J. B. (1974). Continuous distributions of 

ventilation-perfusion ratios in normal subjects breathing air and 100% Oj. /. CUn. Invest., 

54, 54. 

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nitrogen on arterial POj. Anaesthesia, 22, 69. 

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nitrogenation during anesthesia
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Appendix 5: Development of JPL Mask

5.1 Structural Integrity Memo

Lance E. Christensen, Jet Propulsion Laboratory

3/9/2019

1. Background:

The Jet Propulsion Laboratory (JPL) is constructing, in-house, an in-mask carbon dioxide (CO2)

and water vapor sensor (IMCWS) that uses tunable laser spectroscopy to probe the breathable air 

of the pilot. Below are specific details regarding the structural components of this sensor. Much 

of this material was reviewed at the IMCWS design review November 7, 2019.

It should be noted that the entire end-to-end structure, mounted inside the mask with connected 

wiring, has been subject to a windblast test and rapid decompression test. It passed both tests.

2. Specific bondings and housings

A. Bonding of main sensor body to valve

The Delrin surface of the valve is abraded on the surface as shown above. The matching surface

of the aluminum main sensor body is also abraded. Then ScotchWeld Epoxy 2216 A/B is applied 

to these surfaces as shown in Figure 1. The epoxy is cured over 7 days at room temperature. The 

total surface area of bonding is 0.5 in2. The total mass of material on the valve is 35 grams.

Using a loading of 40 G, as prescribed by Armstrong Flight Research Center (AFRC) and based 

on MIL-A-8865 Rev B, outward along the flow axis of the exhalation valve, the force on the 

adhesive bond due to the IMCWS mass is: 40 (0.077 lb)/.50 in2 = 6.2 psi.

B. Bonding of Laser and Detector to main sensor body
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The laser is first bonded to an aluminum laser mount (gold item in Figure 2) via Epo-Tek H20E 

epoxy. The threads (7/16-30 UNF (Unified National Fine)) of the aluminum housing are then 

coated with Solithane 113/300 TYPE B and then the laser mount is screwed in to 2 ft-lb torque.

The detector is bonded directly to housing with 2216 epoxy.

The electrical sockets and wires to the laser and detector are bonded together with housing, 

sockets and wires via Arathane 5753. The worst-case stress in bond at 40 G is 1.4 psi (detector

mass 0.6 g = 0.0013 lb and Bond area = 0.038 in2).

C. Structural integrity of Laser and Detector

The figure shows the laser, which does have a hermetically sealed inside volume. The detector 

does not have sealed air volumes.

There is a borosilicate window attached via Nanoplus epoxy to a Kovar housing. JPL conducted 

two rapid decompression tests and observed that the laser structure (as well as detector) stayed 

intact. That is, rapid decompression did not affect the laser. The rapid decompression test results 

are in a separate document.

D. Bonding of pressure sensor to main body

The pressure sensor is bonded with 2216 epoxy to the main body. The rapid decompression test 

as well as the windblast test demonstrated that all parts of the pressure sensor stayed intact 

during those tests.
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5.2 ICWMS Bill Of Material (BOM): Wetted components

ICWMS BILL OF MATERIAL (BOM): Wetted components

Component part Material

Sensor Housing Aluminum

Gentex Exhalation Valve Delrin Plastic

Laser (hermetically sealed 

package)

Gold/Nickel Plated Kovar, borosilicate window

Detector Gold/Nickel plated Kovar

Pressure Transducer Stainless steel housing, silica gel

PCB Board Fiberglass FR4

Laser and Detector electrical 

sockets

Insulator: Hi-Temp UL 94V-O Terminal: Brass, per ASTM-B16 

Contact: BeCu, Per ASTM-B19, electroplated NiAu coated

Thermistor 3” Solid Nickel Wire Leads, Teflon Insulation, baked-on phenolic 

plastic

Solders SnPb 63/37

Black ink coating on main 

housing

Enthone M-0-C (aka Warnow Ink)

Wire coatings ETFE

Glenair connector Hysol epoxy EE4215 potting compound

Bonding Epoxies ARATHANE 5753A/B (retains PTFE wires)

Scotch Weld EC 2216 A/B (main body to Gentex valve)

SOLITHANE 113/300 TYPE B (threads for heat sink to main body)

Epo-Tek H20E (laser to heat sink)
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5.3 AFRC Laser Evaluation and Recommendation Report
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5.4 Rapid Decompression Test of In-Mask CO2 Water vapor Sensor (IMCWS)

Lance E. Christensen, Jet Propulsion Laboratory

2/11/2020

I. Goals

The main goal is to test the IMCWS in a rapid decompression (RD) environment (differential 

pressure (ΔP) = 5 psia, ≤ 0.1 s) to understand how IMCWS may impact pilot safety in an ejection 

event. IMCWS consists of a sensor head with a laser and detector and a pressure-temperature 

sensor as well as a separate vented electronics box that is linked to the sensor head via an 

electrical cable.

The four components of IMCWS that will be closely studied during this test are the laser, 

detector, pressure sensor, and electronics box. The laser, detector, and pressure sensor (Figure 1) 

are within the mask and it is important to verify that they do not break apart to form hazardous 

pieces (Foreign Object Debris (FOD)) in the event of RD. The electronics box is a potential 

pressure vessel, even though it is vented. In sum, the principle concerns are:

a) FOD generated by the laser hermetically sealed at 1 atm; in particular, the borosilicate glass 

front face of the laser package;

b) FOD generated by the detector; in particular, dislodging of the germanium hyperhemispheric lens over the detector element;

c) FOD from the pressure sensor; in particular, the white porous material over the piezotransducer;

d) The electronics box which could act as a pressure vessel.

II. Test articles

There are two test articles, as shown in Figure 2.

1) The IMCWS sensor head integrated into a Gentex MBU-20P mask.
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2) The IMCWS electronics box which is connected to the IMCWS sensor head via an 

electrical cable (not shown, not to be tested).

The mask weighs 237 g and is 13 × 10 × 14 cm with integrated IMCWS sensor. The mask 

contains the aluminum IMCWS sensor head which in turn holds the laser, detector and pressuretemperature sensor. The sensor head weighs 45 g.

The aluminum electronics box weighs 478 g and is 13 × 10 × 4.5 cm. The electronics box has a 

vent (1/8-NPT, 100-micron 316-ss mesh, max flow 26 scfm at 100 psi). A microSD access port 

of 1.0- × 1.6-inch surface area was epoxied to the front of the electronics box with Devcon 

No.14210 5-min epoxy.

The decompression test will be conducted twice. Each test will have a different IMCWS sensor 

head integrated into its own unique mask: WB#1 and WB#2. Each test will test the same 

electronics box. Table 1 lists the specific test articles for each test.

Table 1. Test Articles for Rapid-Decompression Tests

RD Test 1 RD Test 2

Sensor Head/Mask: WB#1 WB#2

Electronics Box: Science Box #1 Science Box #1

III. Test procedure

The test articles will be tested at KBR San Antonio. Test articles are to be placed inside an 

unoccupied rapid decompression chamber, denoted B-3. The test articles will be unpowered.

Once the articles are placed in the rapid decompression chamber, the pressure-altitude will 

ascend to 22,000 ft at 5000 fpm, then additionally ascend to 60,000 ft at a rate of 5000 fpm.

Once at 60,000 ft, the chamber will be brought back to 22,000 ft at a rate of 5000 fpm. Once at 
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22,000 ft, the explosive decompression test will be performed where chamber will ascend to 

60,000 ft such that 90% of decompression occurred in ≤ 0.1 s. Following this, the chamber will 

slowly de-pressurize to room pressure at a rate of 5,000 fpm. Cameras outside the chamber will 

provide information on the performance of the test articles during the pressurizations/depressurizations. 

III. Test results

Figure 3 documents the pressure-altitude time-series generated from each identical test. A posttest visual inspection was performed with observations documented by JPL.

Videos at 25 fps were taken of both RD tests. In addition, Lance Christensen (JPL) watched the 

test items during the test. Further, after the test was completed, Lance Christensen examined the 

test items and looked for debris inside the chamber.

Video and visual observation revealed no deformation or FOD generation from the RD. The 

white porous material covering the MS5803 pressure-temperature sensor remained in place for 

both optical head units. It is important to note that RD induces mist formation which reduces 

visibility by around 10 to 20% as can be observed in the video.

There were no FOD within the RD chamber but if FOD were generated, there is chance that it 

might have been rapidly sucked into the accumulation chamber connected to the RD chamber too 

quickly to be observed.

The most reliable determination of test results is from post-test examination at JPL on February 

8, 2020, and February 10, 2020. This examination included end-to-end powering of both optical

heads to determine if the laser/detector system worked the same as prior to the RD test. For 

WB#1, the optical head worked exactly the same after the RD test as it did before. However, for 

WB#2, the detector failed to show signal. A second detector was inserted into WB#2, where it 

was discovered that the laser for WB#2 operated the same as before the RD test.
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From visual inspection at JPL of both optical heads, there appears to be no FOD. There appears 

to be no deformation in either optical head or electronics box. There is no indication of loose 

material trapped inside the optical heads or electronics box. The detector in WB#2 appears to be 

intact. It is unclear why the detector on WB#2 does not show signal but it is physically intact.

The conclusions are as follows:

a) The laser package remained intact through RD;

b) The detector package remained intact through RD;

c) The pressure-temperature sensor remained intact through RD;

d) There was no movement, distortion or change in the electronics box through RD.
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5.5 Memos

JPL In-Mask CO2 & Water Sensor (IMCWS)

Memo To Address Hazard Controls for Internal Sensor Components Detaching and Becoming a 

Choking Hazard

Prepared by John Graf (JSC) and Lance Christensen (JPL)

3/2/20

This memo is written to address one of the Hazards identified as part of the AFRC Tech Brief 

Assessment of flight testing the JPL In-Mask CO2 & Water Sensor (IMCWS) as part of the Pilot 

Breathing Assessment. This memo addresses the hazard that improper design and/or manufacture 

could result in an internal component breaking loose and presenting a choking hazard. This 

memo documents the rationale for the project team’s assessment finding and recommendation: 

the JPL In-Mask Sensor effectively controls the choking hazard and it is safe for AFRC flight 

test.

This hazard is controlled by:

Limiting the weight of individual components

Securing the components with appropriate adhesives

Hazard controls are verified by:

Checking the integrity of the bonds and adhesives during assembly

Testing structural integrity under worst case survivable load conditions during windblast 

testing

Testing fracture control integrity under worst cast survivable fracture conditions during 

rapid depress testing.

Order of magnitude structural analysis results in an assessment that components are small and 

well secured – the hazard of choking on loose components is well controlled. Windblast testing 

(the most severe survivable structural load condition) demonstrated structural integrity. Rapid 

depress testing (the most severe survivable fracture condition) demonstrated component 

integrity.

The JPL In-Mask has 6 types on internal components. Each component requires a specific 

assessment. Internal components are:

1. IMCWS Housing

2. Pressure – Temperature Sensor

3. IR Detector

4. Laser

5. Heat Sink

6. Internal Wiring

The largest single component in the IMCWS system that is located inside the mask is the 

IMCWS structural housing. The Housing is shown in Figure 1 and Figure 3. The IMCWS 

Housing is bonded to the Gentex Exhalation valve with 3M Scotch-Weld 2216 epoxy. Because 

the Gantex exhalation valve body is made from Delrin, Scotch-Weld specifications call for an 

abrade and prime surface prep. AFRC specifies a structural analysis following MIL-A-8865, 

RevB – this specifies a 40-g inertial load. The mass of the IMCWS housing is less than 

35 grams, the bond area between housing and valve is 0.50 in2, the calculated force on the 

adhesive bond needs to be 6.2 psi or greater. The rated bond strength of 3M Scotch-Weld 2216 is 
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1700 psi. The IMCWS housing was tested during windblast testing and bond integrity was 

demonstrated.

The pressure-temperature sensor has a mass of less than 0.6 gm. The bond area is 0.038 in2. The 

worst case stress at 40 g is 1.4 psi. The pressure-temperature sensor is bonded using Arathane 

5750 or 5753. The rated bond strength of Arathane 5750/5753 is greater than 500 psi. Sensor 

integrity was tested during windblast testing and bond integrity was demonstrated.

The infrared (IR) detector has a mass of less than 0.6 gram. The bond area is 0.038 in2. The 

worst case stress at 40 g is 1.4 psi. The IR detector is bonded using Arathane 5750 or 5753. The 

rated bond strength of Arathane 5750/5753 is greater than 500 psi. IR detector integrity was 

tested during windblast testing and bond integrity was demonstrated. The detector has an 

additional hazard that needs verification of hazard control. The detector has a Gallium Arsenide 

(GaAs) lens. With sufficient pressure loading the GaAs lens could shatter. The IR detector was 

subjected to rapid depress testing under greatest survivable pressure load conditions, and fracture 

control was demonstrated.

The laser is bonded to the aluminum heat sink using Epo-Tek epoxy. The Laser/Heat Sink 

Assembly is bonded using Arathane 5753. The Laser has a mass of less than 0.6 gram. The bond 

area is 0.038 in2. The worst-case stress at 40 g is 1.4 psi. The rated bond strength of Epo-Tek 

>1200 psi.

The heat sink is bonded to the laser using Epo-Tek epoxy. The Heat Sink / Laser assembly is 

bonded using Arathane 5753. The heat sink has a mass of less than 0.6 gm. The bond area is 

greater than 0.038 in2. The worst-case stress at 40 g is 1.4 psi. The rated bond strength of EpoTek is >1200 psi.

The internal wiring uses 18-gauge wire. The wire weighs less than 0.6 gram. The tensile strength 

of 18-gauge wire is 38 lbs per MIL-T-7928. Acceleration loads needed to impart a 38-lb force on 

less than 0.6 gram of wire are not survivable.

Summary:

The internal components in the IMCWS are robust enough to handle the loads that will be 

experienced during flight testing without any components becoming dislodged.
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Figure 1: Photo showing internal components

Figure 2 Photo Showing External Power Supply and Connecting Cable
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Figure 3 Exploded View Showing the Configuration of the Main Components
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JPL In-Mask CO2 & Water Sensor (IMCWS)

Memo To Address Technical Risk Issues Related to Mask Modifications

Prepared by John Graf (JSC) and Lance Christensen (JPL)

3/2/20

This memo is written to address one of the Technical Review Items identified as part of the 

AFRC Tech Brief Assessment of flight testing the JPL In-Mask CO2 & Water Sensor (IMCWS) 

as part of the Pilot Breathing Assessment. This memo documents the modifications made to the 

mask, and it documents the reasons why the modifications to the mask will not affect mask 

function, performance, or structural integrity. The modifications to the mask are: 1) a connector 

is added to the mask, and 2) internal components with a total weight of less than 35 grams.

The risk issue of leakage through the mask around the connector is addressed by:

The location, mounting technique, and connector type is part of the Gentex mask design –

Gentex manufactures masks that have Glenair connectors in this location.

The mask is fit check, and tested for leaks after manufacture.

The risk issue of leakage around the mask seal, by weighting down the mask is addressed by:

The weight of the internal components inside the mask is less than 35 grams.

The forces on the mask caused by the internal components are substantially less than the 

forces on the mask caused by the addition of the exhalation hose. The exhalation hose has 

been used successfully for all Pilot Breathing Assessment (PBA) flights using the 

VigilOX exhalation sensor block (ESB).

Leakage forces caused by IMCWS are substantially less than leakage forces caused by 

the exhalation hose. Photos of the relevant components are shown in Figures 4, 5, and 6.

Figure 4 Glenair panel mount connector 880-004RB-K19M-M020J5-12
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Figure 5 Glenair plug 880-001PA-K19M-M020J5-48

Figure 6 Photo of Gentex Mask with Glenair Connector
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Memo

To Address Concerns About Oxygen Compatibility and Fire Safety for JPL Mask Sensor 

Components Being Involved in a Fire Triggered by a Spark of Static Electricity

John Graf, NASA JSC

March 12, 2020

Description of Hazard:

The mask environment includes elevated O2 and new components are added to the mask. If the 

new components are susceptible to fire ignition from electrostatic discharge or overheating of an 

electrically energized component – there is a chance that a spark of static electricity could trigger 

a severe O2 fire on the pilot’s face.

Summary Description of Hazard Control:

The materials inside the mask can burn in extreme conditions, but they are hard to ignite in the 

environmental conditions of the mask (>1.1 atm/>50 °C/100% O2). There are no credible 

ignition mechanisms capable of initiating a fire. The lack of ignition mechanism is the primary 

control of this hazard. Note: the laser/detector system used in the IMCWS if flight qualified for 

Crit 1 use – as part of the Laser Air Monitor (LAM).

Review of Ignition Mechanisms:

O2 compatibility assessments survey and assess a set of ignition mechanisms that include:

• Particle Impact

• Heat of Compression

• Flow Friction

• Mechanical Impact

• Friction

• Fresh Metal Exposure

• Static Discharge

• Electrical Arc/Short

• Chemical Reaction

• Thermal Runaway

• Resonance

• External Heating

An assessment of each of these mechanisms is provided below:

• Particle Impact There are no high velocity gases that contain particulate

• Heat of compression There are no compressed gases and no increase of gas 

pressure

• Flow friction Gas velocities are low

• Mechanical Impact There are no large-scale mechanical forces

• Friction There are no moving parts with significant friction

• Fresh metal exposure There is no fresh metal

• Static Discharge Insufficient static energy – see analysis below

• Electrical Arc/Short Insufficient electrical power – see analysis below

• Chemical Reaction No reactive chemicals

• Thermal Runaway No porous materials, no chemical reactions
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• Resonance No acoustic oscillations

• External Heating No external heating, environmental temperatures are low

Static Discharge:

The NASA White Sands Test Facility conducted a set of tests to determine the susceptibility of 

materials to ignition by static discharge. One set of tests were performed at 23.5 psi and 100% 

O2. A fine wire (54 AWG) was placed next to highly flammable materials, including a 100% 

cotton t-shirt, and moleskin. With a fixed voltage of 22.5 V, current levels were increased until 

ignition was achieved, or the limits of the test facility were reached. Cotton t-shirt material did 

not ignite even when maximum power settings were used (8.1 W). Moleskin did not ignite, even 

with maximum power settings were used (6.8 W). These power settings are an order of 

magnitude higher than the power of commonly occurring electrostatic discharge (ESD) pulses.

There is not enough energy in a spark of static electricity to ignite moleskin or cotton t-shirt 

material. The materials used in the JPL sensor are harder to ignite than either moleskin or cotton 

t-shirt material. Static discharge is not a credible ignition mechanism.

From Safe Use of Oxygen and Oxygen Systems: ASTM MNL36. Second Edition, 2007

Electrical Arc/Short or External Heating of Materials to their Auto Ignition Temperature:

Materials can auto-ignite, if they are heated to a sufficient temperature. This analysis considers 

the highest temperature of the materials and compares them to autoignition temperatures. A 

thermal analysis of energized components concluded that the maximum temperature of any 

exposed surface of any material in the JPL sensor (inside the mask) is 50 °C. The power use is 

small, and the device is designed to wick heat away from energized components. The lowest 

rated operating temperature is 400 °C (the rated temperature for Kapton thin film). For the 

purposes of this analysis – the autoignition temperature of gasoline is 247 °C. If components 

were accidently soaked in gasoline – it would be unsafe to expose the materials to 247 °C 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 80 of 260

temperature conditions. Maximum temperature of the JPL sensor materials is 50 °C – well below 

air intake temperature (AIT) (even if the materials were contaminated with hydrocarbons. There 

is too little electrical power to heat JPL sensor components to autoignition temperature.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 81 of 260

Appendix 6: Standardization of Test Flights

Profile H

As a deliverable from this project, the Pilot Breathing Assessment (PBA) team developed a 

“Breathing System Functional Check Flight” profile, called Health check, comprised mainly of 

maneuvers from the other five flight profiles, and designated Profile H. The intent is to offer this 

profile to the flight test community as the first draft of a standard profile, which can be used to 

confirm proper functioning after a ‘problem jet’ has been fixed, to accept a jet out of depot-level 

maintenance, and to test a new or upgraded breathing system. The pilot must be equipped with a 

VigilOX or equivalent data system to record the required parameters.

The profile consists of a short Ground Block before and after the flying portion. The in-flight 

profile was designed with three major climbs and descents of varying speeds as large changes in 

altitude stress the cabin pressurization system as well as the breathing system and pilot. The 

profile is designed to be executed in the order it is written, without deviation if possible, so that 

the data can be compared to earlier check flights of the same aircraft or to any fleet data 

available. 

While designed for the F-18, it should be easy to adapt to other fighter aircraft. The maneuvers 

and profile should be easily learned by any Functional Check Flight (FCF)-qualified pilot. The 

profile requires working airspace from 5,000 ft pressure altitude (PA) up to 30,000 ft PA and 

could be accomplished in airspace with as little as 30 nm between boundaries, though an area 

allowing longer runs would be more efficient. In the F-18, Profile H can be accomplished in a 

single sortie in both single-seat and two-seat aircraft equipped with a centerline fuel tank, if the 

working airspace is nearby. If there is a longer transit to the airspace, wing tanks would likely be 

necessary.

Profile H: Detailed Procedures

Pre-Step:

Approximately 1 hour prior to takeoff, conduct RAD-97 capnography and Spirometry 

measurements seated in a briefing room without flight gear.

Ground Ops:

Strap in normally to the cockpit, but before donning helmet, conduct RAD-97 capnography and 

Spirometry. Don helmet and power on the exhalation sensor block (ESB) and inhalation sensor 

block (ISB). Don the mask and Event Mark. Leave the mask up until after engine shutdown, 

except when specified on a test card. Conduct normal engine start and checklist through Before 

Taxi, ensuring aircraft bus data recorder is powered on.
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Card 1: Ground Block 1

A. Baseline Breathing [ON/NORM/NORM (USAF) / ON (Navy)]: Accomplish this card with 

the canopy UP.

A1. Event Mark and breath normally for 3 minutes, noting any abnormalities with the 

mask valves or regulator.

A2. Event Mark and lower the mask, breathing normally for 2 minutes.

A3. Raise the mask, conduct an Event Mark, and ensure a good seal. Perform a PRICE 

check (USAF only). Breathe normally for 1 minute.

A4. Event Mark and take 3 breaths with Maximum Inhalation and Normal Exhalation 

(fill your lungs as if getting ready to blow up a balloon, but then relax and let the air flow out 

normally). Wait 30 seconds before the next step.

A5. Event Mark and take 3 breaths with Maximum Inhalation and Maximum Exhalation 

(fill your lungs quickly and completely as if getting ready to blow up a balloon and then exhale 

forcibly as if trying to blow out a small fire). Note anything that affects the work of breathing 

during these exercises.

B. Taxi: If outside air temperature permits, close the canopy and Event Mark. If necessary, for 

cockpit cooling, it is acceptable to delay lowering the canopy until just prior to takeoff.

Card 2: Takeoff & Mil Power Climb

A. Mil Power Takeoff: If the canopy is still up, lower the canopy and Event Mark before calling 

for takeoff clearance.

A1. Taking Runway: Event Mark while taxiing onto the runway, shortly before beginning 

takeoff roll. Perform a normal takeoff using Military power.

A2. Weight-off-wheels: Note as accurately as possible the weight-off-wheels time.

B. Set Altimeter – 29.92: As soon as possible, in accordance with local procedures, set the 

altimeter to 29.92.

C. Mil Power Climb: Level off at 5,000 ft PA and stabilize at 350 knots calibrated airspeed

(KCAS). When ready to begin climb, Event Mark and select MIL power while initiating a 

moderate pull to hold 350 KCAS. The initial flight path angle will be approximately 12 degrees, 

depending on weight and configuration. Roll inverted and pull the nose down to level off at 

15,000 ft PA. 

Card 3: 15K Level Systems Operations

A. 15,000 Pressure Altitude Baseline: Conduct the following exercises at 15,000 ft PA and 

250 KCAS in level flight; maintain a constant airspeed. If necessary to turn for airspace or 

traffic, do so between exercises. Do not talk unless required for Air Traffic Control (ATC)

communications or crew coordination.

A1. Event Mark and breathe normally for 3 minutes.

A2. Event Mark and take 3 breaths with Maximum Inhalation and Normal Exhalation 

(fill your lungs as if getting ready to blow up a balloon, but then relax and let the air flow out 

normally). Wait 30 seconds.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 83 of 260

A3. Event Mark and take 3 breaths with Maximum Inhalation and Maximum Exhalation 

(fill your lungs quickly and completely as if getting ready to blow up a balloon and then exhale 

forcibly as if trying to blow out a small fire).

Note anything that affects the work of breathing during these exercises.

Card 4: Mask-On / Mask-Off Comparison

A. 15,000 Breathing Comparison: Conduct the following exercises at 15,000 ft PA and 

250 KCAS in level flight; maintain a constant airspeed. If necessary to turn for airspace or 

traffic, do so between exercises. Do not talk unless required for ATC communications or crew 

coordination.

A1. Time how long it takes for 10 Normal Breaths. Do not attempt to regulate your 

breathing, just relax and breathe naturally.

A2. Verify that the Cabin Altitude is within limits at approximately 8,000’ before the 

next step.

A3. Event Mark and remove the mask.

A4. Time how long it takes for 10 Normal Breaths. Do not attempt to regulate your 

breathing, just relax and breathe naturally.

A5. Event Mark and then don the mask, ensure a proper seal, and breathe normally for 1 

minute.

Card 5: 15K Talking Script

A. 15,000 Talking Script: Set up so the entire script can be read without having to turn the 

aircraft for airspace. Stabilize at approximately 15,000 ft PA and 250 KCAS for the entire card.

Event Mark and give a description over the intercom of the current mission in complete 

sentences, following the prompts on the card. Speak in a normal cadence as if carrying on a 

conversation. Give the following information:

Name

Flight Profile

Date

Altitude

Airspeed

Heading

Cabin Pressure

Card Number

Any notable breathing-related events

Card 6: Mil Power Climb

A. Mil Power Climb: Start level at 15,000 ft PA and stabilize at 350 KCAS. When ready to begin 

climb, Event Mark and select MIL power while initiating a moderate pull to hold 350 KCAS.

The initial flight path angle will be approximately 10 degrees, depending on weight and 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 84 of 260

configuration. When 350 KCAS equals 0.85 Mach, hold the Mach constant. Roll inverted and 

pull the nose down to level off at 30,000 ft PA.

Card 7: OBOGS Descent

A. OBOGS Descent:

A1: Start at 30,000 ft PA and 250 KCAS. Event Mark and smoothly select IDLE power 

while initiating a descent to hold 250 KCAS. Attempt to set up this maneuver in the airspace so 

only small turns are required for traffic or weather; the descent will take approximately 30 nm.

A2. Level off at 7,000 ft PA and Event Mark. Wait 2 minutes before starting the next 

card. 

Card 8: Mil Power Climb

A. Mil Power Climb: Start level at 7,000 ft PA and stabilize at 350 KCAS. When ready to begin 

climb, Event Mark and select MIL power while initiating a moderate pull to hold 350 KCAS.

The initial flight path angle will be approximately 15 degrees, depending on weight and 

configuration. Roll inverted and pull the nose down to level off at 25,000 ft PA.

Card 9: 25K Level Systems Operations

A. 25K Pressure Altitude Baseline: Conduct the following exercises at 25,000 ft PA and 

250 KCAS in level flight; maintain a constant airspeed. If necessary to turn for airspace or 

traffic, do so between exercises. Do not talk unless required for ATC communications or crew 

coordination.

A1. Event Mark and breathe normally for 3 minutes.

A2. Event Mark and take 3 breaths with Maximum Inhalation and Normal Exhalation 

(fill your lungs as if getting ready to blow up a balloon, but then relax and let the air flow out 

normally). Wait 30 seconds.

A3. Event Mark and take 3 breaths with Maximum Inhalation and Maximum Exhalation 

(fill your lungs quickly and completely as if getting ready to blow up a balloon and then exhale 

forcibly as if trying to blow out a small fire).

Note anything that affects the work of breathing during these exercises.

Card 10: Combat Descent and Zoom Climb

A. Combat Descent: Start at 25,000 ft PA and 0.85 Mach. Event Mark and slowly pull the power 

to IDLE while lowering the nose to maintain 0.85 Mach. Continue lowering the nose while 

extending the speed brake to maintain 0.85 Mach. The flight path angle will be approximately 

30 degrees down. When 0.85 Mach equals 420 KCAS, maintain 420 KCAS. Initiate dive 

recovery at 8,000 ft PA using 2 to 3 g and MIL Power. Immediately transition to the zoom climb 

in the next step.

B. Zoom Climb: Continue 2- to 3 g MIL Power pull to achieve a climb at a 30-degree flight path 

angle, wait a few seconds and select IDLE Power. Roll inverted and pull the nose down to level 

off at 12,000 ft PA and 250 KCAS.

C. Normal Breathing: Event Mark and breathe normally for 1 minute in level flight at 

approximately 12,000 ft PA and 250 KCAS.
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Card 11: Level Accel/Decel

A. Level Accel: Start at 12,000 ft PA and 250 KCAS. Event Mark, select MIL Power, and 

accelerate in straight and level flight to 0.95 Mach. If the aircraft stops accelerating at less than 

0.95 Mach, it is acceptable to consider the Accel complete.

B. Level Decel: Immediately upon reaching 0.95 Mach (or max Mach), Event Mark and select 

IDLE power to decelerate in straight and level flight.

C. End Decel: Event Mark upon decelerating to 250 KCAS.

Card 12: G-Exercise

A. G-Exercise:

A1: Set up at 12,000 ft PA and 400 KCAS. Event Mark and initiate a 3- to- 4-g turn in 

MIL power while maintaining altitude. Airspeed will bleed off. Roll out after 90 degrees of 

heading change.

A2: Set up at 15,000 ft PA and 450 KCAS. Event Mark and initiate a 4- to 5-g turn in 

MIL power while maintaining altitude. Airspeed will bleed off. Roll out after 90 degrees of 

heading change.

Card 13: High G Maneuvering

A. 5 g Constant Descending Turn: Set up at 15,000 ft PA and 480 KCAS. Event Mark, select 

MIL power, and roll into a 5-g turn with a -10-degree flight path angle (FPA). Hold the 5-g 

descending turn for a total of 1 min. Descend no lower than 7,000’ PA. At the 1-min mark, return 

quickly to 1-g, roll to wings level, and return to level flight. Wait 2 minutes before starting the 

next card. It is acceptable to slow to 250 KCAS to conserve fuel.

Card 14: Max AB Climb

A. MAX Power Climb: Start level at 7,000 ft PA and stabilize at 350 KCAS. When ready to 

begin climb, Event Mark and select MIL power while initiating a moderate pull to hold 

350 KCAS. As the flight path angle passes approximately 15 degrees, select MAX power and 

continue to increase the flight path angle to maintain 350 KCAS. The initial flight path angle will 

be approximately 30 degrees, depending on weight and configuration. When 350 KCAS equals 

0.85 Mach, hold the Mach constant. Roll inverted at approximately 28,500 ft and pull the nose 

down to level off at 30,000 ft PA.

Card 15: 30K Level Systems Operations

A. 30,000 Pressure Altitude Baseline: Conduct the following exercises at 30,000 ft PA and 

250 KCAS in level flight; maintain a constant airspeed. If necessary to turn for airspace or 

traffic, do so between exercises. Do not talk unless required for ATC communications or crew 

coordination.

A1. Event Mark and breathe normally for 3 minutes.

A2. Event Mark and take 3 breaths with Maximum Inhalation and Normal Exhalation 

(fill your lungs as if getting ready to blow up a balloon, but then relax and let the air flow out

normally). Wait 30 seconds.
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A3. Event Mark and take 3 breaths with Maximum Inhalation and Maximum Exhalation 

(fill your lungs quickly and completely as if getting ready to blow up a balloon and then exhale 

forcibly as if trying to blow out a small fire).

Note anything that affects the work of breathing during these exercises.

Card 16: Cruise Descent

A. Cruise Descent:

A1: Select the DEFOG lever to HIGH.

A2: Start at 30,000 ft PA and 300 KCAS. Event Mark and smoothly select 80% power 

while initiating a descent to hold 300 KCAS. Attempt to set up this maneuver in the airspace so 

only small turns are required for traffic or weather; the descent will take approximately 20 nm.

A3. Level off at 20,000 ft PA and Event Mark. Wait 2 minutes before starting the next 

card. 

A4: Select the DEFOG lever to NORM.

Card 17: Spiral Descent/Defensive BFM

A. Spiral Descent/Defensive Basic Fighter Maneuvering (BFM): This is a series of higher g pulls 

followed by lower g accels to regain airspeed, such as is encountered while conducting defensive 

BFM. Start at 20,000’ PA and 350 KCAS. Event Mark, then select MIL power and roll into a 4-

to 5-g level turn. When the airspeed bleeds off to 300 KCAS, reduce pull to 2 g and descend at 5

to 10 degrees FPA to accelerate back to 350 KCAS. Reaching 350 KCAS, pull 4 to 5 g to bring 

the flight path back to the horizon and hold until reaching 300 KCAS. Continue alternating the 

level pull/decel and descending accel for a total of 3 minutes. Descend NLT 5000 ft PA/2000 ft

above ground level (AGL).

B. Normal Breathing: Event Mark and breathe normally for 3 minutes in straight and level flight 

at the ending altitude from the previous step. Maintain a constant airspeed. If possible, avoid 

having to turn for airspace or traffic. Do not talk unless required for ATC communications or 

crew coordination.

Card 18: Return to Base (RTB)

A. Set local altimeter

B. Perform a normal landing to a full stop and note the time of weight-on-wheels. It is acceptable 

to conduct multiple patterns for training; note only the time of the full stop.

C. Event Mark after exiting the runway and accomplishing the After Landing checklist. Leave 

the canopy down for taxi unless required for cockpit cooling. Keep the mask up until after engine 

shutdown.

D. Taxi to parking.

Card 19: Ground Block II

A. Accomplish this card with the canopy DOWN.

B. Baseline breathing [ON/NORM/NORM (USAF) / ON (Navy)]

B1. As soon as stopped in parking, Event Mark and breathe normally for 3 minutes.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 87 of 260

B2. Event Mark and take 3 breaths with Maximum Inhalation and Normal Exhalation (fill 

your lungs as if getting ready to blow up a balloon, but then relax and let the air flow out 

normally). Wait 30 seconds before the next step.

B3. Event Mark and take 3 breaths with Maximum Inhalation and Maximum Exhalation 

(fill your lungs quickly and completely as if getting ready to blow up a balloon and then exhale 

forcibly as if trying to blow out a small fire). Note anything that affects the work of breathing 

during these exercises.

Shutdown:

A. Event Mark and raise the canopy.

B. Accomplish normal shutdown procedures.

C. Event Mark, lower the mask, and remove helmet.

D. Conduct Post-flight Rad-97 capnography and Spirometry while still fully strapped into the 

seat.

Postflight:

Approximately 1 hour after shutdown, conduct RAD-97 capnography and Spirometry 

measurements seated in a briefing room without flight gear.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 88 of 260

Appendix 7: F-35 Pilot Interviews and Ground Test Data

Table of Contents

Acknowledgements ...................................................................................................................................91

1.0 Introduction.........................................................................................................................................91

1.1 Rationale for In-flight Pilot Breathing Data Acquisition..................................................93

2.0 Introduction..................................................................................................................................94

2.1 Pilot Experiences ..............................................................................................................94

2.2 Pilot Interview Results......................................................................................................95

2.2.1 Pilot Perceptions...............................................................................................................96

2.2.2 Pilot Symptom and Perception Clusters ...........................................................................98

2.2.3 Pilot Interview Conclusions............................................................................................104

2.2.4 Humans and the System-of-Systems Approach..............................................................104

3.0 Motivating Factors for F-35 Breathing Tests..........................................................................106

4.0 Dedicated F-35 Ground Check .................................................................................................107

4.1 Data Collection Setup .....................................................................................................107

4.1.1 Aircraft ...........................................................................................................................108

4.1.2 Subject ...........................................................................................................................108

4.1.3 Data Measurement System Description..........................................................................108

4.1.4 Technical Description of F-35 Life Support System and Data Collection Setup ...........109

4.1.5 Test Procedure and Conditions Description ...................................................................111

5.0 Breathing Dynamics...................................................................................................................112

5.1 F-35 Pilot Interview Comments on Breathing ................................................................112

5.2 Hysteresis: Definition and Examples in the F-35 Breathing System..............................113

5.3 Mask Pressure Dynamics: Mask Pressure versus Line Pressure Graphs........................118

5.4 Pressure Oscillations: F-35 Breathing System During Exhale .......................................121

5.4.1 Disrupted Pressure-flow Relationships during Exhale ...................................................123

5.5 Pressure Spikes and Pressure Drops: The F-35 Breathing System During Inhalation ...126

5.6 Phase Shift: A Metric to Characterize Pressure-Flow Disharmony................................128

5.7 Energy Management: Pressure Oscillation FFT and Dynamic Pressure ........................134

5.8 Perception of Breathing Dynamics.................................................................................134

6.0 System of Systems Interaction ..................................................................................................135

6.1 Standards Review ...........................................................................................................136

6.2 Comparison to Standards................................................................................................138

6.3 Normal Relaxed Breathing .............................................................................................140

6.4 Effects of Maximum Inhale ............................................................................................142

6.5 Effects of Backup Oxygen System (BOS)......................................................................144

6.6 Effects of Defog On........................................................................................................146

6.7 Effects of G-Suit Interaction...........................................................................................148

6.8 PTT, G-suit connection and Mask Off/On......................................................................149

6.9 Effects of Maximum Inhale (without G-suit) .................................................................151

6.10 Effects of Rapid, Deep Breaths (without G-suit)............................................................153

6.11 Effects of Increased Engine Power Setting (without G-suit)..........................................155

7.0 Analysis of Reduction in Minute Ventilation ..........................................................................157

8.0 Physiological and Medical Implications...................................................................................165

8.1 Physiology ......................................................................................................................166

8.2 Analysis ..........................................................................................................................177

9.0 Definition of Terms....................................................................................................................187

Appendix 7.1. Pilot Subjective Report ..................................................................................................189
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 89 of 260

List of Figures

Figure 2.1. The Swiss Cheese Model of Accident Causation (adapted from Reason, 1990)............105

Figure 2.2. Human Factors Model of Person-Task-Equipment System (Czaja & Nair, 2012).........105

Figure 4.1. Locations of the VigilOX Inhalation Sensor Block (ISB) and Exhalation Sensor 

Block (ESB) on the Pilot ................................................................................................108

Figure 4.2. General Layout for F-35 Life Support System (Image from Google) ............................109

Figure 4.3. General Overview of F-35 OBOGS system (Image from Google) ................................110

Figure 5.1. Inhale Flow vs Regulator Outlet Differential Pressure for a NASA F-18 with Diluter 

Demand Regulator; showing nominal hysteresis............................................................114

Figure 5.2. Inhale Flow as a Function of Line-cabin Differential Pressure for NASA F-18 with 

Safety Pressure Regulator; showing pressure and flow hysteresis, with flow lagging 

behind pressure early ......................................................................................................115

Figure 5.3. Breathing Pressure vs Inhale Flow for F-35 Aircraft, demonstrating severely nonlinear path with extreme hysteresis and large amounts of deviation. .............................116

Figure 5.4. Comparison of Hysteresis: (top) F-18 with Diluter Demand Regulator (middle) F-18 

with demand only and safety pressure (bottom) F-35 with demand only and safety 

pressure (figures rescaled to focus on hysteresis effect).................................................117

Figure 5.5. Ground Data from a NASA F-18 Demonstrating Nominal Mask Pressure vs Line 

Pressure Interaction. A CRU-103 Regulator, Pilot Breathing Hose, and Mask are 

shown on the right...........................................................................................................119

Figure 5.6. Mask Pressure (grey) vs Line Pressure (purple) from F-35 Aircraft 1 ...........................120

Figure 5.7. Mask Pressure (grey) vs Line Pressure (purple) from F-35 Aircraft 2 ...........................121

Figure 5.9. Close up view of Oscillations showing Mask Pressure (grey) and Line Pressure 

(purple) on Aircraft 1......................................................................................................122

Figure 5.10. Close up view of Oscillations showing Mask Pressure (grey) and Line Pressure 

(purple) on Aircraft 2......................................................................................................123

Figure 5.11. Exhale Flow versus Mask Pressure from Aircraft 1 and Aircraft 2................................125

Figure 5.12. Mask Pressure vs Line Pressure Close up of Inhale Oscillations for F-35 Aircraft 

compared with nominal performance experienced in F-18. ...........................................127

Figure 5.13. Normal Reynolds Numbers for the Tracheo-Bronchial Tree (Physiological Reviews 

41:314, 1961)..................................................................................................................128

Figure 5.14. Mask Pressure (left), resulting Flow Rate (middle), and correlation when 

superimposed (right), shown using an F-18 sample This is in direct conflict with 

observed F-35 behavior. .................................................................................................129

Figure 5.15. Exhale Breath #4 flow peaks at start of exhale. Mask pressure peaks at the end, as 

flow trends down. Shown using an F-18 sample. Breath #4 is an infrequently 

anomaly...........................................................................................................................129

Figure 5.16. Exhale Breath #5 (top) and Exhale Breath #4(bottom) zoomed in view of breaths 

from Figure 5.15, shown using an F-18 sample..............................................................130

Figure 5.17. Phase Shift Plots of Inhale/Exhale for PBA F-18 and F-35 Aircraft 1 and 2. ................132

Figure 5.18. Sample F-35 Peak Inspiratory Pressure (PIP) and Peak Inspiratory Flow (PIF), where 

the flow should be much closer co-aligned as a response to the driving pressure..........133

Figure 5.19. Fast Fourier Transform comparing F-18 to F-35 mask pressure frequency 

components demonstrating the increased power loading on the pilot breathing 

response in F-35..............................................................................................................134

Figure 6.1. Chart excerpt from MIL-STD 3050 dated 11 May 2015 (top). Oscillator Activity 

excerpt from AIR STD ACS (ASMG) 4039 dated 12 Feb 1988 (bottom).....................137

Figure 6.2. MIL-STD 3050 Trumpet Curves (top), MIL-STD 3050 Mask Pressure Swings 

(middle), Mask Pressure Oscillations (bottom) ..............................................................139

Figure 6.3. Baseline, Normal, Relaxed breathing for both F-35 aircraft during Segment 1 ............141
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 90 of 260

Figure 6.4. Maximum Inhale for both F-35 aircraft during Segment 2.............................................143

Figure 6.5. Backup O2 System activated for both F-35 aircraft during Segment 3...........................145

Figure 6.6. Defog Activated for both F-35 Aircraft during Segment 4.............................................147

Figure 6.7. G-suit (left), black electronic regulator [below a BOS bottle] (right), and mask 

(bottom). .........................................................................................................................149

Figure 6.8. PTT with G-suit connected (top graph), PTT with G-suit disconnected (middle 

graph), Mask doffing with G-suit disconnected (bottom graph) for both F-35 aircraft 

during the respective Segments as labeled. (Top Right graph) also has Mask Doffing 

with G-suit connected. ....................................................................................................150

Figure 6.9. Maximum Inhale for both F-35 aircraft during Segment 8.............................................152

Figure 6.10. Rapid, Deep Breaths for both F-35 aircraft during Segment 9 .......................................154

Figure 6.11. Increased Power Setting for both F-35 aircraft during Segment 12................................156

Figure 6.12. O2 concentrations and regulator pressure schedule for an aircraft flying to 50.000 feet 

with a 5 psi differential pressure cabin. ..........................................................................157

Figure 7.1. Reductions in Minute Ventilation Summary Tables.......................................................160

Figure 7.2. Breath Ratio Comparison Plots of Aircraft 1 and Aircraft 2 ..........................................161

Figure 7.3. Inhale Time-to-50% Volume ..........................................................................................163

Figure 7.4. Pilot Respiration rate from measured data for the two F-35 aircraft. .............................164

Figure 8.1. Diaphragm Anatomy.......................................................................................................168

Figure 8.2. Inspiration and Expiration Muscular Mechanics............................................................168

Figure 8.3. Intercostal Muscles.........................................................................................................169

Figure 8.4. Pulmonary (Lung) Anatomy...........................................................................................170

Figure 8.5. Pulmonary Volumes .......................................................................................................171

Figure 8.6. Reduced TV on Aircraft 1 compared to Aircraft 2 .........................................................177

Figure 8.7. Reduced TV during Maximum Inhale on Aircraft 1 compared to Aircraft 2 .................178

Figure 8.8. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2................................................179

Figure 8.9. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2................................................182

Figure 8.10. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2................................................183

Figure 8.11. O2 concentrations and regulator pressure schedule for an aircraft flying to 50,000 feet 

with a 5 psi differential pressure cabin. ..........................................................................184

List of Tables

Table 3.1. Hand Collected Data of Measured Breath Times and Respective Condition for Hill 

AFB F-35A .....................................................................................................................107

Table 4.1. Timeline and Description of Ground Test Events for F-35 Aircraft 1............................111

Table 4.2. Timeline and Description of Ground Test Events for F-35 Aircraft 2............................112

Table 8.1. O2 Change During Each Segment...................................................................................185
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 91 of 260

This appendix documents a dataset acquired from ground-tests of two F-35 jets that was 

analyzed using the tools developed in Pilot Breathing Assessment (PBA) for 105 scripted flights 

of F-15 and F/A-18. These tests were deliberately run as ground-test only to allow researchers to 

evaluate the F-35 breathing systems without confounding from aircraft parameters such as 

altitude, velocity, G-force, cabin pressure and orientation. Although these findings cannot be 

considered generalizable to all F-35 aircraft, they are sufficiently compelling to indicate the need 

for further investigation.

Acknowledgements

The PBA team would like to express gratitude and appreciation to the pilots who participated in 

this work. Your stories are the impetus for this document and your statements are the backbone. 

We hope this work represents your concerns and insights fairly and justly. We hope this effort 

will elevate the issues faced by the men and women of the F-35 community who stand ready to 

defend us. 

1.0 Introduction 

F-35 pilots interviewed by the NASA Engineering and Safety Center (NESC) Pilot Breathing 

Assessment (PBA) team have stated that perturbations in F-35 breathing systems can present a 

hazard to operations. Some pilots who have suffered Physiological Episodes (PEs) in the F-35 

fault the breathing system for acute and chronic health conditions that have caused impairment 

for days, weeks, months, or longer. Pilot interactions with the F-35 breathing system have 

resulted in symptoms ranging from confusion, distraction, extreme discomfort and persistent 

fatigue, as well as lung inflammation resulting in permanent dysfunction. The breathing system 

may have contributed to ending the career (medical disqualification) of at least one interviewed 

pilot. Pilots regularly label certain aircraft as having consistently more difficult breathing 

systems than others; this appendix explores potential technical issues that contribute to these 

designations.

Pilot interviews prompted the PBA team to explore the behavior/response of the F-35 breathing 

system using the same empirical measurements as for the main study of F-15 and F-18 aircraft 

equipped with liquid oxygen (LOX) systems. PBA data from ground tests of F-35 Tail Numbers 

11-5021 and 12-5042 documented perturbations of within-breath and between-breath flow and 

pressure response from the system. The comparisons show significant differences between the 

two F-35s as well as between the F-35s and the legacy aircraft in terms of breathing dynamics; 

these are of concern as there are potentially severe adverse system interactions between the pilot 

and the F-35 breathing system. Furthermore, comparisons between the two F-35 aircraft show 

differences in breathing dynamics supportive of the subjective labels of certain F-35 aircraft as 

“bad breathers”.

Both F-35’s tested delivered unpredictable flow at the beginning, middle, and end of the breath 

(intra-breath) that changes from breath-to-breath (inter-breath). Such rapid changes in the breathto-breath supply forces the pilot to continually compensate by adjusting breathing rate, volume, 

and exhale/inhale force. When breathing requires conscious adjustments, rather than autonomous 

response, it distracts from the mission. Furthermore, this pilot-jet disharmony could create stress 

on the pilot, and result in discomfort, fatigue, and may ultimately lead to short-term and longterm physiological damage. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 92 of 260

The F-35 data reviewed in this report were obtained from ground tests of two aircraft, one 

qualitatively judged as a ‘bad breather’ compared to the other, however, both jets were 

considered operational and fit for flight. The data measurements consisted of inhalation breath 

pressure, temperature, and flow. Although the ‘bad breather’ jet was found quantitatively worse 

with respect to tidal volume and asynchronous timing, neither was considered good compared to 

the legacy aircraft examined by PBA. Indeed, these data show that both jets exhibited 

asynchrony in flow and pressure that were quantifiably worse than any of those observed in the 

PBA test flights of F-15 and F-18 aircraft. These data, combined with several pilot observations, 

suggest the problem may be systemic to the F-35 breathing system design and not specific to a 

single jet. 

In addition to the asynchronous pressure/flow behavior, the F-35 data from both jets showed 

wide swings (20 to 40%) in the concentration of oxygen (O2) supplied to the pilot. The 

negatively synergistic combination of constantly changing pressure, breathing sequence, and 

inconsistent O2 delivery increase the likelihood of adverse impacts on pilot physiology. PBA was 

able to specifically identify Breathing System Disruptions (BSD), or breathing sequence 

disruptions (BSDs), which have been observed in other aircraft, but are of particular concern in 

the F-35. Continuous breathing disharmony (disrupted inhale/exhale) and pressure/flow 

asynchrony can result in pulmonary micro-trauma (small tears and inflammation) of the alveoli, 

airway damage, and chest wall remodeling. High and/or variable O2 concentrations may 

additionally contribute to cognitive deficits and cumulative trauma resulting in longer-term 

damage.

The human can adapt to abnormal breathing conditions to an extent, but continuous exposure can

inevitably lead to lung injury. At the microscopic level, cumulative pulmonary ‘micro-trauma’ 

results in collapse and loss of function of the alveoli. On the macroscopic level, the body 

attempts to adapt through changes in respiratory volumes and rates, but the machine imposes 

restrictions that limit and eventually exceed the capacity of the body to adapt. Combined with 

high and variable O2 concentrations, all available evidence suggests that cognitive insults and 

cumulative trauma can result in permanent damage. 

In summary, rather than the breathing system responding to a pilot’s physiological needs, the 

pilot is forced to adapt to an unpredictable supply system with potentially adverse consequences. 

One may ask why such events are allowed to continue. Why do the pilots put up with it? In 2012, 

the NESC conducted an assessment of the F-22 pilot breathing problems. It was observed that: 

The F-22 pilot community has come to accept a number of physiological phenomena as a 

“normal” part of flying the Raptor. These include the “Raptor cough,” excessive fatigue, 

headaches, difficulty breathing, and delayed ear blockages. The acceptance of these 

phenomena as “normal” could be seen as “normalization of deviance.” 

This normalization of deviance is part of the F-35 culture as well. Pilots interviewed for this 

report indicate the F-35 community will endure much adversity to be one of the elite that fly the 

nation’s newest fighter. Pilot interviews also highlighted an organizational concern to protect the 

F-35 program, specifying undue pressure to suppress information and ascribe breathing problems 

to pilots rather than the aircraft. Previously we have emphasized that PEs happen to pilots, not to 

planes. The end goal is a breathing system which supports pilot breathing requirements, not 

aircraft-centric provisions. Hence, measuring pilot breathing metrics is the foundational part of 

understanding this complex problem.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 93 of 260

In contrast to the main PBA effort for F-15 and F-18, this small exploratory study was not 

intended to provide statistical significance for all F-35 aircraft. However, the results are 

sufficiently compelling to prompt further testing using the full suite of PBA sensors and 

analytical techniques to further identify and mitigate adverse breathing system behavior in the 

F-35.

1.1 Rationale for In-flight Pilot Breathing Data Acquisition 

When the PBA began in May 2018, aside from the 1987 AGARD study on 3rd generation 

aircraft, very little was known about how a pilot breathes in the cockpit of an advanced modern 

fighter (Harding, 1987). No comparable in-flight physiological data had been collected even 5 

years after the seminal 2013 article lamenting the “tremendous disconnect between what is 

known about the function of the aircraft and the function of the pilot” (West, 2013). After 

gathering both pilot inhalation and exhalation data from over 100 flights at NASA’s Armstrong 

Flight Research Center (AFRC), the PBA was able to understand and characterize pilot breathing 

to a degree that was never available before. The analysis led to new ways of viewing those 

conditions that are detrimental to the pilot. Indeed, metrics have been established which now 

clearly indicate less than favorable conditions for the pilot and importantly, problems in the 

breathing systems as a whole. The PBA team reviewed, discussed, and even argued about these 

results before ultimately coming to a common consensus concerning the methodology and 

metrics used to measure pilot breathing. It was this team, trained, experienced, and ready that 

was offered the chance to review the F-35 data presented in this report. The F-35 data set is not 

statistically significant, but it was thoughtfully acquired, and it was more than enough to give 

this team of pilot breathing experts the evidence to make a number of judgements that will be 

found in this report. Additionally, comments obtained from F-35 pilot interviews are included to 

underline the points being made from the data.

The PBA interviewed five F-35 pilots from that small community. These pilots experienced 

adverse physiological symptoms while flying an F-35, including reported Physiological Episodes 

(PE). Detailed questions put together by NASA Human Factor experts were used to obtain the 

detailed information about these experiences with the F-35 and the individual PEs. 

Some within the F-35 community may disregard the results presented in this report due to 

limited data; that would be a mistake. The importance of listening to what pilots are reporting 

about breathing dynamics cannot be overstated. This report provides detailed, data driven insight 

to help understand subjective pilot concerns about breathing and general stress in the cockpit. 

The NASA NESC team found instances of alarming problems in the F-35 breathing systems that 

should be corrected. It is our hope that this hard-earned knowledge can help our warfighters and 

better enable those responsible for the systems that keep them safe.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 94 of 260

2.0 Introduction

Since the early 2000s, reports of breathing difficulty, adverse cognitive effects, and unusual 

symptoms have increased significantly in fighter and trainer aircraft, the so-called unexplained 

“Physiological Episodes”, also known as PEs. The NESC performed an assessment of breathing 

problems in the F-22 in 2012. Later, they performed an in-depth study of the occurrence of PEs 

in the F-18, which was published in 2017. The Pilot Breathing Assessment (PBA) flight test 

program is a follow-on to the F-18 study using NASA aircraft to gather baseline data on pilot 

breathing. Surprisingly, such baseline data did not exist for advanced fighters, possibly because 

the tools for airborne collection of breathing data have only recently matured to the point of 

enabling collection in the flight environment.

PBA completed 115 documented sortie flights, using six NASA aircraft: four F/A-18s and two 

F-15s. Data collected in the PBA study includes sensors to monitor pressure, temperature, flow, 

and gas concentration during inhalation and exhalation, instrumentation dedicated to jet 

performance (altitude, speed, etc.), and qualitative observations of the pilot. The description, 

data, and analysis of these flights are published in the NESC Pilot Breathing Assessment (NESCRP-18-01320). Ultimately, the analysis of this breathing data has led to significant findings, 

observations, and NESC recommendations which have advanced our understanding of in-flight 

“breathing dynamics” referring to the breathing system performance and the interactions 

between the aircraft breathing system, the flight environment, and the human pilot.

In June 2019, the NESC PBA commenced a further dedicated investigation on the breathing 

dynamics in the F-35 aircraft, facilitated with data and information provided by US Air Force 

Physiological Episodes Action Team (USAF PEAT). The goal of this effort was to examine the 

unique pilot/jet interactions in the F-35 using the tools, techniques and insights amassed during 

the PBA test program, particularly the insights gathered during the investigation of MBU-20/P

mask malfunctions. Thanks to the PEAT and additional data gathered directly by the NESC, the 

PBA analysis of the F-35 uncovered new and compelling evidence that F-35 aircrew are exposed 

to continuous chaotic and disharmonious breathing system dynamics that have the potential to 

cause physiological insults significantly detrimental to both short-term and long-term pilot 

performance and health. 

The conclusions drawn from these data are new, unique, and compelling with respect to the 

F-35, drawn from insights gathered as a result of the detailed analysis of pilot breathing during 

the PBA program. This is not, however, a comprehensive analysis of the F-35 breathing system. 

The tests were limited to two short ground tests of two aircraft. These were deliberately designed 

to assess the breathing system performance without confounding from aircraft flight stressors 

from changes in altitude, cabin pressure, G-force, orientation, and velocity. Although limited, 

these data suggest systemic problems with the F-35 breathing system and life support equipment 

and call for a comprehensive investigation. This investigation should include sufficient aircraft to 

represent fleet characteristics and use appropriate instrumentation to ascertain pilot breathing 

dynamics during representative in-flight conditions. The analysis refined in PBA highlights 

compelling technical and medical concerns that should be cause for investigation and action. 

2.1 Pilot Experiences 

Pilots describe breathing in the F-35 as being significantly, perceptibly different from the 

breathing environment in legacy aircraft, such as the F-16. The flying and combat employment 

of an advanced fighter aircraft is cognitively challenging; breathing should not be a distraction. 
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However, no longer being able to breathe normally or think clearly takes immediate priority over 

any primary task operation. This distraction does not end with that single experience. Pilots 

report that previous negative breathing experiences induce pilots to regularly assess their 

breathing and engage in specific lung exercises while airborne as cautionary protections. The 

most powerful evidence of these breathing discrepancies comes from F-35 pilot reports. 

The importance of listening to what pilots are reporting about breathing dynamics cannot be 

overstated. F-35 fighter pilots are a particularly elite community. They universally like the 

aircraft to the point of being protective advocates and appreciate the F-35 for its advanced 

tactical utility and survivability in combat. Fighter pilot psychology is severely disinclined to 

overreport or exaggerate minimal issues. Additionally, this is strongly disincentivized due to 

concerns about the program, as well as personal career. As such, when a pilot risks highlighting 

herself/himself to discuss an issue, or an emergency is declared, symptoms and physiologic 

effects have surpassed a very high threshold of significance. 

The NESC team has gathered and analyzed reams of flight data, but the subject matter expertise 

of the pilot provided invaluable insight in guiding the interpretation of the relationships and 

dynamics observed during flight. The combination of pilot reports and physiological monitoring 

data is what enabled the findings in this report. The importance of thorough and well-designed 

interviews was emphasized in the NESC F-22 Report, the NESC Report: F/A-18 and E/A-18 

Fleet PEs, and nearly all assessments and investigations conducted. That lesson is applicable in 

the F-35 as well.

During the course of their work, the Pilot Breathing Assessment (PBA) team was made aware of 

safety concerns within the F-35 pilot community. The PBA was informed that data had been 

collected with a testing device similar in build and version to what the PBA was using to 

examine a similar question in a different setting. Thus, the PBA was in a position to examine 

these data using analysis techniques developed to assess the PBA flight data. 

The NESC team conducted a number of interviews during previous aircraft investigations. These 

interviews offered a holistic perspective of the person-task-equipment triad that exists within the 

physical-social-organizational-policy framework. In particular, the individuals who interacted 

most frequently with the system (pilots/maintainers/flight docs) offered the greatest insight 

regarding the variability of the PE problem and by collecting and aggregating these individual 

data points provided routes for further examination. This interview presence established trust 

within the pilot community.

2.2 Pilot Interview Results

PBA conducted five (5) interviews on-record and in an official capacity to capture a range of F35 pilot perspectives concerning the breathing system, common symptoms, and individual 

examples of physiological episodes (PE). The goal of this section is to allow a better appreciation 

of the pilot concerns and to gain an understanding of the cost associated with continued lack of 

response. 

Five F-35 pilot interviews were conducted by a team of three NESC PBA researchers: a flight 

surgeon, an F-35 SME, and a human factors SME. Each interviewee was provided a NASA 

Privacy Act Notice which indicated the protected status of the interview and all materials 

associated with the interview. All interviewees provided explicit consent to video/audio 
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recording, interview transcription, and inclusion in this report. All data are reported in aggregate 

to maintain privacy. 

Each interview began with the pilot account of events related to the flight that induced a reported 

or unreported PE with specific information about the in-flight event, post-flight procedures, and 

recovery. This was followed by a period of question and answers for clarification and expansion. 

Finally, pilots were asked to provide perceptions of overall concepts across all airframes such as 

breathing experience, previous symptoms, common symptomology, and current processes. 

As supported by data in Section 5 of this report, the asynchronous breathing and pressures 

observed in the F-35 breathing system are a significant safety hazard to the pilot. This hazard 

exhibits as causal to acute and chronic health conditions that impact mission performance and 

impair the pilot. 

Pilots report that interactions with the F-35 breathing system generate symptoms ranging from 

mild discomfort, cough, and fatigue, to confusion, distraction, extreme discomfort, and near 

incapacitation. Some symptoms resolved in a range of minutes, hours, or days; others are 

potentially permanent.

Multiple pilot statements indicate an adversarial relationship with the JPO and include statements 

that reflect a) a significant chilling towards pilot reporting, b) an organizational bias to indicate 

non-aircraft related causes, and c) an organizational bias to attribute causation to the pilot such as 

psychogenic/psychosomatic origins, poor motivation, insufficient training, or inappropriate 

biological preparation habits. Pilot statements indicating concerns regarding the safety and 

adequacy of the system were provided to the JPO in verbal and written form, as well as in the 

formal PE reporting process.

2.2.1 Pilot Perceptions 

A physiologic episode (PE) as defined by the U.S. Navy is when a pilot experiences a loss in 

performance related to insufficient O2, depressurization, or other factors during flight. A 

simplified description of human perception provides a basic framework related to pilot subjective 

reporting. The senses provide raw sensation information that requires organization and 

interpretation. Perception is where the conscious experience of sensation is formed, influenced 

by factors such as present context, training, past experience, principles, and cognitive 

heuristics/biases. Ultimately, a pilot experiences sensation information and interprets the 

meaning. 

The complex process of perception is hindered during suboptimal conditions or hazardous states 

of awareness such as hypoxia. The brain goes to extreme lengths to accommodate, but hypoxia 

dulls sensations and obfuscates perception. Onset is typically very slow and flying duties alone 

may distract the pilot enough to delay detection until the hypoxia is advanced. Hypoxia is 

particularly dangerous because the subjective experience of common symptoms can be 

confusing. For example, headache and nausea are uncomfortable, fatigue can induce poor 

decisions, and euphoria is either pleasant or induces a false sense of calm. 

Due to the danger, pilots undergo frequent hypoxia recognition training to learn their individual 

signs and symptoms so as to recognize when intervention may be required. There is a distinction 

between hypoxia signs and symptoms (FAA, 2008). Signs are detectable by others, but are more 

difficult to detect by the hypoxic person. Typical signs of hypoxia include rapid breathing 

(tachypnea), cyanosis, lethargy, poor coordination, and poor judgment. Symptoms are sensations 
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the person can perceive and use to assess their hypoxic state. Hypoxia symptoms are individual 

to the person in terms of appearance and intensity and remain individually consistent over time. 

Typical symptoms of hypoxia include air hunger, fatigue, nausea, headache, dizziness, hot & 

cold flashes, tingling, visual impairment, and euphoria. 

The F-35 pilot population is small. In general, the vast majority of PEs and symptoms are 

unreported since they do not meet the pilot’s threshold to declare an emergency. Thus, out of the 

approximately 40 documented/JPO investigated PEs, the five pilots herein may be identifiable 

simply by revealing particular details of their PEs or interactions within the reporting structure as 

known within the pilot community. Some of these pilots required greater privacy protection, so a 

more restrictive approach was utilized when reporting their incidentals. Some pilots permitted 

more extensive reporting, including summary statements regarding the F-35 and interactions 

with the JPO. Brackets within quotations indicate areas where additional content was provided 

for context, or where sensitive details were omitted. 

Concerns regarding the F-35 breathing system were raised by pilot reports in 2012, during preproduction testing and have continued throughout the program development through to currentday mission flights. The early pilot reports were neither vague nor insignificant and included the 

following statements: 

• “It was trying to kill me”

• “The system was working as designed, but didn’t actually protect me” 

• “Maybe we had some fundamental misunderstandings of what the design of the system needed to be and 

we didn’t have as much physiological understanding of the human/machine system as we needed.”

A pilot noted that, at the time his concerns about the breathing system were raised, there were 

other on-going investigations specifically related to potential breathing gas contamination 

concerns. He stated that his concerns were met with program leadership opposition in the form of 

explicit and implicit rejection and suppression:

• “There was tremendous amount of concern amongst the [F-35] enterprise that the program was vulnerable, 

at the time, and so there was a lot of pressure to continue testing, continue pressing forward. The team as a 

whole, and especially the program office folks who were in charge of the life-support system at the time, 

were fairly motivated to assign [my symptoms] to something that was not attributed to the jet. That was my 

perception that was what they were trying to do, find a way to have it not be the jet so they could press.”

• “They were able to, again, sort of talk themselves into using those words and saying ‘well, maybe it was 

hypercapnia, maybe it was hyperventilation, but in no case is it something we need to change the design.’”

One pilot reported this summary statement regarding the F-35: 

• “It’s the new normal. Breathing in this jet is different than sitting here talking to you and breathing. It 

shouldn’t be, in my opinion, but it is. Talking against positive pressure is different than talking against no 

positive pressure. The schedule of the cockpit pressurization sometimes changes the pressure in the mask, I 

don’t know if it should be doing that or not, sometimes it does do that. The pressure breathing for G is 

slightly, not slightly, it’s different than what I had been previously accustomed to. And so, it is routine for 

me to notice now, put it like this: I NEVER thought about my breathing, EVER, in the Strike Eagle. Never. 

I never, it was not a conscious thought, I didn’t ever, it was never brought forward into my conscious 

thinking about breathing it was just something I was doing and I never considered it. Now it is something 

that I am conscious of, routinely, in flight; I’m conscious of how I’m breathing, conscious of making sure 

I’m controlling my breath, taking a deep breath, to expand my lungs every 10/15 minutes or so, I make sure 

that I do that. That could be a factor of this thing happening to me or it could be a factor of just breathing in 

this jet is different. I think if you were to ask other pilots that they would, my opinion is, of course they 

have their own opinions, is that the breathing in this jet is different than breathing in the Viper, the F-15 C 

or E, the A-10, or any other platforms, F-22, that they’ve come from, even the hornet. We have guys here 

that have flown all of them. It’s just the different apparatus, a different feeling. And so now every sortie I 

am somewhat conscious of how I’m breathing, and how I’m interacting physiologically with the jet.”
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Another pilot reported this summary statement regarding his experience in the F-35. 

• “The overall experience was one of extreme, you know, it’s difficult to convey to other pilots and other 

people how absolutely disconcerting it is to be cognitively bamboozled like that. Because you know there’s 

something wrong with you, you can’t convey it, and you don’t know why, and you don’t even know the 

why to the why. Don’t even know where to begin. ‘Hey, what’s wrong with me?’ ‘I don’t know,’ well, that 

only makes it worse, right? Which, okay, potentially psychologically, is just concerning on all levels, even 

though intellectually you kind of know ‘hey, I’ll be okay. I’ll just go to sleep and this will all...’ But for 

somebody whose entire life you are relying on your brain to be able think, and to fly, and to not be able to 

connect those words causes a level of concern. The jet attacked me. That’s the essence of the way I felt. 

Even though somebody else might go, ‘Oh, you’re just a little bit off, go sleep it off, shake it off, shake it 

off.’ Right? This was an entirely different level going through that experience and if it were to have 

happened while I was still flying, that’s the thing that’s the most concerning. Right? Because now it calls 

into question your ability to handle an emergency. That’s the interesting dichotomy, I think I could have 

flown and landed the aircraft if everything was fine, but now it’s kind of like the insidious where... you 

know... you always hear about the people going to sleep in the car in the garage, right, it’s kind of the 

apathetic, just comfortably go crash, right? That’s the concern. I would just not be able to make a decision, 

not be able to think and connect it airborne. If that had happened, there’s nothing I could do about it. 

There’s no control over it. As a pilot, you like to be able to control and take what actions you can. Nothing 

I can do! Nothing I can do to prevent it, fix it, and potentially maybe it’s causing long-term harm to my 

health. So, that’s the thing to convey. Maybe it’s difficult to convey how that felt. Well, that’s it.

To be clear, all pilots identified the F-35 as an asset to the warfighter. Here are a few summary 

quotes for positivity and perspective:

• “The F-16 had some significant growing pains as it was introduced as far as there were medical factors, it 

was routinely killing pilots with GLOC and spatial disorientation, but that was several decades ago. With 

time, effort, investigation, a merging of aerospace and aeromedical efforts, these were overcome and went 

on to become one of the most successful fighters in history and I’m confident the F 35 will do the same.”

• “The jet is still providing an environment that, although not optimal, I don’t perceive as actually dangerous. 

These UPEs certainly merit further investigation, but they haven’t killed anybody. I’m gambling my life on 

it, so I think that’s one of the more significant endorsements I can provide. 

• “No pilot experienced significant enough symptoms that they have to stop fighting and address that over 

the tactical problem.” [specific to F-35 combat deployment during actual combat, cessation of simulated 

combat has been reported]

• “Overall, pilots trust the jet.”

One pilot would never fully recover and would be medically disqualified from flight shortly after 

his first reported PE. This individual may be identifiable within the pilot community due to the 

number of individuals with such a description. With his permission, his experience was included 

in greater detail and in his own words via transcription, to provide the reader greater insight into 

the pilot’s perspective during this event, his recovery, and the lasting impact. To encourage 

satisfactory document flow, only a small number of direct quotes from pilot interviews are 

provided to support these clusters; however, all relevant quotes have been de-identified and 

included in Appendix 7.1 for further reading.

2.2.2 Pilot Symptom and Perception Clusters

In human subject research, the interview is one type of qualitative research methodology 

frequently used to collect individual instances of subjective experience. Like in quantitative 

research, once the interview data are conducted, the responses are aggregated and analyzed for 

emergent properties that reveal common themes generalizable to the content area in question. 

This analysis method is well-supported in the literature, but does require advanced expertise in 

human subject data collection and the subject matter area to conduct with precision and accuracy 

while avoiding common commission or omission errors. 
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These interviews revealed several pilot symptom and perception clusters. Here, clusters are 

conceptual groupings that emerged after the identification of highly similar statements and the 

subsequent interpretation of shared characteristics. Adverse symptomatology was reported across 

wide spectrum of flight profiles and pilot demographics (e.g., flight hours, age, and expertise). 

These symptomatology does not appear to be specific to individual differences or task 

performance. Pilots reported adverse symptomatology across the spectrum of individual 

differences and characteristics. This range included nascent pilots with low-hour and no previous 

aircraft experience to elite pilots with instructor qualifications, multiple airframes qualifications, 

and many hours of previous experience including extensive combat experience. Pilots reported 

adverse symptomatology across the spectrum of flight regimes ranging from straight and level, 

administrative, non-demanding phases of flight, to flight that is physically and cognitively 

intense. Quotes for this cluster have been excluded as detailing individual-specific characteristics 

of demographic and flight profile would compromise the privacy of our pilots. 

The remainder of this section will consist of a cluster title, a summary or description of this 

cluster, and relevant quotes to demonstrate sufficient support for the grouping.

Cluster 1: The F-35 breathing environment and physiological experience is dissimilar to a) 

other aircraft flown and b) normal physiologic breathing. The F-35 breathing system 

noticeably discourages normal breathing function via high-pressure, pressure surges, and 

hyperoxia.

Pilots reported the breathing mechanics specific to the F-35 as readily detectable and distracting. 

Pilots negatively compared the F-35 breathing environment to any previous aircraft experienced, 

and to the normal environment defined as that found external to the aircraft. The 

characterizations involve the increased exhalation pressure, the difficulty inhaling, inter/intrabreath pressure surges, and the latency in the cycling of the pressure. In particular, when 

considered in aggregate, the pilot statements suggest the hyperoxic environment and high 

exhalation pressure modulates in-aircraft breathing patterns to be distinctively different from 

normal ambient physiological environment. High exhalation pressure causes an inability to fully 

exhale without intentional and forceful exhalation. The hyperoxic environment perceptibly 

reduces the respiratory drive. A perceptible and pervasive aberration in breathing is a sensation 

of lung hyperinflation relative to normal respiration (due to increased Functional Residual 

Volume and/or due to increased mask exhalation pressure). Other perceptible differences are 

paroxysmal sudden intra breath pressure changes, difficulty exhaling completely, latency in 

gaseous supply from the aircraft, and reduced respiratory rate.

Quotes include: 

• “The respiratory environment is not, still, is not optimized for normal human physiology”

• “F-35 is known to produce erratic oxygen output both in concentration and in pressure. Some latency in the 

pressure delivery, or a lag in the system, as far as the pressure delivery. It’s perceptible.”

• “What I do know is that breathing in the F-35 is different. Breathing in [Strike Eagle] off of an MSOGS 

was a different experience than it is breathing out of the F-35. The F-35 is different in the fact that it has 

positive pressure all the time, not just pressure breathing for G but positive pressure in the mask. It’s 

different in the fact that the ECS environmental control system in the F-35 sometimes surges, sometimes 

pulls back. It’s a different physical environment that you’re in and the breathing is different. The cockpit 

pressurization schedule above 25,000 feet is different, it feels different on your body. It’s like hard for me 

to describe quantitatively the difference, but it’s different enough that you feel different.”

• “You kind of have to begin the exhale as an event, and then once that all starts, and the flow begins, then 

kind of exhale normally. So, I guess another way to describe it, and this is not an accurate mechanical 

description, but the feeling was kind of that it was like a sticky valve, both directions. You, kind of, have to 
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pull to get the inbound air going and then once the valve is flowing that I could breathe in with big 

continuous motion. And the same thing, I had to initiate the exhale, so a sticky valve feeling in that sense, 

and then once the exhale began, I could just go ahead and exhale normally.”

• “The positive pressure isn’t really, in my thinking, isn’t so positive. It can be annoying.” 

• “Sometimes the F-35 just provides a whole bunch of pressure into the mask for unknown reasons, I don’t 

know why but it does, it makes exhalation difficult”

• “Sometimes even in a single exhalation there could be a change in the pressure. So there’s like a kick back 

and it can actually bite off a radio call.” 

• “Occasionally, especially on startup, you’ll get a sudden decrease in pressure, so it’s actually like a sudden 

choking from the jet, - there will be a sudden decrease in flow, pressure that might last like 10 seconds or 

something like that but then it resolves. But it will get your attention.”

Cluster 2: There is a distinct breathing system disparity across F-35 aircraft with no clear 

explanation or solution. 

Pilots detect a very clear difference in the breathing system between aircraft. This differential is 

most related to the pressure cycling throughout the respiratory cycle. This difference was 

reported and met with no solution nor reinforcement to continue in reporting. Pilots report that 

detecting stark deviations has become normalized such that pilots commonly refer to aircraft as 

“easy breathers” and “bad breathers” which has led to early notice of hardware failure or nonannunciated failure. 

Quotes include: 

• “There is noticeable change between jets, and some are easy breathers versus more difficult breathers.”

• “Difficulty breathing off the oxygen system which led to, kind of, a mild shortness of breath symptom that 

would come and go, based on how cooperative the breathing system was at the time.”

• “It was just a hard-breathing day. And the thing that just stuck in my mind that it was just way harder than 

normal to breathe without any definitive smoking gun as to what was causing it. I [informed the program 

office and the head of the maintenance] said ‘hey, I just want to give you a heads up, this just breathes 

strange and it was very hard and it just really caught my attention, but there’s nothing... I can’t say 

anything one way or the other for you guys to go fix... So I just wanted to kind of let you know, and just 

talk it over him you’ and they’re ‘oh, alright, well, just let us know if you think of anything else.’ So that 

was the end of that.” The next day I flew an entirely different jet. Same mission, profile, same rough 

temperature, same place, pretty much everything the same except different jet. Another F-35A. Another Air 

Force variant. And flew and the breathing was just night and day. So, I went from probably the worst 

breathing jet that I’ve ever flown in my life in terms of, it just struck me, that ‘hey this is really, really, 

really, difficult’ to nice, easy, breathing, and the contrast between the two of them was just what really 

caused me to highlight it. So, I thought, ‘alright, this is... this is something there. This is real.’”
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Cluster 3: Symptoms are frequent and variable among pilots and tend to mimic pilot-specific 

hypoxia symptoms. However, there are additional individual symptoms that are F-35 specific 

and learned exclusively from flying the F-35 that suggest additional pathophysiology.

A PE report is not an exclusive indicator of symptomology among the pilot population. Pilots 

often indicate experiencing symptoms that are detectable, but not as significant as to require a 

change in the sortie, a knock it off, or early return to base. Pilots may report a significantly 

increased level of fatigue after sorties. This fatigue is reported as unlike any post-flight fatigue 

experienced in other aircraft. The fatigue is so severe, some pilots report being unable to conduct 

a normal day following some flights. This fatigue may even last several days. 

Common symptoms that are formally reported and/or informally discussed with flight medicine 

personnel include pronounced/idiosyncratic post flight fatigue, post flight cough, mild nausea. 

Other significant symptoms observed include cognitive slowing, confusion, lightheadedness, and 

dizziness. Pilots report a significantly increased level of fatigue after sorties. This fatigue is 

reported as unlike any post-flight fatigue experienced in other aircraft. The fatigue is so severe, 

some pilots report being unable to conduct a normal day following some flights. This fatigue 

may even last several days. 

Some symptoms are predictable and considered to be related to inflight maneuvers or 

environment (e.g., tingling in the distal extremities). Some pilots report consistently experiencing 

symptoms for high altitude flights – flights with portions at or above 38k - 40k feet above mean 

sea level (MSL). The symptoms are lightheaded and dizzy, consistent, do not reach severity to 

declare an emergency, and resolve upon descent below 40. As these symptoms are so 

predictable, these pilots will preemptively go on the backup O2 system before going above 40k 

ft.

Other symptoms are secondary to aggressive changes in altitude (e.g., climb or descent). Some 

pilots report consistently experiencing symptoms in an aggressive max performance climb such 

as climbing from administrative airspace transition altitude, around 10k ft up to about 20k feet. 

This maneuver results in 1-2 minutes of numbness and tingling in the hands and fingers which 

these pilots expressed as similar to those experienced in the hypoxia chamber. 

Many pilots report several hours of nonproductive dry cough after every sortie with no other 

symptoms. For some, the cough begins late in the sortie, persists for 3-4 hours after landing, and 

gradually resolves. Another cough symptom presentation begins with a sudden onset coughing 

fit following rapid tactical descents and onset of G such as: dropping from 20k MSL down to 5k 

MSL. The cough would become sudden with severe onset and then persist though landing and 

into postflight, nonproductive, dry cough for several hours in duration. Similar symptomology 

was previously reported in investigations of the F-22 “raptor cough”.

Prominent quotes:

• “Pilots experience symptoms in the jet, they notice, but they’re not at the threshold that they consider 

necessary to declare or that they’re willing to flag themselves, highlight themselves, over.”

• “There’s been a lot of questioning with these events as far as whether or not it is psychogenic but out in the 

aircraft, I felt no anxiety whatsoever”

• “I think somebody asked me if I was hyperventilating or something, which was ridiculous, I was not 

anxious, there was no increased respiratory rate.”

• “After about 10 seconds or so, I felt my hypoxia symptoms from the altitude chamber get to the point 

where they were now part of my consciousness. So, in hindsight, I would’ve probably said that they had 

been gradually coming on, but it became part of my consciousness at that point.”
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• “Lightheadedness and the blurred vision”

• “I was experiencing nausea, call it low-grade. It’s actually something I get in the jet fairly routinely.”

• “At one point I noticed [the numbness in my extremities] all the way up to the top of my calf towards my 

knee on both of my legs. I had only been in the flight for 10 minutes when that onset began. And that’s not 

a normal symptom.

• “I didn’t feel like there wasn’t physical air being brought into my body, I felt like in the ROBD, I’m

breathing but I’m not getting that satisfaction of breathing, I’m not being fulfilled, my breathing isn’t doing 

anything. That’s why wanted more. I was air hungry.”

• “I couldn’t fully inflate my lungs [For several hours post-flight]. I’d get that pressure and burning sensation 

in my lungs, trying to expand my lungs”

• “It’s worth noting, after landing, I felt, again, pretty out-of-it, fatigued, and even a little bit confused.”

• “I was inappropriately confused at that point. Nothing manifested in the air, but I could definitely detect a 

cognitive slowing and confusion on the ground, after landing.”

• “I’m looking at the switch and I can’t remember which direction, which is telling that I’m not cognitively 

with it, I can’t remember which direction to turn the switch. I’m looking at it. I don’t know which way to 

turn it.”

• “You’re still dragging for a solid two days afterwards.”

• “I tend to experience more post-flight fatigue in the F35 than I have in previous jets. That’s actually really 

common, among F 35 pilots, previously experienced. Definite postflight fatigue.”

Cluster 4: Hypoxia recognition training as it currently exists is not a sufficient match with the 

respiratory environment in the F-35 when compared to the symptom exhibition and mitigation 

needs experienced during actual flight. 

Some pilots reported inconsistencies in the symptomology between hypoxia awareness training 

and the actual onset of physiological symptoms in the aircraft. Pilots reported this 

expectation/reality mismatch caused a delay in enaction of appropriate response. Pilots suggested 

that increased ROBD training would be insufficient as that only induces simple hypoxia which 

does not capture the complexity of symptom exhibition. Furthermore, ROBD training was 

reported as counterproductive as symptoms in training were resolved in seconds while in several 

observations, symptoms remained much longer. This kind of training conditions the pilot to 

anticipate an immediate resolution of symptoms when engaging the BOS which is inaccurate. 

Quotes include:

• “People figure out their F-35 symptoms, essentially by flying it, as odd as that sounds.” 

• “This isn’t the hypoxia that you were trained to in UPT, you pull your green ring, or you turn the BOS on, 

it’s a green knob in this aircraft, and you’ll instantly feel better, kind of like you get in the altitude chamber, 

but this may be a - then kind of let things settle out for a few minutes and then you should feel better over 

time but it might require minutes to address the situation and feel better.”

Cluster 5: Normalization of deviance.

There is a large body of literature on normalization of deviance (Vaughan, 2016). In operation, 

deviation from planning, expectations, procedures, and execution is common in most 

environments. Ideally, these deviations are detected and assessed for acceptance into operation. 

If acceptable, these deviations are used to improve the standard and folded into new policies and 

procedures that better suit the needs of the person-task-environment. Alternatively, these 

deviations are considered unacceptable, adherence will be emphasized. The failure to address 

identified deviations allows expectations to become informally set and influenced with an 

inherent cost of an unquantified risk acceptance. 
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Some areas listed above included elements that were potentially contributory in increasing the 

threshold for pilots detecting deteriorating conditions that could have served as early warning of 

an unstable system prior to pilot injury or physiological event. Several pilots used word choices 

such as “new normal”, “normal normal”, and “nonevent” to describe the different sensations and 

impact perception capabilities.

First, previous software versions in the F-35 yielded prevalent OBOGS fails. The report refrains 

from comment regarding the accuracy of these notifications nor sensitivity of the system. The 

prevalence of these ICAWS were addressed in a software change, but in the interim, program 

guidance to the pilots modified pilot perception of the severity of this warning. In one incident, a 

pilot indicated he was notified of an OBOGS problem and went on the BOS, as dictated by the 

procedure. Unfortunately, as reported earlier, hypoxia recognition training does not accurately 

provide the expectation that symptoms may continue for some time before improvement. So, 

during the first few minutes on BOS, his condition continued to degrade. He misattributed his 

issue to be with the BOS and went back on the (actually) faulty OBOGS. 

Second, the known deviations in perceived breathing characteristics within the F-35 aircraft fleet 

reduced pilot identification of unsafe breathing conditions. The example provided is an aircraft 

with a 50% kinked OBOGS hose. The test pilot detected this aircraft as a “very bad breather” 

during the fit-to-fly check flight and reported concerns to relevant individuals. However, the 

pilot had no threshold guidance to identify when a bad breather should be considered an 

unacceptable breather. Although the breathing experience was undesirable, it did not result in a 

PE; therefore, the pilot had no choice, but to sign off on the aircraft as fit to fly. There was no 

way to quantify the subjective sensation which might have led to the detection of the reduced 

functionality of the breathing line. 

Quotes include: 

• “Now thinking back and knowing how I respond in the jet now, how I feel in the jet now, that may also be 

incorrect. That may be something that’s happening all the time now, and I’m just used to it with 500 hours 

or so now in the F-35.”

• “It’s important to emphasize these ICAWs, these OBOGS fails in the 2B software that we were flying at 

the time, these happened all the time like it was considered a nonevent. In fact, depending on what software 

subset you had of the software subset you could actually just continue the sortie [after the ICAW cleared].”

Cluster 6: Pilots expressed several concerns related to the organizational or leadership 

elements related to the F-35.

The beginning of this section contains comprehensive statements made by the pilots. These 

statements typically included significant concerns related to responsiveness and considerations 

for the pilot. Human are typically able to adjust and compensate for a wide range of flawed 

designs. Unfortunately, this accommodating feature can obfuscate the importance of the human 

element, attributing the successes, instead, to the technological development. Accurate and 

sufficient testing much be conducted to determine likelihood of success for any system. With 

large, dissociated programs, unintended outcomes can occur even from small, simple, or 

seemingly meaningless modifications to design or protocol as occurred in Apollo 1. Close 

examination by individuals with appropriate expertise is required for modifications.

Quotes include: 

• “The F-16 had some significant growing pains as it was introduced as far as there were medical factors, it 

was routinely killing pilots with GLOC and spatial disorientation, but that was several decades ago. With 
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time, effort, investigation, a merging of aerospace and aeromedical efforts, these were overcome and went 

on to become one of the most successful fighters in history and I’m confident the F 35 will do the same.”

• “There was tremendous amount of concern amongst the [F-35] enterprise that the program was vulnerable, 

at the time, and so there was a lot of pressure to continue testing, continue pressing forward. The team as a 

whole, and especially the program office folks who were in charge of the life-support system at the time, 

were fairly motivated to assign any, or my symptoms, I guess, my actual reaction, to something that was 

not attributed to the jet, I guess was their aim. That was my perception, was that that was what they were 

trying to do: find a way to have it not be the jet so they could press.”

• “We talked our way through it and I advocated for an investigation of the design of the system, because, at 

least it seemed clear to me that, the system even if it had functioned as designed... and that was a rapid 

conclusion, that they evaluated how everything worked; all the equipment in the chain from OBOGS and 

BOS through the PIC through my mask to me everything had functioned as it was designed to and so my 

concern was if they had designed it to do THIS and not protect me from hypoxia in this sort of a scenario, 

then we had a problem with the design that we should evaluate where those problems were. At the time 

there was a significant amount of resistance to doing that, again, their assessment was: it worked as 

designed, the oxygen system wasn’t broken, it was a bleed air problem. No need to continue any 

investigation into the design of the system, as far as it being available in an emergency where there’s no 

bleed air available for pressurization air or for the pilot.”

• “I learned a lot of words that I didn’t know before. Besides hypoxia, they discussed that they thought 

maybe it was hyperventilation. And maybe not hyperventilation in the sense that I was breathing too often 

and too shallow, but because I was actually actively trying to control my depth and rate of breathing that I 

had over-controlled and therefore induced hypoxia symptoms by a sort of self-induced hyperventilation. 

That was one theory. They also, I learned a word called hypercapnia... they were able to, again, sort of talk 

themselves into using those words and saying “well, maybe it was hypercapnia, maybe it was 

hyperventilation, but in no case is it something we need to change the design.” 

2.2.3 Pilot Interview Conclusions

The excerpts from F-35 pilot interviews, above, suggest that there a number of problems with the 

F-35. A more comprehensive record of the pilot interviews is included in Appendix 7.1. The 

breathing experience in the aircraft is unlike anything these pilots had experienced before. The F35’s breathing system noticeably discourages the normal breathing function via high-pressure, 

pressure surges, and hyperoxia. However, the pilots’ desire to fly this new fighter, despite the 

abnormal breathing experience, has led them to try and adapt as best they can both autonomically 

and cognitively. A mismatch between pilot expectation of the performance of a system and that 

system’s actual performance can provide warning of a potential problem. However, if the 

observed system performance continues to deviate from expected without formal assessment or 

protocol correction, expectations will recalibrate to consider the deviated performance as normal. 

This modifies the importance assigned to the system deviation and reduces the effectiveness of 

the warning system. This normalization of deviance can undermine the safety of mission, a pilot, 

and an entire program. Even flying the F-35 on routine sorties has led to symptoms that include 

dizziness, cognitive confusion, and severe fatigue. Some pilots who report the onset of hypoxia 

indicate that is markedly different than hypoxia awareness training. As difficult as the F-35 

breathing system is, it can vary significantly between aircraft as described later in this report. 

Finally, despite highlighting these issues and requesting that the design of the F-35 breathing 

system be investigated, a number of the pilots interviewed believe that there is undue pressure to 

ascribe breathing problems to pilots and suppress information about these problems.

2.2.4 Humans and the System-of-Systems Approach

Disciplines such as Human System Integration and Human Factors are used to ensure safe and 

effective performance outcomes of tools, systems, interfaces, and/or procedures through the 

comprehensive application of the limitations, expectations, and tendencies of the intended 
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user/population (Sharit, 2012). Human error is frequently cited as the cause when performance is 

judged as unsafe or ineffective. Responsibility for that error is commonly assigned to the 

“closest” individual related to that error, the person at the “sharp end,” rather than examining the 

situation within which the human was required to operate to understand the why and how. 

Error does not occur in a vacuum (Reason, 1990). 

Figure 2.1. The Swiss Cheese 

Model of Accident Causation 

(adapted from Reason, 1990)

Figure 2.2. Human Factors Model of Person-TaskEquipment System (Czaja & Nair, 2012)

A system-of-systems approach enables the exploration of the interaction of many components. 

The person-task-equipment triad exists within the physical, social, organizational, and policy 

environment (Czaja & Nair, 2012). Contributing factors for any event must address every point 

between the dull end and the sharp end including “those responsible for conceptualizing and 

designing the artifact; those responsible for installing, maintaining, or providing instruction on its 

use; those who determine and oversee the rules governing its use; or those who actually use it.” 

(Sharit, 2012). This framework clarifies that the unwanted event or outcome considered as 

human error was simply the natural outcome of the culmination of events. That, given the 

situation, there is increased likelihood that any person would perform the same way. Numerous 

frameworks are available to assist during this decision-making process with proper training for 

implementation. One such is the Human Factors Analysis and Classification System (HFACS) 

by Shappell & Wiegmann. This technique is currently used by the services for accident 

investigation, but are also extremely beneficial to identify the root of the problem.

Avoiding further undesirable events cannot be accomplished without addressing latent factors 

that induced the undesirable event. Outcomes assigned absent context will result in error 

management techniques that do not address the latent factors, do not improve the error rate, and 

potentially even yield unintended consequence (e.g., overpressurization). The thorough 

integration and application of disciplines related to the human in an operating environment will 

provide better solution identification. Without including the interaction of the human to the 

machine and environment numerous areas for improvement remain unidentified. Other 

comments not included in these clusters were related to the current flight crew equipment. In 

particular, that the combat AFE complement when flown in the long missions in combat, is 

reported as extremely problematic. Statements such as these along with bodily pain associated 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 106 of 260

with the ergonomics of the F-35, though not within the scope of this report, are stressors on the 

body. A serious examination of the pilot experience in this aircraft should be conducted with the 

understanding that the human can be pushed beyond the ability to perform by numerous small 

insults as easily as a few large insults. No task is without disadvantage, but there is a limit to the 

reasonable expectation of a pilot to compensate and proceed without impact to task operation.

References:

Czaja, S. J., & Nair, S. N. (2012). Human factors engineering and systems design. Handbook of 

human factors and ergonomics, 4, 38-56.

Federal Aviation Administration (FAA) (2008). Introduction to Aviation Physiology, Civil 

Aerospace Medical Institute (CAMI). 

www.faa.gov/pilots/training/airman_education/media/IntroAviationPhys.pdf, accessed 

April 25, 2020.

Reason, J., & Newport, R. I. (1990). Human Error. International Journal of Man-Machine 

Studies, 39, 1056-1057.

Sharit (2012). Human Error and Human Reliability Analysis. In G. Salvendy (Ed.), Handbook of 

Human Factors and Ergonomics (4th ed., pp. 734-800).

Swift Report (2017). https://www.navy.mil/local/pes/Comprehensive%20Review%20PE.pdf

Vaughan, D. (2016). The Challenger Launch Decision: Risky Technology, Culture, and 

Deviance at NASA, Enlarged Edition. University of Chicago Press. pp. 30–1.

3.0 Motivating Factors for F-35 Breathing Tests

In 2017, an F-35A at Hill AFB, Utah, was impounded and grounded after a Functional Check 

Flight (FCF) for difficulty breathing, cognitive disability, and breath times reportedly doubled 

from an average 5-second interval to a noticeably longer and repeatable 10-second interval. 

Lingering physiological symptoms including feelings of cognitive disability and extreme fatigue 

were present post flight. After investigation, the aircraft was discovered to have a significantly 

kinked tube delivering breathing O2 from the On-Board Oxygen Generation System (OBOGS) to 

the breathing regulator. In the course of troubleshooting this problem, the regulator in this 

aircraft was replaced twice, followed by replacement of the kinked OBOGS feed line, when the 

faulty hidden line was discovered. 

Extensive procedural and maintenance checks were accomplished with the aircraft running, and 

the aircraft was released from impound after the new breathing line was installed and checked. 

During these checks, it was noted that multiple factors appeared to be repeatedly affecting 

breathing dynamics, most noticeably breathing times. This observation prompted further 

investigation to characterize and understand the phenomenon of varied breathing times. 

Measured breath times at various settings can be seen in Table 3.1. Note that these measurements 

were taken in-flight after the identified kinked line had been replaced, the regulator had been 

replaced, and all maintenance checks performed during a dedicated FCF flight. The aircraft 

breathing system was fully “operational” during these measurements, and as such was expected 

to be representative of a nominal F-35.
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Table 3.1. Hand Collected Data of Measured Breath Times and Respective Condition for Hill AFB 

F-35A

Approximate 

Altitude in MSL

Cabin 

Pressure 

Altitude Condition Setting

Measured Time to Complete 10 

Breaths

39,000 MSL 15,800 CP Military Power 63 seconds

38,000 MSL 14,900 CP Military Power + Defog 82 seconds

38.000 MSL 15,300 CP Idle Power 59 seconds

30,000 MSL 11,200 CP Military Power 66 seconds

30,000 MSL 10,700 CP Military Power + Defog 76 seconds

30,000 MSL 11.500 CP Idle Power 56 seconds

20,000 MSL 08,100 CP Idle Power 56 seconds

20,000 MSL 08,100 CP Military Power + Defog 68 seconds

15,000 MSL 14,500 CP 250 KCAS + Defog 45 seconds (Cabin Pressure Dump)

15,000 MSL 14,500 CP 250 KCAS 40 seconds (Cabin Pressure Dump)

15,000 MSL 08,200 CP 250 KCAS 37 seconds (No Mask/Mask-off)

15,000 MSL 08,100 CP 250 KCAS 58 seconds

11,000 MSL 10,900 CP 240 KCAS 40 seconds (BOS/Cabin Press Dump)

A typical mask-off breathing time for 10 breaths was 37 seconds (highlighted in yellow), and for 

the purpose of Table 3.1, this value was considered the baseline nominal breathing time. Also, 

note that many of these conditions show significantly longer measured times to complete 10 

breaths, in some cases more than doubling the baseline 37 seconds. This is indicative of the 

aircraft significantly altering the pilot’s breathing. The Defog setting (detailed later) was a 

consistent factor in significantly increasing breathing time, correlated with a significant 

backpressure sensation reported by the pilot. The cabin pressure dump setting ameliorated the 

prolonged breathing dynamics, again correlated with pilot reported decrease in backpressure 

sensation. These data were unexpected and led to the collection of the higher fidelity data 

presented in this report, which were intended to help understand and characterize those factors. 

4.0 Dedicated F-35 Ground Check

In January 2018, Colonel Kevin “Sonar” Hall received permission from the appropriate 

authorities (including the local JPO representative) to take pilot breathing measurements using a 

VigilOX in two F-35s. Colonel Hall, an F-35 pilot, developed the measurement regime and took 

data with himself in the cockpit. The data was taken while both aircraft were on the ground with 

engines running. The data was subsequently embargoed by the Air Force. Later, in May 2018, 

the PBA team stood up with Colonel Hall as a member serving as a subject matter expert. 

Approximately one year later, the PBA Lead (C H Cragg) requested this F-35 data from the Air 

Force be made available to the PBA team for analysis. After some delays, the Air Force provided 

the requested data. The analysis in this report comes from this data.

4.1 Data Collection Setup

In January 2018, data were collected to compare two F-35A aircraft during performance of a 

scripted ground profile. The aircraft configuration, pilot, day, and measurement device remained 

constant between the two test observations to ensure adequate comparison capability. This setup 

was approved for ground testing only and no airborne data was available for examination. 
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4.1.1 Aircraft

Two F-35A aircraft, tail numbers 11-5021 (Aircraft 1) and 12-5042 (Aircraft 2), were used for 

the data collection effort. Both aircraft were airworthy with no grounding maintenance pending. 

4.1.2 Subject

One pilot was used for this data collection. The pilot was male, 41 years old, and in good health 

on flying status. He was an F-16 and F-35A/B/C test pilot current and qualified for flight on the 

F-35A at the time with 4 years/300 hours flight experience in all variants of the F-35. He was 

also an Instructor Pilot and Functional Check Pilot with approximately 18 years flying 

experience in 35+ aircraft with 2,400 hours of high-performance aircraft flight time.

4.1.3 Data Measurement System Description

The pilot breathing data was collected using a Cobham VigilOXTM Integrated Aircrew 

Equipment Physiologic Monitoring System prototype. VigilOX is an integrated suite of 

synchronized measurement sensors and represents the third generation of device development. 

The development of these sensors was guided by USAFSAM (USAF School of Aerospace 

Medicine), the US Navy, and NASA to meet the performance parameters and attributes required 

for in-flight physiologic measurements and safety of flight. 

The ISB was connected in-line with the existing breathing supply hose going to the pilot’s O2

mask (Figure 4.1). The ISB attached to the front of the F-35 flight jacket and situated at the 

center of the chest between the Life Preserver Units. The breathing supply hose from the mask 

connects to one end of the ISB, in-line with the breathing hose that connects down to the PIC 

(Pilot Interface Connection) at the regulator. This position provided no interference with cockpit 

operations and no impact to breathing gas flow. The ESB was attached on the side of the vest 

using existing attachment points aft of the main vest pocket (Figure 4.1). A flexible breathing 

tube is attached to the exhalation valve on the O2 mask and connected to the ESB sensor location 

on the side of the vest. The stock mask exhalation valve vents directly to the aircraft cockpit 

cabin, however, the ESB uses a hose to capture the exhaust breathing gas flow and redirects it 

down the ESB block for measurement. 

Figure 4.1. Locations of the VigilOX Inhalation Sensor Block (ISB) and Exhalation Sensor Block 

(ESB) on the Pilot 

The PBA’s use of prototype versions prior to delivery of production versions during the 

assessment allowed comparisons of data quality between F-35, production, and prototype data 
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quality. The F-35 data was comparable to PBA data. For this assessment, the ISB build version 

used was: ISB_DEV003, Software V0.24. The ESB build version used was: ESB_EDEV05, 

Software V0.12. 

The majority of this analysis relies on mask differential pressure, ISB and ESB line pressures and 

flows, the most reliable sensors. Despite the extensive consideration given to known data 

reliability issues, some individual data artifacts presented in this report may be due to signal or 

processing errors. Due in part to the noted limitations in measurement and small sample of two 

aircraft, it is the intent of the NESC team that this analysis be treated as a compelling preliminary 

identification of potential problems in the F-35’s breathing system which should serve as a 

motivation for more comprehensive testing.

4.1.4 Technical Description of F-35 Life Support System and Data Collection Setup

The F-35 is the most advanced fighter in the United States aircraft fleet, and as such has many 

new systems which are unique to the F-35. For aircraft in general, the systems directly 

responsible for a pilot’s breathing may be divided into two categories, the Environmental Control 

System (ECS), responsible for maintaining the cabin environment, and Life Support System 

(LSS), responsible for delivering breathable air to the pilot via a mask. For the F-35, the ECS 

exists as a subset of the Power and Thermal Management System (PTMS), and provides 

pressurized air to the aircraft cockpit via engine bleed air, as shown in Figure 4.2.

Figure 4.2. General Layout for F-35 Life Support System (Image from Google)
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The Life Support System (LSS) of the F-35 consists of multiple components, starting with an 

On-Board Oxygen Generation System (OBOGS) which is fed engine bleed air. The OBOGS 

then uses a dual-bed 13X zeolite sorbent to remove nitrogen (N) and concentrate O2 via a swing 

cycle shown in Figure 4.3. 

Figure 4.3. General Overview of F-35 OBOGS system (Image from Google)

O2-enriched breathing air is fed through a Breathing Regulator and Anti-G (BRAG) system, 

through a Pilot Interface Connection (PIC), and into a mask fitted to the pilot’s face. The BRAG 

system in the F-35 is of particular note since it is a completely electronic regulator (as opposed to 

mechanical), representing the very first of its kind to be fielded in an American fighter aircraft. 

The BRAG feeds both the breathing air to the pilot’s mask and the air supply to inflate/deflate 

the pilot’s G- suit. The F-35 uses Positive Pressure Breathing (PPB) for G and for altitude, but 

does not utilize a chest counter-pressure garment. The F-35 OBOGS attempts to keep O2

concentration within the range specified in MIL-STD-3050, and schedules O2 enrichment based 

on altitude. This BRAG is manufactured by Air Liquide, a French partner company. These 

systems have undergone extensive centrifuge and altitude testing. From discussion with F-35 

engineering and maintenance personnel, the internal workings of the BRAG are not well 

understood, and were not declared as a contract deliverable at the time that the BRAG was 

designed and integrated. 
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The pilot mask used in the F-35 is the same basic mask (MBU-20/P) which is used in all other 

fighter/trainer aircraft and uses the same inhalation/exhalation valve set. Differences include the 

addition of an anti-suffocation valve and a different microphone. 

4.1.5 Test Procedure and Conditions Description

The two F-35 ground tests were performed on January 18, 2018 and January 22, 2018 at Hill 

AFB, UT. Breathing data was gathered by the same experienced F-35 test pilot, at the same 

location, with the same climate conditions, the same flight equipment, and the same basic script 

from two different stationary F-35 aircraft with their engines running during normal ground 

operations. 

The pilot collected data under intentionally relaxed breathing, at pre-determined conditions 

(Tables 4.1 and 4.2) for approximately 1 minute each. The Aircraft 1 vs. Aircraft 2 test 

conditions were made as similar as possible, so the primary variable was the aircraft. Talking and 

physically moving around inside the cockpit influences nominal breathing patterns; therefore, 

activities that change breathing patterns were intentionally avoided during the one-minute 

acquisition intervals. Additionally, effort was made to avoid “fighting” the aircraft by 

intentionally modifying nominal breathing; though, breathing impacts as forced by the aircraft 

systems cannot be entirely avoided. 

The script in Tables 4.1 and 4.2 were performed in each aircraft at the indicated times, and the 

VigilOX data recorded.

Table 4.1. Timeline and Description of Ground Test Events for F-35 Aircraft 1
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Table 4.2. Timeline and Description of Ground Test Events for F-35 Aircraft 2

5.0 Breathing Dynamics

5.1 F-35 Pilot Interview Comments on Breathing

The F-35 interviews were consistent in their observation that the breathing dynamics of the F-35 

are different compared to other aircraft. Pilots described breathing in different ways: 

• “It does breathe differently. It was something that I got used to relatively quickly, and poor test piloting on 

my part because I adapted to the airplane, and didn’t make good note of that adaptation. In hind sight there 

certainly is a threshold of initiation of the breath that the pilot has to do. So it kind of doesn’t do anything 

until you breathe in past some certain threshold, and then you begin getting flow, so there’s this general 

breathing technique I learned, and it was more subconscious than learned, where I would initiate the breath, 

then breathe while I have flow, and then you kind of have to exhale a little bit more forcibly and then that 

sort of stops and resets the valves, and then you can finish the exhale process. It definitely takes more 

attention, whether subconscious or conscious to breathe in the F-35 than it does in any of the other 

airplanes that I flew, including ones that I did fly the F-15 with OBOGS and F-18 with OBOGS, and I 

don’t remember those having any need to adapt my breathing like I had to in the F-35.”

• “What I do know is that breathing in the F-35 is different. Breathing in [Strike Eagle] off of an MSOGS 

was a different experience than it is breathing out of the F-35. The F-35 is different in the fact that it has 

positive pressure all the time, not just pressure breathing for G but positive pressure in the mask. It’s 

different in the fact that the ECS environmental control system in the F-35 sometimes surges, sometimes 

pulls back. It’s a different physical environment that you’re in and the breathing is different.

• "It's the new normal. Breathing in this jet is different than sitting here talking to you and breathing. It 

shouldn’t be, in my opinion, but it is."

• “Sometimes the F-35 just provides a whole bunch of pressure into the mask for unknown reasons, I don’t 

know why but it does, it makes exhalation difficult”
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• “And then sometimes [the expiratory pressure] will change in the same expiration, like you’ll be expiring, 

against a certain expiratory pressure and then it’ll kick back at you sometimes or sometimes it’ll go away 

and it can be somewhat variable, even within the same respiratory cycle. 35 things.”

• “When you’re breathing off the mask in the F-35 you feel like you have to work a little bit harder so you’re 

a more forceful inhalation, sometimes, you have to more forcefully exhale” 

• “The positive pressure isn’t really, in my thinking, isn’t so positive. It can be annoying.” 

• “Sometimes even in a single exhalation there could be a change in the pressure. So there’s like a kick back 

and it can actually bite off a radio call.” 

• “You’re exhaling against a constant pressure but then it’ll kick back whatever pressure you’re using to 

exhale and speak, and that pressure is equalized ceasing your exhalation and ceasing your vocalization for 

the radio transmission.”

• “you’ll be talking and then as you’re talking your expiring and you’re anticipating certain expiratory 

pressure as you’re talking but within the same exhalation while you’re talking, sometimes it will kick back 

and it will literally just like (mimes inability to exhale) like stop your expiration and it’ll just, like, cut off 

your exhalation and talking concurrently, or as a secondary effect, and then you have this oddly clipped 

radio call.”

5.2 Hysteresis: Definition and Examples in the F-35 Breathing System

Hysteresis is usually understood as a lag in a mechanical system, or the reluctance/inability of a

dynamic system to return to a previous state once perturbed. A system demonstrates hysteresis if 

it does not return to its original rest state along the same path on which it went out, or if the 

system takes longer to return from a dynamic state than it did to reach the state initially. Test 

pilots may be familiar with the concept of hysteresis in aircraft controls or avionics systems. The 

analysis of PBA data shows that the concept of hysteresis applies and is a significant factor in the 

performance of pilot breathing systems.

Ideally, the aircraft breathing system will respond to pilot demand pressure quickly, reliably, and 

in proportion to the demand. Figure 5.1 shows three consecutive inhale breaths from a PBA test 

flight in an F-18 aircraft. The jet breathing system was in a USAF CRU-73 diluter demand panel 

mounted configuration, so it did not have safety pressure. The graph shows the inhale flow as a 

function of the differential pressure of the regulator outlet and the cabin,

ΔPl−c = ሺPline − Pcabinሻ.

The start and end of an inhale are indicated by the open triangle and closed circle, respectively. 

Each datum point represents a time increment of 0.05 sec and the arrows represent the path taken

from the start of the breath to the end of the breath. The three breaths are from a part of the test 

while the F-18 aircraft was still on the tarmac before takeoff.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 114 of 260

NASA F-18 with diluter demand regulator

Figure 5.1. Inhale Flow vs Regulator Outlet Differential Pressure for a NASA F-18 with diluter 

demand regulator; showing nominal hysteresis. Note the linear relationship between flow and 

demand pressure over time.

There are several significant points of note from these breath plots. First, breathing flow varies 

very linearly with the pressure differential. The flow from the regulator is in direct proportion to 

the demand signal on the regulator. Further, the regulator response is the same from breath to 

breath; the system reliably produces the same flow for a given demand. Finally, there is no 

appreciable lag or hysteresis in the system; the flow from the regulator is only a function of the 

demand signal and not dependent on whether it is in the beginning, middle, or end of the pilot 

inhale cycle. 

Figure 5.2 shows a sequence of three breaths during relaxed breathing (the same pilot as in 

Figure 5.1) in an F-18 only this time in a USN chest mounted CRU-103 configuration with 

safety pressure. In this case there is an offset in the differential line pressure (x-axis) 

corresponding to the safety pressure. The data show that the path for the inhale is now oblong 

(non-linear) rather than a line (linear), and does not trace the same path back and forth as the 

pilot’s breath pressure changes with time, with a different return path than the “out” path; in 

other words the breathing pattern is displaying hysteresis.
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NASA F-18 with Safety Pressure Regulator

Figure 5.2. Inhale flow as a function of line-cabin differential pressure for NASA F-18 with Safety 

Pressure Regulator; showing pressure and flow hysteresis, with flow lagging behind pressure early. 

Later in the breath, flow exceeds demand.

While the breathing system in Figure 5.2 is not ideal, the path traced by the breath is still very

smooth. There is a predictable relationship between the flow of air supplied to the pilot, and the

pilot’s demand. The PBA data show that pilots can breathe on a demand safety pressure system

like that in Figure 5.2 safely as long as the hysteresis in the system remains relatively low.

This oblong path is symptomatic of the lag inherent in a demand system without a diluter

function. Because the regulator is sensing the signal at a finite distance from the pilot (the length

of the mask and hose), and has mechanical springs and bellows regulating the mass flow

response, it cannot respond instantly to changes in the demand signal. In a demand regulator

system, unlike normal breathing on the ground, there will always be a delay between the

initiation (the request for air), the regulator response, and the resulting flow reaching the pilot’s

mask. Diluter systems minimize this problem. The dilution allows instant access to a large

volume of unrestricted (cockpit) air to backfill for the delay or compensate for regulator

restriction. Significant loss or delay in this process results in flow that is not directly

proportional. During the first half of the breath where demand is increasing, a delay results in

less flow than demanded in any given instant, which is why the oblong path is lower at first.

Conversely, when a pilot is decreasing their breathing demand during the second half of the

breath, the lag causes the regulator to proportionally provide more flow until the responding flow

drops to match the decreased demand for flow. The pilot has to work against the aircraft, being

slightly undersupplied during the first part of the breath, and being slightly oversupplied during

the second half of the breath. In summary, hysteresis makes it more difficult to start the flow

(flow lags demand), and more difficult to stop the flow (flow exceeds demand). The hysteresis as

seen above is too subtle for pilots to notice (i.e., the pilot is unaware that a machine is reading

and responding to the pilot’s needs, and subconscious respiration functions normally). However,

past some point the divergence from open air breathing becomes apparent to the pilot, such as in

the F-35, according to pilot reports.

• “You kind of have to begin the exhale as an event, and then once that all starts, and the flow begins, then 

kind of exhale normally. So, I guess another way to describe it, and this is not an accurate mechanical 

description, but the feeling was kind of that it was like a sticky valve, both directions. You, kind of, have to 

pull to get the inbound air going and then once the valve is flowing that I could breathe in with big 
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continuous motion. And the same thing, I had to initiate the exhale, so a sticky valve feeling in that sense, 

and then once the exhale began, I could just go ahead and exhale normally.”

Figure 5.3 shows a set of three consecutive breaths during a relaxed breathing period collected

from a pilot breathing on an F-35. There is a chaotic relationship between pressure, flow, and

time that is neither linear nor smooth. This demonstrates the pilot fighting the machine. The path

is now extremely oblong, indicating a severe lag in the response of the breathing supply system.

On each breath, the pilot demand increases at the beginning of the inhale, but the system does

not initially respond at all. During the end of the breath when the pilot demand decreases, the

supply remains high and the jet overcompensates pushing air into the pilot. Also notice that the

path has become jagged, irregular, and inconsistent from breath to breath. This flow is highly

inconsistent throughout each breath, with large variations in flow as the pilot breathes in and out.

Because each breath is so variable and different it is difficult for the pilot’s subconscious

breathing to seek out and find a consistent breathing solution.

F-35 with electronic demand regulator

Figure 5.3. Breathing Pressure vs Inhale flow for F-35 aircraft, demonstrating severely non-linear 

path with extreme hysteresis and large amounts of deviation. 

Early in a breath, there is no flow despite increasing demand. In the middle of the breath, flow 

is complex and overshoots demand. At the end of the breath, pressure remains high as demand 

drops.
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Figure 5.4. Comparison of hysteresis: (top) F-18 with diluter demand regulator (middle) F-18 with 

demand only and safety pressure (bottom) F-35 with demand only and safety pressure (figures 

rescaled to focus on hysteresis effect)

The data in Figure 5.4 show that at the beginning of each F-35 inhale the line is flat in stark 

contrast to the two F-18 regulators. The pressure drops with no corresponding increase in flow. 

The pilot is not just being undersupplied, as in the middle case with mild hysteresis, but is not 

receiving any flow at all initially. During the middle part of the breath the slope goes the wrong 

way, with both pressure and flow increasing. Normal human-generated breathing results in flows 

that increase proportionately with demand, and decrease with reduced demand. That is the 

environment for which our respiratory system is adapted and that is how diluter demand 

breathing systems operate. Simultaneous increases in flow and pressure result in overshoots of 

pressure during the second half of the breath. Note that pressure and flow are never in the top 
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right corner for the F-18 data (Figure 5.4). The F-35 pressure and flow (Figure 5.4) in the top 

right corner of the graph signifies high flow with no pressure demand signal; this is 

physiologically important and will be discussed more in Section 8, medical, as excessive 

inspiratory pressure. 

Consider this pressure-flow pattern in the context of lung physiology. The alveoli are tiny thin 

membrane balloons. Imagine the pressures in of these thin membrane balloons experience during 

inhalation. In the patterns observed, each breath is similar to sucking against a closed valve (no 

flow initially), followed by the valve quickly opening (rapid onset of flow), and the valve 

remaining open when it should be closing (over pressurization). This thin membrane in the lungs 

is exposed initially to outward pulling forces from all sides to open-up, however with no flow, it 

is as if the opening of the balloon were pinched off. The rapid onset of flow is analogous to a 

balloon popping open with air rushing in to fill the balloon to the position where the natural lung 

demand had been pulling it open. Lastly, the air continues to flow past the point of natural 

demand, forcibly overinflating the analogous balloon at an unnaturally high pressure for that size 

of balloon. This breathing pattern of starved flow initially, followed by a non-linear discontinuity 

(a pop or snap during opening), and then rapid over pressurization tending towards over inflation 

occurs with regularity in the F-35. This pattern is a stark contrast to the much smoother and near 

linear flow observed in the F-18/F-15. The energy imparted to these thin membrane balloons by 

rapid changes in pressure (up to 2 mmHg) without a change in flow, changes in flow (up to 20 

LPM) with minimal change in pressure, and pressure oscillations exceeding the AIR-STD 4039 

limits (see discussion in 9.2 & Figure 9.2) contain energy up to 2 orders of magnitude higher 

than observed in the F-18 (see discussion in 5.8 & Figure 5.19). The physiologic ramifications to 

the sensitive alveoli in the lungs, and potential for injury from continuous asynchronous pressure 

flow patterns are discussed in Section 8. 

The data in Figures 5.1 through 5.4 are for relaxed breathing where the pilot’s metabolic demand 

is minimal. The fact that the F-35’s breathing system does not respond to pilot demand 

proportionally, quickly, or reliably for relaxed breathing should be considered highly concerning. 

The clear lack of a predictable, proportional relationship between demand and supply in (Figure 

5.3) shows a system that will be very difficult to breathe on in general, and may introduce 

random and unpredictable effects. Breathing pressure-flow relationships in the F-35 are very 

different from those analyzed in the F-18. Whereas F-18 pressure and flows exhibit a linear 

relationship with a diluter demand regulator, or exhibit a small hysteresis with a safety pressure 

regulator, the F-35 in comparison to the F-18 undersupplies flow at the beginning of the breath, 

compensates abruptly, oversupplies flow at the end of the breath, and does not have a 

corresponding monotonic relationship between pressure and flow. 

5.3 Mask Pressure Dynamics: Mask Pressure versus Line Pressure Graphs 

Every time a pilot breathes, the mask cavity pressure changes and the mask/regulator system has 

to keep up with the changes. Figure 5.5 is an example of the mask pressure and line pressure 

from the regulator on the F-15. There are many nuances, and understanding the following figures 

are essential to understanding the issues discussed herein, so it is discussed here in detail. 
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Figure 5.5. Ground data from a NASA F-18 demonstrating nominal mask pressure vs line pressure 

interaction. A CRU-103 Regulator, Pilot Breathing Hose, and Mask are shown on the right.

Figure 5.5 data was collected during PBA Flight #63, in an F-18 with the CRU-103 Regulator. 

The grey line in figure 5.5 represents mask pressure, which varies in this example from 0 to 4 

mmHg during each breath. The purple line represents the line pressure from the regulator. The 

straight black line at 3 mmHg represents the nominal value (positive offset) of the CRU-103 

safety pressure. Note the line pressure plateaus at approximately 3 mmHg (nominal safety

pressure value) in-between each drop in pressure. 

Note on Safety Pressure: Safety pressure is a regulator design (or option on some) where the 

pressure delivered by the regulator is set at a value higher than the cabin pressure. Safety pressure 

is integral to the design of the regulator, set at a fixed value, and the mechanisms which maintain 

that pressure are involved in breathing dynamics. Higher safety pressures make inhale easier by 

pushing air into the lungs and make exhale more difficult as that same pressure must be overcome 

to exhale. 

Each drop in pressure indicates inhalation, which results in a drop in the mask pressure and a 

corresponding drop in the line pressure. Note that the drop in mask pressure and line pressure 

track closely and smoothly together in a rounded-off “V” shape. As inhalation demand ends, and 

the inhalation valve is closed, the line pressure returns to its nominal safety pressure value and 

remains there at a relatively smooth and stable plateau. As the pressure rises during exhalation, 

the mask pressure increases to approximately 4 mmHg in this example and has a relatively 

smooth shape resembling an upside-down “U”. The x-axis of Figure 5.5 is time; each graph 

displays a one-minute segment of breathing. The mask pressure is measured in the mask by the 

ESB. The line pressure is measured just after the regulator in the ISB. Note that the line pressure 

shown on the graphs is a differential pressure calculated by subtracting the ESB line absolute 

pressure from the ISB line absolute pressure (ISB-ESB line pressure). This is done in order to 

match the scale of the mask pressure, which is a differential pressure. In other words, the line 

pressure displayed is essentially the ISB line pressure minus the ambient cabin pressure. This 

derived measurement is fairly precise, but less accurate than the mask pressure. In this example, 

the breathing system is regulating and exhausting airflow in a relatively smooth, linear, and mmHg

Pilot Breathing

Hose
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proportional manner as the pilot breathes, exhibiting the same characteristics as shown in Figures 

5.1 and 5.2.

The breathing patterns of the two F-35 aircraft are shown in Figures 5.6 and 5.7. Both aircraft 

show distinctly abrupt waveforms, with significant oscillations and high rates of change during 

pressure swings. Additionally, the two aircraft traces are very different from one another. 

Oscillations exist during both inhalation and exhalation. The distinct troughs and peaks displayed 

in the F-18 (Figure 5.5) are notably absent. The exception is the mask pressure (grey) which 

occasionally exhibits a distinct trough. This trough has a rough “V” shape with frequent negative 

sharp downward pressure spikes at the beginning of inhalation. The degree of change between 

troughs is notable, having considerable variation in the magnitude of pressure drop due to the 

spike at the beginning. 

Figure 5.6. Mask Pressure (grey) vs Line Pressure (purple) from F-35 Aircraft 1
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Figure 5.7. Mask Pressure (grey) vs Line Pressure (purple) from F-35 Aircraft 2 

The lack of separation between mask pressure and line pressure during exhalation (above 3 

mmHg) indicates that the pressure domains are communicating pressure when they should be 

isolated and independent. These rapid oscillations can also be considered as shock loading the 

alveoli, and they are observable by the pilots. 

• “These oscillations are extremely prevalent and observable. If you unclick the mask from the helmet and 

hold the mask to your face, you can watch the mask move several millimeters during these pulses both 

during inhale and exhale. While the pulses during exhalation are annoying, and can cut you off midsentence, the pulses during inhalation are much more disturbing and have at times had an immediate 

physiological impact during deeper breaths. Kind of like a slight blow to the chest, which I guess the surge 

of air actually is in some respects. Said another way, it’s like mid breath having the wind knocked out of 

you, or sucked out of you momentarily. Not pleasant.”

• “Sometimes the F 35 just provides a whole bunch of pressure into the mask for unknown reasons, I don’t 

know why, but it does. It makes exhalation difficult.”

• “The backflow valve would get stuck sometimes. In fact, I remember there would be times I would reach 

up into the mask and punch the backflow valve if it got jammed. And then that would kind of leave you 

sometimes with a momentary shortness of breath sensation, I would say, maybe 1 in 10 flights you’ll see 

that.” 

5.4 Pressure Oscillations: F-35 Breathing System During Exhale

The extreme oscillations present during exhale are circled in red in Figure 5.9. Both the mask 

pressure and the line pressure oscillate 1 to 2 mmHg. Normal breathing uses very little pressure 

differential: Exhale takes approximately 1 mmHg and inhalation approximately 2 mmHg. The 

pressure budget for moving air is very small. These oscillations superimposed upon otherwise 

normal breathing in Figure 5.9 are of the same magnitude as the pressures generated during 

normal breathing. 
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Exhale Oscillations: During exhale, the inhale valve should be closed, and pressure in the mask 

should be separate and independent from the line pressure, which itself should be constant at the 

safety pressure value. However, both the mask pressure and line pressure track the oscillations 

almost identically. This indicates the pressure domains are interacting and are not isolated from 

each other. Due to the compensation tube referencing the line pressure, the exhalation valve 

opens and closes as the pressure inside the compensation chamber increases and decreases with 

changes to the line pressure. Every increase in line pressure effectively increases exhalation 

resistance as the valve inflates and closes, and every decrease in line pressure effectively 

decreases exhalation resistance as the valve deflates and opens. The magnitude of change is large 

enough to be impactful: by comparison, in this segment, line pressures actually drop more during 

exhale than during inhale. Because the valve has a finite response time, as the pressure is 

changing, the valve is slightly more closed than it would be at steady state. However, as 

designed, the converse is not true; the exhalation valve cannot have less resistance than when 

fully deflated. This results in a one way ratchet effect, where oscillations can only increase 

resistance, but can never decrease resistance. These oscillations induce a breathing dynamic of 

highly variable exhalation resistance and higher than designed resistance with pressure changes 

exceeding the normal inhale and exhale pressures. This requires the pilot to consciously change 

breathing patterns to compensate, distracting them and contributing to fatigue or pulmonary 

micro-trauma.

Figure 5.9. Close up view of oscillations showing mask pressure (grey) and line pressure (purple) on 

Aircraft 1.

Note that the data comes from two physically separate sources; the mask pressure from the ESB 

and the line pressure from the ISB. The use of the ESB and ISB signals in correlation with each 

other is supported by over 100 PBA flights, and is an example of the redundancy and error 

checking available with the multiple overlapping redundant sensors. This data are further 

Oscillations during Inhale
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supported by pilot interviews on the difficulty of exhalation, as well as reports of being cut-off 

mid-sentence by increased exhalation pressure.

Figure 5.10. Close up view of oscillations showing mask pressure (grey) and line pressure (purple) 

on Aircraft 2.

The exhale oscillations on Aircraft 2 while less frequent, are still pervasive. The middle 

exhalation in Figure 5.10 is the closest to a normal exhale with line pressure staying close to 

safety pressure, minimal oscillations, and only one drop of less than 1 mmHg. However, the first 

exhale has very sharp swings approaching 3 mmHg. Generally, Aircraft 1 has larger oscillations 

during exhalation and Aircraft 2 has larger oscillations during inhalation. A small sample of the 

F-18 data from Figure 5.5 is shown at right for comparison.

5.4.1 Disrupted Pressure-flow Relationships during Exhale

Pressure-flow relationships during exhale are again so disjointed that they have no discernable 

functional relationship to each other. The first thing to notice after exhale starts at the red triangle 

(Figure 5.11) is that pressure increases with no flow. The point at which flow starts is called the 

cracking pressure, which in an ideal system would be at the nominal safety pressure of the 

regulator. Here we see cracking pressures on both jets ranging from 4 to 5 mmHg, which is much 

higher than the nominal safety pressure of 3 mmHg. Once flow begins, the traces are 

characterized by a decline in pressure. This is a backwards relationship. Flow initially starts at a 

higher cracking pressure than designed, and then gradually decreases until peak exhale flow is 

reached on every breath. After peak flow, pressure increases markedly (approximately 1 mmHg) 

accompanied by a decrease in flow. Again, this is an inverted relationship. Higher exhale 

pressure should result in more flow if the exhale resistance is constant. If the resistance is 

increasing, however, flow will be choked off and pressure will increase concordantly. 

Unlike the inhale breath where the regulator is directly involved in the dynamics of flow, exhale 

flow is only a function of the exhale valve mechanics and pressure balances. The compensation 

bladder in the exhalation valve references the inhale line pressure, and is the mechanism which 

Middle 

Exhale

First

Exhale
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stops or restricts flow. Because the exhale compensation valve is connected to the line pressure, 

pressure oscillations in this line can cause variable flow restrictions during exhale, which can be 

observed happening in virtually every exhale. 

The Exhale Breath Number 116 (EBN 116, Figure 5.11, top right) is notable as it has the most 

linear relationship during the second half of the exhale (denoted by the green line). An ideal 

exhale flow would track closer to the green line, which serves as a comparative reference to how 

far removed from linear these flows are.

Aircraft 1 has higher cracking pressures, larger flow restrictions, and more pressure fluctuations 

(compared to Aircraft 2; and together they have characteristics exceeding those found in F-18’s 

during the PBA study. These observations corroborate pilot reports of increased backpressure 

sensation, and restriction to flow. 

The flow patterns in Figure 5.11 are consistent with a restrictive exhalation pattern with 

breathing dynamics characterized by an unpredictable and variable pressure-flow relationship 

marked by excessive cracking pressure and intermittent flow restrictions. This combination can 

result in flow reductions of 15 to 20%. Remembering that the only mechanism the human body 

has to control breathing is pressure, an inverted or non-existent pressure-flow relationship like 

this will have physiologic consequences for the natural ability to properly control exhale volume.

Additionally, an unpredictable and constantly changing average resistance, further complicates 

compensatory efforts. 
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Figure 5.11. Exhale Flow versus Mask Pressure from Aircraft 1 and Aircraft 2.
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5.5 Pressure Spikes and Pressure Drops: The F-35 Breathing System During Inhalation

The pressure spikes and oscillations present during inhale are circled in red in Figure 5.12. Both 

the mask pressure and the line pressure oscillate continuously at less than 0.5 mmHg. Aircraft 1 

exhibits an unusual pattern with minimal pressure drop during inhale. These oscillations 

superimposed upon otherwise normal breathing (Figure 5.12) are of the same magnitude as 

pressures generated during normal breathing. 

During inhale, the exhale valve should close, the inhale valve should open, and pressure in the 

mask should track closely with the line pressure. During PBA flights in the F-18 (Figure 5.12, 

bottom) this concurrent drop in both line and mask pressure can be seen clearly. Notice that the 

line and mask pressures rise and fall together smoothly, indicating that the aircraft is breathing 

“with” the pilot. The strong (normalized) correlation (R=0.9) on PBA flights between mask 

pressure and resultant flow is discussed below and also shown in Figure 5.14 and 5.17. The mask 

pressure is essentially the demand signal from the pilot, while the line pressure is essentially the 

demand signal seen by the regulator.

Inhale line pressure on F-35 Aircraft 1, however, stays at safety pressure the entire time. Line 

pressure drops to its lowest, not during inhalation, but during exhalation due to oscillations! The 

line pressure and mask pressure show a major disconnect and frequently trend in opposite 

directions; this is a profound disconnect between pilot and machine. We have already seen that 

the correlation between mask pressure and flow was very low at R=0.42 for Aircraft 1 (Figure 

5.16). Here we see that the correlation between mask pressure (the demand signal closest to the 

pilot) and the line pressure (the demand signal closest to the regulator) is very low or anticorrelated. Looking at breathing dynamics, a pilot’s breathing should be supported by the aircraft 

according to the following pattern:

1. Both line pressure and mask pressure start at the same point, or with a fixed offset in 

the case of safety pressure.

2. The pilot inhales, the pilot’s chest wall expands causing a pressure drop in the lungs.

3. The mask pressure begins to drop to follow the pressure drop in the pilot’s lungs.

4. The mask pressure drop is communicated down the line by the regulator.

5. The regulator increases feed flow to the pilot until the inhale flow peaks.

6. The pilot’s chest wall is harder to expand and lung pressure becomes less negative.

7. The mask pressure rises to follow pressure rise in the pilot’s lungs.

8. The mask pressure rise is communicated to the line by the regulator, the regulator 

reduces flow until they match again at (1)
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Figure 5.12. Mask Pressure vs Line Pressure Close up of inhale oscillations for F-35 Aircraft 

compared with nominal performance experienced in F-18.
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The F-35 regulator has been compared to a racehorse, a very powerful thoroughbred, because it 

can send the air racing to the pilot. The energy necessary to overcome the pressure drops in 

modern fighter systems necessitates a strong regulator that can respond very quickly. Indeed, the 

F-35 has a very powerful regulator that responds quickly. This is necessary, however, when a 

finely tuned system capable of fast and large pressures is out of sequence or out of harmony with 

the pilot’s demands, the same increased power and speed have an equally increased ability to 

cause disruption. 

5.6 Phase Shift: A Metric to Characterize Pressure-Flow Disharmony

Is there a simple way to characterize significant pressure-flow disharmony? Nearly 100 PBA 

flights show that overlaying Mask Pressure and Flow Rate squared is a sensitive indicator of 

anomalies. The analysis of resulting anomalies suggested a simple way to characterize 

disagreements between pressure and flow by comparing the respective pressure-flow peaks. In 

order to define disharmony, the nominal pressure-flow relationship must be defined first.

Ideal Pressure-Flow Relationship: Flow is resultant as pressure changes at one point of a system 

compared to another. The relationship between flow and pressure is defined by the nature of the 

flow, which can be laminar (streamlined), transitional, or turbulent, and is characterized by the 

Reynolds number (Re).

Figure 5.13. Normal Reynolds Numbers for the Tracheo-Bronchial Tree (Physiological Reviews 

41:314, 1961).

Turbulent flow transitions when Re > 2000. Reynold's number increases with the increase in 

linear velocity of gas (flow rate), density of gas, or radius of tube. As an example, breathing in 

quickly (which occurs during G-breathing) creates more turbulent flow throughout the 

tracheobronchial tree and significantly increases the work of breathing. From Figure 5.13, in the 

Nasal Canal, flow starts to become turbulent at flow rates greater than 30 L/min. Pilot air supply 

flow rates measured with instruments such as VigilOX (at altitudes under 23 kft) are lowerbound at 40 to 50 LPM, with flow arriving via tubes with radius larger than in the human system, 

thus the supply flow is above the Reynold’s number for turbulent flow at peak flows.
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For pilot breathing, the square of Q (supplied turbulent flow) is approximately proportional to 

the differential pressure ΔP. While not an exact solution to the pressure-flow relationship due to 

transitional flow dynamics, this relationship is useful and documented during the more than 100 

flights analyzed by the PBA on specially instrumented F-18 LOX supplied aircraft.

Figure 5.14. Mask Pressure (left), resulting Flow Rate (middle), and correlation when 

superimposed (right), shown using an F-18 sample. This is in direct conflict with observed F-35 

behavior.

As shown in Figure 5.14, when mask pressure is superimposed with flow squared, it yields a 

strong correlation. We note that the rising edge of the flow is preceded by the mask pressure 

signal by 1 sample time (1/20th second). The smooth pressure contour is a stark contrast in 

comparison to the jagged contour of the F-35 mask pressure

Pressure-Flow Disharmony is a mismatch between the pressure profile and the flow profile, 

including start/stop and time it takes to reach the peak (maximum). In the example (Figure 5.15), 

a mismatch between the Grey Mask Pressure and the Blue ESB (Exhalation) Flow2is shown (in 

the red box).

Figure 5.15. Exhale Breath #4 flow peaks at start of exhale. Mask pressure peaks at the end, as flow 

trends down. Shown using an F-18 sample. Breath #4 is an infrequently anomaly.

Time (sec) Time (sec)

Time (sec)
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Ideally, exhalation flow is an instantaneous response to a pressure signal. During exhalation 

through a mask, flow is expected to lag slightly behind pressure due to exhalation valve 

cracking-pressure and finite valve resistance. For real life examples we enlarge exhale breaths #4 

and #5 from Figure 5.15.

Figure 5.16. Exhale Breath #5 (top) and Exhale Breath #4(bottom) zoomed in view of breaths from 

Figure 5.15, shown using an F-18 sample.

Phase Shift in degrees builds on finding the optimal time-shift between 2 signals and takes in 

consideration the length of the exhale. Since breath time varies every breath, we can normalize 

each breath by its length such that the breath phase length totals 180 degrees. The red arrows 

(Figure 5.16) point to the peak flow and peak mask pressure, and between themselves, have a 

corresponding phase angle between 0 to 180 degrees. 20Hz sample times mean that if the peaks 

are off by one sample, there will be a phase shift of around 6 degrees (depending on the breath 

time).
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Both of these sensations are experienced regularly in mask breathing, and pilots adjust to small 

phase shifts routinely. 

Quantitative Results: Inhalation was analyzed in the same manner as exhalation, and 

distribution plots prepared for comparison. These histograms show the distribution of phase 

shifts present on the F-18 and F-35 for comparison. Dozens of PBA flights were characterized 

regarding phase shift. F-35 data was limited to the 2 tests described in this report.

For the F-18, Figure 5.17 (top 4 plots) shows how well pressure and flow match, with minimal 

delay on inhale (minimal negative phase shift). It may be helpful to think of negative phase shift 

as a valve that is “Slow to open”. During exhale, there is no positive phase shift, no pressure 

build-up in the mask, and minimal lag (minimal negative phase shift). 

For the F-35, Figure 5.17 (bottom 4 plots) shows far greater ratios of negative phase shifts 

compared to the F-18s and major mismatches in pressure/flow during both inhale and exhale. 

Both F-35 also have far greater ratios of positive phase shifts during exhale. It may be helpful to 

think of positive phase shift as a valve that is “Early to close”. Positive phase shift only occurs 

on exhale with few exceptions.

On Aircraft 1, the mode of Phase Shift is skewed to the left -30 degrees (in contrast to 0 degrees 

for other jets evaluated), with a wide distribution of phase shifts to the left and right. This 

graphically illustrates the preponderance and magnitude of the pressure-flow mismatches at a 

glance. While pressure-flow mismatches can be seen clearly in other data products showing 

select breaths, this metric is powerful because it creates a view of the pressure-flow 

characteristics over the entire data set in order to see systemic effects. Aircraft 1 has a 

quantifiable delay during inhale where the flow lags the pressure demand by 30 degrees on 

average, and very few breaths with 0 degrees phase shift. In comparison, Aircraft 2, the “normal 

breather,” had approximately half of the inhales with 0 degrees phase shift. This is an indication 

of a systematic inhale flow restriction and a perfect example of a pervasive breathing sequence 

disruption or disharmony. 

Correlation Results: Normalized Correlation (R) compares paired points of the signals. This 

method can be used to characterize how the shape of the pressure and flow of the entire exhale 

compare, not just the timing of the peaks as is done with phase shift. The output is between 0 and 

1. A value greater than 0.9 is a high correlation. Values for R in the F-18 are found to be around 

.9, as the teaching example from Figure 5.5 graphically showed earlier with pressure and flow 

closely matching the entire time. Values for R of 0.5 indicate a very large mismatch in 

correlation.

Correlation numbers are 0.42 to 0.59 for the F-35 aircraft over the course of 20 minutes on the 

ground only, contrasted with 0.9 for the 1-hour flights on the F-18 aircraft. Correlation indicates 

the magnitude of the overall match or mismatch of pressure with flow, whereas phase shift 

provides insight into the direction and source of the mismatch in pressure and flow. These low 

correlations numbers are in agreement with, and a good quantitate measure of the visual 

hysteresis shown in the breath-by-breath examples of Figure 5.3. 

Phase shift and correlation results are in agreement with the breath-by-breath hysteresis plots and 

are a useful quantitate measure of the breathing sequence disorder that is visually apparent on 

plots of individual breaths. From the above phase shift analysis and side-by-side analysis of the 

F-35 pilot breathing, it appears that the inhalation positive pressure scale and timing is over-
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 132 of 260

compensating for lag (pressure-flow response time) in the system. Lag during inhale can result in 

reduced inhale flow and lag at the end of inhale can result in reduced exhale flow. 

Figure 5.17. Phase Shift Plots of Inhale/Exhale for PBA F-18 and F-35 Aircraft 1 and 2. 

F-35 aircraft exhibit increased phase shift and more erratic breathing behavior than legacy F-18 

aircraft with or without safety pressure. In ideal breathing all flow should correlate to the driving 

pressure, and all instances should be in the (-10, 0] bin, even if we consider the slight delay breathing 

through a system.
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A micro look at the F-35 Inhale: The demand safety pressure regulator system in the F-35 is 

aggressive at maintaining/restoring a high safety pressure. Peak Inspiratory Pressure (i.e., the 

peak drop in pressure, since inspiration decreases pressure) (Figure 5.18, top) is predominantly at 

the very beginning of the breath where the pressure drop from an inhaled breath is steepest. This 

can also be seen in Figure 5.3 where pressure drops, but there is no flow. This explains why 

inhale phase shift is almost always negative. Also, the Peak Inspiratory Flow (Figure 5.18 

bottom) has jagged pressure plateaus due to inspiratory pressure oscillation. This is the source of 

the large variation in distribution of the phase shift; flow reaches its peak in a chaotic fashion, 

but always late. Whether the timing and pressure response of the F-35 regulator is designed this 

way intentionally is unknown: the regulator is a black box.

Figure 5.18. Sample F-35 Peak Inspiratory Pressure (PIP) and Peak Inspiratory Flow (PIF), where 

the flow should be much closer co-aligned as a response to the driving pressure

Phase Shift Analysis is a numerical tool to quantify disharmony between pilot breathing pressure 

and the breathing system flow (pilot demand vs. aircraft supply). The test results on the F-35 are 

corroborated by independent pilot observations. The F-35 presents quantifiable phase shift 

disharmony during inhale and exhale through its breathing system. The values of these shifts 

indicate significant deviations from the ideal pressure-flow relationship, and are much greater 

than in the F-18. Phase shift analysis addresses specific causes and outcomes, but does not 

address the entire breadth of the issues at work. 

For flights where both mask pressure and flow are available, apply phase shift and hysteresis 

analysis for early detection of equipment issues, or validation of pilot reports. Flights or 

segments can be collapsed into single numbers of Phase Shift Mean, Standard Deviation, Lag 

time, and correlation coefficients.
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5.7 Energy Management: Pressure Oscillation FFT and Dynamic Pressure

Pressure oscillations, like all waves, carry energy. After taking the Fast Fourier Transform (FFT) 

of the mask pressure signal to analyze the frequency in comparison to the F-18 (Figure 5.19), a 

concerning observation emerges. Between 2 to 6Hz there is an order of magnitude more 

frequency content in F-35 Aircraft 1 compared to the F-18. Power is the coefficient of the FFT 

squared. So a factor of 10 in the coefficient is a factor of 100 in the power. That could be a lot of 

energy the pilot basically has to absorb or fight. These oscillations are not present on PBA flights 

and in the F-35 these oscillations exceed the AIR-STD 4039 as discussed in Section 6.

Figure 5.19. Fast Fourier Transform comparing F-18 to F-35 mask pressure frequency components 

demonstrating the increased power loading on the pilot breathing response in F-35.

This is a single comparative data point and should be viewed accordingly. What is clear is that 

the F-35 exceeds the standards for allowable pressure oscillations and exhibits a profound 

disconnect in its attempts to quickly and forcefully respond. 

5.8 Perception of Breathing Dynamics

Exhalation is difficult in the F-35. Mask pressure swings resembling a saw tooth during 

exhalation (in contrast to a smooth peak or a steady plateau) are characteristic of the chaotically 

changing pressure conditions inside the mask due to sources other than the pilot. Constantly 
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changing mask pressures during exhalation predominantly track the changes of inhale supply line 

pressure, which should not be changing. 

Pilot feedback, observations of the raw data, and processing of the data as in the frequency 

curves (Figure 5.19) all agree. Excessive exhale pressure is a common complaint in pilot 

feedback, the raw data exhibit extensive pressure fluctuations equal to or greater than pressures 

used during normal exhale, and the hysteresis plots demonstrate disruptive dynamics consistent 

with mid-speech pressure kickbacks that can stop vocalization.

The transition between inhalation and the start of exhalation is inconsistent. Sometimes F-35 

breathing transitions are smooth and seamless; sometimes there is a staggered transition between 

the end of inhalation and the start of exhalation. Breathing challenges during inhalation and 

exhalation routinely come to the pilot’s conscious attention and exceed the ability of human 

physiology to compensate without conscious effort.

6.0 System of Systems Interaction

It has been previously stated by the NESC that aircraft act as a “System of Systems”. That is, the 

aircraft is a conglomerate of individual systems, with small changes in the behavior of any one 

system potentially contributing to an aggregate effect which may lead to large impacts on the 

aircraft as a whole. This effect was widely discussed in the previous 2017 report by the NESC F18 PEAT. The F-35 exhibits “system of systems” behaviors. 

The F-35 data shown in Figure 2.1 were the main input to the design of this test. The changes in 

breathing dynamics with the selection of defog and removal of the G-suit were so remarkable 

and noticeable that it drove the design of experiment selected. At the time, little was known 

about the F-35 breathing dynamics, so large repeatable changes were targeted on systems known 

to cause noticeable changes for comparative analysis. The selected points were not conditions an 

operational pilot would be expected to encounter for extended periods of time under normal 

circumstances. They were intended to elicit the underlying dynamics responsible for the 

subjective exhale backpressure which has been extensively documented. The points listed below 

are selected the full data set of script points (Tables 7.1 and 7.2). 

First, however, breathing standard requirements are shown for understanding of how the F-35 

data compares to established standards:

Military Standard 3050 / Air Standard ACS (ASMG) 4039 [Figure 6.1]

Trumpet Curve Plots / Mask Pressure Swing Plots / Oscillatory Activity [Figure 6.2]

System of System Comparisons:

Normal Relaxed Breathing (Baseline Breathing) [Figure 6.3]

Effects of Maximum Inhale (2x Max Inhale/Relaxed Exhale) [Figure 6.4]

Effects of Backup Oxygen System (100% Oxygen) [Figure 6.5]

Effects of Defog On (Defog Full On – Hi Flow/Hi Temp) [Figure 6.6]

Effects of G-Suit Interaction [Figure 6.7]

Push-to-Test (PTT), G-suit Connection, and Mask Off/On [Figure 6.8]
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Effects of Maximum Inhale (without G-suit) [Figure 6.9]

Effects of Rapid, Deep Breaths (without G-suit) [Figure 6.10]

Effects of Increased Engine Power Setting (without G-suit) [Figure 6.11]

6.1 Standards Review

Military Standard 3050 

Trumpet curves are one of the traditional tools for of analyzing a regulator’s performance during 

peak inspiratory and expiratory flows in order to ensure that mask pressure does not become 

excessive. The specifications of these curves are detailed in MIL-STD 3050 (Figure 6.1, top). 

The values for aircraft with safety pressure are different than for aircraft without safety pressure, 

and the values applicable to the F-35 are circled in red. Note that in addition to the minimum and 

maximum mask cavity pressure for each given flow value, there is also a maximum swing value. 

These values are plotted for Segment 1 (Figure 6.1, top and middle).

Air Standard ACS (ASMG) 4039

The standards for oscillations are detailed in AIR STD ACS (ASMG) 4039 (Figure 6.1, bottom). 

The general requirement is straightforward: Not produce significant oscillations of pressure 

within the mask cavity. Note that this requirement applies to the “complete breathing system”, 

but is measured at the nose and mouth of the user, termed the ‘mask cavity’. The detailed 

requirement for what is termed ‘Oscillatory Activity’ is circled in red, prohibiting any 

oscillations lasting longer than .25sec from exceeding a double amplitude (peak to peak) pressure 

of .25inWG (or .25 in H2O). These values are plotted for Segment 15 on Aircraft 1 and Segment 

1 on Aircraft 2 respectively (Figure 6.2, bottom). It has been noted that the curves have not been 

evaluated or corrected for any potential influence (added resistance) of adding the VigilOX ISB 

and ESB into the breathing loop; these effects are believed to be small. 

Applicability to the F-35

The F-35 program represents one of the largest military acquisition programs in history and 

discussion of the process for accepting risk, notably the decision to forego dedicated 

developmental testing of the breathing system, is well outside the scope of this paper. However, 

we note that while the standards shown in Figure 6.1 significantly predate this acquisition, the F35 aircraft were not required to meet these standards. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 137 of 260

Figure 6.1. Chart excerpt from MIL-STD 3050 dated 11 May 2015 (top). Oscillator Activity excerpt 

from AIR STD ACS (ASMG) 4039 dated 12 Feb 1988 (bottom).
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6.2 Comparison to Standards

Trumpet Curve Plots

The mask pressures for Aircraft 1’s trumpet curve are all well within the limits. Note that both 

the inhale and exhale peaks are clustered below the 50 LPM (Figure 6.11, top left), as opposed to 

the more even distribution seen on Aircraft 2.

The mask pressures for Aircraft 2’s trumpet curve are mostly within the limits with a few 

exceptions that are clustered around the lower limit during inhale (Figure 6.11, top right).

We acknowledge that the introduction of any measuring device adds a factor (in this case 

expected to be a minimal offset) to the phenomena being measured. It is not our intent

characterize any offset here, but to focus on dynamics and differences between these data and 

those taken similarly in other PBA aircraft.

Mask Pressure Swing Plots 

The mask pressure swings on Aircraft 1 are clustered between 30 to 50 LPM and below 1inH2O 

(2 mmHg). These values are unusually low, and well within the specification. The cluster 

corresponds with the trumpet curve values clustering, and together with the pilot’s report of 

feeling as if breathing was constrained, raise an unexpected issue. Mask pressure is typically 

thought of as a demand signal from the pilot because of the linear relationship between pressure 

and flow in the absence of a regulator. However, under conditions where pressure-flow 

relationships are non-linear, as was shown previously, that relationship breaks down. Here the 

low mask pressures appear to be a distinct indicator of a broken pressure-flow relationship, and 

not an indicator of reduced demand from the pilot. 

The mask pressure swings on Aircraft 2 exceed the MIL-STD-3050 specification limit 

approximately 50% of the time. Note that the values plotted for the mask pressure swings are just 

peak values, not all data points. Hence, points are expected near the limit line. Ignoring the 

outliers, mask pressure swings routinely exceed 3 inH2O. These are big pressure swings for such 

low flow. These pressure swings are concerning because they can contribute to several undesired 

physiological outcomes, not the least of which is barotrauma. These outcomes were unexpected. 

The mask pressure on the ESB and the flow sensor on the ISB are the two most trusted sensors. 

Both the data exceeding spec on Aircraft 2, and the unusually low values from Aircraft 1 should 

be cause for follow up investigation, especially given the difference dynamics between the two 

systems.

Oscillatory Activity

Exhale flow on the F-35 is characterized by extreme oscillations that exceed Air Standard ACS 

(ASMG) 4039 limits almost continuously. The magnitude of oscillations greatly exceeds the .25 

inH20 limit on almost every single exhale. Several oscillations 5 times larger than the .25 limit 

can be seen (Figure 6.11, bottom). Segments in red exceed .25s from peak to trough on a single 

half-cycle swing. Most oscillations are about .2s peak to trough (2 to 3Hz), but continue to 

oscillate for much longer than .25 sec. This is extremely concerning due to the energy that can be 

contained in high frequency pressure oscillations in addition to any breathing sequence 

disruption. The potential harm from these exceedingly large and overwhelmingly pervasive out 

of spec pressure oscillations should not be discounted.
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Figure 6.2. MIL-STD 3050 Trumpet Curves (top), MIL-STD 3050 Mask Pressure Swings (middle), 

Mask Pressure Oscillations (bottom).
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6.3 Normal Relaxed Breathing

Prior to beginning the system level comparisons a few concepts must be defined. The following 

terms are discussed in more detail in the section on Physiology, but are defined here for 

reference since they are used extensively in this section.

Tidal Volume: The volume of air that is moved with each breath is defined as the Tidal Volume 

(TV). At rest TV is approximately 0.5 Liters or 500 mL, which can increase greatly with 

exertion. In this section, TV refers specifically to the volumes calculated from the ISB/ESB 

flows.

Minute Ventilation: Pulmonary ventilation is the volume of gas per unit time entering the lungs, 

often defined as Minute Ventilation (MV) in units of Liters/min abbreviated in this section as 

LPM. MV is a direct product of TV (liters/breath) and respiration rate (breaths/minute). MV 

requirements are driven by the body, as metabolic rates increase, MV will correspondingly 

increase to match the current physiologic needs. During a period of rest, and absent any acute 

changes in metabolic needs such as exertion, MV should demonstrate minimal variance over a 

short sampling period. Changes in MV observed within this data set are reflective of external 

forces on the human limiting physiological needs.

Inhale Flow (Red) and Exhale Flow (Blue) Conventions

Like the pressure versus time plots (Grey/Purple) that have already been introduced, the 

inhale/exhale flow versus time plots (Red/Blue) in Figure 6.1 are used extensively in this section. 

The convention for inhale flow is red, shown below the axis to line up with the pressure drops 

during inhale. The convention for exhale flow is blue, shown above the axis. The flow and 

pressure charts are time synchronized and shown one above another for easy comparison. 

Normal Relaxed Breathing (Baseline Breathing)

Initial data was taken for resting, relaxed breathing in F-35 Aircraft 1 and 2 with no additional 

activities such as talking or body movement in the cockpit. This data are shown for Aircraft 1 

and 2 in (Figure 6.3). TV and MV are significantly lower in Aircraft 1 than in Aircraft 2; with 

MV a full 50% lower, and TV 25% lower. The average mask pressure swing is lower on Aircraft 

1 (the “bad breather”) than Aircraft 2. Pilot interview stated that “The experience is one of 

breathing being constrained or limited”. These lower mask pressure swings are indication of a 

flow limitation in the system, which would be interpreted by the pilot as restriction and limited 

air available to breathe. Breathing is inherently stochastic, and the reduction in variability on 

Aircraft 1 (the sinusoidal appearance of the flows with peaks all clustered near 30L/min) in 

comparison to the flows of Aircraft 2 (variable peak flows from 30 to 70L/min) is also an 

indication of a constraint.
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Figure 6.3. Baseline, Normal, Relaxed breathing for both F-35 aircraft during Segment 1.
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6.4 Effects of Maximum Inhale

The positive pressure supplied by the F-35 system leads to unexpected dynamic behaviors when 

the pilot attempts a “maximum inhale”, or a sudden strong intake of breath followed by a relaxed 

exhale.

Maximum inhale maneuvers for the two F-35 aircraft allowed means of 3.4 L and 4.l0 L in 

single breath tidal volumes; although this represents a significant difference between the two 

aircraft, these numbers are not considered restrictive as the 99th percentile is 2.04 L for realworld flights from the main PBA study.

However, the breathing dynamics during exhale are characterized by a sharp exhalation shape 

and transitions from inhale to exhale that are not smooth (Figure 6.4). In Aircraft 1 there is 

significant lag as pressure increases during exhale before flow begins. Once flow begins, it rises 

rapidly to a sharp peak and then declines rather than maintaining a steady flow. Peak expiratory 

flow is higher at 117.9 LPM and occurs during the beginning of exhale due to the pressure buildup prior to flow occurring and the continuous decline throughout the remaining exhale. 

Breathing dynamics during inhale are characterized by a drop in line pressure that sharply 

reverses near 0 mmHg and returns to a nominal safety pressure during the remainder of the 

inhalation flow with overshoots of approximately 1 mmHg. The peak (negative) mask pressure is 

-6.5 mmHg, and the Peak Inspiratory flow is lower at 235.2 LPM.

In Aircraft 2, the breathing dynamics during exhale are characterized by a smoother transition 

from inhalation with a declining inhale flow rate that seamless transfers with no delay into an 

exhale flow rate that matches the rate (slope) of declining inhalation. The peak expiratory flow is 

slightly lower at 97.8 LPM due to the rounded distribution of flow over the duration of the 

exhale with the peak flow occurring near the middle of exhale. However, breathing dynamics 

during inhale are characterized by a line pressure oscillation of approximately 8 mmHg during 

both breaths. The peak negative mask pressure of -10.50 mmHg is more negative due to the 

oscillation; note that the peak negative mask pressure coincides with a 7 mmHg drop in line 

pressure, immediately followed by a rapid increase of 8 mmHg. The high frequency nature of 

these oscillations is not attributable to human input. Peak Inspiratory flow is higher for these two 

breaths at 279.6 LPM.

O2 concentration is dropping for both aircraft (green line in the top graph of Figure 6.4) during 

the maximum inhalation test. While the aircraft is supporting the pilot’s increased breathing, 

instability of O2 concentrations during maximum breathing are not desirable. Stability of O2

concentration during maximum breathing is desired because decreases in O2 concentration 

during increased breathing demand for O2 are counterproductive.

While exhale was more impacted in Aircraft 1, inhale was more impacted in Aircraft 2. Notice 

that Aircraft 2, which was anecdotally described to be the “normal breather” aircraft, while 

exhibiting overall smoother exhale features, still has undesirable pressure fluctuations during 

exhale and more importantly demonstrates the largest line pressure oscillations seen in all of the 

data. Breathing dynamics depend on many different factors, and this exemplifies the importance 

of the testing all aspects of a system since the inhale/exhale dynamics can have problems both 

dependently and independently of each other and can vary from system to system (aircraft have 

personalities).
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Figure 6.4. Maximum Inhale for both F-35 aircraft during Segment 2.

Note 1: In order to keep the time scale the same, both segments are 15 seconds long; that results 

in more than 2 breaths in the second window, lowering the average TV in the window. When 

recalculated with only the two Maximum Inhale breaths as is the intent of this segment, the TV is 

4.00L

Exhale 

Spike 

Shape

Not 

smooth 

transition

Exhale 

Shape 

Round

Smooth 

transition

Inspire 

sharp 

reversal

Line 

pressure 

oscillations
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6.5 Effects of Backup Oxygen System (BOS)

The Backup Oxygen System in the F-35 is a high-pressure bottle completely independent of the 

OBOGS designed to supply 100% O2 to the pilot for emergency use. This bottle supplies the 

pilot through the same regulator, which does not change functionality during normal operations. 

There is a failsafe which allows the BOS to bypass the regulator in a trickle flow mode, but that 

is not testable under normal circumstances. 

A very important takeaway here is that many disruptive breathing patterns persist despite 

OBOGS being removed from the system. While this suggests that the OBOGS is not the primary 

source of the observed breathing anomalies, the OBOGS contribution to these dynamics should 

not be neglected. This is especially true for failure modes in the F-35 since the BOS waits to turn 

on until the plenum depletes, which makes for a very abrupt transition. The primary backup 

supply of O2 is a critical part of the life support system, and its dynamics (including transitions) 

should not be neglected in testing. Drops in supply pressure are known to challenge regulators in 

general, and the worst time to have a disruptive breathing dynamic is at the very time when 

primary O2 supply is at or near failing. OBOGS DEGD advisories and transition to and from 

BOS have occurred frequently in reported F-35 physiological events immediately prior to the 

onset of symptoms. It is also notable that BOS often does not resolve the symptoms immediately 

based on F-35 PE reports, indicating the primary problem is not O2 concentration.

On Aircraft 1, (Figure 6.5) the mean TV is higher at 1.2L, yet the MV is lower at 11.98L due to a 

much lower respiration rate of 10 Breaths/min. Breathing dynamics are characterized by larger 

mask pressure swings and minimal separation between mask pressure and line pressure; mask 

pressure swings are from 5.2 to 2.9 mmHg in the green circle on Figure 6.3. As noted above, 

even with BOS activated and the OBOGS out of the loop, oscillations are present. In this case, 

vibrations during exhale predominate at 1.8 to 2Hz. Note the mask pressure does not have good 

separation from the line pressure and they frequently track together with swings of 1 to 2 mmHg 

several times during each exhale.

On Aircraft 2, (Figure 6.5) the mean TV is .84L (nominally the same as baseline in segment 1), 

and MV is 15.04L with a respiration rate of 18 Breaths/min. Breathing dynamics are 

characterized by less frequent oscillations of lower magnitude. Note the mask pressure has good 

separation during exhale with the line pressure staying at a nominal safety pressure of 3 mmHg. 

The mask pressure and line pressure occasionally track changes together, but the pressure swings 

are predominantly less than 1 mmHg.
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Figure 6.5. Backup O2 System activated for both F-35 aircraft during Segment 3.

2Hz Vibrations
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6.6 Effects of Defog On

The Defog system in the F-35 controls the temperature, distribution, and flow of the air entering 

the cockpit. The air temperature is increased the maximum amount, diverted to the canopy, and 

the flow is increased in order to defog the canopy or preheat it in order to prevent canopy 

fogging. Selecting defog has a noticeable impact on the pressurization of the cockpit with cabin 

pressure transiently exceeding several hundred feet of pressure change. There is a pronounced 

sensation of more difficult exhalation in the mask and breathing being constrained considerably, 

even after the transients stabilize. 

The changes in breathing dynamics with the selection of defog were so remarkable and 

noticeable to the test pilot collecting this data that it drove the design of experiment selected for 

the segments and emphasis on system of systems interactions. Perception of differences in 

breathing are difficult enough for pilots given the overwhelming number of sensory inputs 

present in the cockpit and flight environment. A repeatable and reversible system interaction that 

causes a marked and noticeable change in breathing offers a unique opportunity to correlate 

sensations with data. As is discussed in more detail in the medical section, “Fit pilots are poor 

perceivers of decline in lung function hence need objective measures”. These sensations, though 

pronounced, are easy to miss, as their magnitude is dwarfed by far greater sensory inputs 

experienced continually such as the mild roll and G-forces present during every turn made in 

flight, let alone high G maneuvering. 

On Aircraft 1, the mean TV is .62L (same as baseline), MV is 7.41L (lower than baseline), and 

Respiration Rate is 12 Breaths/min (lower than baseline). Breathing dynamics are characterized 

by low variability in peak flow and the smallest pressure swings of all segments with mask 

pressure never dropping below 1.2 mmHg, and usually not dropping below 2 mmHg. While the 

exhale mask pressures show good separation from the line pressure, inhale pressure drops are 

only slightly larger in magnitude than the pressure oscillations themselves.

On Aircraft 2, the mean TV is 1.00L (greater than baseline), MV is 13.02L (lower than baseline), 

and Respiration Rate is 13 Breaths/min (much lower than baseline). Breathing dynamics are 

characterized by sharp spikes down during inhalations and larger pressure swings.

For both aircraft, MV during this segment was impacted, and the lowest of all segments. While 

the sensation of difficult exhalation was pronounced and MV did decrease in both cases 

compared to baseline (1.4L and 3.3L, respectively), that decrease is not nearly as large compared 

to the difference between aircraft of 5.6L (43% decrease) in segment 4 (Figure 6.4). Despite the 

overall impression that Aircraft 1 was a “bad breather”, it should be noted that there was no 

particular sensation or indication to the pilot of the magnitude of differences between MV, 

underscoring the silent and unnoticed nature of many of these changes in breathing dynamics. It 

is troubling to consider the possibility that a potential decrease in minute ventilation up to 50% 

could present to a pilot as a sensation that was just a little bit off. 
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Figure 6.6. Defog activated for both F-35 aircraft during Segment 4.
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6.7 Effects of G-Suit Interaction

The G-suit on the F-35 connects to the same regulator as the pilot’s breathing system. The 

combined BRAG in the F-35 has a Press-To-Test function (PTT) which is used during normal 

checklist procedures to manually test the G-suit and mask prior to flight in order to ensure proper 

function. The PTT function is scheduled to deliver 18 inches of water gauge pressure to the 

pilot’s mask, and 55.4 inches of water gauge pressure to the pilot’s G-suit. 

G-suits are not normally disconnected in an operational squadron; however depot operations 

require routine cross country sorties during deliveries of aircraft around the country, and these 

are generally accomplished without G-suits. During the course of flying dozens of these 

deliveries, with plenty of uninterrupted time for observation, it was noted by one pilot that the 

absence of the G-suit caused a material change to the breathing experience. The G-suit 

connection should not impact the breathing experience as the systems should operate 

independently; however, even though the pressure supply to the G-suit is entirely separate from 

the OBOGS pressure supply to the mask, they both connect through the BRAG in close 

proximity. 

During PTT without a G-suit, the BRAG still attempts to inflate the non-existent G-suit. This 

causes air intended to inflate the G-suit to flow into the cockpit by the pilots left hip. PTT takes 

several seconds to reach full strength steady state pressure in the mask and G-suit, and since air 

was streaming out of the G-suit port, usually only held momentarily. Holding PTT for much 

longer than usual until pressures stabilized in this condition resulted in an observation of 

markedly lower mask pressure than was customary. This was found to be repeatable. Therefore, 

the G-suit was disconnected during these segments in order to understand why aircraft breathe 

different without a G-suit connected. PTT was also intended as a benchmark pressure since it is 

supposed to deliver 18 inches of water gauge pressure. If the G-suit port (Figure 6.5, Red Cap) is 

covered (essentially plugged with a thumb) during PTT while air is attempting to inflate the 

missing G-suit, the mask pressure instantly increases, and conversely, when the G-suit port is 

uncovered allowing air to flow freely, mask pressure decreases. This pressure difference is so 

large, that it is very easy for a pilot to sense. 

“The possibility of reduced bleed air pressure at the OBOGS generator reducing pressure 

at the regulator (due to G-suit air freely flowing into the cabin) appears to be ruled out 

entirely since the same effect occurs in BOS, which has nothing to do with OBOGS 

pressure. The second possibility appears to be what is happening; the regulator baseline 

or reference pressure appears to be skewed by the G-suit venting/plumbing” [Test Pilot 

original write-up submitted to F-35 program office]

Segments from this point on in the report are without the G-suit connected. Without the G-suit 

connected the pilot reported that the breathing dynamics were significantly improved on Aircraft 

1.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 149 of 260

Figure 6.7. G-suit (left), black electronic regulator [below a BOS bottle] (right), and mask (bottom). 

Both G-suit and mask connect to the BRAG [note that the mask shown is not an F-35 mask; the 

figure only depicts proper system connection locations].

6.8 PTT, G-suit connection and Mask Off/On

In Segment 5 (Figure 6.8, top), PTT was accomplished with the G-suit connected. The mask 

pressures stabilized at and slightly above 20 mmHg.

In Segment 7 (Figure 6.8, middle), PTT was accomplished without the G-suit connected on the 

left side of both graphs. The mask pressures stabilized 5 to 7 mmHg lower at 13 mmHg and 15 

mmHg respectively for Aircraft 1 and Aircraft 2. When the G-suit port was covered and 

uncovered repeatedly, mask pressure in both aircraft increased by approximately 1 0 mmHg 

(Figure 6.6, middle). The right sides of the graphs, which show quick successive spikes and 

valleys, are where the port is plugged (covered) repeatedly. 

In Segment 11 (Figure 6.8, bottom), after doffing the mask (middle section flat lines), it was 

difficult to restart flow on Aircraft 2. The first attempt to take a breath after donning the mask 

resulted in approximately -10 mmHg in both mask pressure and line pressure without any flow 

initially causing a distinct “sucking rubber” sensation. Negative 10 mmHg without any flow is a 

significant respiratory insult. This was true for Aircraft 2 in both Segment 5 and Segment 7

(circled in green). In both cases there was an unsuccessful attempt to initiate breathing during the 

first drop in pressure and a subsequent successful attempt on the second drop in pressure. Note 
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that on Aircraft 1, this breathing dynamic was not present, and although the line pressure drops 

to -7 mmHg during the first attempt to inhale, the mask pressure remains steady at 0 mmHg, and 

flow started with minimal delay. 

While Aircraft 2 was anecdotally described as the “good breather”, this is one of several 

breathing dynamics during inhale that were less than desirable. 

Figure 6.8. PTT with G-suit connected (top graph), PTT with G-suit disconnected (middle graph), 

Mask doffing with G-suit disconnected (bottom graph) for both F-35 aircraft during the respective 

Segments as labeled. (Top Right graph) also has Mask Doffing with G-suit connected. 

A note about F-35 data quality: The developmental VigilOX units used in this test have been 

known to suffer occasional errors that introduce spikes into one recorded channel (not all 

simultaneously). In the present case, the data quality is helped by evaluating multiple channels 

together and explaining particular features in the data alongside pilot notes of perceptions during 

these short acquisition windows. Additionally, when compared to the subset of 24 F-18 and F-15 

PBA flights taken with the same type of developmental unit, again, the differences between 

patterns in the three jet types are clear.

Hard to draw first 

breath (-10mmHg) 

Plugging G-suit 

port repeatedly 

Pressure lower 

without G-suit 

Hard to draw first 

breath (-10mmHg) 

First breath only 

drops mask 

pressure to 0mmHg 

G-suit disconnected F35 Aircraft 1 G-suit disconnected F35 Aircraft 2

Hard to 

draw first 

breath 

(-10mmHg) 
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6.9 Effects of Maximum Inhale (without G-suit)

When the G-suit is disconnected the dynamic behaviors are different and it is easier to breathe. 

The intent of the maximum inhale segment followed by a relaxed exhale was to create a 

repeatable point (maximum inhale volume at maximum effort is fixed) in order to show 

comparative differences in breathing dynamics between aircraft and in the presence of system 

interactions. While changes in breathing dynamics without a G-suit are not operationally relevant 

and normally fall into the category of degraded systems operations, the underlying change in 

dynamics are an important pointer to fundamental systems dynamics and their impact on 

breathing.

On Aircraft 1, the mean TV was 3.72. TV increased .32L above the 3.4L mean in Segment 2 

with the G-suit connected. This was consistent with the pilots report of easier breathing with the 

G-suit disconnected. The breathing dynamics during exhale more closely resemble Aircraft 1. 

They are no longer characterized by a sharp exhale flow shape with an immediate peak. Flow 

still peaks early during exhale, but the shape is more rounded. While transitions from inhale to 

exhale are still not as smooth as Aircraft 2, the delay in transition to flow is decreased, and mask 

pressures are more commensurate with the resulting flow. Breathing dynamics during inhale are 

characterized by a drop in line pressure near -2.5 mmHg without a subsequent sharp return back 

to safety pressure as before.

On Aircraft 2, the mean TV was 3.98. This was essentially unchanged from 4.0L in Segment 2 

with the G-suit disconnected. Aircraft 2 exhibited the largest line pressure oscillation in all of the 

data during Segment 2, and while oscillations are still present during inhale, they are now of 

lesser magnitude and lower frequency.

O2 concentration drops during the first breath for Aircraft 1 by 28%, but increases on the second 

breath. On Aircraft 2 O2 concentration remains steady, actually increasing after the two 

maximum inhales (green line in the top graph of Figure 6.9).

In comparison to Segment 2 with the G-suit connected, the data without the G-suit connected had 

overall better breathing dynamics; increased TV and decreased exhale resistance on Aircraft 1 

and decreased inhale oscillations on Aircraft 2.

Until the development of hysteresis and phase shift metrics, a repeatable measure with the same 

pilot such as this was the closest substitute for an objective breathing metric. In addition, it is 

important to test the “corners of the envelope”, as aircraft breathing systems have a requirement 

to support breathing in the entire breathing envelope. Maximal Inhales elicited breathing 

dynamics not observed elsewhere in the data and should be considered essential for any end to 

end system testing.
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Figure 6.9. Maximum Inhale for both F-35 aircraft during Segment 8.
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6.10 Effects of Rapid, Deep Breaths (without G-suit)

During Segment 9, the goal was not a maximal and repeatable inhale, but rather a continuous 

demand for a maximum amount of air. Instead of a relaxed exhale, here the exhales are forceful. 

Basically the goal was maximum effort, continuous breathing for 15 seconds, with no attempt to 

have a specifically defined rate or depth of breathing. Accordingly, no attempt is made to 

compare TVs, MV, or peak flows because of the inherent variability of the design.

On Aircraft 1, the breathing dynamics are characterized by spikes in the line pressure during 

exhale (Figure 6.10, bottom left) and inhale flow continuing during the beginning of exhale flow 

(Figure 6.10, top left) Conversely, this can be viewed as exhale flow beginning before inhale 

flow ends. Either way it appears as a gap of white space during the transition in flow from inhale 

to exhale. While this resembles an inhale valve malfunction, this only happens during this 

segment, and only on Aircraft 1. These points were accomplished with the same mask. 

As discussed previously, when the inhale valve does not sequence closed properly, pressure can 

flow back down the line. That usually causes an ISB DFLR bit, but in this case there is no DFLR 

bit associated with Aircraft 1 during this Segment. The alternative is that both the flow from the 

regulator and the flow from the pilot can exit the exhale valve at the same time due to the 

overshooting pressure from the regulator. The second scenario is more consistent with the data, 

especially considering that the peak expiratory flow of 340LPM is the largest peak flow in all the 

data, and would not likely result with flow going back down the inhale line. Regardless, if both 

valves are open simultaneously as the data strongly suggest, neither breathing dynamic is 

healthy. Note the mask pressure in excess of 30 mmHg immediately after the line pressure 

dynamically overshoots past 10 mmHg. These values are in the range capable of causing 

barotrauma. 

In addition, the extended high demand causes the O2 concentration to drop 43% during this 

segment. Unfortunately from a physiology standpoint, poor breathing dynamics often line up 

with rapid changes in O2 concentration. This is a good example.

On Aircraft 2, the breathing dynamics are characterized by smoother transitions to exhale with 

no gap (Figure 6.10, top right). Line pressures only marginally overshoot during exhalation 

(Figure 6.10, bottom right) and stay close to the nominal safety pressure value. The ISB DFLR 

bit was set for a period of less than .4 seconds during one exhale in this segment. O2 drops 25% 

during this segment. 
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Figure 6.10. Rapid, deep breaths for both F-35 aircraft during Segment 9.

N/A N/A

Line Pressure 

spikes during 

exhale! 

Line Pressure 

closer to safety 

pressure 

Exhale Flow starts early 

(gap) 

Exhale Flow seamless 

transition 
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6.11 Effects of Increased Engine Power Setting (without G-suit)

In the F-35 at idle power, the OBOGS is supplied with a lower pressure from the ECS compared 

to increased power settings. The pressure of the air supplied to the OBOGS determines the 

pressure the OBOGS is able to supply the regulator, and hence can change the response 

characteristics of the regulator and resulting breathing dynamics. The F-35 does not use RPM or 

EGT to manage thrust like legacy aircraft, but rather uses the Expected Thrust Request (ETR). 

This is basically how much thrust is expected at the selected throttle position compared to the 

total thrust available as measured on a scale with 100% being Military Power (full power without 

afterburner). At idle on the ground, ETR is usually 10%, or 10% of total available thrust. 15% 

ETR is a significant increase in power as far as ECS bleed air is concerned, but still a nominal 

value used during normal ground operations. 15% ETR was selected during this segment. 

On Aircraft 1, the mean TV was .93L, the MV was 13.92L, and the Respiration Rate (RR) 15 

breaths/min. The breathing dynamics are characterized by significantly improved separation 

between mask pressure and line pressure during exhale, and a concomitant increase in TV and 

MV. Compared to baseline in Segment 1, TV is increased from .63L, respiration rate is slightly 

increased from 14, and overall MV is increased from 8.79L (a 58% increase). 

On Aircraft 2, the mean TV was .91L, the MV was 13.58L, and the RR was 15 breaths/min. 

Compared to baseline in Segment 1, TV is increased from .82L, respiration rate is decreased 

from 20 breaths/min, and overall MV is decreased from 16.36L. See Figure 6.11.

In comparison, Aircraft 1 and 2 are now essentially equal. Breathing dynamics result in TV, MV, 

and RR which are all nominally the same. Compared to baseline, Aircraft 1 has TV and MV 

which have increased approximately 50%. Compared to baseline, Aircraft 2 has MV and RR 

decreases of approximately 20%, with a TV increase of approximately 10%. 

While Segment 1 was intended as a reference point for comparison and is herein called a 

baseline, it should not be confused with a true baseline of physiological values. Such a baseline 

does not currently exist and is not currently possible to ascertain accurately in the cockpit of any 

fighter or trainer aircraft. One of the main goals of the PBA is to create a database of pilot 

breathing on legacy aircraft for comparison. In other words, help answer the question, “What is 

normal breathing in a fighter?”

The trends and comparisons present in this data, however, still provide an information on system 

behavior. 
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Figure 6.11. Increased Power Setting for both F-35 aircraft during Segment 12.

Exhale separation better 

Mask Pressure > Line 

Pressure 
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Figure 6.12. O2 concentrations and regulator pressure schedule for an aircraft flying to 50.000 feet 

with a 5 psi differential pressure cabin.

7.0 Analysis of Reduction in Minute Ventilation

The goal of a breathing system is to provide an appropriate volume of air over time; hence the 

volume of air over one minute, minute ventilation (MV), is one of the most relevant parameters 

in analyzing breathing system impact on physiology. The data presented in Figure 7.1 (top three) 

were discussed in Section 6. Tables are presented here for ease of comparison. In summary, 

aircraft environmental settings and G-suit connections changed respiratory patterns (MV, TV, 

and respiration rate) by variable, inconsistent, and often physiologically significant amounts. On 

two separate F-35 aircraft, ground tests with the same pilot using similar scripts and with 

expected similar metabolic loads resulted in ~50% differences in measured MV and exhibited 

dissimilar breathing profiles. 

There is a consistent pattern across all sensors and across all metrics (from individual breaths to 

distributions for the entire run) of lower TV, lower flows, longer times, lower respiration rates, 

and lower MV on Aircraft 1 compared to Aircraft 2. There are also considerable changes in 

observed MV corresponding with changes in aircraft systems; in other words system interactions 

between man and machine (Figure 7.1). While limited to two aircraft and several 1 minute 

segments of interest, which of necessity cannot be statistically representative of all aircraft or all

pilots, the F-35 data refute the widely held assumption that aircraft breathing systems do not 

significantly affect pilot breathing physiology. Furthermore, these observations point to overall 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 158 of 260

differences between the F-35’s with respect to the legacy F-15 and F-18 aircraft in the main PBA 

report.

The differences between jets are not particularly surprising. Several pilots reported these 

differences: 

• “I talked to [JPO individual] afterward, and they crunched the data in the jet...it was ascertained that during 

the period of time encompassing the physiologic event enumerated my minute ventilation, so the amount of 

oxygen/air I had consumed from the jet was about half of what would had been predicted.”

• “I do think that the jet breathes differently or each tail number did at least have some subtle variations.”

• “There is noticeable change between jets and some are easy breathers versus more difficult breathers.” 

• “[Aircraft 1] was definitely a ‘Bad Breather’, but nowhere close to the worst I’ve flown.” 

Pilot reports and data suggesting that decreases in MV up to 50% can occur almost without 

notice, present to a pilot as feeling just a little off, and be of such magnitude in an aircraft 

“nowhere close to the worst”, is troubling from a detection and reporting viewpoint.

Metabolic demand is ordinarily the primary determinant of MV (e.g., muscular exertion causes 

elevated breathing and higher MV), so data were taken during relaxed breathing while sitting, 

where metabolic demand is minimal. Changes to MV in the absence of changes to metabolic 

demand are indicative of aircraft breathing systems exerting influence on pilot breathing. A 

useful reference point may be the CPAP (Continuous Positive Airway Pressure) breathing 

machine which many people are familiar with for sleep apnea. In the absence of metabolic 

changes, changes to MV while breathing on a CPAP are due to the influences of the breathing 

system controlling breathing. In other words, large breathing changes in one minute while sitting 

(no increase in exertion) are not likely caused by metabolic changes; they are caused by the 

aircraft breathing system. Therefore, MV was calculated as the simplest approximation of 

breathing system performance with changes to this value considered an indication of impact to 

pilot breathing physiology.

The data in Figure 7.1 (bottom) is a summary table of results that will be analyzed in this section. 

MV is a convenient summary statistic; however, it has inherently more error as a calculated 

value as it is not measured directly. Therefore, it is important to view this data in the context of 

all the available evidence, which as will be seen, all show trends pointing at the same conclusion. 

The redundant sensors on the VigilOX allow for multiple comparative analyses of slightly 

different aspects of breathing parameters: Two independent flow sensors, two independent line 

pressure sensors, a mask pressure sensor, two different time clocks all combine to allow for a 

robust comparison of flow rates, tidal volumes, and breath times. This redundant, multi-faceted 

approach gives greater confidence to the analysis than can be placed in one single calculated 

value of MV. Ultimately, the goal of a breathing system is to provide an appropriate volume of 

air over time; hence it is still the most relevant value in analyzing and summarizing the impact of 

breathing dynamics on pilot physiology. 
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MV per Aircraft per Segment 

[Segment Number – Name]

Aircraft 2

“Normal 

Breather”

Aircraft 1

“Bad 

Breather”

Difference

#1 – Normal Relaxed Breathing 

(Baseline)

16.4L/min 8.8 L/min 7.4L (-

47%)

#3 – Backup Oxygen System (100% O2) 15.0L/min 12.0 L/min 3.0L (-

20%)

#4 – Defog Full On (with G-suit) 13.0L/min 7.4 L/min 5.6L (-

44%)

#10 – Defog Full On (w/o G-suit) 13.2L/min 11.5 L/min 1.7L (-

13%)

#12 – Engine Thrust ETR 15% (w/o Gsuit)13.6L/min 13.9 L/min 0.3L (+2%)

Aircraft 1 MV Increases with systems 

changes

Aircraft 1

System

Aircraft 1

Baseline

Difference

#1 – Normal Relaxed Breathing 

(Baseline)

8.8 L/min 8.8 L/min N/A

#4 – Defog Full On (with G-suit) 7.4 L/min 8.8 L/min 1.4L (-16%)

#10 – Defog Full On (w/o G-suit) 11.5 L/min 8.8 L/min 2.7L (+31%)

#3 – Backup Oxygen System (100% O2) 12.0 L/min 8.8 L/min 3.2L (+36%)

#12 – Engine Thrust ETR 15% (w/o Gsuit)13.9 L/min 8.8 L/min 5.1L (+58%)

Aircraft 2 MV Decrements with systems 

changes

Aircraft 2

System

Aircraft 2

Baseline

Difference

#1 – Normal Relaxed Breathing 

(Baseline)

16.4 L/min 16.4 

L/min

N/A

#3 – Backup Oxygen System (100% O2) 15.0 L/min 16.4 

L/min

1.4L (-9%)

#12 – Engine Thrust ETR 15% (w/o Gsuit)13.6 L/min 16.4 

L/min

1.8L (-17%)

#10 – Defog Full On (w/o G-suit) 13.2 L/min 16.4 

L/min

2.2L (-20%)

#4 – Defog Full On (with G-suit) 13.0 L/min 16.4 

L/min

3.4L (-21%)
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 160 of 260

Consistent pattern of lower TV on Aircraft 1 compared to Aircraft 2

Shown by multiple metrics calculated from individual breaths up to the entire run

Histograms of TV Relaxed TV/Flow Maximum TV Peak Inspire Flow

Inhale Tidal

Volume Peak

Average TV

Per Breath

Maximum TV 

(with G-suit)

Peak Inspire Flow

(with G-suit)

Aircraft 1 .7 L Aircraft 

1

.63 L Aircraft 1 3.40 L Aircraft 1 235 LPM

Aircraft 2 .9 L Aircraft 

2

.82 L Aircraft 2 4.00 L Aircraft 2 280 LPM

Exhale Tidal

Volume Peak

Inhale

Flow Peak

Maximum TV

(w/o G-suit)

Peak Inspire Flow

(w/o G-suit)

Aircraft 1 .6 L Aircraft 

1

30-40 LPM Aircraft 1 3.72 L Aircraft 1 246 LPM

Aircraft 2 .8 L Aircraft 

2

30-70 LPM Aircraft 2 3.98 L Aircraft 2 280 LPM

20% Lower TV

on Aircraft 1

25% Lower TV on 

Aircraft 1

15% Lower Max TV 

on Aircraft 1

15% Lower Peak 

Flow on Aircraft 1

Figure 7.1. Reductions in Minute Ventilation Summary Tables

Tidal Volumes

Unlike laboratory settings where many of the equipment check-out procedures are initially 

performed, the cockpit environment (air and ground) is much more demanding. As such, realworld measurement data tend to exhibit more variability than their bench-test counterparts. This 

requires visualizing data within the context of larger trends rather than individual point 

comparisons. Histograms (frequency distributions) represent a valuable tool in describing 

complex measurements. 

Breathe Time Distributions

Changes to breath times indicate breathing system impact on pilot physiology. Longer exhale 

times correspond with higher cracking pressures and flow restrictions. Longer inhale times 

correspond with lags in flow during the start of breathing demonstrated in Section 5.

Breathe Ratio (Inhale time compared to Total Time)

Breath Ratios are another general metric, applied here to assess changes in the pilots breathing 

dynamic. These breath ratios show a systematic difference between the two aircraft characterized 

by longer exhale times on Aircraft 1. Inhale to Exhale ratios are commonly used in respiratory 

physiology. They are important during mechanical ventilation as a control parameter and are also 

discussed in Section 5. Normal I/E ratios are 1:2 (.33 Breath Ratio), and with safety pressure I/E 

ratios of 1:1 (.5 Ratio) are not unexpected. As a point of reference, during mechanical 

ventilation, abnormal I/E ratios are uncomfortable and often require sedation of the patient. In 

room air, it takes about twice as long to exhale as it does to inhale due to the passive nature of 

exhalation, resulting in the common I/E ratio of 1:2 found in respiratory literature. A mask with 

safety pressure is similar to a mechanical ventilator, so similar changes to the I/E ratio are 

expected.

The Breath Ratio calculated here is not strictly speaking an “I/E ratio”, instead it is the inhale 

time compared to the total time of that breath. Exhale times as discussed above are unreliable 
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due to the pervasive disruptions to exhale flow present in the F-35. However, inhale times are 

distinct, and the time from one inhale to the next can be reliably calculated. Here the inhale time 

is divided by the total time from the start of one inhale to the start of the next inhale, or the total 

time of the breath. These values approximate an I/E ratio and become exactly equivalent when 

the exhale time is equal to the time between inhales.

Comparing the two aircraft in Figure 7.2, we see that the breath ratio is much lower for Aircraft 

1, up to 50% longer exhalation time than inhale time. Aircraft 1 has an average ratio of 

approximately .43 (43% inhale/57% assumed exhale) and drops as low as .3 (30% inhale/70% 

assumed exhale) on two separate occasions. Given the longer exhale times in Figure 7.3, this 

result is not surprising, but helps put into context the relationship between the two times and 

relative differences at a glance over the entire duration of the test.

In contrast, Aircraft 2 has an average ratio of approximately .5 (50% inhale/50% exhale). In the 

context of safety pressure, the higher ratios are not unexpected due to the relative ease of effort 

during inhale (safety pressure), and relative difficulty of exhale prohibiting the normal passive 

mode of exhalation. The ratio on Aircraft 2 went as high as .6 (60% inhale/40% assumed exhale) 

on three occasions. 

This is another metric showing a systemic difference between the two aircraft, and points 

towards significantly longer comparative exhale breathe times on Aircraft 1.

Figure 7.2. Breath Ratio Comparison Plots of Aircraft 1 and Aircraft 2
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Inhale Time-to-50% Volume

Like the concept of phase shift, inhale time to 50% of final volume is another way to see the 

relative time sequence of events during the course of a single particular breath. The longer it 

takes to get to 50% volume the less air is received when the muscles are at their greatest 

mechanical advantage during the first half of inhalation. 

Because of the extensive lag in the flow seen during the start of inhale, and the overshoot in 

pressure/flow during the second half of the breath, this was considered as a way to gauge the 

overall impact of those dynamics and quantify their magnitude and frequency. Simply timing 

how long it takes to get to 50% volume (Figure 7.5, left side graphs) is one potential measure of 

delay, but suffers from the difficulty inherent in stochastic breathing. Is the longer time due to 

lag or due to a longer breath? To alleviate that concern, the times are normalized by the time of 

the breath (Figure 7.3, right side graphs)

For sinusoidal breathing half way through a breath, the volume should be half way to its total as 

well. This equates to a relative Time-to-50% of .5 (dimensionless ratio of Time-to-50% divided 

by total breath time). Values below .5 indicate the flow arrives early in the breath and 50% 

volume is reached before 50% of the time. There are relatively few instances significantly below 

the .5 ratio and the distribution tails off quickly for front loaded breaths past a .4 ratio. 

Conversely, values above .5 indicate the flow arrives late in the breath and 50% volume is 

reached, in many cases, well after the half way point in the breath.

PBA ground testing of pilots with a medical-grade spirometer has shown an average value 

slightly greater than 0.4 with a tight distribution around the mean and values rarely, if ever, 

exceeding 0.50. This was accomplished during preflight in room air with no flight equipment. 

Flight data from PBA for nominal tests (no reported breathing difficulties, mask anomalies, etc.) 

show values that have a mean of approximately 0.50 with a tight distribution about the mean and 

values rarely, if ever, exceeding 0.60.

On Aircraft 1, 40% of the distribution is above a .6 ratio, with flow arriving very late in 

breathing sequence), and the distribution continuing well past a .8 ratio. On Aircraft 2, only 20% 

of the distribution is above a .6 ratio, and the tail of the distribution is much smaller, tapering off 

just after a .7 ratio. Concordantly, the actual time-to-50% volume was longer for Aircraft 1 at 

1.15 seconds than the comparable time in Aircraft 2 of .85 seconds. 

This is a significant indication of delayed flow response during the time sequence of a normal 

breath. Both aircraft exhibit this delayed flow (phase lag), as detailed in Section 5 on breathing 

dynamics. By this measure, Aircraft 2 is 20% worse with more breaths having flow arrive 10-

30% later during the time sequence of an inhaled breath than a comparable evenly distributed 

breath would be expected to arrive.
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Figure 7.3. Inhale Time-to-50% Volume
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Respiration Rate

Pilot respiration rate was measured and is displayed in Figure 10-4. The respiration rate for the 

pilot in Aircraft 1 was consistently less over the course of the profile compared to Aircraft 2. 

This is consistent with the individual histograms of inhale/exhale times. Longer inhale/exhale 

times result in a lower respiration rate since longer total time per breath results in fewer breaths 

per minute. This presentation gives a better overview of the entire sortie at a glance for direct 

comparison as opposed to individual breaths. This metric also indicates a systematic trend with 

Aircraft 1 having significantly lower respiration rate (longer time per breath on average) than 

Aircraft 2.

In summary, the anecdotally reported differences between jets and reports of reduction in minute 

ventilation are supported by this data. There is a consistent pattern across sensors and metrics 

indicating significantly lower TV, lower flows, longer times, lower respiration rates, and lower 

MV on Aircraft 1 compared to Aircraft 2 for breathing profiles that were expected to be similar.

On two separate F-35 aircraft, ground tests with similar scripts and expected similar metabolic 

loads resulted in >50% changes in minute ventilation and dissimilar breathing profiles. This is 

consistent with a pilot interview statement regarding the calculations made by the program office 

on data observed during a physiological event.

Figure 7.4. Pilot Respiration rate from measured data for the two F-35 aircraft. Note that the pilot 

in Aircraft 1 consistently has less respiration than in Aircraft 2.
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8.0 Physiological and Medical Implications

The engineering design consideration of any breathing gas system of a high-performance aircraft 

is simple in concept: provide sufficient O2 (flow, volume, concentration) to prevent hypoxia and 

other hypoxia-like symptomology. In practice, the dynamic range and response characteristics of 

those systems may be insufficient to sustain optimal physiologic function during highperformance flight, and hypoxia is not the only adverse pathology to be avoided. To match the 

highly variable human physiology to the machine is a highly complex process which, as will be 

demonstrated in this discussion below, is deceptively difficult and may inadvertently evoke 

unforeseen technical issues capable of compromising intended function of the breathing systems 

of the F-35. The current breathing system imposes an excessive burden of physiological 

adaptation on the pilot, resulting in adverse and undesirable physiological changes which often 

will go unnoticed or barely perceived. The body will attempt to respond within the confines of 

the system, but the response may be inadequate.

The numerous and pervasive technical issues discussed previously can contribute significantly to 

the acute emergence of adverse in-flight performance-degrading physiological symptoms. Up to 

50% of F-35 pilots have experienced undesirable symptoms at least once, according to 

interviews. Many milder cases often go unreported, with abnormal in-flight symptoms dismissed 

or marginalized as ‘normal’. The accounts of normalization of deviation for in-flight symptoms 

during interviews are similar to what has been documented in the F-22 community. In more 

severe cases, the aircraft is actively causing acute injury, which, in rare but concerning instances, 

has demonstrated the potential for permanent disability. At least one pilot has been medically

retired from military service with demonstrated pulmonary changes from his service entry. 

The subtle, but pervasive nature of these physiological impacts, should not be overlooked, as in 

the following reports from Pilot interviews:

• “The most important observation to convey is the impact on pilot performance. Cognitive ability, 

fatigue, and overall performance are degraded without acute symptoms...repeated firsthand experience 

with excessive/chronic fatigue over 4 years of flying F-35s leads me to the conclusion that pilots are 

subjected to a physiologically compromising environment on a frequent basis resulting in sub-optimal 

performance and excessive fatigue not just during flight, but also cumulatively over time and over many 

flights.”

• Pilot: “...I was experiencing nausea, call it low-grade. It’s actually something I get in the jet fairly 

routinely...”

Interviewer: “Going over that, how often do you get it? Every flight? Or, is there any associated 

symptoms with getting nausea in the aircraft?”

Pilot: “Not every flight, frequently enough that it doesn’t surprise me that a low-grade, call it 2 or 3 out 

of 10, nausea after a longer flight. It happens enough that I’m not surprised by it.”

Interviewer: “You said you had Viper [F-16] experience; any of that symptomatology in the Viper?

Pilot: “No, none whatsoever. Never had it [nausea] in any other aircraft.” 

In order to forge a more thorough understanding of the implications of pilot reports above, it is 

crucial to develop a foundational model of normal human respiratory physiology and how this 

physiology reacts when exposed to the cockpit environment: high altitude, high O2 tension, and 

high forces of acceleration. The effects that any one of these in-flight conditions has on 

diminishing respiratory function cannot be understated. It may be best to think of human 

respiration, particularly in-flight, as a dynamically dynamic system; it can tolerate and adapt to 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 166 of 260

certain deviations to a limit. The body will respond to changes to restore homeostasis through a 

multitude of mechanisms to be discussed, but its ability to compensate is finite. The goal of a 

breathing system is to stay safely away from the boundaries of these finite limits. However, any 

breathing system built around ground-tested, best case breathing parameters will always fail to 

account for the omnipresent effects that any in-flight perturbations away from ‘normal’ can have.

The aforementioned data collected during ground testing demonstrates the concerning changes in 

physiological parameters that the F-35 breathing gas system has on the human respiratory system 

across all phases of flight. These respiratory changes can cause a variety of often non-specific 

symptoms like those of Pilot 4 above. In more severe cases, these respiratory changes will result 

in reduced performance or incapacitation.

It should be noted that this section draws heavily from the Aircrew Breathing main report, but

does concentrate on the specifics and findings in the F-35 ground runs.

8.1 Physiology

Skeletal-Muscular injuries due to the effects of G’s, seat position, and helmet weight, especially 

on the back, spine, and neck are well documented. Pilots and Flight Surgeons are rightfully 

sensitive to minor trauma from these issues because of the known cumulative damage and longterm disability that historically results. There is currently very little sensitivity to respiratory 

muscular trauma or insults to lung function. Unlike a sore neck, pilots and general physicians are 

not accustomed to recognizing the symptoms nor thinking about the physiological consequences 

of muscular trauma to respiratory function. While a sore neck is painfully obvious and makes 

head movement difficult, traumatized respiratory muscles rarely draw notice, yet have a far 

greater impact on our ability to function at peak performance. Poor lung function affects all 

aspects of physiology; poor function due to muscle trauma elsewhere does not. As the Air Force 

Chief of Pulmonary Medicine said, “Fit pilots are poor perceivers of decline in lung function 

hence need objective measures” (Lt Col Dara Regn, MD, USAF, MC, FS, Chief, Pulmonary and 

Sleep Medicine Aeromedical Consultation Service. E-mail correspondence April 9 2020) and 

studies note that elite athletes “symptoms have been shown to be poor predictors” of breathing 

problem diagnosis (Couto et. al) It is difficult to mitigate imperceptible, minimally perceptible 

but unaddressed, and unrecognized declines in physiological function. Hence it is critically 

important for those developing standards and requirements to have a thorough understanding of 

respiratory physiology.

Breathing, or more precisely, ventilation, is an automatic, rhythmic, and neutrally-regulated 

mechanical process. The contraction and relaxation of the skeletal muscles of the diaphragm, 

abdomen, and rib cage cause gas to move into and out of the alveoli of the lung. The human 

respiratory cycle is tightly controlled by central and peripheral nervous system chemoreceptors 

which respond to local concentrations of carbon dioxide (pCO2), oxygen (pO2) and acidity (pH). 

At rest, an averaged sized male will consume 0.34 L (STPD)/min of O2. Through 

chemoregulatory control, this will increase to 1.00 L (STPD)/min of O2 consumption during 

strenuous tasks such as air combat maneuvering. (Loer et al) To provide this drastic increase in 

O2 requirement and to offload all the resultant CO2 produced, the body will alter volumes and 

rates to achieve desired ventilation, or movement of air. 

Inspiration is the active phase of breathing and is initiated by neural influences from the 

respiratory control centers in the brainstem. During inspiration, the diaphragm along with the 

intercostal muscles contract which, in turn, cause the thoracic cavity to expand. As the thoracic 
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cavity expands, the distensible lungs passively expand. The surface of the lung is coupled to the 

thoracic cavity by a thin layer of liquid. The liquid coupling allows the lung to “move” during 

breathing and to adapt to the shape of the thorax.

As the thoracic cavity expands, the pressure in the terminal air spaces (alveolar ducts and alveoli) 

decreases. Once the pressure in the thorax decreases to a subatmospheric level, fresh air flows 

down the branching airways and into the terminal air spaces. As the pressure in the airways 

equalizes with the atmospheric pressure, inspiration ends.

The inspiratory muscles work against resistance: the elasticity of the lungs, the airway resistance, 

and the resistance of the chest wall. All of these are altered in the cockpit environment; the shape 

of the lungs adapts to the same shape as that of the thoracic cavity. If thoracic size is temporarily 

reduced, (e.g., cockpit posture, flight gear, harness, etc.) lung size is also reduced. This will alter 

the natural breathing rhythm or cadence and increase the work of breathing and can lead to a 

variety of symptoms such dyspnea or breathlessness. Impedance to inspiration will increase the 

negative pressure inside the lung and result in under-ventilation.

Expiration is generally more passive compared to the active muscle recruitment during 

inspiration. During expiration, the elastic recoil properties of the lung and decreasing size of the 

thoracic cavity cause pleural and alveolar pressures to rise to greater than atmospheric level. 

Consequently, gas flows out of the lung and continues to do so until the pressure in the alveoli 

equilibrates with atmospheric pressure. Expiration is relatively passive at rest, but at higher 

levels of ventilation some expiratory muscles do contribute to the expiratory process. 

Muscle groups enabling ventilation: The combined efforts of muscles of the chest wall, 

principally the diaphragm, expand the volume within the thoracic cavity, leading to inspiration. 

Of these, the diaphragm is the primary muscle of ventilation. 

The diaphragm (Figure 8.1) is a dome-shaped muscle that separates the thoracic from the 

abdominal cavity. It is a thin, sheet-like muscle that originates on the lower rib cage (costal 

diaphragm) and lumbocostal spine (crural diaphragm) and inserts on the central tendon. 
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Figure 8.1. Diaphragm Anatomy

The diaphragm can be considered as a cylinder capped by a dome (Figure 8.2). During 

inspiration the muscle fibers of the diaphragm shorten, but the dome of the diaphragm does not 

change shape. 

Movement of diaphragm acts to increase thoracic volume by several mechanisms. During 

contraction, the diaphragm is directed downwards with a piston like action. As the diaphragm 

descends down from the thoracic cavity and into the abdominal cavity thoracic volume 

concomitantly increases. Due to its insertion on the lower ribs, the diaphragm imposes a 

cranially-directed force on the lower rib cage, lifting the ribs and rotating them laterally. 

Figure 8.2. Inspiration and Expiration Muscular Mechanics
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In addition to the diaphragm, the intercostal muscle group contributes to inspiratory portion of 

ventilation. The intercostal muscles can be divided into three groups: the parasternal intercostals, 

and the external and internal intercostals. 

The parasternal intercostals originate on the lower rib, adjacent to the sternum, and then insert 

onto both the sternum and the rib directly above. The parasternal intercostals have an inspiratory 

mechanical action. The external and internal intercostals are located more laterally between the 

ribs. Due to their fiber orientation and pattern of activation during breathing, the external 

intercostals also tend to produce an inspiratory action. In addition to the above muscles, several 

muscles in the neck (scalens, sternocleidomastoid) elevate the sternum and upper two ribs during 

deep inspiration, aiding in the inspiratory action on the thorax. During inspiration, enlargement 

of the upper rib cage is due to actions of the neck and intercostal muscles, but enlargement of the

lower rib cage is due to the actions of the diaphragm and intercostal muscles.

While the parasternal and external intercostals are concerned with inspiration, the internal 

intercostals tend to produce an expiratory action on the rib cage during quiet breathing (Figure 

8.3). An additional rib cage muscle, the triangularis sterni, originates on the inner aspect of the 

sternum and inserts on the ribs adjacent to the sternum and also has an expiratory action on the 

rib cage.

Figure 8.3. Intercostal Muscles

Additionally, four expiratory muscles are located in the anterolateral abdominal wall: the 

transversus abdominis, internal and external obliques, and rectus abdominis. They reduce 
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thoracic size by increasing abdominal pressure which moves the diaphragm back into the thorax 

cavity. Those movements, in conjunction with their action of pulling down on the rib cage, 

decrease thoracic volume to facilitate exhalation. 

The diaphragm, parasternal intercostal, and external intercostal muscles are the most consistently 

active during resting breathing in humans. Consequently, these are considered to be the primary 

ventilatory muscles while the others can be considered as accessory ventilatory muscles. Their 

activation occurs when ventilatory demands increase, for example, with exercise. Respiratory 

muscle fatigue and reductions in ventilation are reported during use of inspiratory and expiratory 

positive pressure. 

These breathing muscles enable ventilation through the conducting airways (the nose, mouth, 

pharynx, larynx, trachea, bronchi, and bronchioles) before entering the alveoli.

Figure 8.4. Pulmonary (Lung) Anatomy

Dead Space: Air that does not undergo gas exchange is referred to as physiologic dead space. 

The total dead space volume is made up of alveolar and anatomical dead space. Alveolar dead 

space is the gas that remains in the individual air sacs or alveoli to keep the alveoli open. 

Anatomic dead space refers to airs in the conducting passageways of the respiratory system, 

including the nose, mouth, pharynx, larynx, trachea and airways up to the terminal bronchioles. 

O2 and carbon dioxide do not significantly exchange between gas and blood while in the 
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conducting airways. This physiologic dead space, or residual volume, is typically approximately 

150 mL in an average adult. 

Dead space will increase with use of aircrew equipment, the largest contribution coming from 

the mask. Mechanical dead space (e.g., in a mask) can become rebreathed air that increases in 

carbon dioxide if not completely replaced with each breath volume delivered to the mask. This 

will increase the content of CO2 to the lungs. Mechanical dead space can also become additional 

retained (unexhaled) air with excessive expiratory pressure. This dead space does not participate 

in gas exchange, and can lead to increased alveolar CO2. Increased physiologic dead space (e.g., 

atelectasis or retained air) limits gas exchange and can contribute to hyperinflation. Furthermore, 

following a rapid decompression event, dead space volume will cause an immediate reduction in 

available inspired O2, potentially leading to hypoxia. As a principle, added dead space volume by 

aircrew equipment should be no more than 150 mL 

Pulmonary Volumes are the volume of air present in the lungs and airways at different phases 

of the respiratory cycle. 

Figure 8.5. Pulmonary Volumes

The volume of air that is moved with each breath is defined as the Tidal Volume (TV). At rest 

TV is approximately 0.5L or 500 mL, which can increase greatly with exertion. Resting lung 

volumes are defined by the relationship between the inward elastic pull of the lung tissue and the 

outward expansile force of the chest wall. When relaxed, the lung has a volume of air within 

defined as Functional Residual Capacity (FRC). This is made up of the Expiratory Residual 

Volume (ERV) and Residual Volume. The expiratory reserve volume (ERV) is the additional air 

that can be forcibly exhaled after the expiration of a normal TV. The residual volume (RV) is 

made up of physiological dead space. This residual volume is typically fixed for an individual in 

the range of 1.2L or 1200mL. Active inhalation will expand the lungs to a volume greater than 

FRC, and passive exhalation will return lungs to FRC. 

Vital capacity (VC) is the maximum volume of air that can be moved in the lungs – a maximum 

effort inhalation followed by a maximum effort exhalation. Typically, VC is on the order of 5L 

or 5000mL. Total lung capacity (TLC) is the sum of VC and RV. Inspiratory capacity (IC) is the 

maximum volume of inhale from FRC. Inspiratory reserve (IRV) and expiratory reserve (ERV) 

represent the volumes of air that can be moved at end inspiration and end exhalation, 

respectively. 
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Numerous features of the breathing gas system and aircrew equipment can serve, often 

synergistically, to adversely affect resting lung volumes. If expansion of the chest wall is limited, 

as is the case when strapped in to the aircraft, this will limit lung volumes including VC. By 

decreasing the natural outward pull of the chest, or outright resisting chest expansion, more 

inspiratory force is required for breathing. If lung elasticity is also increased, as is the case with 

unequal ventilation due to atelectasis or collapse of alveoli or segments of alveoli, this will 

further increase the effort of breathing. Chest wall restriction also limits the body’s natural 

defense mechanisms against atelectasis. 

Ventilation Rates: Pulmonary ventilation is the volume of gas per unit time entering the lungs, 

often defined as MV in units of L/min. Alveolar ventilation is the volume of gas per unit time 

that functions in for gas exchange, accounting for dead space. The alveolar ventilation rate 

(AVR) is the expression of this functional exchange of air, defined and illustrated below: 

The normal respiratory rate at rest is variable between individuals and within a given individual. 

Normal rates for the pilots range from 12 to 18 breaths per minute. The breath structure at rest 

characteristically has an inspiration to exhalation time ratio of 1:2 to 1:3, with more time spent in 

exhalation – a passive process. This will increase toward 1:1 inhalation/exhalation under 

exertion. Safety pressure also changes the I/E ratio closer to 1:1 due to higher pressures causing 

exhalation to become more active instead of passive. During anti-G straining maneuvers, breath 

structure is radically different, notable for rapid, maximum exhalation and inhalation efforts in a 

very short period of time. Flow limitations, pressure variations and dyssynchrony in 

demand/supply will alter the breath structure forcing the pilot to attempt to adapt. This will be 

explained in detail.

Flow of gas across the capillary wall into the blood stream within individual alveoli is influence 

by the partial pressures of gasses in the alveoli. An effective breathing gas system would be 

tailored to maintain the O2 content within the alveoli at physiological levels (about 104 mmHg) 

while minimizing the toxicity associated with high inspired O2. The general alveolar gas 

equation describes the partial pressure of O2 within the alveoli as a function of the inspired O2

concentration:

PAO2 = PIO2 – PACO2 * (FIO2 + [ 1 – FIO2 ] / R )

PAO2 = partial pressure of oxygen in the alveoli, normally 100 mmHg

PACO2 = partial pressure of carbon dioxide

FIO2 = fractional inspired oxygen content

R = respiratory quotient, approximately 0.8 in the healthy aviator

Alveolar ventilation rate is negatively influenced by decreased TV and increased dead space. The 

body has many mechanisms to alter ventilation in response to fluctuations in gas exchange and 

composition in the blood stream. In response to increased PCO2, the body will increase 
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ventilation in a linear fashion. For every 1 mmHg increase in PCO2 above normal (range 35 to 45 

mmHg), ventilation will increase by 2 to 3 L/min. Ventilation will increase first elevating the TV 

and then by raising the respiratory rate. In an otherwise healthy adult, this drive will increase to a 

point past which central respiration fails, usually in the arterial range of 60 to 80 mmHg PCO2. 

The ventilatory response to high PCO2 is increased in the presence of hypoxia. The ventilatory 

response to hypoxia is based on Hemoglobin saturation and the provision of adequate blood flow 

to the lungs. Compensation to hypoxia occurs when the O2 saturation is below about 95-96% or a 

drop in arterial O2 contraction of 10-20 mmHg. This is done by various combinations of 

increased lung volume and respiratory rate. Maximal compensation is reached at an arterial O2

pressure of 50 to 60 mmHg.

Gas exchange at the alveoli is connected to capillaries and is influenced by and has impacts on 

the cardiovascular system. This ratio of ventilation (V) of the lung to perfusion with blood (Q) is 

referred to as ventilation-perfusion ratio or V/Q. It is normal for the upright lung under the force 

of gravity to have more blood flow to the lower regions of the lung, and lesser blood flow near 

the apices. These regional differences are physiologic. Conditions which alter local ventilation or 

perfusion will adversely impact the function of the lung and the efficiency of respiration.

Airway resistance: which limits flow rates of gas into or out of the lung, is generated by 

aerodynamic forces of air movement within the lung. It is principally a function of airway 

diameter. Airway resistance is optimized at normal, resting FRC. Under conditions of increased 

airway resistance, the body will slow respiratory rates to provide more efficient respiration. It 

will be increased by changes in lung volumes and numerous additional conditions present in the 

breathing gas system of the F-35, including high O2 concentrations and the atelectasis that will 

ensue. Any increased airway resistance is undesirable. Impedance to expiration will reduce 

average and peak flow rates, prolong exhalation and, over time, lead to lung hyperinflation.

Cardiac Output: Breath dynamics, lung volumes, and ventilation pressures are intrinsically 

linked with cardiac output and vascular function. This is particularly of consequence in the 

demand regulator system currently In normal, resting physiology, active inhalation occurs with a 

decrease in intrathoracic pressure, which helps draw low pressure venous blood into the right 

heart, increasing right heart output and filling the pulmonary arteries and capillaries. This leads 

to an intra-breath increase in blood volume in the pulmonary circulation, facilitating gas 

exchange. During exhalation, intrathoracic pressure will increase, helping to push oxygenated 

blood back through the left heart and into systemic circulation. Output is limited by net blood 

flow from the right side of the heart through the pulmonary circulation, which may be reduced 

with excessive airway pressures. For reference, the right atrial pressure normally is 

approximately 2 – 6 mmHg and normal right pulmonary artery pressure during contraction 

(systole) is 15 – 25 mmHg. Positive airway pressures which exceed the low pressure venous and 

pulmonary circulatory systems will impact cardiac output.

Flight Related Pathophysiology

Atelectasis is the term applied to describe collapse of alveoli, the functional end-units of the 

lung. Atelectasis is another lung decrease in ventilation that also affects circulation. There are a 

multitude of medical causes of atelectasis, but to the healthy aviator, the etiologies of high 

prevalence and concern are acceleration and absorption atelectasis. Atelectasis of any kind will 

result in reduced lung function and can cause symptoms of chest pain, irritation, or cough. 

Normally, the pressure of nitrogen within the alveolus will maintain patency through the breath 
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cycle. If nitrogen is removed from the alveolus, as is the case when breathing concentrated O2, 

the body will rapidly absorb available O2 within the alveolus. This will decrease the pressure of 

gas within the alveolus and potentially leading to collapse. There is a critical point at which 

inspired oxygenated gas entering the alveolus is balanced by O2 uptake by the bloodstream, with 

atelectasis becoming increasingly likely with inhaled gasses composed of 60% or more of 

concentrated O2. Collapsed alveoli will cease to participate in gas exchange until reopened, 

perhaps by coughing or deep breathing. However, even after being reopened by such a 

maneuver, these alveoli will be unstable and more likely to collapse again. Referred to as 

denitrogenation absorption atelectasis, this can cause significant and cumulative changes in lung 

ventilation and perfusion over time. 

Acceleration Atelectasis: Under vertical acceleration forces, + G, there will be regional changes 

in blood flow in the lungs which will lead to the formation of acceleration atelectasis. Under 

sustained + Gz, the lower regions of the lung segments will see increased blood flow, and the 

apices will see progressive decreased flow. At + 5 Gz and greater, the upper half of the lung will 

effectively be nonperfused. This nonperfused lung is effectively ventilated dead space. 

Conversely, the lower regions of the lung will receive increased blood flow to the point of 

becoming engorged and collapsed, having then no ventilation, but high perfusion. This can result 

in shunting of deoxygenated blood to mix with oxygenated blood in circulation, lowering the O2

content in arterial circulation. This can also lead to the formation of atelectasis in the lower 

portions of the lung – portions which normally play a more significant role in ventilation due to 

higher perfusion. 

Acceleration atelectasis will begin to occur by + 3 Gz, and be prominent from + 5 to + 9 Gz. Use 

of anti-G suits will exacerbate this exposure, causing restriction of the diaphragm and fall in 

FRC. Sustained, this can result in a shunt of deoxygenated blood on the order of 20 to 25% of 

total blood flow. Acceleration atelectasis will be exacerbated with inspiration of high O2

concentrations. Atelectasis will reduce the functional capacity of the lung, limiting and whenever 

feasible measures should be taken to minimize the causal forces. 

Hyperoxia: Inspiration of higher O2 concentration, necessary with increases in altitude with less 

O2, has a multitude of undesirable adverse effects as concentrations increase, including 

atelectasis. O2-enriched air can lead to the production of reactive O2 species which can directly 

cause inflammation, alveolar damage, and respiratory distress, concurrent with and in addition to 

atelectasis, as previously discussed. 

O2-induced changes in neurovascular tone: A topic of ongoing interest, the inhalation of high 

concentrations of O2 has been found to cause regional blood flow changes in the brain and 

changes in brain function. Damato et al. demonstrated reduced blood flow by Magnetric 

Resonance Imaging, with some preservation of cognitive function. (Ref Section F-35 

Hyperoxia). Although memory may not be affected, some areas of reasoning and judgement may 

be affected. These vascular changes are under investigation and may prove insightful in 

delineating the pathophysiology of Hyperoxic cerebrovascular changes and cognition.

Rapidly Oscillating Hyperoxic Concentrations: During the T-6 Safety Investigation Board for 

unexplained PEs, it was determined that fluctuating O2 can cause hypoxic like symptomology 

(19AF OBOGS Summit Jun 2019 After Action Report and report of Safety Investigation Board, 

Jun 2019, Randolph AFB.) While there are currently no formal studies on humans or pilots to 

reference, the medical literature on animals does support this. Boehme et al. demonstrated that 
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oscillating O2 induced release of proinflammatory cytokines in the lung, followed by onset of 

inflammation. 

Oscillation Pressure Effect on Surfactant: Surfactant is the coating that helps to prevent 

alveolar collapse. Pressure oscillations facilitate atelectasis formation by displacing surfactant. In 

combination with decreased nitrogen and/or acceleration, this further increases the amount of 

atelectasis in the lungs. Higher pressure oscillations can also cause barotrauma to the airways 

and alveoli. High pressure oscillations potentiate lung damage through a variety of mechanisms. 

High pressure oscillations cause mechanical stress and strain within the lungs, as the mechanical 

force applied to the pulmonary epithelium lining the airway and the alveoli initiates a resultant 

inflammatory response within the lungs. An inflammatory response can spread to other organs 

causing secondary barotrauma.

Asynchrony: is a pervasive problem in mechanical ventilation of critical patients, but it also is a 

contributing factor in aircrew breathing systems. One form of asynchrony (dyssynchrony) 

involves timing of mechanical triggering of the system to the pilot’s individual breaths. 

Asynchrony is defined as the triggering or cycling of a breath that either leads or lags the pilot’s 

inspiratory effort. Regarding the size of a breath, asynchrony means the inspiratory flow or TV 

does not match the pilot’s demand (too much/little, too early/late). Asynchrony will lead to 

increased work of breathing, excessive fatigue of respiratory muscles, and non-specific 

respiratory discomfort. Volume and flow mismatches can cause micro-trauma in the form of 

barotrauma due to alveolar over distention even if the pressures are not excessive in the 

traditional sense of high PIP/PEEP. Asynchrony is a subtle problem for which patients have no 

way to perceive or communicate its presence directly. (Ref sec -Asynchrony)

Inspiratory Over Pressure: In the F-35 electronic safety pressure regulator, we have seen that 

the response is not proportional to the demand from the pilot and varies at the beginning, middle 

and end of the response. High safety pressure in combination with sudden and unexpected 

inhalation flow towards the end of inhalation can lead to inspiratory overpressure. With even 

small amounts of safety pressure, inhalation will become more passive while exhalation becomes 

an active process. This inhalation/exhalation reversal will change chest wall and lung dynamics, 

usually resulting in expansion of the lung and increase in FRC. Exhalation becomes prolonged, 

and indicative of increased effort needed to breathe out against pressure. The work of breathing 

will increase, and even the small safety pressures utilized in modern demand regulator systems 

can contribute to hyperinflation and fatigue over time. Higher levels of positive pressure, 

particularly above the intrapleural pressure of -4 mmHg, can have adverse consequences. 

Trained individuals can tolerate pressure breathing up to 30 mmHg for very short periods, to 

compensate with a high altitude cabin pressure decompression. In the high Gz regime, combined 

with G-straining maneuvers, a pressure of 60-90 mmHg can be tolerated in short durations. If left 

for longer it will lead to hyperinflation, hypoventilation, fatigue, and respiratory failure. Positive 

pressures of 4-10 cm H2O can be well tolerated, but require a constant, uninterrupted flow with 

no oscillations. Higher pressures to 20 cm H2O are also tolerated, but do result in higher rates of 

dry mucous membranes and nose bleeds. In a system with increased peak pressures or flow, the 

addition of continuous airway pressures serves to worsen hyperinflation. Hyperinflation due to 

asynchrony results in insufficient exhalation time preventing the respiratory system from 

returning to its normal resting equilibrium volume between breaths. So in using safety pressure, 

it must use in the light of normal inspiratory flow rates, peak pressures, a synchronized breathing 

system, and normal tidal volumes. 
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Excessive expiratory pressure: This has physiological consequences and can cause perfusion 

problems. Normal respiratory dynamics function as a negative pressure system during inhalation. 

As described previously, the diaphragm descends and produces a negative pressure in the 

airways that draws air for gas exchange in. This same negative intrathoracic pressure decreases 

the right atrial pressure and draws blood from the inferior vena cava and increases venous return 

to the heart. An increased airway exhalation pressure is reflected in the airways and alveoli. That 

negative pressure in turn is transmitted to the thoracic cavity and decreases the negative 

pressures from the diaphragm (creating a positive pressure). This increases right atrial pressure, 

decreasing venous return. This affects the pulmonary flow and decreases overall heart volume. 

This has a doubling effect of decreasing cardiac output as well as less effective cardiac function. 

This can result in overall drop in mean arterial pressure, which extrapolated to a fighter aircraft 

pilot can result in brain hypoxia.

Hyperinflation: Inappropriate and excessive exhalation pressures will lead to dynamic 

hyperinflation. This condition is the increase in lung volume (over inflation) that occurs 

whenever insufficient exhalation time prevents the respiratory system from returning to its 

normal resting end-expiratory equilibrium volume between breath cycles. This results in trapped 

air, inability of the pilot to initiate a breath, and an increased work of breathing. Hyperinflation 

also results in limited inhalation volumes, as the excessive exhalation volume is not displaced. 

This increases the physiologic dead space. In the case of dynamic expiratory hyperinflation, 

volumes of both inspiration and exhalation are decreased, TV is diminished and a state of 

hypoventilation results. Persistent breathing dysfunction (oscillations, lung over-inflation, and 

forceful exhalation) can cause long term changes to pulmonary function. 

Barotrauma: If peak inspiratory pressure is too high, the compensatory reaction is to limit TV 

so as to prevent excessive pressure on the airways and alveolus. An excess pressure can cause 

over distention of the alveoli to the point that they lose structural integrity and collapse. High 

alveolar pressures can be due to excessive TV, gas trapping, excessively high expiratory 

pressures or low compliance (“stiff lungs” or lung tissue that has limited elasticity). This may 

result in hypoventilation of the patient and hypoxia. Chronically high airway pressure may cause 

micro-barotrauma to the alveoli and accumulates over time. 

Discussion

The above framework can serve as a brief guide to develop an understanding of some of the 

vulnerabilities of the respiratory system that may be affected in the F-35. Many of the 

physiological properties of the lung will vary between breaths or within an individual breath to 

maintain the proper balance of O2 and carbon dioxide within the blood. This highly tuned, highly 

responsive system will respond consciously and subconsciously to external forces. The body will 

make efforts, consciously or subconsciously, to attempt to restore alveolar ventilation. If there 

are external forces at work limiting this physiological response, there may be undesirable 

symptoms of dyspnea, nausea, cough, or worse. If the human’s physiologic reserve is depleted, 

and, without awareness, the pilot may acutely become incapacitated. Within the turbulent nature 

of high-performance aircraft, small, consistent perturbations can have cumulative effects, even if 

the breathing gas systems are functioning within current design specifications. The human 

system is constantly responsive to pressure, volume and time, and so any fluctuation will result 

in changes that affect the function of the entire system.
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8.2 Analysis

The data from the F-35 display patterns of mismatch and dyssynchrony between the pilot and 

breathing gas system. These impacts are pervasive and can cause undesirable respiratory changes 

which align with pilot reports and interviews. 

In-flight investigations into respiratory physiology and pathophysiology are nascent, but the 

patterns observed thus far are cause for concern. 

• “And so there was this, kind of, general kind of breathing technique that I learned, like I said, I guess it was 

more subconscious than I initially said just then. Where it was kind of: initiate the breath, then breathe 

while I have flow, and then you kind of have to exhale a little bit more forcibly, and that sort of stops and 

resets the valves, and then you can exhale and finish the exhale process. It definitely takes more attention, 

whether it subconscious or conscious, to breathe in the F 35 than it does in any of the other airplanes that 

I’ve flown, including ones I did fly, and I’m trying to remember right, I did fly a couple of other airplanes; 

F-15s with the OBOGS and a flew an F-18 with the OBOGS, and those I don’t remember having any need 

to adapt my breathing like I had to in the F 35.”

Pulmonary consequences: Loss of minute ventilation is reflective of an inability of the pilot to 

adequately adapt to the breathing environment. Many of the patterns in the data, including 

dyssynchrony, increased impedance to airflow, and undesired dynamic pulmonary changes, can 

interact synergistically to reduce minute ventilation. 

Dyssynchrony is the product of mismatch between pilot demand and supplied flow. The data 

demonstrate that pilot demand and airflow supply are disjointed. Early in the breath demand 

exceeds supply, whereas later at the end of the breath by supply exceeds demand. When supply 

exceeds demand at the end of a breath, this will result in an excess volume of air being forcibly 

delivered to the pilot, with a number of concerning effects. The data consistently demonstrates 

metrics of hysteresis and phase shift. Increasing time to 50% inhalational volume will 

physiologically result in reduction in tidal volumes, consistent with trends observed. 

Dyssynchrony is also facilitating dynamic hyperinflation of the lungs in conjunction with 

increased impedance to airflow.

Reduced Minute Ventilation: Starting with the TVs (Figure 8.6), the pilot in Aircraft 1 shows a 

0.7L peak inhale TV, vs. 0.9L for Aircraft 2. This represents a significant reduction in TV, 

recalling that these TV measured include dead space ventilation. The reduction in TV in Aircraft 

1 also thus reflects a decreased proportion of each breath which participates in alveolar 

ventilation – breathing is much less efficient in Aircraft 1 vs Aircraft 2.

Figure 8.6. Reduced TV on Aircraft 1 compared to Aircraft 2
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In addition to being 20% lower on Aircraft 1, the histogram shows a more variable and skewed 

distribution. Breathing is stochastic in nature, and the distributions analyzed in the F-18/F-15 

during PBA flights are more normally distributed. The skew in the distribution away from a 

normal distribution indicates an influence on the normal physiological distribution of breaths, 

which is larger in Aircraft 1. Evidence indicates that this can be imperceptible or barely 

perceptible to pilots in flight, despite the significance. Compensation occurs readily provided the 

pilot does not require an increase in TV to contend with an increased metabolic demand. Many 

perceptions related to in flight compensation may be manifested post flight with fatigue and 

malaise due to the increased work of breathing. 

The inhale breath time distributions are shifted longer on Aircraft 1 by 20%. This increased 

inhale breath time is a physiological adaptation to undesired and problematic flow restriction. It 

reflects also a disruption of the natural breath structure, with increased inhale/exhale time. The 

body naturally will try to preserve volume by slowing down the respiratory rate and increasing 

breath to breath volume, provided the restriction is not variable. This clearly shows variable 

restriction and asynchronous patterns.

Another indicator of restriction during inhalation is the reduction in Inspiratory Capacity (IC) 

indicated by a reduction in maximum achievable TV. Both the peak flow rate and resulting 

maximum TV are lower on Aircraft 1 by 15%. Aircraft 1 is limited to 3.4L compared to 4.0L on 

Aircraft 2. Both reflect a decreased TV compared to a healthy aviator, who would be expected to 

achieve approximately 5L. Some degree of loss is known to occur with the added weight and 

restriction of flight equipment. However, if this was a volume restriction was chiefly due to 

aircrew flight equipment such as flight jacket, seat harness, or other physical restrictions to actual 

expansion of the chest itself, there should be minimal difference as the configuration worn was 

identical in both cases. 

Figure 8.7. Reduced TV during Maximum Inhale on Aircraft 1 compared to Aircraft 2

When the G-suit was disconnected on Aircraft 1, the maximum TV increased to 3.7L, which is 

inconsistent with purely external chest wall restriction. Also, chest wall restriction, which causes 

decreased breath volumes, would be expected to elicit an increased breath rate to maintain 

adequate ventilation. However, longer duration inhalation reflects the expected compensation for 

a flow restriction, and we can visibly observe it here again with Aircraft 1 taking approximately 

20% longer to get 15% less air (Figure 8.7). 
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Figure 8.8. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2

Physiologically, the lower TVs and longer breath times in Aircraft 1 result in reduced MV 

(Figure 7.1). Aircraft 1 had TV’s of .63L and MV of 8.79L compared to Aircraft 2 which had 

TV’s of .82L and MV of 16.36L. These data were collected during periods of relaxed breathing 

without exertion, and it would be highly irregular to have such grossly different MV in the 

absence of medical condition. The pilot in Aircraft 1 received approximately 50% less air with 

TV’s 25% lower. On Aircraft 1, the inhale and exhale flows are much less stochastic than on 

Aircraft 2, further indicative of flow limiting effects. Analysis of mask pressure reveals a number 

of concerning trends which show pathological changes in pulmonary function and all but certain 

impact to cardiac function. Average mask pressure swings for each breath are lower on Aircraft 

1. In Segment 1 (Figure 8.8), Aircraft 2 has mask pressure which drop down to 0 to 1 mmHg 

regularly, whereas Aircraft 1 rarely drops below 2 mmHg. These reduced pressure swings are 

also seen in (Figure 8.8) in comparison to MIL-STD 3050. These lower mask pressure 

excursions are a concerning finding, suggesting inadequate compensation of the human to flow 

limitations and reduced TV. 

Alveolar overdistension: Overdistention is likely occurring as a result of pressure demand/flow 

mismatch. The data demonstrates that pilot demand and airflow supply are disjointed. Early in 

the breath demand exceeds supply, whereas later at the end of the breath by supply exceeds 

demand. When supply exceeds demand at the end of a breath, this will result in an excess 

volume of air being almost forcibly delivered to the pilot, with a number of concerning effects. 

This undesired and excess airflow will be directed to more patent and highly ventilated regions 

of the lung, with the possible end result of regional alveolar over distention forcible exhalation is 

initiated during peak regulator flow, as is frequently the case, this may further increase 
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transpulmonary pressures and stress on the alveoli. On the microscopic level, the alveolar over 

distension will lead to inflammation, disrupted blood flow, collapse and loss of function. Alveoli 

that have been over distended may be subsequently stretched open on successive breaths; they 

will be unstable and prone to collapse again. The cyclical atelectasis that results will lead to 

further injury and the shear forces will be transmitted locally, causing neighboring alveoli to also 

collapse. The end result is somewhat of a ‘micro-tear’ in lung tissue, with cumulative progressive 

injury, inflammation and loss of tissue function, which will continue as long as the mask and 

regulator are on. (Ref sec– Barotrauma) 

The pathological effects of breathing gas system hysteresis or dyssynchrony will be most 

pronounced during periods of high metabolic demand with large lung volumes and rapid breath 

rates, but can also impact function during quiet breathing due to the disproportionately large 

magnitude of the dyssynchrony compared to the small pressures used at rest. The body may 

attempt to compensate for this process with slowed respiration with feedback from lung stretch 

receptors via the Hering-Breuer reflex, but this will pose yet another risk for hypoventilation. 

Once activated, the receptors send signals to the inspiratory area in the medulla and brain stem. 

In response, the inspiratory area is suppressed directly and inhalation is inhibited allowing 

expiration to occur. With constant pressure against the chest wall and airways this can result in 

slowing of respiration.

Circulatory consequences: Pulmonary circulation may be affected by the demand regulator 

safety pressure system, with downstream changes in cardiac output and systemic vascular 

function. These changes can be classified by their principle etiology: effects of safety pressure, 

pressure and flow hysteresis, and pressure oscillations. Safety pressure in combination with 

pressure alterations and flow restrictions, may increase pressure within alveoli, reducing 

capillary perfusion pressure, although this potential effect has not been demonstrated to be 

physiologically relevant. The effects of low safety pressure (3 mmHg) alone are small, but are 

additive with other increases or fluctuations in inspired gas pressures. Higher positive airway 

pressures will increase pulmonary artery pressure and right heart loading, decreasing right heart 

output and exacerbating any underlying shunting or V/Q mismatch. 

Pressure oscillations alter pulmonary blood flow. These oscillations can be transmitted to alveoli, 

and the resultant physiological effects depend on a host of factors. These factors include the 

frequency and magnitude of the oscillations, the time during breath when the oscillations occur, 

and the current physiologic state of the lung (lung volumes, atelectasis, etc.). If the magnitude of 

the oscillations are large, they may be additive with safety pressure to cause pathological 

reductions in pulmonary capillary perfusion, increase right heart strain and worsening any 

existing V/Q mismatch. If the oscillations occur in the presence of regulator hysteresis, the 

effects may be magnified, with significant changes in regional blood flow. Airway and thoracic 

pressures above venous or right heart pressures will be transmitted to the systemic venous system 

and cerebral veins, limiting flow and reducing perfusion. High thoracic pressures will also 

trigger the baroreceptors in the aortic arch with reflexive slowing of heart rate and reducing 

cardiac output. Reduced cardiac output, in conjunction with reduced cerebral perfusion pressures 

and coexisting reflexive hypoventilation are a recipe primed for hypoxic insult. 

Mask Pressure Swings: Low mask pressures and lower swings in mask pressure are usually 

thought to denote a system that is performing as designed, but that is not necessarily true when a 

flow restriction is present. Conversely, elevated mask pressures and larger mask pressure swings 

are traditionally considered to denote a poorly performing system, but pilots do not perceive that 
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those pressures as large or objectionable when the flow adequately responds in tune with large 

demands. In either of the previous cases, pilot perception of breathing performance may not 

correlate well with the magnitude of mask pressure. Rather, pilot perception of breathing 

dynamics appears to depend more upon receiving a flow commensurate with demand and 

without delay. Excessively high inspiratory and/expiratory pressures will cause a commensurate 

decrease in TVs. The higher the peak pressures, the more the TVs will be restricted to prevent 

barotrauma. However, fast oscillations as seen in the data can cause barotrauma before reflexes 

can protect against excessive pressures. At higher metabolic demands the protective restriction 

on TV will result in hypoxia over time. 

It is likely that breath dynamics studied here have been significantly disrupted leading to 

increased ventilatory effort and lower demand mask pressures. Subconscious physiological 

adaptive measures to flow restriction are being exacerbated by the inefficient pulmonary 

dynamics of lower breath volumes. In other words, it is becoming clear that the pilot is fighting 

the machine to maintain normal homeostatic breathing, but in the case of Aircraft 1, the pilot is 

losing. 

Oscillating Pressure consequences: 

The level of flow restriction observed in Aircraft 1 by estimate appears to be moderate, as judged 

by the adaptive nature of the pilot’s response and the lack of any reported resistance by the pilot. 

Trained aircrew, under most circumstances, would be unaware of the effects of increased 

resistance in the breathing gas system, highlighting the insidious nature of some of these issues. 

High flow resistances would be expected to cause slower, deeper breathing, which does not here 

appear to be the case. However, with any degree of flow or inhale resistance, the body will adapt 

to preserve MV, which, as above, is not the case. Again, it appears the pilot is fighting the 

machine, and the pilot is losing the battle. 

Traditional thinking equates large drops in mask pressure as deleterious due to the increased 

work of breathing associated with large mask pressures. Traditionally, when a flow restriction 

was present, there was a corresponding increase in large negative pressures, which intuitively 

makes sense when breathing against an insufficient flow, such as a pinched off mask or straw. 

However, we do not see large negative mask pressures associated with Aircraft 1’s flow 

restriction. 

This combination of airflow restriction without a corresponding drop in mask pressure would be 

less prominent in a diluter demand breathing system, wherein flow response is proportional to 

the mask pressure demand and limited principally by regulator function. However, in this 

electronic safety pressure demand regulator, we have seen that the flow response is not in 

synchrony with the demand from the pilot. Rather, the supplied air varies at the beginning, 

middle and end of the inhalation demand. This hysteresis or dyssynchrony between the pilot and 

regulator can cause significant changes in respiratory dynamics, akin to trying to drink water 

from a faucet unpredictably varying its output from a dribble to high pressure stream. Air is not 

being adequately provided at the beginning of the breath, and too much is being delivered after 

demand ceases at the end of the breath. This is very different physiologically from a proportional 

or nearly linear response of demand for which the body is accustomed. 

Physiologically, demand regulators with safety pressure create numerous issues and alter normal 

breathing dynamics. As previously discussed, at rest, inhalation is active and effort-driven, while 

exhalation is passive. With even small amounts of safety pressure, inhalation will become more 
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passive while exhalation becomes an active process. This inhalation/exhalation reversal will 

change chest wall and lung dynamics, usually resulting in expansion of the lung and increase in 

FRC. Exhalation will become prolonged, indicative of the effort needed to breathe out against 

pressure. The work of breathing will increase, and even the small safety pressures utilized in 

modern demand regulator systems will lead to some degree of hyperinflation and fatigue over 

time. Higher levels of positive pressure, particularly above the intrapleural pressure of 

-4 mmHg, have adverse consequences although trained individuals can tolerate pressure 

breathing up to 30 mmHg for very short periods. This is usually associated with high Gz

maneuvering or high altitude. If left unabated, this condition could lead to hyperinflation, 

hypoventilation, fatigue, and respiratory failure. (Ref sec - Aviation, Gz, and Hypoxia - AIN). 

Hyperinflation: The following depictions are designed to illustrate the influences of overpressurization leading to hyperinflation (i.e., continued increases in FRC). Reported instances of 

hyperinflation and increased FRC during the interviews correspond to the higher exhale 

pressures, decreased exhale flows, longer exhale times, saw tooth exhale pressure oscillations, 

and lower tidal volumes detailed in this report. Together these suggest the pathology of increased 

FRC occurs regularly with significant potential to cause harm. (Ref Sec – Barotrauma and 

Overpressure). 

Figure 8.9. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2
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Figure 8.9 shows a normal alveolar volume distribution with the respective volumes labeled and 

defined. The individual alveolus is used to help visualize what happens to the lung volumes as a 

whole as residual volume increases (regional differences occur in the lungs, but the principle 

concept is the same). 

There is a progression of effects due to excessive inhalation or exhalation pressures. During 

inspiratory overpressure, the natural compensation mechanism is decreased tidal volumes to 

prevent barotrauma (this compensation is not depicted) (Ref Sec - Overpressure) 

Decreased tidal volume increases the airway and alveolar dead space by the amount of decreased 

tidal volume. A complete reduction in tidal volume to zero (breath hold) results in no 

barotrauma, but also no air exchange, as the entire lung becomes dead space. 

In expiratory hyperinflation the residual volume expands as depicted in Figure 8.10. Normal 

residual volumes (left) become larger through a combination of higher inhale pressure (i.e.,

larger breath due to being stuffed with air) and higher exhale pressure (i.e., incomplete exhale 

due to reduced exhale flow or time). Passive exhale is no longer sufficient to return the lungs to 

their starting residual volume, and the residual volume gradually expands (middle). A complete 

expansion of the residual volume (right-like blowing up a balloon and tying off the end) results 

in no air exchange, as the entire alveolus becomes dead space. Natural compensation is for 

exhale to become active (requiring the use of muscles not normally engaged) with a significant 

increase in the work of exhaled breathing. The higher lung volumes associated with increased 

residual volume result in muscles having to work from a position of mechanical disadvantage, as 

they are already stretched out.

Figure 8.10. Normal Relaxed Breathing for Aircraft 1 and Aircraft 2

Persistent breathing dysfunction from breathing sequence disruptions (e.g., oscillations, 

restricted exhalation, and attenuated inhalation) can lead to decreased Inspiratory Capacity. 

Medical literature, multiple pilot reports, and data here indicating lower TVs all suggest F-35 

pilots experience lung over-inflation and increased Functional Residual Volumes (Ref F-35 

References).

Demand regulators also introduce dyssynchrony or asynchrony into the breathing system. As a 

general principle, demand regulators can be tuned for responsiveness or maximum flow. As 

previously discussed, however, breathing patterns in flight are highly variable and will 

simultaneously require instantaneous response and high flow rates, as is the case with anti-G 

straining maneuver breathing. Current regulations, based on pressure and flow rate 

specifications, do not account for the synchrony with human respiration or the hysteresis that 
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they inherently produce. This is not unique to the F-35 and would apply to other aircraft using 

demand regulators.

O2 concentrations varied by 20 to 40% over one-minute intervals in several occurrences. Large 

breaths produced a precipitous drop in breathing gas O2 concentration. The concentration values 

were calculated from the regulator supply gas pressures measured at the ISB. There is currently 

no standard regarding magnitude or duration for O2 concentration swings, and the values for the 

O2 concentration fall within the current MIL-STD-3050 Figure 1 envelope (below), which only 

requires O2 concentration to be above or below the minimum or maximum thresholds in order to 

prevent hypoxia (lower bound) and to prevent acceleration atelectasis (upper bound), depending 

on altitude. This legacy standard was conceived for liquid O2 dilution style breathing systems 

(LOX) where O2 concentrations did not vary significantly, nor did they oscillate continuously. 

The adsorption swing process in OBOGS makes the output inherently cyclical. The O2 output 

can be stabilized, as is done on the F-15E MSOGS, by continuously producing sufficient near100% O2 and diluting to the appropriate schedule, much as LOX does, however this results in a 

larger and heavier system with excess O2 production under most circumstances. There are no 

known studies showing that swings in O2 concentration are safe. 

Figure 8.11. O2 concentrations and regulator pressure schedule for an aircraft flying to 50,000 feet 

with a 5 psi differential pressure cabin.

As mentioned previously, too much O2 can be toxic to tissues, including the brain. The body has 

natural mechanism to alter blood flow to limit the development of inflammation or reactive O2

species in highly metabolic tissues including the central nervous system. These mechanisms 

generally involve a restriction in blood flow on the order of 10 to 40% depending on the study. 

Furthermore, this vasoconstriction generally will persist for a period of time after removal of the 

hyperoxic gas, usually on the order of several minutes or longer. This can create a vulnerable 

period, if the body is compensating to the hyperoxic gas and the hyperoxic gas is suddenly 

removed, it places the more metabolically active tissues at risk, especially the Central Nervous 
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System (CNS). This vulnerable period also appears to include increased inflammation and 

cellular lung damage when oscillations in hyperoxic concentrations exceed the ability of the 

homeostasis mechanisms to compensate and keep pace with the continuous changes. Decreases 

of > 20% O2 concentration also increase absorption atelectasis. If this occurs sequentially and 

rapidly in less than 5 minutes, the atelectasis is worsened with each swing, due to lack 

insufficient time to re-inflate with nitrogen Evidence for these conclusions is included in medical 

literature examined in the Hyperoxia addendum of the PBA report.

Table 8.1. O2 Change During Each Segment

O2 change by Segment Aircraft 1 Aircraft 2

1) Normal Relaxed Breathing 23% 17%

2) 2x Max Inhale/Relaxed Exhale 20% 17%

3) Backup Oxygen System (100% O2)

[Expected due to system change to BOS]

[51%] [58%]

4) Defog Full On (Hi Flow/Hi Temp) 17% 16%

5) 2x Press-to-Test (Mask Off in #2 only)

(*Due to Mask Off free flow)

18% 39%*

6) Disconnect G-suit 13% 10%

7) 2x Press-to-Test (w/o G-suit) 17% 16%

8) 2x Max Inhale/Relaxed Exhale w/o G-suit 29% 8%

9) Rapid Deep Breaths (w/o G-suit) 43% 25%

10) Defog Full On (w/o G-suit) 17% 27%

11) Mask Off (w/o G-suit) 38% 25%

12) Engine above Idle (15% w/o G-suit) 18% 20%

13) Connect G-suit

[Expected due to system change to 100%]

17% [49%]

14) Press-to-Test (BOS – 100% Oxygen) 63% 56%

15) 10 timed breaths in #1 / Mask Off in #2

(*Due to Mask Off free flow)

15% 25%*

Average oxygen change per Aircraft

[Excluding 3, 13 due to 100% oxygen]

(*Excluding 5, 15 due to dissimilar points)

27% 22%

The O2 swings shown are neither comprehensive, nor representative of airborne performance. 

The O2 was always well above 100mmHg, providing sufficient O2 for minimum requirements at 

all times. The O2 variability of the F-35 has been well documented and independently confirmed 

by the Joint Program Office and Dr. Miller’s OBOGS Lab. These data reinforce the importance 

of understanding and mitigating the physiological impact of high and rapidly varying O2

concentrations in pilot breathing gas. They also reinforce the importance of end to end systems 

testing as the data show variations due to systems interactions and aircraft differences.

Compensation: The ability of the human body to compensate for dynamically changing 

circumstances is remarkable, but finite. Pilot in the Loop Oscillations (PIO) may provide a 

helpful analogy to explain the pilot/plane interactions in the F-35. In both cases, there are control 

systems on both sides. The pilot can make adaptive changes either consciously or subconsciously 

to the system to accommodate system disruptions. These adaptive changes and compensations 

may be short term (sigh, yawn, etc...) or long term, may take time to work (slightly slower 

respiration rate), have a response lag, and may have additional side effects of their own over 

time. These adaptive changes are effective in accommodating small changes, but ineffective in 

accommodating large changes past some undefined critical point. The decay in effectiveness 
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occurs gradually, and then suddenly. The danger with unrecognized compensation is that 

breathing margins can be so small that any further breathing challenge can result in rapid 

decompensation, which will look out of proportion to the proximal cause.

Breathing Distraction: In the F-35, according to interviews, the breathing system forces pilots 

to think about their breathing. This is a cognitive distraction that divides attention away from 

mission tasks. 

• "...it is routine for me to notice now, put it like this: I NEVER thought about my breathing, EVER, in the 

Strike Eagle. Never. I never, it was not a conscious though...it was never brought forward into my 

conscious thinking about breathing, it was just something I was doing and I never considered it. Now it is 

something that I am conscious of, routinely, in flight; I'm conscious of how I'm breathing, conscious of 

making sure I'm controlling my breath, taking a deep breath, to expand my lungs every 10/15 minutes or so, 

I make sure that I do that."

8.3 Physiological Conclusions

Observed breathing dynamic changes and O2 swings of up to 40% are consistent with interview 

reports that 50% of pilots have experience mild physiological symptoms at some point in their F35 flying, and with some pilots experiencing them on a regular basis. The synergistic 

combination of breathing sequence disruptions (constantly changing pressure, flow, and 

synchrony) and inconsistent O2 concentrations may lead to pervasive respiratory dynamics 

changes, but this is as yet undocumented in flight. Continuous breathing disharmony and 

pressure/flow asynchrony are consistent with pulmonary Micro-trauma of the alveoli, airways, 

and chest wall remodeling. The effects of these many disparate physiological responses, in 

aggregate, can predispose to pathological hypoxia. These factors are all present on the F-35 in 

this study at levels capable of causing short-term dysfunction or even longer-term harm from 

chronic inflammation. The physiological changes in response to fluctuations in inspired O2

concentrations on the order of 40% are not well understood, but highly concerning for 

contributing to individual PEs or long-term cumulative damage. The destructive synergy of these 

factors are consistent with the documented permanent damage to lung physiology responsible for 

the medical retirement of at least one F-35 pilot (based on pilot interview and medical record 

review), consistent with pilot complaints over the last 8 years, and consistent with interview 

accounts of symptoms experienced by pilots. 

The human is a pressure differential generator, and controls breathing with pressures; however, 

the breathing system does not appear responsive to a pilot’s pressure signals with appropriate 

flows. The pilot is being forced to adapt physiologically to an unpredictable and oscillatory flow. 

The result is compensation in the form of lower MV, lower TV, increased functional reserve 

capacity, and likelihood of atelectasis, increased dead space, micro-trauma, hyperinflation, and 

an increased predisposition to hypoxia.

Summary of Potential Pulmonary Insults

Pilots flying the F-35 are subjected to various alterations in the breathing dynamics that can 

cause directly or contribute to distinct respiratory pathophysiology.

Hyperoxia

a. Absorption atelectasis resulting in decreased lung volumes and altered lung circulation

b. Cerebrovascular constriction in specific brain regions placing these regions at risk for regional hypoxia

2) Acceleration atelectasis 

a. Decreased tidal volumes, diminished cardiac volume with higher Gz, and chest wall increased work of 

breathing
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3) Rapid Oscillating Hyperoxic concentrations

a. Accelerated cerebrovascular constriction in specific brain regions resulting in regional hypoxia

4) Breathing System Asynchrony

a. Asynchronous timing – mechanical triggering of breath lags or leads the pilots breathing cycles. 

Lagging a breath diminishes tidal volumes delivered to the pilot. Leading a breath (oversupply) 

induces restricted volumes physiologically to prevent hyperinflation.

b. Asynchronous volumes or flow – The inspiration flow or volume does not match the pilot’s inspiratory 

effort. Too much volume causes a physiological reaction to limit the volume to prevent hyperinflation 

or to little reduces TVs

c. Asynchrony leads to increased work of breathing, excessive fatigue of respiratory muscles, and nonspecific respiratory discomfort. Excessive flow or pressure will result in alveolar micro-trauma

5) Inspiratory overpressure 

a. Results in an increase in dead space volume over time 

b. Chest wall muscular remodeling

6) Expiratory overpressure 

a. Results in dynamic hyperinflation, air trapping (increased dead space), and decreased inspired TV

b. Decreased venous return to the heart causing deceased cardiac output and reduced circulatory pressure 

and volume (decreased blood pressure)

7) Inspiratory and expiratory overpressure combined

a. Results in increased dead space volume more rapidly than just inspiratory or expiratory overpressure

i. Expiratory dynamic hyperinflation results in worsened air trapping (increased dead space) by 

additional decreased inspiratory TV.

ii. Higher likelihood of larger areas of micro-trauma and barotrauma

b. Chest wall muscular remodeling

c. Combined effect further worsens the individual decreases in venous return to the heart. Substantial 

reduction in cardiac output and reduces circulatory pressure and volume (decreased blood pressure)

The general hypothesis, as yet untested in the flight environment, is that F-35 breathing 

dynamics insults, singularly or in combination, could result in cerebral (brain) hypoxia and 

cognitive disorders, and/or create conditions of increased work of breathing, excessive fatigue of 

respiratory muscles, and non-specific respiratory discomfort. Furthermore, excessive flow or 

pressure may lead to alveolar micro-trauma and/or induce alveolar and airway barotrauma. An 

ongoing program of pulmonary function testing would lead to a better understanding of these 

results.

9.0 Definition of Terms 

Chest Wall Remodel Inspiratory over pressure usually does not produce hyperinflation unless 

the peak pressure is excessively high. Because you can breathe out if the 

expiratory pressure is not excessive, then hyperinflation by itself will not 

hyperinflate. If tidal volumes are limited to compensate, this is another 

matter and can lead to hypoxia

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 
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Appendix

Appendix 7.1. Pilot Subjective Report
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Appendix 7.1. Pilot Subjective Report

This appendix is a companion to Section 5 to include all recorded pilot comments related to the 

scope of this exploration. Please see Section 5 for the full explanation and interpretation of the 

pilot report clusters. 

Collection Summary: Five F-35 pilot interviews were conducted by a team of three NESC PBA 

researchers: a flight surgeon, an F-35 SME, and a human factors SME. Each interviewee was 

provided a NASA Privacy Act Notice which indicated the protected status of the interview and 

all materials associated with the interview. All interviewees provided explicit consent to 

video/audio recording, interview transcription, and inclusion in this report. All data are reported 

in aggregate to maintain privacy. Each interview began with the pilot account of events related to 

the flight that induced a reported or unreported PE with specific information about the in-flight 

event, post-flight procedures, and recovery. This was followed by a period of question and 

answers for clarification and expansion. Finally, pilots were asked to provide perceptions of 

overall concepts across all airframes such as breathing experience, previous symptoms, common 

symptomology, and current processes. Brackets within quotations indicate areas where additional 

content was provided for context or to omit and substitute sensitive details. Grayed out pilot 

comments were included in the main body of Appendix 7.

Pre-production testing and program development through to current day mission flights. The 

early pilot reports:

• “It was trying to kill me”

• “The system was working as designed, but didn’t actually protect me” 

• “Maybe we had some fundamental misunderstandings of what the design of the system needed to be and 

we didn’t have as much physiological understanding of the human/machine system as we needed.”

A pilot noted that, at the time his concerns about the breathing system were raised, there were 

other on-going investigations specifically related to potential breathing gas contamination 

concerns. He stated that his concerns were met with program leadership opposition in the form of 

explicit and implicit rejection and suppression:

• “There was tremendous amount of concern amongst the enterprise that the program was vulnerable, at the 

time, and so there was a lot of pressure to continue testing, continue pressing forward. The team as a whole, 

and especially the program office folks who were in charge of the life-support system at the time, were 

fairly motivated to assign [my symptoms] to something that was not attributed to the jet. That was my

perception that was what they were trying to do, find a way to have it not be the jet so they could press.”

• “they were able to, again, sort of talk themselves into using those words and saying ‘well, maybe it was 

hypercapnia, maybe it was hyperventilation, but in no case is it something we need to change the design.’”

One pilot reported this summary statement regarding the F-35: 

• “It’s the new normal. Breathing in this jet is different than sitting here talking to you and breathing. It 

shouldn’t be, in my opinion, but it is. Talking against positive pressure is different than talking against no 

positive pressure. The schedule of the cockpit pressurization sometimes changes the pressure in the mask, I 

don’t know if it should be doing that or not, sometimes it does do that. The pressure breathing for G is 

slightly, not slightly, it’s different than what I had been previously accustomed to. And so, it is routine for 

me to notice now, put it like this: I NEVER thought about my breathing, EVER, in the Strike Eagle. Never. 

I never, it was not a conscious thought, I didn’t ever, it was never brought forward into my conscious 

thinking about breathing it was just something I was doing and I never considered it. Now it is something 

that I am conscious of, routinely, in flight; I’m conscious of how I’m breathing, conscious of making sure 

I’m controlling my breath, taking a deep breath, to expand my lungs every 10/15 minutes or so, I make sure 
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that I do that. That could be a factor of this thing happening to me or it could be a factor of just breathing in 

this jet is different. I think if you were to ask other pilots that they would, my opinion is, of course they 

have their own opinions, is that the breathing in this jet is different than breathing in the Viper, the F-15 C 

or E, the A-10, or any other platforms, F-22, that they’ve come from, even the hornet. We have guys here 

that have flown all of them. It’s just the different apparatus, a different feeling. And so now every sortie I 

am somewhat conscious of how I’m breathing, and how I’m interacting physiologically with the jet.”

Another pilot reported this summary statement regarding his experience in the F-35. 

• “The overall experience was one of extreme, you know, it’s difficult to convey to other pilots and other 

people how absolutely disconcerting it is to be cognitively bamboozled like that. Because you know there’s 

something wrong with you, you can’t convey it, and you don’t know why, and you don’t even know the 

why to the why. Don’t even know where to begin. ‘Hey, what’s wrong with me?’ ‘I don’t know,’ well, that 

only makes it worse, right? Which, okay, potentially psychologically, is just concerning on all levels, even 

though intellectually you kind of know ‘hey, I’ll be okay. I’ll just go to sleep and this will all...’ But for 

somebody whose entire life you are relying on your brain to be able think, and to fly, and to not be able to 

connect those words causes of level of concern. The jet attacked me. That’s the essence of the way I felt. 

Even though somebody else might go, ‘Oh, you’re just a little bit off, go sleep it off, shake it off, shake it 

off.’ Right? This was an entirely different level going through that experience and if it were to have 

happened while I was still flying, that’s the thing that’s the most concerning. Right? Because now it calls 

into question your ability to handle an emergency. That’s the interesting dichotomy, I think I could have 

flown and landed the aircraft if everything was fine, but now it’s kind of like the insidious where... you 

know... you always hear about the people going to sleep in the car in the garage, right, it’s kind of the 

apathetic, just comfortably go crash, right? That’s the concern. I would just not be able to make a decision, 

not be able to think and connect in airborne. If that had happened, there’s nothing I could do about it. 

There’s no control over it. As a pilot, you like to be able to control and take what actions you can. Nothing 

I can do! Nothing I can do to prevent it, fix it, and potentially maybe it’s causing long-term harm to my 

health. So, that’s the thing to convey. Maybe it’s difficult to convey how that felt. Well, that’s it.

To be clear, all pilots identified the F-35 as an asset to the warfighter. Here are a few summary 

quotes for positivity and perspective:

• “The F-16 had some significant growing pains as it was introduced as far as there were medical factors, it 

was routinely killing pilots with GLOC and spatial disorientation, but that was several decades ago. With 

time, effort, investigation, a merging of aerospace and aeromedical efforts, these were overcome and went 

on to become one of the most successful fighters in history and I’m confident the F 35 will do the same.”

• “The jet is still providing an environment that, although not optimal, I don’t perceive as actually dangerous. 

These UPEs certainly merit further investigation, but they haven’t killed anybody. I’m gambling my life on 

it, so I think that’s one of the more significant endorsements I can provide. 

• “No pilot experienced significant enough symptoms that they have to stop fighting and address that over 

the tactical problem.”

• “Overall, pilots trust the jet.”

Pilot Symptom and Perception Clusters

These interviews revealed several pilot symptom and perception clusters. Here, clusters are 

conceptual groupings that emerged after the identification of highly similar statements and the 

subsequent interpretation of shared characteristics. Adverse symptomatology was reported across 

wide spectrum of flight profiles and pilot demographics (e.g., flight hours, age, and expertise). 

These symptomatology does not appear to be specific to individual differences or task 

performance. Pilots reported adverse symptomatology across the spectrum of individual 

differences and characteristics. This range included nascent pilots with low-hour and no previous 

aircraft experience to elite pilots with instructor qualifications, multiple airframes qualifications, 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 191 of 260

and many hours of previous experience including extensive combat experience. Pilots reported 

adverse symptomatology across the spectrum of flight regimes ranging from straight and level, 

administrative, non-demanding phases of flight, to flight that is physically and cognitively 

intense. Quotes for this cluster have been excluded as detailing individual-specific characteristics 

of demographic and flight profile would compromise the privacy of our pilots. 

The remainder of this section will consist of a cluster title, a summary or description of this 

cluster, and relevant quotes to demonstrate sufficient support for the grouping.

Cluster 1: The F-35 breathing environment and physiological experience is dissimilar to a) 

other aircraft flown and b) normal physiologic breathing. The F-35 breathing system 

noticeably discourages normal breathing function via high-pressure, pressure surges, and 

hyperoxia.

Quotes include: 

• “The respiratory environment is not, still, is not optimized for normal human physiology”

• “That was the first time the jet had attacked me”

• “F-35 is known to produce erratic oxygen output both in concentration and in pressure. Some latency in the 

pressure delivery, or a lag in the system, as far as the pressure delivery. It’s perceptible.”

• “What I do know is that breathing in the F-35 is different. Breathing in [Strike Eagle] off of an MSOGS 

was a different experience than it is breathing out of the F-35. The F-35 is different in the fact that it has 

positive pressure all the time, not just pressure breathing for G but positive pressure in the mask. It’s 

different in the fact that the ECS environmental control system in the F-35 sometimes surges, sometimes 

pulls back. It’s a different physical environment that you’re in and the breathing is different. The cockpit 

pressurization schedule above 25,000 feet is different, it feels different on your body. It’s like hard for me 

to describe quantitatively the difference, but it’s different enough that you feel different.”

• “I adapted to the airplane and didn’t make good note of that adaptation. There is a threshold of initial 

initiation of the breath that the pilot has to do. It doesn’t do anything until you breathe a bit past some 

certain threshold and then you begin getting flow. There was this, kind of, general kind of breathing 

technique that I learned, like I said, I guess it was more subconscious than I initially said just then. Where it 

was kind of: initiate the breath, then breathe while I have flow, and then you kind of have to exhale a little 

bit more forcibly, and that sort of stops and resets the valves, and then you can exhale and finish the exhale 

process. It definitely takes more attention, whether it subconscious or conscious, to breathe in the F 35 than 

it does in any of the other airplanes that I’ve flown, including ones I did fly, and I’m trying to remember 

right, I did fly a couple of other airplanes; F-15s with the OBOGS and a flew an F-18 with the OBOGS, 

and those I don’t remember having any need to adapt my breathing like I had to in the F 35.”

• “Tighten specifications on the delivery schedule to reduce that frequent rapid oxygen cycling, pressure and 

the concentration”

• “You kind of have to begin the exhale as an event, and then once that all starts, and the flow begins, then 

kind of exhale normally. So, I guess another way to describe it, and this is not an accurate mechanical 

description, but the feeling was kind of that it was like a sticky valve, both directions. You, kind of, have to 

pull to get the inbound air going and then once the valve is flowing that I could breathe in with big 

continuous motion. And the same thing, I had to initiate the exhale, so a sticky valve feeling in that sense, 

and then once the exhale began, I could just go ahead and exhale normally.”

• “When you’re breathing off the mask in the F-35 you feel like you have to work a little bit harder so you’re 

a more forceful inhalation, sometimes, you have to more forcefully exhale” 

• “The backflow valve would get stuck sometimes. In fact, I remember there would be times I would reach 

up into the mask and punch the backflow valve if it got jammed. And then that would kind of leave you 

sometimes with a momentary shortness of breath sensation, I would say, maybe 1 in 10 flights you’ll see 

that.” 

• “There’s a cross valve in the mask, that sometimes if that thing gets gummed up it can be difficult to 

exhale, I’ve had that happen.” 
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• “Every time after 100% oxygen you always have kind of a standard Valsalva (mimes a Valsalva, plugs 

nose and blows), you know, you’ll still have the standard post-high oxygen issues. That’s just normal, 

normal.”

• “You can hold your breath a lot longer in the 35 because it’s got 100% oxygen. I could go like 50 seconds, 

a minute, without air hunger.”

• “The positive pressure isn’t really, in my thinking, isn’t so positive. It can be annoying.” 

• “Sometimes the F-35 just provides a whole bunch of pressure into the mask for unknown reasons, I don’t 

know why but it does, it makes exhalation difficult”

• “Increased exhalation pressure. It’s very much perceptible to the pilot as a positive pressure ventilation 

system”

• “I mostly tend to notice it as expiratory pressure.”

• “Flying the jet you notice, ‘oh, I could actually forcibly exhale more and that would actually be closer to 

how I would normally breathe.”

• “The amount of oxygen/air I had consumed from the jet was about half of what would had been predicted 

for someone with my body weight. I had consumed about half the oxygen just due to decreased respiratory 

rate.” 

• “I noticed I tend to have a significantly decreased respiratory rate in this jet.”

• “I think somebody asked me if I was hyperventilating or something, which was ridiculous, I was not 

anxious, there was no increased respiratory rate.” 

• “The ECS does surge. It pulls. So, there’s a single outlet for air that’s between your legs on the center 

pedestal, if you will. Sometimes that is really providing a lot of flow, as well as the sound of the ECS 

around you is providing a significant amount of air into the cockpit, and in other times it is not. And it gets 

warm. Sometimes it pulls the amount of air it’s blowing through the vent and into the cockpit, sometimes 

that decreases for a couple of minutes. I don’t know what the jet is doing during that time, there’s no 

change I can tell, it doesn’t really have anything to do with the throttle position, sometimes it does. For 

instance, on takeoff MIL-power, even into afterburner, the cockpit will surge and then settle. It used to be 

more significant on the older software suites, where it would, sometimes on takeoff, it would almost 

completely die in the cockpit and even now, every now and then, even at altitude, it’ll just decrease the 

volume of air being provided into the cockpit for whatever reason and that’s different. The Eagle did that 

too, but it was more throttle control. That is something that is unique. It’s not internal to the OBOGS that I 

can tell. It’s just outside the ECS”

• “Sometimes even in a single exhalation there could be a change in the pressure. So there’s like a kick back 

and it can actually bite off a radio call.” 

• “You’re exhaling against a constant pressure but then it’ll kick back whatever pressure you’re using to 

exhale and speak, and that pressure is equalized ceasing your exhalation and ceasing your vocalization for 

the radio transmission.”

• “you’ll be talking and then as you’re talking your expiring and you’re anticipating certain expiratory 

pressure as you’re talking but within the same exhalation while you’re talking, sometimes it will kick back 

and it will literally just like (mimes inability to exhale) like stop your expiration and it’ll just, like, cut off 

your exhalation and talking concurrently, or as a secondary effect, and then you have this oddly clipped 

radio call.”

• “And then sometimes [the expiratory pressure] will change in the same expiration, like you’ll be expiring, 

against a certain expiratory pressure and then it’ll kick back at you sometimes or sometimes it’ll go away 

and it can be somewhat variable, even within the same respiratory cycle. 35 things.”

• “Occasionally, especially on startup, you’ll get a sudden decrease in pressure, so it’s actually like a sudden 

choking from the jet, - there will be a sudden decrease in flow, pressure that might last like 10 seconds or 

something like that but then it resolves. But it will get your attention.”

• “not too long after startup after the OBOGS has come online, sometimes it will just dramatically decrease 

its production and you’re left sucking rubber, so to speak, against the mask, but then it generally clears up 

in a few seconds. It’s not unique, it’s just an F-35-ism.”
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Cluster 2: There is a distinct breathing system disparity across F-35 aircraft with no clear 

explanation or solution. 

Quotes include: 

• “There is noticeable change between jets, and some are easy breathers versus more difficult breathers.”

• “I do think that the jet breathes differently, each tail number did at least have some subtle variations”

• “Little bit of difficulty getting a deep breath or difficulty breathing off of the OBOGS system. I felt like it 

was more work than usual but I had seen that before in other flights. Over the course of flying F-35, some 

days it seems a little bit harder to breathe off the oxygen than others.”

• “Difficulty breathing off the oxygen system which led to, kind of, a mild shortness of breath symptom that 

would come and go, based on how cooperative the breathing system was at the time.”

• “It was just a hard-breathing day. And the thing that just stuck in my mind that it was just way harder than 

normal to breathe without any definitive smoking gun as to what was causing it. I [informed the program 

office and the head of the maintenance] said ‘hey, I just want to give you a heads up, this just breathes 

strange and it was very hard and it just really caught my attention, but there’s nothing... I can’t say 

anything one way or the other for you guys to go fix... So I just wanted to kind of let you know, and just 

talk it over him you’ and they’re ‘oh, alright, well, just let us know if you think of anything else.’ So that 

was the end of that.” The next day I flew an entirely different jet. Same mission, profile, same rough 

temperature, same place, pretty much everything the same except different jet. Another F-35A. Another Air 

Force variant. And flew and the breathing was just night and day. So, I went from probably the worst 

breathing jet that I’ve ever flown in my life in terms of, it just struck me, that ‘hey this is really, really, 

really, difficult’ to nice, easy, breathing, and the contrast between the two of them was just what really 

caused me to highlight it. So, I thought, ‘alright, this is... this is something there. This is real.’”

Cluster 3: Symptoms are frequent and variable among pilots and tend to mimic pilot-specific 

hypoxia symptoms. However, there are additional individual symptoms that are F-35 specific 

and learned exclusively from flying the F-35 that suggest additional pathophysiology.

Prominent quotes:

• “Pilots experience symptoms in the jet, they notice, but they’re not at the threshold that they consider 

necessary to declare or that they’re willing to flag themselves, highlight themselves, over.”

• “There’s been a lot of questioning with these events as far as whether or not it is psychogenic but out in the 

aircraft, I felt no anxiety whatsoever”

Quotes include:

• Hypoxia

o “I thought immediately that I was hypoxic and that’s a big deal.”

o “I noticed hypoxia-like symptoms, but, within about 5 to 10 seconds of noticing those, I received 

an OBOGS fail caution. I was about 50 miles [away], I turned my emergency oxygen on, I felt 

better, I landed.”

o “Several minutes into the situation, I realized I was experiencing lightheadedness, which, is kind 

of my primary symptom”

o “I noticed that my chest was rising and falling then I realized my heart rate was increased and right 

about that same time I got a warm sensation in my ears, right around the ear cups. Right about 

then is when I started to get the general graying, in my experience with hypoxia which is limited 

to the ROBD and the oxygen chamber, symptoms of hypoxia in my mind”

o “it’s hard to tell if the onset was due to the actual exiting the aircraft or if everything had built up 

to that point and just went over the edge. You have a higher oxygen concentration in the jet and 

then when you come out of that, that protective measure is gone and it just felt to me like 10 

minutes or so after getting out of the jet, everything just crashed. All. Everything. Onset of 

cognitive disability and fatigue occurred pretty much at the same time. When it happened, it 

happened within the span of a minute or two.”

o “about 10 minutes into the flight. I started noticing some numbness in my hands and feet. Kind of 

some blurred vision as well and I basically just didn’t feel right. I felt a little bit off. But I had no 
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OBOGS cautions, nothing that would indicate that the jet was have any types of pressurization 

issues, cabin pressure was on schedule as it should be at that altitude. So, everything was fine, in 

terms of what jet was indicating to me, it’s just I didn’t feel right. Initially attributed it to 

potentially some, maybe fatigue, just kind of accumulating from the operational tempo or maybe 

from working out that morning.”

o “After about 10 seconds or so, I felt my hypoxia symptoms from the altitude chamber get to the 

point where they were now part of my consciousness. So, in hindsight, I would’ve probably said 

that they had been gradually coming on, but it became part of my consciousness at that point.”

o “When I assessed myself and said ‘I’ve got my hypoxia [symptoms], I think I said ‘Preliminary 

symptoms of hypoxia’ is what I said, because again, at that point, me and everybody else were 

trying everything we could to not cancel the F 35 program so I didn’t want to declare the radio that 

it was trying to kill me, so I said “preliminary symptoms of hypoxia” over the hot mic just to the 

control room.

• Lightheadedness

o “Headache, some occasional in-flight or postflight headache.”

o “Lightheadedness and the blurred vision”

o “Just, like, lightheadedness, not even dizziness or vertigo but just a... it’s difficult to characterize 

lightheadedness beyond feeling lightheaded... slightly spaced out, depending on what you want to 

call it.”

o “A pronounced lightheadedness in the aircraft which actually resolved in the aircraft on the way 

back, and post landing confusion with nausea. The confusion probably resolved within, this is an 

estimate, 30 minutes to an hour later, the nausea probably persisted about an hour and then 

everything resolved.”

• Nausea

o “It’s like a mild upset stomach kind of feeling”

o “I was experiencing nausea, call it low-grade. It’s actually something I get in the jet fairly 

routinely. I think it’s an OBOGS thing.”

• Numbness

o “At one point I noticed [the numbness in my extremities] all the way up to the top of my calf 

towards my knee on both of my legs. I had only been in the flight for 10 minutes when that onset 

began. And that’s not a normal symptom.

• Vision

o “My vision grayed even more, my heart rate was high, and I was, I guess, oxygen-hungry would 

be the word to describe the way I was breathing.”

o “It started on the periphery and it was a graying. Think of it like a color fading, that’s really what 

it was, it was like a color fading on the outside that started to move in with this general, kind of, I 

wouldn’t call it tunneling, it is not a G, like when you’re under G, a tunneling like that, where it’s 

kind of, like, dark and then all you see is a tunnel, it was a graying or loss of color that kind of 

moved in and then stayed. The center of my field of view really was all right and then everything 

on the outside was grayish/loss of color, I wasn’t really able to focus really outside of that inside 

field of view.”

• Air Hunger

o “I didn’t feel like there wasn’t physical air being brought into my body, I felt like in the ROBD, 

I’m breathing but I’m not getting that satisfaction of breathing, I’m not being fulfilled, my 

breathing isn’t doing anything. That’s why wanted more. I was air hungry.”

o “It wasn’t like there was a huge amount of pressure or a lack of supply, it wasn’t a supply issue, it 

was what was in the supply basically. I felt like I wasn’t getting enough air; or enough oxygen.”

o “I’m in a pretty regular relaxed resting heart rate when I’m just cruising so I realized that my 

volume was significantly increased as was the rate of my breathing was increased.”

o “‘I’m feeling like there’s less air than I want’ kind of a thing, but a mental not a physical breathing 

air hunger.”

o

• Anxiety

o “No, [there was no elevated sense of anxiety] I was stereotypically confident that I was handling it 

just fine”
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o “Preemptively going on the BOS at higher altitudes. Which probably sounds like a lack of 

confidence in the system... maybe it is... but I think it is people being cautious.”

o “I think the symptoms can be, let me just give you an example. Let’s just say that if you stand up 

too quickly you get lightheaded and you have the same exact symptoms as if you’re hypoxic in 

ROBD chamber. There’s no difference in the symptoms, but there’s a difference in the cause. The 

physiological response might be the same, what you feel might be the same, but the cause of it 

may be different. I just think that I know how my body responds even now to cautions, warnings, 

things that are going on in the jet, and I have a physiological response to that.” 

o “I don’t think that the jet was restricting oxygen content in the breathing gas to me. I DO think 

that the way that the mask feels, the way that the breathing in the jet feels, the way that the 

environment feels, I think that all of that was a factor into how I responded physiologically.”

o “As I went through from the beginning of the emergency, I consciously was controlling my 

breathing. There was, some breathing data available to us as far as rate and depth of breathing, 

and... they didn’t identify that is being grossly unusual.”

o “As a pilot you’re concerned because ‘I don’t feel right’ you’re worried about your medical 

license, and your ability to fly more, and you’re telling the world that ‘hey, something’s wrong, 

with me, because of this plane.’”

• Lungs

o “[It hurt to breathe] at the top of an attempt to breathe in.”

o “There was no smoking gun, but it’s just generically difficult to breathe and now I’ve flown three 

days in a row and the upper chest soreness just stood out as ‘I feel like something is wrong there 

and again I can’t put a finger on it.’”

o “I couldn’t fully inflate my lungs [For several hours post-flight]. I’d get that pressure and burning 

sensation in my lungs, trying to expand my lungs”

o “you can hold your breath a lot longer in the 35 because it’s got 100% oxygen. So, I tried to hold

my breath and when I tried to do that, I had like instant air hunger. So normally, I could go like 50 

seconds, a minute, without air hunger... about 10 seconds to 15 seconds just extreme air hunger, 

like you’re a little kid whose breath is... your face is red and you’re ready to burst. I found that I 

could progressively hold my breath longer and longer. What was noticeable is when you take a 

breath in to hold your breath, that pain in the chest would become more prominent, almost like a 

burning sensation, and it was gradually going away as I would do that more and more. And so, 

after I did that, I think probably five or six times, I actually started feeling much better, much 

clearheaded and the fatigue lessened. 

o “I couldn’t take an entire, full, deep breath because the pain... I mean, it wasn’t excruciating or 

anything, but it was just uncomfortable to the point your body is like “no, no, let’s just stop there.”

And then, if you’ve ever stretched a muscle and felt that kind of burning sensation that if you hold 

it there for 10, 15 seconds it just kind of relaxes and let’s go a little bit. You can do that several 

times and it just gradually opened up.”

• Cognitive issues

o “During the flight, I probably could’ve run a checklist. In the heat of it, when it was the worst it 

was in that decent, probably not. I pretty much had my hands full just maintaining aircraft control 

and getting the jet down. I was fully committed to that. I was not, at that point, going to be able to 

run a check list. It would take me a little bit of time to get down, get the jet under control, get to a 

position below 10,000 feet, and then run a checklist.”

o “It’s worth noting, after landing, I felt, again, pretty out-of-it, fatigued, and even a little bit 

confused.”

o “I was trying to log into the [computer] to document the standard post-maintenance debrief. It’s an 

incredibly basic thing, just logging into a computer that I’ve done over and over again. But I kept 

logging in over and over again with the wrong password. I had to ask the maintenance individual 

there, why I couldn’t log in and they were like ‘you’re straight up using the wrong login name and 

password.’ It was really obvious to them that it was kind of an inappropriate. I was inappropriately 

confused at that point. Nothing manifested in the air, but I could definitely detect a cognitive 

slowing and confusion on the ground, after landing.”

o “When I was looking at it, I couldn’t remember, I could see that it said [label] and [label] but I 

couldn’t remember if [label] or if [label] was the one that [did what I wanted] and I was 

contemplating like “which one is it, which one is it, which one is it, I can’t remember” and I 
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couldn’t think, I couldn’t remember [laughs] which is odd because I KNOW that it’s [label]. But I 

couldn’t, I couldn’t put together.”

o “Not tunnel vision per se, but focus lock was the only thing I noticed. You’d stare at something. 

Your attention will remain locked on something but not vision wise. 

o “I’m looking at the switch and I can’t remember which direction, which is telling that I’m not 

cognitively with it, I can’t remember which direction to turn the switch. I’m looking at it. I don’t 

know which way to turn it. And I couldn’t really read the, because I couldn’t see well, couldn’t 

really read the labels on it.”

o “Again, I wasn’t really with it, and I considered [committing a hazardous error], if you will. And 

just about the time I get my hand [in position to commit the action], I start to feel little bit better. 

At this point I had dropped my mask and I start to feel little bit better.”

o “I’m just slow. It’s like you just stare at something for however long, for 20, 30 seconds and 

you’re like ‘I’m just staring at it... and I need to... I know what I need to do... but,’ [I can’t].”

o “No [I could not fly again immediately after]. Well, I wouldn’t have wanted to. I don’t think I 

could have. I would never would have tried to.”

o “The cognitive deficit was one of being able to concentrate and I specifically remember not being 

able to find words. Like the words... I knew the concept or whatever, but I couldn’t, you know, I 

would just sit there, looking somebody for like 20 seconds trying to pull the word out. Never in 

my life have I had the wheel spin where I knew what it was, and it just never engaged to find the 

word, is the best way I could describe it.”

o “No trouble standing or walking or gross motor skills. Just kind of like, maybe, slow and 

uncoordinated, would be one way to describe it”

o “I had to fill out a questionnaire is the closest thing I can think to it and just look at the 

questionnaire and you’d stare at the question for a little while, right, trying just to make heads or 

tails of it. But I think that was more of a... I didn’t have trouble manipulating objects or seeing 

what they were, it was just the conceptual linkage. The judgment was all fine, all the thoughts 

were there. That’s what struck me the time, is the fact that the thoughts were there, but the linkage 

was not.”

o “I had been lightheaded, by the time I noticed I was experiencing some confusion, I was already 

on the ground so that didn’t elicit any anxiety.”

o [Could you have followed a procedure so the T?] “If you gave me 20 minutes to do a 2-minute 

task, sure.”

o I just had a mental block [in flight]. It immediately struck me like, “why can I not remember this?”

o “The cognitive benchmark I remember feeling” “I got in the van after getting out of the aircraft 

and my [commander]... made a joke... I didn’t really think anything of it, but... about three hours 

in the chamber... I thought was pretty funny. The only real recognition was that, along with a 

couple other things that people have said between getting out of the jet and then getting back kind 

of hit me at that point in the chamber.”

• Fatigue

o “I was only airborne 15 minutes but I was probably in the jet for an hour total. Get out. And now 

I’m feeling just like dog crap. Now everything hits me like a train. And I have cognitive disability, 

extreme fatigue, and I’m just like out of it. To the point where it’s kind of scary. I can’t form 

words properly. People that know me are kind of a little bit scared. That this is just not the normal 

me. And it’s just like you’ve run a marathon, but worse, and you can’t think straight and just want 

to go home and sleep.”

o “I felt that way for hours: three, four hours”

o “Fatigued over sleepiness. Fatigued like physically drained, not needing sleep.”

o “You’re still dragging for a solid two days afterwards.”

o “[After landing] 1) I was relieved to be back. 2) I was, I was just out of it. Like if you go for a 

really, really, hard run, I mean like a hard run, and you’re done with that workout or you’re done 

with exerting yourself significantly and you feel just a little off, little out of it, tired, that’s kind of 

how I felt. Like I had really exerted myself, which I may have, that may have been the adrenaline 

wearing off, or it could have been a number of different factors, certainly there was some 

adrenaline involved in that, my body’s reaction to what had just happened, and how I felt, so that 

was definitely part of it, but I just felt mentally kind of... not sharp. For a while.”
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o “I tend to experience more post-flight fatigue in the F35 than I have in previous jets. That’s 

actually really common, among F 35 pilots, previously experienced. Definite postflight fatigue.”

o “I could fly a really intense F-16 sortie, land, and then go to the gym. In the F35, I’ll fly even a 

fairly relaxed F35 sortie and then I’ll land, and I’ll just be like ‘wow, I really don’t have the 

energy to go to the gym.’ And that is pretty close to universal. Most pilots experience that. This jet 

fatigues the pilot more than previous airframes have.”

o “My general stress and workload was so much higher during an F-35 test sortie during those years 

that if there was any physiological effect, it was rolled into it. I mean, I was exhausted, I was 

always exhausted. I attributed it at the time, and probably would still... was more because of the 

mental effort that was involved with just doing the job...the concentration and workload that was 

associated with the actual testing we are doing.”

o “One of the things that I remember specifically was that the caffeine failed to help at all.”

o “I had excessive fatigue during the mission and after the mission for a while”

o “[After the night’s sleep] I’d say 90% recovered in terms of, you know, no longer super, super, 

slow like you’re in molasses but just you know you’re dragging.”

o “[I] was tired for the next few days. It took a while to recover from that. It’s very disturbing, both 

from a personal and psychological point to go through.”

o “It’s not like you just ran a marathon and you’re just super tired. It’s more than that. And it’s 

different. So, it’s not like being super, super, tired, like you woke up in the middle of the night 

type of thing, it’s more like sheer exhaustion. And so, I went to bed well before my kids and slept 

like through the whole night and didn’t feel refreshed in the morning. And caffeine didn’t help.”

o “the fatigue was different than any other fatigue that I’ve ever had”

o “the other thing that was disturbing was how long it took to recover. And this wasn’t the first, and 

definitely wasn’t the last time, where I go several days after a, what I would have before called, 

“the hard flight”, you know, just some days were harder than others... people who have been 

flying. You just wake up, even though you thought you got a good night sleep and you slept for a 

really long time and you’re still almost as tired when you get up as when you went to bed”

o “the thing that kind of sticks out in my mind, is that, the motivation... it’s like somebody just 

killed all motivation. I’d be happy to sit behind, you know, play a videogame or just sit there and 

do something and I wouldn’t be like I was falling asleep, but I had no motivation, to do anything. 

Like, I’d have a task and it would seem kind of, like, insurmountable.”

o “At least 2.5 - 3 days [to recover]. I don’t specifically recall feeling when I was 100% back. Like I 

said, this wasn’t the only time, this was probably the most extreme where I had an acute event and 

took several days to recover, but there were other times where I was just fatigued and still the next 

day or the day after I was still dragging a little bit. So, at least two days, may have been longer, up 

to a week. There were a couple of times, where I would, like I remember, I even had a workup 

after this at some point for chronic fatigue, because I was just dragging week after week with no 

getting better in sight and this was a perfect example of that. After this for several days.”

o Kind of just hitting a fatigue wall. That first week I would notice that I would hit a point, even if I 

was doing an isolated exercise or something that everything felt tired, not just what I was working, 

you know, what I was doing at that time. So, that is the only difference I really noted. But then 

after about a week, that fully subsided.

• Other People

o “People were just like ‘you’re not your normal self.’”

o The flight doctor, multiple times, mentioned that I was not myself until coming out of the 

chamber. He said “you were low-energy, not a lot of eye contact, there is a persistent nystagmus 

that he that he noticed.” 

Cluster 4: Hypoxia recognition training as it currently exists is not a sufficient match with the 

respiratory environment in the F-35 when compared to the symptom exhibition and mitigation 

needs experienced during actual flight. 

Quotes include:

• “People figure out their F-35 symptoms, essentially by flying it, as odd as that sounds.” 

• “The thing about having a problem with your body while you’re flying is that I don’t have a switch, I don’t 

have anything I can do to, you know, fix it. I don’t have it checklist to go fix, you know, me. So it can be 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 198 of 260

very disconcerting, that, you know, I don’t have any action I can do. I can descend, you know, but in my 

mind I’m like ‘dude, something is not right with me’ and I was also thinking ‘if this keeps getting worse at 

the rate that it is getting worse I am not going to be able to stay conscious’ was my thought. That if it 

continued in the direction it was going I was in trouble.”

• “This isn’t the hypoxia that you were trained to in UPT, you pull your green ring, or you turn the BOS on, 

it’s a green knob in this aircraft, and you’ll instantly feel better, kind of like you get in the altitude chamber, 

but this may be a - then kind of let things settle out for a few minutes and then you should feel better over 

time but it might require minutes to address the situation and feel better.”

• “Ironically enough though, the canopy up, rush of fresh air, did not make me feel better. I basically stayed 

the same. I didn’t really feel normal until about two hours later.”

• “I also didn’t know what to do with my initial symptoms. Honestly I had never, I mean, you train to it but 

you don’t really think about hypoxia as something that you’re going to, I mean you train all the time to 

other emergency procedures, you know, hydraulic failures things like that, but training to what your body is 

going to feel like and what you need to do in that situation is different.”

• [upon canopy open] “there was really no change. It was not like when I’m in the ROBD and I’m not feeling 

well and I gang load, and I get that [snaps fingers] rush of oxygen or a new supply, you pretty much right 

away regain your color vision and all that, it wasn’t like there was a huge rush of feeling, “oh my gosh, 

now the canopy’s open and I feel great,” it wasn’t that. It was, I felt pretty much the same, which was much 

better than I had in the cockpit 15 minutes earlier but, it wasn’t, when I popped the canopy and I got the 

Oxygen mask, it wasn’t like “Oh my god, thank God, I feel so much better” it was “okay, I feel the same as 

I did about five minutes ago.”

• “normally you would go on backup oxygen system, when you are experiencing hypoxia symptoms, or 

physiologic symptoms, I should say, however, the symptoms occurred after several minutes while 

breathing the backup oxygen system provisioned so I made a pilot decision and elected to discontinue that 

and go back onto the OBOGS” 

• “I think it’s important to talk a little bit about perception versus reality here, so in my mind what I was 

perceiving was an OBOGS fail, the BOS came on, I did not feel well with the BOS on in my mind, started 

the decent, now I noticed the OBOGS is back on, I’m below 7000 feet, the BOS is off, and I am now 

feeling better. I’m confused. I think at that point that there was something wrong with the BOS, the backup 

oxygen system, maybe a bad supply of air in the bottle and so I did not trust the backup oxygen system at 

that point so I did not turn the BOS on and I did not drop my mask. I just get my mask up thinking that the

OBOGS was okay and that the BOS was bad.”

• “I start to have the same symptoms again so: air hungry, not feeling well, and now in my mind I start to 

think that backup oxygen system is bad and so I don’t really have any actions that I can do. I’m kind of at 

the end of my rope, if you will. The only thing I can do is drop my mask.”

• “I had a caution in an aircraft that I was not that familiar with, with 100 hours, in an environment that I was 

not used to knowing what my body would feel like, and I think that that, again just my opinion, I think that 

that caution caused a physiological response with me, because the caution system in the F 35 is pretty... 

loud.”

• “I didn’t think of controlling that inhale and controlling the exhale. I didn’t think about trying to control my 

heart rate or control my body. I never thought that I would have to think about that. I had not really 

prepared myself, I guess, to be in that situation to control my rate and depth of breathing so it was kind of 

on its own. I was doing it subconsciously. I was not consciously controlling my breathing.”

• “if a pilot experiences symptoms, than obviously they particularly want to descend to try to mitigate that, 

but that may or may not be possible [due to the active location].”

• “it was about two and a half later, two hours or so later, that I felt normal”

• “While I did feel the same symptoms, my opinion is that I don’t think that there’s anything wrong with the 

jet, I think it was the way that I reacted. Now the [second event in same flight], I don’t know. I don’t know 

if there was another physiological response or whatever, but the initial one, I don’t know, my guess is that I 

hyperventilated or I was breathing too much or I had, I don’t know, some response where I breathed myself 

into that, I don’t know. It’s just what I think.”

• “No, [vision problems]. I do have, even now, I have a mental picture of looking at the BOS handle and it 

looks very clear in my mind’s eye now.”

• “I had trained on legacy equipment so my expectation, when I flipped the switch for it to go BOS was that 

there would be a big push of 100% oxygen in my face. Now I know that’s not how the system is designed, 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 199 of 260

so it’s not a surprise that I didn’t get that. And the reason that I flipped the switch, even though I actually 

had the icon that said BOS on, I still flipped the switch because I wasn’t getting any sort of excess pressure 

and so I was concerned that ‘Crap, I had an erroneous indication and it hadn’t flipped over’ so I was 

wanting it, but I’m not sure, I think that may have been just negative training from physiological with 

legacy equipment.”

• “When I started evaluating myself and realized I had exactly the same hypoxia symptoms as I had 

experienced... in the altitude chamber. There were always the same, and they were consistent in the jet at 

that point with what I experienced in the altitude chamber. A little bit of an overall queasiness and then it 

was that clammy kind of feeling and a bit of a hot skin sort of feeling, and then it would get gradually a 

little bit more tingly, but still that hot skin, clammy feeling, and then I would get lightheaded, was kind of 

the progression each of the times”

• “I think there was a discussion at some point whether we need to go to the flight doc and I’m like “Well, I 

don’t have any symptoms other than I’m just extremely tired, right? And I just can’t quite think straight, 

you know, what’s the Doc gonna do?” So we didn’t go over to the doc because there was really nothing to 

report.”

• “In the hypoxia training, everything seems to escalate very quickly. So, you go from normal to hypoxic in 

about a minute or two minutes. This seemed to be a very slow onset, a very slow ramp-up over the course 

of about 20 to 30 minutes. I think that’s why it delayed recognition on my part. And then again, usually in 

our hypoxia training it’s also accompanied by some type of warning or caution that the aircraft gives you or 

a pressurization failure that you can also correlate to your symptoms. I didn’t have any of those.”

• I was basically constantly trying to let myself know, “Hey, you have no warnings or cautions, your cabin 

altitude is fine, you know, you’re fine, you’re just fatigued. This isn’t a hypoxia thing, everything looks 

fine, there’s nothing the jet is telling you is off.” That, contributed with the symptoms being relatively mild 

and then slow onset is why I delayed recognition on my part.”

• “I didn’t know what it was but I just had this feeling that something wasn’t right. And I kept on trying to 

essentially reason out the symptoms I was having based on the fact that I had no warnings, cautions or 

cabin pressure malfunctions.”

• “I opened the canopy but it was nothing immediately noticeable. I noticeably felt better when I was in the 

ambulance and I got put on oxygen.

Cluster 5: Normalization of deviance.

Quotes include: 

• “OBOGS sensor had failed, so there actually was a system failure that had generated that ICAW, it wasn’t 

just a spurious ICAW that we tended to get back then.”

• “Now thinking back and knowing how I respond in the jet now, how I feel in the jet now, that may also be 

incorrect. That may be something that’s happening all the time now, and I’m just used to it with 500 hours 

or so now in the F-35.”

• “I have observed there are a lot fewer OBOGS fails. It used to be a fairly common occurrence in the old 

block 2 software and it was kind of like a ‘well, just hang out, breathe the BOS, reset your OBOGS, and if 

it resets, fine,’ and you’d actually just continue with the sortie. I don’t think anybody’s gotten any OBOGS 

fails recently. It’s certainly a less common occurrence in this software variant.”

• “There generally have been episodes while flying block 3, in fact, when I had my episode, it was a brandnew 3F so early block three jet. So, it certainly doesn’t correlate absolutely, but there were more jet 

OBOGS issue annunciations”

• “It’s important to emphasize these ICAWs, these OBOGS fails in the 2B software that we were flying at 

the time, these happened all the time like it was considered a nonevent. In fact, depending on what software 

subset you had of the software subset you could actually just continue the sortie [after the ICAW cleared].”

• “It was just a hard-breathing day. And the thing that just stuck in my mind that it was just way harder than 

normal to breathe without any definitive smoking gun as to what was causing it. I [informed the program 

office and the head of the maintenance] and said ‘this just breathes strange and it was very hard, and it just 

really caught my attention, but there’s nothing... I can’t say anything one way or the other for you guys to 

go fix... So, I just wanted to kind of let you know, and just talk it over him you” and they’re “oh, alright, 

well, just let us know if you think of anything else.’ So that was the end of that. I passed it on its FCF 

checks and signed it off. It required me to actually sign off the jet and say ‘hey, it’s good to go.’ The 

OBOGS hose was kinked 50%.”
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Cluster 6: Pilots expressed several concerns related to the organizational or leadership 

elements related to the F-35.

Quotes include: 

• “Pilots experience symptoms in the jet, they notice, but they’re not at the threshold that they consider 

necessary to declare or that they’re willing to flag themselves, highlight themselves, over.”

• “I’m firmly of the belief that it is not a psychogenic phenomenon, for a variety of reasons. These are some 

very skilled, aggressive pilots. They have no inclination to generate symptoms for themselves.”

• “There was tremendous amount of concern amongst the enterprise that the program was vulnerable, at the 

time, and so there was a lot of pressure to continue testing, continue pressing forward. The team as a whole 

that the program was vulnerable, at the time, and so there was a lot of pressure to continue testing, continue 

pressing forward. The team as a whole, and especially the program office folks who were in charge of the 

life-support system at the time, were fairly motivated to assign any, or my symptoms, I guess, my actual 

reaction, to something that was not attributed to the jet, I guess was their aim. That was my perception, was 

that that was what they were trying to do: find a way to have it not be the jet so they could press.”

• “There is going to be some hard work that we are going to have to do because I think our assumptions are 

wrong; that your system worked as designed and this is what the outcome was.” “This is a fight I don’t 

have the resources to continue to fight, at the time.”

• “We talked our way through it and I advocated for an investigation of the design of the system, because, at 

least it seemed clear to me that, the system even if it had functioned as designed... and that was a rapid 

conclusion, that they evaluated how everything worked; all the equipment in the chain from OBOGS and 

BOS through the PIC through my mask to me everything had functioned as it was designed to and so my 

concern was if they had designed it to do THIS and not protect me from hypoxia in this sort of a scenario, 

then we had a problem with the design that we should evaluate where those problems were. At the time 

there was a significant amount of resistance to doing that, again, their assessment was: it worked as 

designed, the oxygen system wasn’t broken, it was a bleed air problem. No need to continue any 

investigation into the design of the system, as far as it being available in an emergency where there’s no 

bleed air available for pressurization air or for the pilot.”

• “I was advocating that we needed to do some research and understand if, maybe, the fact that the system 

was working as designed, but didn’t actually protect me, maybe we had some fundamental 

misunderstandings of what the design of the system needed to be and we didn’t have as much physiological 

understanding of the human/machine system as we needed.” 

• “I learned a lot of words that I didn’t know before. Besides hypoxia, they discussed that they thought 

maybe it was hyperventilation. And maybe not hyperventilation in the sense that I was breathing too often 

and too shallow, but because I was actually actively trying to control my depth and rate of breathing that I 

had over-controlled and therefore induced hypoxia symptoms by a sort of self-induced hyperventilation. 

That was one theory. They also, I learned a word called hypercapnia... they were able to, again, sort of talk 

themselves into using those words and saying “well, maybe it was hypercapnia, maybe it was 

hyperventilation, but in no case is it something we need to change the design.” 

Cluster 7: Other Comments – Not included in Chapter

Other comments the pilots included covered areas such as aircraft ergonomics, aircraft heat 

signature, mask discomfort, and proposed non-contributory elements. 

Quotes include: 

• “The flight equipment and the ergonomics of the jet. 80% of us are getting severe back pain 30% are 

getting severe left leg pain.”

• “For comfort, a lot of people will drop the mask for maybe, call it, out of an eight-hour sortie, maybe two 

hours... three hours with the mask down”

• The heat signature on the jet is pretty significant. Some places... that becomes actually a very significant 

issue, physiologically, “hey, I was super dehydrated... at takeoff” 

• The thermal burden of the flight equipment you’re wearing is pretty significant, more significant than 

previous generations of aircraft

• “Ground ops take a lot longer. F-35 ground ops are notoriously longer, so I was in the jet and breathing off 

of the system for probably a good 40/35 min. [just on the ground].”
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• “The equipment the pilot wears to execute the mission, people mentioned that as a potential causation for 

the symptoms, but I consider that to be noncontributory. There’s some weight, but it doesn’t actually result 

in [any restriction].” 

• “Thank you, guys, truly, for doing the work, like I said, that we probably should have started in ’12. This is 

going to be useful for very long time.”

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VI. Barotrauma

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Trials Network. Higher versus lower positive end-expiratory pressures in patients with 

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13. Gattinoni L, Pesenti A, Avalli L, Rossi F, Bombino M. Pressure-volume curve of total 

respiratory system in acute respiratory failure. Computed tomographic scan study. Am. 

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14. Tsuno K, Prato P, Kolobow T. Acute lung injury from mechanical ventilation at 

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18. Lu Q, Rouby JJ. Measurement of pressure-volume curves in patients on mechanical 

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X. Aviation, Gz, and Hypoxia - AIN

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Ernsting's Aviation and Space Medicine. 5th ed. Boca Raton: CRC Press; 2016. 

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regional lung ventilation determined by functional EIT during parabolic flights. Journal 

of applied physiology (Bethesda, MD: 1985). 2001;91(1):39-50. 
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6. Green ND. Lung volumes during +Gz acceleration and the effects of positive pressure 

breathing and chest counter-pressure. Journal of gravitational physiology: a journal of the 

International Society for Gravitational Physiology. 1994;1(1):P41-4. 

7. Haswell MS, Tacker WA, Jr., Balldin UI, Burton RR. Influence of inspired oxygen 

concentration on acceleration atelectasis. Aviation, space, and environmental medicine. 

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Induction and prevention of acceleration atelectasis. Aviation, space, and environmental 

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aircrew demand breathing systems. Air Standard 61/101/6A. Washington DC: Air and 

Space Interoperability Council (ASIC); 2000. 
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Appendix 8: Pilot Breathing Assessment (PBA) Considerations on 

NESC’s F/A-18 PE Report (2017) and Other Issues

The PBA was undertaken to use NASA aircraft as testbeds to instrument, measure, analyze study 

and document the physical phenomena behind pilot physiological episodes. As previously stated 

by the NESC in 2017, the physiological episodes experienced by F/A-18 pilots were found to be 

the result of system-level weaknesses and failures to consider the entire aircraft and pilot as an 

integrated whole, as well as the aircraft-pilot interactions as integrated components in a critical 

human life-sustaining system. The details of these findings were documented in the NESC report 

on the F/A 18 and E/A-18 Fleet Physiological Episodes (NESC, 2017; Chapter 10). The work 

done by the Pilot Breathing Assessment as documented in this report is an effort to further that 

work by showing a path forward for instrumenting and measuring dynamic system interactions 

such that the inherent shortcomings may be identified and addressed at a system level.

2017’s Key Recommendations and PBA Updates

The 2017 report presented a number of key recommendations. These recommendations are 

discussed below, updated with knowledge gained from the PBA:

1. 2017 Recommendation #1: Pilot Breathing Needs to Be Measured

a. F/A-18 Report Key Recommendation: Measure parameters that directly assess 

human health and performance. Make measurements in the cabin environment 

whenever possible.

b. Related PBA actions: The entire PBA project was focused on measuring human 

health and performance in the cabin environment.

c. PBA 2020 assessment: PBA developed a standard method for measuring human 

health and performance in the cabin environment. PBA developed a standard 

method for assessing data.

2. 2017 Recommendation #2: Cabin Pressure Needs to be Measured

a. F/A-18 Report Key Recommendation: Measure cockpit pressure and compare 

pressure profiles for PE flights to pressure profiles for non-PE flights. 

b. Related PBA actions: PBA measured cockpit pressure, mask pressure, delta 

pressure.

c. PBA 2020 assessment: PBA identified breathing system interactions related to 

pressure, especially cabin pressure fluctuations in the 0.1 – 1.0 Hz frequency 

range.

3. 2017 Recommendation #3: Investigate the statistical connection between VOC levels at 

OBOGS outlets and PE rates

a. F/A-18 Report Key Recommendation: Establish VOC testing at OBOGS outlet on 

all F/A-18 model aircraft to further evaluate the association between VOC level 

and PE rate found in the Growler samples.

b. PBA actions: PBA did not measure VOC levels at OBOGs outlets. PBA testing 

was performed on legacy Hornets with LOX. PBA did identify Breathing System 

Disruptions (BSDS) for jets with low and variable ECS supply pressure

c. PBA 2020 assessment: Elevated levels of VOCs at OBOGS outlet correlate with 

low and variable ECS pressure. PBA recommends closer look at VOC trends.
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VOCs should not be evaluated from the perspective of chemical toxicity – VOCs 

should be evaluated as in indication of low and variable ECS pressure. If ECS 

pressure is directly measured, evaluating VOCs is not necessary

4. 2017 Recommendation #4: Learn why some jets have more cabin pressure fluctuations

a. F/A-18 Report Key Recommendation: Recommend taking further steps to 

validate and apply the NASA cabin pressure model.

b. PBA actions: PBA found breathing system interactions occur involving cabin 

pressure fluctuations.

c. PBA 2020 assessment: PBA recommends reducing the number and magnitude of 

cabin pressure fluctuations, especially those with a frequency similar to 

inhalation/exhalation cadence. PBA additionally recommends avoiding safety 

pressure if possible, because cabin pressure fluctuations cause greater disruption 

to systems that maintain positive pressure in the mask.

5. 2017 Recommendation #5: Collect and analyze data in a systematic way – make data 

sets as comprehensive as possible

a. F/A-18 Report Key Finding: Determine the full necessary capability and optimal 

organizational relationships and alignments to support fleet-level data analysis 

throughout the operational life of all F/A-18 variants.

b. PBA actions: The entire PBA project was focused on developing a database that 

is comprehensive and can be analyzed in a systematic way

c. PBA 2020 assessment: The Pilot Breathing Almanac (Technical Section 11 of this 

report) did not exist when F-18 report was written. It is a new tool, and it can be 

helpful. PBA team encourages USN and USAF to use, and add new data to this 

database.

6. 2017 Recommendation #6: Assess the causes of PEs in a systematic way – with 

sufficient data – with relevant mechanisms

a. F/A-18 Report Key Finding: A structured data-driven causal analysis effort 

extending to the organizational level should be launched immediately upon 

recognition of the existence of both severe and widespread safety hazards.

b. PBA actions: PBA identified the importance of timing and sequence. PBA 

collected in-flight data necessary to make a data-driven assessment of pilot 

breathing system interactions

c. PBA 2020 assessment: With in-flight measurements and quantitative 

measurements of breathing system timing and sequence, the causes of PEs can be 

more precisely identified.

7. 2017 Recommendation #7: PEs involve many systems and requires expertise in many 

areas, form a multi-disciplinary team and make sure pilots are included

a. F/A-18 Report Key Finding: Form a multi-disciplinary working group to conduct 

a dedicated physiological investigation. The primary focus should be the human 

physiological basis and root cause, which in turn can drive engineering changes 

and modifications.
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b. PBA actions: PBA team was multidisciplinary, and the interactions between 

pilots, data analysts, medical doctors, human factors researchers, and 

instrumentation SMEs resulted in new insights.

c. PBA 2020 assessment: Continued support of the idea that small, dedicated, 

multidisciplinary teams are needed to address PEs. 2020 emphasis is on the pilot –

include pilots on your team – listen to all of your pilots.

8. 2017 Recommendation #8: Develop and maintain a clinical practice guideline

a. F/A-18 Report Key Finding: A dedicated Clinical Practice Guideline (CPG) for 

PEs must be developed and implemented. 

b. PBA actions: Spirometry, pulse oximetry, and capnography were major parts of 

PBA.

c. PBA 2020 assessment: Continued support of clinical best practices and providing 

the medical diagnostic infrastructure to collect necessary medical data in a timely 

way.

9. 2017 Recommendation #9: Standards should be reviewed and updated where 

appropriate

a. F/A-18 Report Key Finding: The U.S. Navy should review:

i. Standards described in this review, especially the most recent MIL-STD3050 and determine how those specifications can be incorporated into the 

current F/A-18 

ii. Workforce capability and billets regarding Human-Systems Integration to 

determine if they meet the requirements and intent of the applicable 

sections of DoD Instruction 5000.02

b. PBA actions: PBA identified issues related to timing and sequence. PBA 

identified breathing system interactions between pilot breathing, cockpit pressure 

fluctuations, regulator, and mask components. PBA especially identified a gap in 

regulator performance specifications – the most difficult and important aspects of 

regulator performance involve sudden changes and asymmetric breathing 

patterns. Standards involve constant flow, or sinusoidal waveforms.

c. PBA 2020 assessment: PBA strongly recommends reviewing and updating 

standards. Standards related to trumpet curve measurements do not involve timing 

or sequence. PBA recommends that pilot breathing system standards include 

specifications of timing and sequence. Hysteresis and Phase Shift are quantitative 

measures of breathing system timing.

The Navy and the Causes of PEs

Navy data reviewed by NASA in 2017 revealed Hypoxia in approximately 80% of the PE cases 

examined. Objective data has now given credence to that hypotheses of hypoxia as the root 

cause. Hypoxia results from the complex systematic breathing dissociations as discussed and 

demonstrated by PBA. Increased work of breathing is a result of these interactions and not a 

cause of PEs, but an indicator (symptom) of breathing system erosion on physiological capacity.

Unfortunately, some F/A-18 systems are not designed well enough. Cabin Pressure fluctuations, 

caused by the F/A-18’s ECS, have been shown to cause BSDs. Pilot Regulators adjust to these 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 220 of 260

cabin pressure changes in ways not anticipated by the designers. PBA has shown that the Pilot 

Masks have major BSD issues. 

In the 2017 report, the team concluded that pilot breathing issues are not simply due to 

differences in individual pilots’ susceptibilities, shortcomings in pilot crew equipment, or lack of 

adequate preparation and training of pilots; rather these issues stem from a basic lack of 

understanding of human breathing in dynamic flight environments, and the lack of 

comprehension for the multiple potential complex dynamic behaviors which are possible 

between the interconnected systems of a modern tactical military aircraft. While addressing 

flight crew equipment and crew training may help to reduce the rate of PEs, these episodes will 

certainly never be eliminated without undertaking a fundamental re-evaluation of the 

institutional establishment of basic requirements and integration for the aircraft life support and 

pressure control systems. The data collected by the PBA team re-iterates the conclusion that 

these fundamental behaviors are endemic to the basic design of current fleet aircraft, and that 

PEs will not and cannot be eliminated without undertaking a fundamental re-think of current 

established requirements, hardware integration, flight testing, and continued in-flight monitoring 

of military pilots in real-world field environments.

Examining the Navy’s PEs Through Knowledge Gained from PBA

When the NESC started their investigation of PEs for the Navy in 2017, the PE rates from 2007 

were continuing to climb, despite the Navy’s best efforts to understand their cause and 

implement corrective actions. Now that the NESC has concluded PBA in 2020, it may be helpful 

to examine the PE problem from a new perspective and offer opinions about the PE problem in 

general. 

One of the biggest revelations the NESC team came to during the PBA study was that pilot 

breathing difficulties happened at all. This was surprising for the following reasons: 

• PBA was designed to study pilot breathing over a wide variety of flight conditions. It was not 

designed, nor was it ever expected, that these conditions would ever produce breathing 

difficulties. (PBA Volume I, Technical Section 1)

• PBA used highly experienced PBA test pilots (PBA Volume I, Technical Section 1) to fly 

benign flight profiles (PBA Volume I, Technical Section 1) with aircraft that had liquid 

oxygen (LOX) delivery systems (PBA Volume I, Technical Section 1). As a closed system, 

LOX provides a more reliable source of oxygen (O2) at more consistent pressures and flows

than OBOGs-equipped jets. In fact, prior to the NESC 2017 investigation, conventional 

wisdom by many in the flying community was that OBOGS was the root cause of PEs. By 

flying with LOX-equipped jets, it was assumed that the O2 delivery system would eliminate 

most of the confounding effects observed with OBOGS in the F/A-18 study. 

• PBA test pilots flew in a non-hostile, less stressful research environment than the typical 

sorties flown by their Navy and Air Force counterparts.

Despite these more favorable conditions, PBA has proven for the first time, with in-situ data, that 

in the best of conditions, F/A-18 breathing systems can interact in non-deterministic 

unpredictable ways (mask – regulators – cabin pressures) and have deleterious effects on 

breathing. 

PBA found that even when a flight is nominal – even when the breathing system is functioning 

well – even when the pilot reports no perceptible effects, pulse oximetry measurements show 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 221 of 260

that O2 levels in the pilot’s blood are routinely reduced. Hypoxia is defined as reduced O2 levels 

in blood and tissues.

Logically, then, if PBA gathered evidence of breathing irregularities and mild hypoxia in benign 

conditions, using the same masks, with the same regulators, and with a more stable O2 delivery 

system than those used in the USN fleet, then PEs have undoubtedly been occurring all along. 

Fortunately, PBA has evidence that supports this case (PBA Volume I, Technical Sections 6 and 

7) and has developed a way forward to diagnose bad breathing aircraft and AFE with the 

prospect of eliminating them.

Considerations

This section provides reminders to the Navy based on the NESC 2017 F/A-18 assessment and 

offers updated recommendations based on new experiences and insights developed in PBA. 

1. Defining the problem is key to solving the problem 

Consider the Oxygen Transport Model first presented in the NESC F/A-18 report (and in PBA 

Volume I, Technical Section 1) and now revised to include new insights gained from PBA (PBA 

Volume I, Technical Section 12). It is important to frame the problem as a complex Human 

System Integration (HSI), identify available evidence, identify missing evidence and fill the gaps 

with objective scientific data. 

2. The approach to the problem is key to solving the problem

NESC team recommended in the F/A-18 report to use a multi-disciplined team, consisting not 

only of engineers and managers, but also of medical doctors, physiologists, and pilots. PEs occur 

in the complex human systems interacting with complex aircraft systems. They are governed by 

principles of engineering systems design interacting in complex ways with human anatomy and 

physiology. 

Throughout the course of PBA, pilot observations were used to guide data analysis. PBA 

analysts asked pilots about their breathing, and their experiences in the cockpit. PBA medical 

doctors and physiologist were highly integrated on the team. They listened carefully to what the 

pilots had to say and worked closely with data analysts to develop a fuller, more complete 

understanding of the measured data.

Also recommended in 2017 F/A-18 study was to use a structured form of causal analysis to help 

understand the complex systems interaction problem. Fault tree methods and other linear, 

progressive techniques can approach the problem using methodologies not well-suited for the PE 

problem and dismiss possible sources of interacting branches on a fault tree. 

3. Focus on flying, not just bench testing

“Fly like you test and test like you fly” is an adage that reinforces the concept that the connection 

between test and flight goes in both directions. Tests need to be conducted in a flight 

environment, or the results may not be valid. Operational profiles should be restricted to the 

envelope where valid test data are available. A pilot breathing system is a complex system, with 

interactions that only occur in the flight environment.
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4. Use/adapt PBA-developed flight test methods to gather missing data and establish 

performance baselines

Profile H - PBA has developed a standardized flight profile, data analysis technique, and 

database of comparison data to serve as a standard diagnostic flight test (PBA Volume I, 

Technical Section 10). If the US Navy accepts new aircraft or makes corrective changes to 

existing aircraft systems, this standard diagnostic test can provide a quantitative way to verify 

that breathing system performance is healthy. If the US Navy suspects that a specific tail number 

aircraft has problems, this test can provide a quantitative score to compare one aircraft to fleetwide standards.

5. Use/adapt PBA-developed metrics to evaluate the quality of the aircraft/aircrew 

breathing systems 

PBA can describe the methods to calculate hysteresis, phase shift, and pressure, no-flow (PNF) 

(PBA Volume I, Technical Section 6). Additionally, PBA can describe ways to calculate an 

inhalation flow score, and exhalation flow score, an inhalation O2 score, and an exhalation CO2

score. (PBA Volume I, Technical Section 10)

6. Cabin pressure fluctuations are important

PBA has gained insight about how cabin pressure fluctuations can cause breathing sequence 

disruptions. Cabin pressure fluctuations can cause the diaphragm muscle movement response to 

inhalation pressure and flow to become completely negated, or delayed, or amplified. Available 

data, data analysis techniques, and insights about cabin pressure effects could assist US Navy 

efforts to identify and correct problems with breathing systems.

7. Pressure and flow in the O2 delivery system are important, but the timing of air 

delivery during breathing (BSDs) is equally important

Cabin pressure fluctuations can cause breathing sequence disruptions. The changes in lung 

pressure caused by pilot diaphragm muscle movement are relatively small. In regular breathing 

in open air, chest cavity expansion/contraction is the only thing causing a pressure difference 

between lungs and open air. There is a direct correlation between diaphragm muscle movement 

and inhalation/exhalation flow. In a jet with cabin pressure fluctuations, that direct correlation is 

lost. The magnitude of cabin pressure fluctuations can be greater than pressure changes caused 

by diaphragm muscle movement. The effect of diaphragm muscle movement can be completely 

negated, or delayed, or amplified – by cabin pressure changes. Hysteresis and phase shift provide 

quantitative ways to measure this effect.

8. Reconsider the cost/benefit of demand regulators with safety pressure. 

PBA has gained insight about sequence disruptions affecting regulators, and how diluter-demand 

regulators may be better suited to provide large and sudden demands for air with less delay. 

Available data, data analysis techniques, and insights about the differences between demand 

regulators and diluter-demand regulators could assist US Navy efforts to identify and correct 

problems with breathing systems. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 223 of 260

9. Use and continue development of current and new in-flight breathing measurement 

systems – VigilOX and JPL Mask

Making measurements of pilot breathing in-flight is difficult. PBA has gained a substantial 

amount of experience (>100 flights) with the VigilOX instrumentation system. The detailed 

methods and procedures can improve the quality of in-flight data, and can make in-flight testing 

more consistent.

The JPL – In Mask CO2 and Water Sensor. The JPL-IMCWS can make fast and accurate 

measurements inside the mask. AFRC has qualified the sensor as certified flight-test equipment.

The JPL – IMCWS has been successfully demonstrated in-flight. 

Use of the JPL sensor can be used with mask flow and pressure to isolate the particular valve 

dysfunctions. If used solely, the JPL may be used as an early warning or as a diagnostic tool for 

the mask. The new in-mask CO2 sensor offers significant capability by producing a capnograph 

with system diagnostic or testing insight. The benefits from having a CO2 measurement at the 

mouth and mask exhale valve are substantial. High fidelity data with insight as close to the 

mouth as possible of pressure, valve function, and CO2 is a revolution in pilot sensor capability. 

(PBA Volume I, Technical Section 9)

10. Use/adapt PBA test methods to develop a comprehensive baseline of pilot breathing 

for a wide variety of conditions (PBA Volume I, Technical Section 11- Almanac) 

PBA has developed a database of pilot breathing measurements from >100 scripted flights.

Results from US Navy and USAF in-flight testing can be compared to this database.

11. Use/modify our Ground Test Methods to gather missing data

Conducting pre- and post-flight spirometry, capnography, and pulse-oximetry testing was an 

important part of PBA. If the USN gathers a large volume of data, then effects of flights can 

accurately assessed with statistical rigor. Even with the relatively small number of PBA flights 

(compared to USN fleet-wide implementation), pulse oximetry measurements of PBA pilots 

showed that O2 levels in the pilot’s blood were routinely reduced.

References

• NESC F/A-18 Final Report, NESC-RP-17-01205, 2017
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 224 of 260

Appendix 9: Results of Pilot Questionnaires and Interviews

Subjective Pilot Report Questionnaires and Interview Summary

Introduction

Self-report data and techniques

To appropriately examine a phenomenon in the world requires adherence to a set of rules and 

procedures that help ensure the accuracy and generalizability of the findings. Following 

standardized research methodology for design, collection, and analysis through a study ensures 

the accuracy and reliability of the collected data and defines the scope of those findings beyond 

the study (Jones & Thissen, 2007; Stout 2002). This is easily and readily known for objective or 

quantitative data, but might seem less well-defined for subjective or qualitative data such as 

those observed with human subject data. This is not the case. Indeed, these standards must 

remain extremely strict due to the fluctuations expected due to the natural variance of data within 

and between humans. 

As the name suggests, human subject research examines aspects related to the human. This might 

include directly measurable components such as those related to the functions within the human 

body (e.g., heart rate, skin temperature, reaction time) with other objective measurements more 

useful when derived (e.g., heart rate variability). However, many aspects of human cognition 

remain impossible to assess with a sensor and instead require the subject to answer a query. This 

subjective information is incredible valuable and can inform the objective measurements. For 

example, the General Duty Clause, section 5(a)(1) of the Occupational Safety and Health Act 

(OSHA), states that employers must provide employees a place of employment that "is free from 

recognizable hazards that are causing or likely to cause death or serious harm to employees.” 

This is fairly straight-forward for temperature exposures that reliably result in damage and death, 

however, when it comes to temperature for office workspaces, OSHA can only provide guidance 

(Lang, 2004). This is due to the high variability of the individual physiological differences and 

preference. An objective measurement of an 81-degree room temperature with 40% humidity 

provides information, however, the real question is if these environmental conditions cause an 

impact to the workforce during task operation. These conditions may be acceptable to some 

workplace populations and unacceptable to others. If the workplace is peopled with mostly 

individuals who find this unacceptable then there will be an impact to productivity and job 

turnover. The only way to know the human perception of the objective information is to ask. The 

goal is to gain an understanding of this perception with as little error as possible.

The traditional outcome of human interaction examination is to develop probabilistic findings 

that generalize from a sample to a larger population using empirical evidence rooted in theory. 

Another outcome can be to simply provide descriptive information about specific individuals in a 

group. Subjective human cognitive or psychological state data are most frequently assessed with 

a self-report questionnaire (“on a scale of 1-10 how do you feel?”) or structured/unstructured 

interview (“tell me what happened). This subjective data can be collected either quantitatively 

through the use of survey scales developed with psychometric methods, or qualitatively via a 

structured or unstructured interview. The quality and accuracy of these techniques relies heavily 

on the training and ability of the researcher. A simple word choice can unintentionally produce 

unusable, misleading, or inaccurate data. In the above example, the employer demands all 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 225 of 260

employees fill out a questionnaire about the temperature of the workplace. The results will go 

directly to the employer without anonymity. The main question of interest is written as “I 

produce poor quality work in this temperature environment, and I do not want to come to work.” 

The employer may have intended to gauge the severity of the temperature impact, but the 

employee may perceive this to be a threat to his or her continued employment. Next, failing to 

clearly indicate these responses will be de-identified can provide further cause of concern for an 

employee in terms of potential future backlash or adverse impact. In this case, an employee may 

simply not respond but, in this example, the employer made questionnaire completion 

mandatory. The employer only wanted to make sure everyone filled out the questionnaire, but 

because poor wording, no concern for privacy, and instituting a mandatory response, an 

employee might report data that aligns with his or her perception of what might be considered 

more preferable to the employer, but does not align with his/her actual state. Inaccurate data 

reduces the ability of the employer to accurately understand areas of concern in the workplace 

and to enact positive changes to address those issues. Closely adhering to well-defined 

psychometric principles and techniques can increase response rates and reduce conditions that 

encourage false data. 

The interview is another type of qualitative research methodology frequently used to collect 

individual instances of subjective experience. Like in quantitative research, once the interview 

data are conducted, the responses are aggregated and analyzed for emergent properties that 

reveal common themes generalizable to the content area in question. This analysis method is 

well-supported in the literature, but also requires advanced expertise in human subject data 

collection and the subject matter area to conduct with precision and accuracy while avoiding 

common commission or omission errors. In the previous workplace temperature example, now 

the employer uses an unstructured interview style to discuss temperature with each employee. In 

this case, the employer will not ask the question in the same way to every employee and the 

employee response will likely be influenced by style of the question, the preexisting 

interpersonal relationship with the employer, or the power differential. This can produce equally 

poor results. 

Exploring phenomena using established and appropriate scientific procedure can empirically 

separate fact from fiction by utilizing evidence-based observations designed to falsify or support 

a theory. The goal of empirical research is to realize understanding through the targeted 

observation and experience, and analysis. The scientific method is designed to encourage the 

validity and reliability of those findings in terms of the phenomenon being examined. The 

scientific method is a multi-stage “procedure starting from observations and description of a 

phenomenon and progressing over formulation of a hypothesis which explains the phenomenon, 

designing and conducting experiments to test the hypothesis, analyzing the results, and ending 

with drawing a conclusion” (Andersen & Hepburn, 2016).

However, in operational environments the traditional scientific method may appear impractical 

as hypotheses development may be difficult and nearly impossible to statistically test in the early 

stages. Unclear expectations may yield solutions based on inaccurate assumptions that produce 

unintended consequences when implemented. Gaining a greater understanding of the depth and 

breadth of the problem space can provide guidance towards the development of highly controlled 

studies designed to formally assess variable correlations and to increase the success of those 

future data collected. This is the case for PBA. This early investigation examines both new 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 226 of 260

objective measurements and subjective measurements to determine lessons learned for future 

application.

Method. Survey Design, Development, and Interpretation

Prior data regarding physiological episodes (PE) primarily relied on subjective report. The 

absence of objective measurements of breathing data taken from inside the cockpit and/or 

breathing loop limited any way to validate the pilot experience. For PE Data, parts A/B/C are 

data collected primarily through self-report via questionnaire. These physiological event reports 

consist of an individual’s experience and the medical team’s perception of symptoms with no 

objective data collected from within the aircraft. Objective data collected by medical often 

occurs after lengthy time delay when the situational conditions that lead to cognitive impairment 

may no longer present. To ensure appropriate interpretations and meaningful conclusions can be 

drawn from these data, this questionnaire or survey must be psychometrically sound and address 

features such as dimensionality, reliability and validity (Jones & Thissen, 2007; Stout 2002). 

To identify the parameters of a subjective experience, data must be gathered, analyzed, and 

interpreted. This includes using an accepted metric and method by which to collect those data. A 

questionnaire is the primary method of measurement for self-report psychological phenomena. 

These data are subjective as they are based entirely on the individual’s perspective. Objective 

data are those collected using an outside measurement. For example, an individual might 

subjectively report experiencing symptoms they perceive to be a high fever. The actual state of 

the body temperature can be measured objectively by way of a temperature measurement. This 

temperature measurement can confirm or refute the subjective report of high fever which can be 

used to govern the appropriate mitigation strategy. 

A fundamental concept of measurement is that measurement is often imperfect. This 

imperfection means the observed value reflects some amount of error therefore the true value 

cannot be not known. The True Score Theory states that the observed value (X) equates to the 

true value (T) and the addition of an unknown amount of error (e). The goal is to increase the 

accuracy of the measurement by reducing or explaining the error term. This goes for groups of 

individuals as well. 

Questionnaire development must be appropriately conducted otherwise the data could end up 

meaningless or uninterpretable. The kind of questions must be appropriate for the desired data 

such as dichotomous (yes/no), rank order, level of agreement (continuous or likert - strongly 

agree to strongly disagree), cumulative score, and text box. The wording of the question must be 

thoughtful and carefully examined to craft questions that help the researcher identify information 

about the individual taking the questionnaire. Questions that are leading, double-barreled, out of 

context, confusing, biased, loaded, embarrassing, revealing, or offensive can reduce the accuracy 

of those data. A leading question is one that suggests an answer more than another. A doublebarreled question asks two questions that could have different answers. The appropriate approach 

to developing questions is to ask subject matter experts such as pilots, flight medical, and first 

responders to provide questions that help discriminate useful information from non-useful 

information. After initial development, psychometric testing and analysis should be conducted 

on a sample to assess if these questions garnered the expected responses and iterate as needed.

Conceptual Reasoning 
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The pilot is an extremely valuable source of information on the health and safety of the aircraft. 

Although standardized and objective machine data gathered from an aircraft is desirable for 

numerous reasons, at this time, the aircraft data cannot provide all information of the subjective 

experience of the pilot flying the aircraft. The fighter pilot is an expert user and can provide 

insight into the aircraft in terms of performance changes. Unfortunately, pilot opinion has 

become less sought.

The NESC team learned that no questionnaires/surveys were conducted with the F/A-18 

community with respect to PEs. Interviews with pilots and maintainers, and discussions with 

USAF scientists indicated that a survey could be very useful in determining the scope of the 

problem, and the community’s main concerns. Such information could help guide the Navy’s 

response to the problem and its communication with the fleet.

Discussions with USAF scientists who participated in the USAF’s resolution of the F-22 

problems have revealed the importance of Surveys that were conducted in 2011 and 2012. 

Surveys afforded a number of benefits to the Air Force:

1. Provided the leadership with: 

a. pilots’ views on the F-22 breathing problems

b. pilots’ confidence in the aircraft, and 

c. pilots’ views on efforts by the Air Force to correct the breathing problems.

2. Spoke to the rank and file that the leadership had a vested interest in solving the problem

3. Provided important data that became part of the F-22’s problem resolution

4. Led to a concerted effort by senior leadership to fix the communication problems

The NESC recommended that a survey be conducted throughout the fleet to capture the overall 

context of PEs within the envelope of unremarkable flights, as well as to gauge the general 

attitude regarding the PEs and the Navy’s efforts to address them among pilots and maintainers.

The information gained from a well conducted survey could help guide the Navy’s response to 

the problem and its communication with the fleet.

PBA Questionnaire and Interview Responses

The subjective query portion of PBA had two main goals. First, to gain insight on the individual 

differences, experiences, and demographics of the subjects used in this study as they relate to the 

PBA study variables. Second, as an opportunity to collect information related to previous inflight experiences from five elite pilots with high expertise, high flight hours, numerous airframe 

exposures, and extensive training that includes verbalization of experience. These two goals were 

attempted using a combination of written questionnaire and pseudo-structured post-experiment 

interview. Recommendations for process improvements follow. 

PBA Questionnaire Responses

The questionnaire portion included three parts, each targeting a portion of the reporting that 

would provide insight into the subjects for the study. The first questionnaire (Pre-Test) involved 

demographics that would not change frequently such as previous flight experience, airframes, 

common symptoms experienced as a pilot, PE history, and normal diet and exercise routines. 

This questionnaire was taken only once. The second questionnaire (Pre-Flight) involved the 

subjects reporting about factors that change daily and might impact the study variables of interest 

such as recent altitude exposure, diet, fluid intake, and any current symptoms. This questionnaire 

was completed prior to each flight. The third questionnaire (Post-Flight) involved the subjects 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 228 of 260

reporting about the experience during flight and any usual events that might impact the collected 

data. 

Pre-Test Questionnaire Summary 

The pre-test questionnaire was designed to collect individual differences and demographics of 

the subjects used in this study and to collect information of previous in-flight experiences. This 

survey contained questions that pertained specifically to PBA and more general questions related 

to overall flight history.

Demographics:

The PBA pilots all served in the military, obtained instructor pilot training on one or more 

airframes, and completed test pilot training. All pilots had prior experience breathing on LOX, 

some had prior experience with gas oxygen (O2), and some had prior experience with OBOGS. 

No pilots reported ever using emergency O2 in-flight. 

Mask wearing:

All pilots reported mask wearing any time in the cockpit except when eating or drinking. Pilots 

reported the primary reasons for mask removal in a non-heavy aircraft as due to facial 

discomfort, eating/drinking, difficulty exhaling, scratch/wipe nose, to adjust microphone, 

Valsalva.

Altimeter:

As reported in the F-18 study, the sole method for a pilot to gather objective information about 

the cabin altitude is to look at the cabin altimeter gauge absent the use of any wrist-worn altitude 

display devices. The P/E reporting structure for the USN included a directed question asking for

cabin altitude and pilots reported having difficulty reporting the actual value and often reported 

the expected value based on the flight plan. For PBA, an open-ended question was included to 

collect individual perceptions related to the usability and readability of the altimeter design and 

location in the F-18. Comments include:

• Tough to read accurately. Didn't notice issues earlier in my career but now I notice 

pressure changes a lot more in my ear and on the gauge. When it changes schedule or has 

an issue, I can feel it my ears before I see something on the gauge...or it alerts me to look 

at the gauge. 

• Some, such as the F-18 and T-38 have cabin altimeters that are out of the pilot's field of 

view (more difficult to monitor). 

• F-18 gauge is in a difficult location. Fighter type aircraft don't display differential 

pressure. 

• They are generally small, difficult to read, and located out of the way such that it is 

difficult to check quickly and regularly. They don't have aural or illuminated warnings 

that the cabin altitude is too high. Some of the aircraft would illuminate a Master Caution 

light and associated Cabin Press light, but these warnings generally come on after the 

cabin pressure has climbed too high and hypoxia could already be present. 

Pressure and Flow:

Pilots reported previously experiencing sensations related to overpressure on a scale ranging 

from never, one flight, some flights, half the flights, most flights, and every flight. No pilots 
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reported most or every flight for any of the questions provided. All PBA pilots reported having 

lost cabin pressurization at some point in their flying career. 

When flying, have you ever noticed any of these 

sensations related to overpressure? (Check all that 

apply)

Never One 

flight

Some Half 

the 

flights

Most 

flights

Every 

flight

Air forced down your throat 1 0 4 0 0 0

Lungs blow up like a balloon 5 0 0 0 0 0

Lazy breathing (pressure does the work) 3 0 2 0 0 0

Mystery breath (effortless) 5 0 0 0 0 0

High pressure in mask (during inhalation) 0 2 3 0 0 0

Too much flow rate (Emergency oxygen) 5 0 0 0 0 0

Too little flow rate 2 1 2 0 0 0

Air hose vibrating 2 0 2 0 0 0

Airflow stops a little before inhalation complete 3 1 1 0 0 0

Sticky exhalation valve (large initial pressure, then relief) 0 0 4 1 0 0

Backpressure feeling (pressure working against you) 1 1 3 0 0 0

When flying, have you ever noticed any of these 

sensations related to reduced flow?

Never One 

flight

Some Half 

the 

flights

Most 

flights

Every 

flight

Breathing through a straw 1 3 1 0 0 0

Sucking rubber in mask 3 1 1 0 0 0

Generating negative pressure in mask 3 1 1 0 0 0

Can’t get enough air/unsatisfactory feeling (slight air 

hunger)

2 3 0 0 0 0

Labored breathing (extra effort/extra work of breathing) 2 1 2 0 0 0

Cracking the seal to get the flow started 4 1 0 0 0 0

Taking deeper breaths to compensate 3 0 2 0 0 0

Restricted breathing/straining against gear 2 1 2 0 0 0

Too little flow rate 2 2 1 0 0 0

Want to take the mask off to breathe deeper, more freely & 

fully

1 3 0 1 0 0

Air pressure pulsing slightly (low frequency) 3 0 2 0 0 0

Air suddenly rushing out, speeding up momentarily 5 0 0 0 0 0

Symptomatology and Sensations:

The symptomology questions were based on multiple symptomatology questionnaires used 

through the USAF, USN, and medical checklists. These were included to examine which 

wording might garner the best subjective response. Three of five PBA pilots reported previously 

experiencing air sickness. Three of five PBA pilots reported previously experiencing g-induced 

loss of consciousness (g-LOC) during flight training.
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During or after flying, have you ever noticed any of 

these sensations? (Please check all that apply)

Never One 

flight

Some Half 

the 

flights

Most 

flights

Every 

flight

Extraordinary breath holding (3-4 minutes) during ground 

ops

5 0 0 0 0 0

Tinnitus (ringing ears) 1 0 4 0 0 0

Valsalva overnight / oxygen absorption / ear pain 0 0 4 1 0 0

G-induced Atelectasis (need to re-inflate lungs with 

cough)

1 1 3 0 0 0

Trachea irritation (Raptor cough) 4 0 1 0 0 0

Lung inflammation / irritation 5 0 0 0 0 0

Slight pain in “lungs” post flight 4 0 1 0 0 0

“Lungs” tired next few days, flat/tortured workouts 4 1 0 0 0 0

Excessive fatigue (asleep before your little kids) 1 2 2 0 0 0

How frequently do you experience the following 

symptoms after a flight (unrelated to a previous 

condition)?

Never One 

flight

Some Half 

the 

flights

Most 

flights

Every 

flight

Ringing in ears 1 1 2 0 0 0

Pressure in ears 0 0 4 0 0 0

Sinus pressure 1 0 3 0 0 0

Headache 1 1 2 0 0 0

Feeling sick to your stomach 4 0 0 0 0 0

Feeling faint 4 0 0 0 0 0

Feeling annoyed or irritated (not attributable to an 

annoying in-flight event)

3 0 0 0 0 0

Feeling sweaty 2 0 1 1 0 0

Feeling queasy 3 0 1 0 0 0

Feeling lightheaded 4 0 0 0 0 0

Feeling drowsy 1 0 3 0 0 0

Feeling clammy or developing a cold sweat 4 0 0 0 0 0

Feeling disoriented 4 0 0 0 0 0

Feeling nauseated 3 0 1 0 0 0

Feeling dizzy 4 0 0 0 0 0

Feeling as if you are spinning 4 0 0 0 0 0

Feeling as if you would vomit 4 0 0 0 0 0
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During or after flight, have you ever noticed any of 

these sensations? (Check all that apply)

Never One 

flight

Some Half 

the 

flights

Most 

flights

Every 

flight

Tingling, prickling in extremities 4 0 1 0 0 0

Face tingling, prickling sensation 5 0 0 0 0 0

Ear pain, ear blocks 0 2 3 0 0 0

Pain in fingers, toes, or other joints 5 0 0 0 0 0

Chest pain 5 0 0 0 0 0

Cough 2 0 3 0 0 0

Nausea 4 0 1 0 0 0

Cyanosis/blue coloring of lips or nails 5 0 0 0 0 0

Headache 1 0 4 0 0 0

Sinus squeeze, tooth pain 2 2 1 0 0 0

Abdominal pain 4 0 1 0 0 0

Mental confusion 5 0 0 0 0 0

Disorientation 5 0 0 0 0 0

Vision changes 4 1 0 0 0 0

Lack of focus, difficulty concentrating 5 0 0 0 0 0

Dizziness 5 0 0 0 0 0

Vertigo, tumbling or spinning sensation 5 0 0 0 0 0

Cognitive; struggling with basic tasks 5 0 0 0 0 0

Euphoria 5 0 0 0 0 0

Tired, need to fall asleep 2 0 3 0 0 0

Shortness of breath 5 0 0 0 0 0

Behind the Jet 3 1 1 0 0 0

Feeling "Not Normal" 3 0 1 0 0 0

Hypoxia Symptom Recognition Training:

ROBD has become the standard for Hypoxia Recognition Training across the services. All PBA 

pilots had hypoxia training in the chamber due to the time period they completed their training. 

Three PBA pilots also had ROBD training. Two PBA pilots indicated their hypoxia symptoms 

differed between a hypoxia event in the chamber, the ROBD, and during flight. One pilot 

reported the same symptoms in both. Another pilot reported the majority of the same symptoms 

in both with an additional symptom in the ROBD alone. The third pilot reported two symptoms 

in both and two additional symptoms in the chamber alone. 
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Which hypoxia symptoms do you feel while in these training settings? (If in 

both, check both boxes)

In Chamber ROBD

Shortness of breath / air hunger 2 0

Drowsiness 1 0

Headache 0 0

Euphoria 1 1

Aggression 0 0

Poor judgement 2 0

Incoordination 2 1

Difficulty with simple tasks 4 1

Diminished vision 5 3

Tingling 5 3

Numbness 2 2

Hot/Cold flashes 2 2

Blue nail beds 2 1

Dietary, Hydration, Supplement, and Exercise Habits:

Modified eating habits and tactical dehydration was identified as commonly practiced in the pilot 

community. Questions were included related to those issues. All PBA pilots indicated some 

modification to eating habits on days scheduled to fly. Most pilots indicated hydration habits that 

met and maintained hydration recommendations while some pilots indicated an intentional 

reduction in liquid to reduce need to urinate in flight. Two pilots reported to regularly bringing 

water on a flight under two hours, one pilot reported consuming water on a flight less than two 

hours. Three pilots reporting bringing and consuming water on a flight greater than 2 hours. 

Some dietary habits or supplements can influence physiological resilience, so questions related 

to dietary habits were included. Most PBA pilots reported regular caffeine intake with coffee 

listed as most frequently consumed, followed by tea. (Note: soft drinks were unintentionally 

omitted from this list and should be included on future caffeine questionnaires.) No PBA pilots 

identified following a specific diet plan (e.g., paleo, ketogenic, etc.) beyond low carb/low calorie. 

Some PBA pilots reported taking supplements such as vitamin C, creatine, protein, and amino 

acids. PBA Pilots reported approximately 2-5 hours a week of cardiovascular exercise and 

occasional strenuous exercise within the 2 hours before or following a flight. 

Pre-Flight Questionnaire Summary

The second questionnaire (Pre-Flight) involved the subjects reporting about factors that change 

daily and might impact the study variables of interest such as recent altitude exposure, diet, fluid 

intake, and any current symptoms. This questionnaire was designed to be completed immediately 

prior to each flight to determine the baseline for each pilot as comparison for the post-flight 

survey. Questions included recent altitude exposure (e.g., chamber, flights, scuba diving), food 

and liquid intake, and fatigue. Data from 82 pre-flight questionnaires have been compiled and 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 233 of 260

examined in aggregate. Pilots were permitted to skip questions, so blank answers were omitted 

from interpretation as indicated in the question totals. 

Pre-Flight Altitude Exposure

Pilots were asked to report any altitude exposure in the 48 hours prior to the test flight.

Approximately 79% (65) of the pilots reported no altitude exposure. The 21% (17) of reported 

altitude exposure was due to flight. A wide range of aircraft and flight profiles were included 

with a range from high-performance AeroBatics to long-haul extended cruise, with 6 flights 

being other PBA flights. Altitude exposure details provided the maximum cruise altitude ranged 

from 6000 ft to FL450. The max cabin altitude range of 6k to 18k. The durations ranged from 30 

minutes to 9.8 hours. The time between the previous altitude exposure and PBA test flight 

ranged from 3.25 hours to 47 hours. 

Pre-Flight Diet, Hydration, Exercise, and Sleep

Approximately 95% (78) of the pilots indicated normal eating and hydration habits prior to 

flight. The 5% (4) of non-normal eating habits was due to a skipped breakfast. All pilots 

indicated a normal hydration habit. Approximately 85% (69) of the pilots indicated a normal 

exercise routine prior to flight with the 15% (12) non-normal reports were a reduction in exercise 

typically related to time constraints.

Approximately 90% (74) of the pilots indicated normal sleeping habits prior to flight with the 

10% (8) non-normal responses listed as due to a required shift to a night schedule, an extended 

period of wakefulness in the night, and an earlier than usual morning wake time. Approximately 

16% (13) of the pilots indicated feeling unusually tired or fatigued in the last week. PBA pilots 

were asked to rate fatigue (weariness, tiredness) by checking the one number that best describes 

his fatigue on a scale from 0 (no fatigue) to 10 (as bad as you can imagine) at the different time 

periods ranging from the past week, to the time of the survey. The overall majority of the 

responses indicated very low fatigue. High fatigue was explained in the comments as due to 

working long hours temporarily and to multiple time zone changes in the previous few days. 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 234 of 260

No Fatigue As bad as you can imagine

0 1 2 3 4 5 6 7 8 9 10

Please rate your fatigue (weariness, tiredness) by checking the one number that best describes your fatigue 

right NOW.

36% 35% 22% 5% 2% 0% 0% 0% 0% 0% 0%

Total Responses = 81 29 28 18 4 2 0 0 0 0 0 0

Please rate your fatigue (weariness, tiredness) by checking the one number that best describes your 

USUAL level of fatigue during past 24 hours.

11% 48% 27% 9% 5% 1% 0% 0% 0% 0% 0%

Total Responses = 82

9 39 22 7 4 1 0 0 0 0 0

Please rate your fatigue (weariness, tiredness) by circling the one number that best describes your WORST 

level of fatigue during past 24 hours.

2% 10% 43% 22% 14% 1% 6% 1% 0% 0% 0%

Total Responses = 81 2 8 35 18 11 1 5 1 0 0 0

Does not interfere Completely Interferes

0 1 2 3 4 5 6 7 8 9 10

Circle the one number that describes how, during the past 24 hours, fatigue has interfered with your:

A. General Activity 96% 1% 0% 2% 0% 0% 0% 0% 0% 0% 0%

79 1 0 2 0 0 0 0 0 0 0

B. Mood 91% 4% 1% 2% 0% 1% 0% 0% 0% 0% 0%

75 3 1 2 0 1 0 0 0 0 0

C. Walking Ability 99% 0% 0% 1% 0% 0% 0% 0% 0% 0% 0%

81 0 0 1 0 0 0 0 0 0 0

D. Normal Work (Work 

outside the home and daily 

chores)

93% 6% 0% 0% 0% 1% 0% 0% 0% 0% 0%

76 5 0 0 0 1 0 0 0 0 0

E. Relations with other people 94% 5% 0% 0% 1% 0% 0% 0% 0% 0% 0%

77 4 0 0 1 0 0 0 0 0 0

F. Enjoyment of Life 93% 6% 0% 0% 0% 1% 0% 0% 0% 0% 0%

76 5 0 0 0 1 0 0 0 0 0

Total Responses = 82

Pre-Flight Symptom Reporting

A brief symptom questionnaire was included to establish differences in symptoms pre-flight to 

post-flight. Further reasoning for this metric including the typical pre-flight status for a pilot 

during normal operation over an extended time window. Similar to the pre-test questionnaire, 

this symptom list is a composite from commonly used symptom reporting questionnaires. A total 

of 82 pre-test surveys were at least partially completed. The most reported symptom is slight 

sleepiness (17), followed by slight muscular fatigue (10). The only symptoms reported as 

moderate include sleepiness (1), muscular fatigue (1), and vertigo (1).
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To what extent are you currently feeling these symptoms? None Slight Moderate Severe Total

Low blood sugar 82 0 0 0 82

Cough (not attributable to pre-flight condition) 78 3 0 0 81

Extreme Hunger 82 0 0 0 82

Sleepiness 64 17 1 0 82

Fatigue (muscular) 71 10 1 0 82

Fatigue (mental / cognitive) 77 5 0 0 82

Blurred vision 82 0 0 0 82

Dizziness 82 0 0 0 82

Mood disturbance 82 0 0 0 82

Irritability 81 1 0 0 82

Appearing slowed or "off" 81 1 0 0 82

Light-headedness 82 0 0 0 82

Difficulty breathing 82 0 0 0 82

Anxiousness / Nervousness 82 0 0 0 82

Personality changes 82 0 0 0 82

Appearing euphoric or elated 82 0 0 0 82

Vertigo 81 0 1 0 82

Vision changes 82 0 0 0 82

Disorientation 82 0 0 0 82

Memory difficulties 82 0 0 0 82

Difficulty communicating 82 0 0 0 82

Pain anywhere (e.g., joints) 78 4 0 0 82

Skin rashes 80 2 0 0 82

Nausea 82 0 0 0 82

Dizziness 82 0 0 0 82

Headache 80 2 0 0 82

Clumsiness 82 0 0 0 82

Lack of coordination 82 0 0 0 82

Confusion 82 0 0 0 82

Forgetfulness 82 0 0 0 82

Difficulty Concentrating 82 0 0 0 82

Post-Flight Questionnaire Summary

The third questionnaire (Post-Flight) involved the subjects reporting about factors during flight 

that might provide subjective insight on the objective data collected during flight. This 

questionnaire was designed to be completed immediately following each flight. As the pre-flight 

questionnaire received 82 responses, the post-flight questionnaire received 60. This is due to 

factors such as a scrubbed flight or the competing schedule demands post-doff. The final 

observations yield a sample mixture of profiles, pilots, and configurations such that preclude the 

majority of analyses on the questionnaire data set due to overly sparse, unequal or absent 

samples, as shown below. The below graph already does not include considerations for seating 
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position, or aircraft model. The remainder of the questionnaire data section will continue with 

simple data descriptions of the pilot responses

Figure 9.1. Graph of the observation counts for pilot by profile, by crew equipment configuration.

When asked if the flight went as planned, pilots reported “no” for 5 of 53 responses. The 

comments listed factors related to the aircraft, the mask, and inconsistencies in profile 

performance due to time pressure. Only three abnormal physical or cognitive experiences were 

reported. One pilot reported a dull ache in the chest midway through acrobatic performance that 

was described as the feeling of needing to cough. To elaborate further, this pilot commented 

being unsurprised of this feeling because this was an elevated G profile, in USN aircrew flight 

equipment, and on 100% O2 with safety pressure. The pilot stated, “I normally would not notice 

it except we've been asked to look for small changes with our bodies.” Two reports consisted of 

the crew stating they were very hot with likely dehydration. Of 58 reported flights, only 6 pilots 

(10%) brought water with them for the flight and 0 consumed the water during the flight. 

All pilots reported wearing the mask the majority of the time in the jet with any removal due to 

test card directive, to scratch an itch, to wipe away sweat, or discomfort. Of 56 flight 

observations, the pilots reported 5 (9%) flights as having cabin pressure anomalies. These 

included intentional RAM DUMP, nominal ECS fluctuations, noticeable ECS surges, inaccurate 

(low) cabin altimeter, slow cabin pressurization during a Max AB climb, noticeable cabin 

fluctuations during AeroBatics unrelated to throttle setting.

Comparisons of pre/post flight symptoms 

A brief symptom questionnaire was included to examine pre and post flight differences and 

examine the post-flight experiences for pilots during normal operation over a long-term period. 

Similar to the pre-test questionnaire, this symptom list is a composite from commonly used 

symptom reporting questionnaires, please see below. The most reported symptom is pressure in 
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the ears (19 mild, 2 moderate), followed by feeling sweaty (4 mild, 6 moderate, 1 severe), and 

headache (4 mild). The pilots recorded 15 comments. Pilots expressed the pressure in the ears 

related to the performance of the flight profiles including “normal” ear pressure during cabin 

pressure changes and two comments supporting the moderate pain as during combat descent and 

nose slice maneuvers. Seven comments indicated that that feeling sweaty was due to the 

temperature both outside and inside the aircraft during flight and expressed the temperature 

inside the cockpit as problematically high One pilot indicated he had the feeling of slight lightheadedness after a sustained 5 G turn with an abrupt roll-out.

Did you feel any of these symptoms today (during or 

after the flight)? None Mild Moderate Severe Total

Ringing in ears 58 0 0 0 58

Pressure in ears 37 19 2 0 58

Sinus pressure 57 1 0 0 58

Headache 54 4 0 0 58

Feeling sick to your stomach 58 0 0 0 58

Feeling faint 58 0 0 0 58

Feeling annoyed or irritated (not attributable to an 

annoying in-flight event) 57 1 0 0 58

Feeling sweaty 47 4 6 1 58

Feeling queasy 58 0 0 0 58

Feeling lightheaded 57 1 0 0 58

Feeling drowsy 57 1 0 0 58

Feeling clammy or developing a cold sweat 58 0 0 0 58

Feeling disoriented 58 0 0 0 58

Feeling nauseated 58 0 0 0 58

Feeling dizzy 58 0 0 0 58

Feeling as if you are spinning 58 0 0 0 58

Feeling as if you would vomit 58 0 0 0 58

At the end of the questionnaire, pilots were provided open-response opportunities to comment on 

breathing experience, symptomology, and any additional thoughts following that specific flight. 

There were 48 additional comments across 4 open-response questions in 58 completed postflight questionnaires. The responses were aggregated and analyzed for emergent properties that 

reveal common themes generalizable to the content area. Here, clusters are conceptual groupings 

that emerged after the identification of similar statements and the subsequent interpretation of 

shared characteristics. This analysis method is well-supported in the literature, but does require 

advanced expertise in human subject data collection and the subject matter area to conduct with 

precision and accuracy while avoiding common commission or omission errors. The remainder 

of this section will consist of a cluster title, a summary or description of this cluster, and relevant 

quotes to demonstrate sufficient support for the grouping.
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The first, and largest, emergent cluster is related to the human breathing system interface in the 

aircraft and the impact to breathing. One pilot reporting “After flight feels like you can't take as 

full of a deep breath as before flight.” As this study was known to be an exploration of pilot 

breathing in flight, it is unsurprising that this cluster contains the most responses. Concerns 

related to the mask valve malfunction had the most comments of the study with 7 of 58 flights 

with reported noticeable valve inconsistencies.

• Feeling as if inhalation valve collapses during maximum inhales. No problem during 

maximum exhale. Happened at all altitudes.

• During maximum inhalation, noticed one of the valves in my mask seemed to collapse, 

restricting O2 flow by about 50%. This happened multiple times during Max inhalation 

both on the ground and in flight. It did not happen during normal breathing or even 

during "heavy breathing"...only during max inhalation. I didn't experience any symptoms, 

however.

• After collapsing mask valves on inhalation a few times, Maximum Exhalation also 

resulted in a collapse or stuck valve reducing exhalation by about 50%. No symptoms as 

a result, though.

• O2 mask exhalation valve sticky after max exhale events. Annoying, but not restrictive.

• When I first put my mask up, the inhalation valve seemed to stick a little; it is somewhat 

common for the exhalation valve to stick but rare that the inhalation valve sticks. This 

stickiness went away after a few minutes and no additional issues were noted.

• During the Max Inhalation/Normal Exhalation event, on two of three maximum 

inhalations, it felt as if the valves (2) in my mask collapsed or otherwise partially closed.

This slightly restricted the volume of air coming in but did not completely restrict it.

Valves immediately opened upon exhalation and worked fine otherwise. Later in the 

flight during the 'heavy breathing' exercise, I again noticed the valves on my mask appear 

to collapse or close during max inhalation.

• I experienced periodic breathing "in" stoppages with the mask, regulator, or hoses. I 

needed to relax for a couple breaths (breathe out) before the O2 would flow again.

The second most relevant cluster was the subjective experience between the USAF and USN 

configuration of the flight crew equipment and the non-safety/safety pressure. The majority of 

the comments that included reference to the fit of the USN AFCE and the USN safety pressure 

were negative. 

• Navy gear makes you feel more restricted, especially when doing spirometry in the 

cockpit.

• I flew the AF and NAVY configurations back-to-back. I noticed I could "max out" the 

amount of air provided by the Navy regulator during maximum inhalation events. I could 

never "max out" the AF regulator (CRU-73.) There was always plenty of airflow no 

matter how hard I inhaled or exhaled through the CRU-73. On the Navy regulator, if I 

inhaled extremely hard, I would experience what felt like a hose collapse or momentary 

valve stick (or regulator running out of air). Max breathing through the Navy system 

seemed more restrictive than the AF system.

• During the forcible exhales, the mask would inflate like a balloon and if I didn't hold it to 

my face, the seal around my nose would blow out.
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• With the USN gear it required more effort to breathe than with the USAF/NASA gear.

Under relaxed breathing the positive pressure assisted inhalation, but exhalation requires 

some effort against the pressure.

• When exhaling forcefully, the mask filled up as if resisting the airflow. Normal for USN 

regulator. 

• After first High g event (5g) rear pilot asked me how I felt and I said I felt like I needed 

to cough, so I did. Dry cough probably due to 100% O2 of the Navy gear.

As noted above, the symptoms questionnaire included a specific query for sweating. Pilots rated 

sweating highly and provided comments. Here, pilots made simple statements about sweating 

because of the environmental factor of elevated temperatures rather than an adverse internal 

reaction to the flight. However, among these comments are several negative references to 

temperature exposure including indications that the temperature was a detriment to the 

pilot/mission. Prior interviews with pilots (e.g., PBA, F/A-18, F-35, T6) indicated that operating 

in a high temperature environment is an incredibly common hazard faced by flight crew. 

Operational procedures and guidelines exist (such as water intake), but reports of “tactical 

dehydration” are equally common within the fleet. Heat exposure is a serious hazard resulting in 

heat related illnesses ranging from simple dehydration to heat stroke and death with 

compounding effects over multiple events. Hydration level does impact the body’s ability to 

withstand high performance flight parameters. Furthermore, a pilot who is already borderline 

dehydrated from tactical dehydration and then exposed to high temperature environment for an 

extended period of time is unlikely to remain mission ready. Heat should be more consciously 

considered including the elapsed exposure time and heat accumulation with the appropriate 

mitigations deployed. Although impossible to remove all heat risk, other strategies should be 

examined to reduce the risk of thermal dangers prior to and during mission performance. See 

workplace standards via the Occupational Safety and Health Administration (OSHA) and/or 

National Institute for Occupational Safety and Health (NIOSH) for further insight (OSHA, 

2017). 

• The outside temperature during the flight was around 110o F.

• Cockpit was very hot 

• With canopy down, sun made the cockpit very hot

• It was hot, especially on the ground with the canopy closed

• Back-seater got pretty sick. I'm not sure the risk of flying demanding maneuvers in an unair-conditioned cockpit is worth the data. Why don't we limit these flights to cool 

mornings with OATs < 15 C?

• With no pressurization there is no AC so the plane was hot, especially with the ATAGS 

G-suit and Navy gear

• During High G portion sweat in my mask caused it to slide down slightly

• Feel slightly dehydrated

• It was hot; I was sweating

• It was hot so I was sweaty.

Another symptom on questionnaire was related to pressure in the ears. Cabin pressure 

fluctuations in the F/A-18 fleet were commonly reported and could range from very small to 

very large in magnitude. Elsewhere in this report discusses the importance of small and large 

oscillations in cabin pressure and the impact on the flight equipment. Here, pilots note being 
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made aware to pressure changes by the ear. This is frequently used as the primary indicator of an 

unusual change in pressure. A period during operation when a pilot might be performing 

maneuvers such as these may not benefit from the additional distraction of ear pain or eardrum 

rupture. 

• The pressure in the ears occurred during the nose slice maneuvers.

• Since this flight was entirely below 8000', I did notice the change in pressure in my ears 

during the mild maneuvers (nose slice). I did not notice it during the more demanding 

maneuvers like the pop-up attack.

• Felt the pressure change with the pressurization system in my ears. 

• My ears will register a cabin pressure change faster than the Cabin altimeter.

• When Cabin pressure changes (flux) you can feel it in your ears first.

• Pain in left ear just prior to level-off on combat descent.

• Normal ear pressure when the Cabin pressure changes.

The next symptom cluster was discussion of G and the new versus the old G-suit. This is an 

important note as during this study three of the five pilots received new G-suits mid-data 

collection. These new G-suits were commented both in discussion and in questionnaire 

comments as a noticeable improvement from the older versions. This is not an ideal occurrence 

to happen mid-study as a subjectively detectable improvement likely has objective differences. 

These were not able to be examined in the time allotted. 

• Slight light-headedness after a sustained (1-min), 5+ g turn followed by an abrupt rollout.

• Occasionally needed L-1 for extended G maneuvers

• I have a new ATAGS G-suit which is different than our old G-suits. It seems to 

function/fit better than the older G-suits and I noticed I didn't really need to strain (L-1) 

under sustained 5 G like the old G-Suit.

• With new ATSGS G-suit, did not have to strain at all during 5g or other g maneuvers, 

even to the point where I noticed other things under g that I usually don't. Changed bingo 

bug while pulling 5g and could feel the weight of my feet on the rudder pedals.

The specialized equipment for use in experimental observation during operation is ideally 

unobtrusive and unnoticeable. The pilots reported only two instances when the VigilOX was 

noticeable during the flight performance. Other interviews with the pilots confirmed that the 

VigilOX did not impede performance, though using the VigilOX itself to gather data was 

problematic related to difficulty in determining device recording or power status. 

• Had a slight mask leak near my nose due to the heaviness of the VigilOX gear. Mask 

adjusted to full tight.

• The VigilOX O2 seemed to be twisted causing the hose to rise up blocking my view of 

the instruments

Post-Test Interview 

Ultimately, although valuable, the structured nature of the questionnaire was insufficient to 

gather more directed and long-form questions related to PBA experience and previous flight 

experience in general. In human subject research, the interview is a qualitative research method 

frequently used to collect individual instances of subjective experience. At the end of the 

experiment, pilots completed a one-on-one debrief to engage in a summary discussion and 
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overall opinion of the entire experiment, profiles, equipment, and perceptions of breathing in 

flight across numerous air frames. These interviews upheld the aggregate pilot 

symptom/perception clusters from the questionnaires and provided contexts to better examine 

and explore the data. In-depth exploration of these data should be conducted in the future.

Future Questionnaire Development

A structured questionnaire was not sufficient for the exploratory nature of this study and the 

subject population. Although a questionnaire is possible, it must be developed using 

psychometric standards and practices. For this study, a better alternative would have been an 

immediate in-person debrief using a semi-structured interview with audio recording for

transcription and database entry. 

The use of a semi-structured interview would enable expanded pilot commenting and permit 

follow-up questions related to any potential unexpected events or symptomology that were not 

previously considered. This method would permit the interviewer (researcher) access to 

interview techniques such as non-verbal communication and speech patterns to identify lines of 

questioning that should be worded more clearly or engaged in more directly to generate the best 

data collection. These data could be used to develop a structured questionnaire for future 

deployment.

Like the creation of a flight profile to specifically assess the Pilot Breathing System components 

of an aircraft requires pilot and engineer expertise, the development of a questionnaire to 

determine the subjective experience of breathing in-flight must be completed using SMEs and 

formalized psychometrics principles expertise. The goals of an early post-flight interview should 

be to identify the optimal question and wording to gain the desired information, and to identify 

the most common statements and responses related to the subjective experience of breathing 

systems in flight. These are a few thoughts for consideration. 

1. During the semi-structured interview, there will be a few structured questions designed 

to begin the creation of a structured questionnaire.

2. The goal is to iterate and develop these questions to create a formal questionnaire 

designed to capture the subjective experience of the pilot during the flight to improve 

the health of the aircraft in the fleet.

3. Successful questionnaire development requires professional expertise in formal 

psychometric methodology, funding, time, and access to numerous members of the 

target audience and subject matter experts. Expertise could be available via onboarding, external contract, or university partnership. This individual or company 

should have expertise in survey design such as knowledge elicitation and psychometric 

methodology including factor analysis (exploratory and confirmatory). 

4. The resulting questionnaire should have with high sensitivity to detect mild increases in 

flight symptomology and high discriminability to distinguish among the many potential 

related causes. For example, pressure fluctuations can be cabin, mask, or line and can 

have different causes. Factor analysis will permit question improvement, help 

determine how many factors are present, identify the factors that explain the most 

variance, identify any needed weighting, and provide a down-selection opportunity to 

only include as many factors as necessary to understand the data. 

5. Target questions should include elicitation of specific observations about their flight, 

the aircraft, comparisons to other aircraft, or specific segment of the flight. Responses 

may include a dichotomous response (yes/no) or numeric response (e.g., Likert-Type 1-
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7, or scale 1-100) followed by an open-ended response period for the pilot to provide 

any specific examples or comments. If Likert-Type scales are used, they should have a 

range of no less than 1-7 to permit more rigorous statistical interpretation.

6. All subjective statements require operationalization. This is the process of defining a 

nebulous concept variable in terms of a measurable factor. The subjective term 

“Difficult” needs to be defined into something measurable and so described to ensure 

equal perspective across subjects. This operationalization will be generated throughout 

the psychometric questionnaire development process. One method involves the use of 

SMEs to generate descriptions associated with a term, then another set of SMEs 

conducting a card-sorting of those previously generated terms to confirm and downselect, and then multiple administrations of the questionnaire with yet another group of 

SMEs. All, of course, with factor analysis to ensure accurate loading across the 

expected terms and consistent mapping. For example: what is the subjective 

performance of the breathing system during flight. Is a breathing system “difficult” or 

“easy”? Is a 1 like sucking rubber? or filling up like a balloon? Is a 7 like no pressure 

fluctuations? Or are pilots just conceptualizing “easy” as “normal”?

7. Suggested areas for exploration and required operationalization:

a. The subjective performance of the breathing system during flight. 

b. The inhalation and ability to easily take in full deep breaths.

c. The exhalation and ability to fully and easily exhale.

d. Extent to which the pilot perceived being distraction related to thinking about 

breathing

e. Extent to which the pilot was able to focus entirely on the flight

f. Extent to which any adverse physical or cognitive symptoms or discomfort 

occur during flight 

Near-Miss Data Accumulation

To use data for prediction, the data need to be relevant, reliable, and representative. A major 

problem surrounding the prediction of PEs is an underreporting of symptoms. The primary goal 

is the acquisition of a true baseline and accurate trends of minor disruptions before an increase of 

mission impacts such as a rash of mission aborts. An internal fleet-wide survey has low 

likelihood of success due to the presence of elements that negatively influence survey response 

behaviors. A successful implementation has been the inclusion of a neutral third party. Aviation 

Safety Reporting System (https://asrs.arc.nasa.gov/) is an example of such a strategy. ASRS is a 

non-jeopardy reporting system for the commercial aviation industry to enable the Federal 

Aviation Administration to watch for an increase in dangerous trends prior to an aviation 

incident. ASRS has the capability to conduct specialized surveys to specific focus matters and 

has for other industries (e.g., rail) and this technique could be implemented to increase pilot 

participation in reporting and to improve the quality of data and subsequent predictions.

References

Andersen, H. & Hepburn, B. (2016). Scientific Method, In E. N. Zalta (Ed.), The Stanford 

encyclopedia of philosophy (Summer 2016 ed.). Retrieved from

https://plato.stanford.edu/archives/sum2016/entries/scientific-method/
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Jones, L. V., & Thissen, D. (2007). A history and overview of psychometrics. In C.R. Rao and S. 

Sinharay (eds). Handbook of Statistics, 26: Psychometrics (pp.1-27). Amsterdam: North 

Holland.

Lang, S. (2004). Study links warm offices to fewer typing errors and higher productivity. Cornell 

Chronicle. Retrieved from http://news.cornell.edu/stories/2004/10/warm-offices-linkedfewer-typing-errors-higher-productivity

Occupational Safety and Health Administration (OSHA) (2017). Section III: Chapter 4. Heat 

Stress. OSHA Technical Manual (OTM) TED 01-00-015. 

https://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_4.html

Stanton, N. A., Salmon, P. M., Walker, G. H., Baber, C., & Jenkins, D. P. (2005). Human 

Factors Methods: A Practical Guide for Engineering and Design. 2005. Farnham, UK: 

Ashgate.

Stout, W. (2002). Psychometrics: From practice to theory and back. Psychometrika, 67(4), 485-

518. 
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Appendix 10: PBA Machine Learning

Machine Learning Evaluation Overview

The ability to predict pilot breathing behavior with respect to the many stressors of flight could 

have implications towards developing hardware and software systems that can help manage 

those stressors with physiological coping mechanisms. For instance, if it is possible to predict 

how such factors such as altitude, acceleration, and aircraft orientation affect pilot breathing 

activity, breathing regulators could be programmed to adapt proactively as soon as the aircraft 

engages in any complex maneuver. These smart systems would therefore employ the same state 

of the art computational technology that is integrated throughout autonomous systems applied at 

the Autonomous Integrated Systems Research Branch and NASA Langley Research Center, the 

Autonomous Systems Lab at NASA Stennis Space Center, and the Autonomous Systems Project 

at NASA Ames Research Center, among others. These projects involve using machine learning 

and artificial intelligence in a variety of applications throughout the mission of NASA where 

there can be successfully integrated to supplement or enhance human activities or performance.

For the Pilot Breathing Assessment research, an application of artificial intelligence (AI) was 

developed through advanced machine learning (ML) methodologies to determine if pilot 

breathing activity could be predicted based on learning from the data collected from the pilot and 

the aircraft during flight operations. This data driven approach to predictive modeling focused on 

two flight profiles, one conducted at high altitude and another conducted at low altitude. The 

resultant mixed dataset allowed the model learn from a diversity of environmental conditions for 

the aircraft and examine the breathing activities on the pilot in those different environments.

Key Findings:

• Pilot mask pressure can be reliably predicted over 90% of the time

• Peak flow inhalation can be reliably predicted over 85% of the time

• AI can learn from multiple flight profiles and still achieve over 90% accuracy in 

predictions

• Including USN and USAF aircraft does not decrease model performance; model 

performance improves with the inclusion of multiple aircraft

• Modeling from multiple pilots is possible with reliable accuracy, albeit lower than the 

model performance for predicting breathing activity for a single pilot

Data Characteristics and Feature Engineering

Modeling was conducted using a single pilot, and utilized data obtained from flights in a U.S.

Navy and U.S. Air Force aircraft, both of which were the F18. Two flight profiles were 

evaluated, Profile A that was conducted at high altitude, and Profile D that was conducted at low 

altitude. A summary of the four flights included in this evaluation are as follows:

Flight Number Profile Aircraft

50 A (high altitude) USN

56 A (high altitude) AF

76 D (low altitude) AF

83 D (low altitude) USN
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Additionally, three additional datasets were constructed to model data from all pilots who 

participated in Profile A and D flights: all pilots for Profile A, all pilots for profile D, and all 

pilots for both Profile A and D together. This included 5 pilots for Profile A and 4 pilots for 

Profile D.

Flight Number Profile

26, 27, 39, 55, 56, 61, 85, 85 A (high altitude)

72, 75, 76, 81, 83, 84 D (low altitude)

Data recorded from the above flights included pilot specific and aircraft specific variables, or 

features, that described the both the pilots breathing activity at each time interval as well as the 

aircraft’s physical and environmental state at that time interval. These data were then subjected 

to a feature engineering approach whereby there was an aggregation performed across the 

continuous measurements of pilot breathing to yield the following features:

These features were then evaluated for examine any correlations. As seen in the chart below, the 

correlation coefficient for any combination of two features is rarely above 0.50. This suggests 

that the data should be modeled with a nonlinear approach. 

Cabin Pressure (mmHg) 

Altitude (ft) 

Acceleration (g) 

Longitudinal Acceleration 

Lateral Acceleration 

Normal Acceleration 

Pitch Angle 

Roll Angle 

True Heading 

RMS Velocity (fps) 

Peak Flow (lpm) 

Mean Pressure O2 (mmHg)

Mean Cabin Line Pressure (mmHg)

Tidal Volume (l) 

Inhale Duration (s) 

Total Breath Time (s) 

Maximum Mask Pressure (mmHg) 
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ML Methodology

Employing ML for predictive studies has the advantage of being able to process large volumes of 

data to mine for patterns that humans cannot detect. The patterns can represent underlying trends 

that the AI can discover that may otherwise go unnoticed. For instance, there could be non-linear 

relationships among the measured variables that could influence the predictive ability of a model 

(as seen in the correlation matrix chart above). This is often the case with a particular approach 

to artificial learning, known as the multi-layer perceptron model, also known as a feed forward 

artificial neural network (ANN). ANNs have an input layer and an output layer, each consisting 

of a set of nodes. Additionally, there is at least one layer of nodes in between, known as a hidden 

layer. Initially, each node is assigned a weight value at random. The random model leads to a 

prediction value, which is then compared to the observed value in the training data. This 

comparison is made using the error function. Generally, a deep feed-forward (DFF) neural 

network has many hidden layers in between the input layer and the output, with these layers 
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trained by backpropagation – which is short for “backward propagation of errors”. Additionally 

the ANN employs a graph-like structure that produces an efficient learning process for the model 

parameters, as employed with backpropagation. The algorithm uses gradient decent (a first order 

iterative optimization algorithm that finds the minimum of a function) to calculate the error of 

the function with respect to the function’s weight, and proceeds backwards through the network 

from the output back to the input layer. This backwards directionality of the computations for 

error allows for an efficient calculation of the gradient of each layer, compared to a simpler 

approach where the gradient of each layer is calculated separately. Using backpropagation as the 

training algorithm, the activation function for the ANN was logistic regression. The activation 

function determines the output for each node based on nonlinear mappings from the given inputs.

The ANN model architecture that was used for this analysis is shown below. Two hidden layers 

were used with 16 nodes in each layer, to match the number of inputs. In the graph below, 

Maximum Mask Pressure was chosen as the target, however, the same model architecture was 

used to also model Peak Flow. The values in black represent the inputs going into the next layer, 

while the values in blue represent the weight for that node.

Each dataset used for modeling was separated into two groups, 80% for training and 20% for a 

holdout validated set. Each model was therefore trained on 80% of the data and then tested with 

the remaining 20% (which the model did not evaluate during training) to examine model 

performance. Each model was run exactly 100 times, and the average model performance value 

is used in the following discussion.

Results: Single Pilot, Single Flight Profile

The model performance for both Flight A and Flight D data are consistent in terms of predicting 

Peak Flow and Mask Pressure based on the other measured flight and breathing variables. 

Initially tested on a single flight, to compare model performance for a USN and AF aircraft, the 

models show no significant difference in making predictions for either of the two aircrafts. 

However, when a combined dataset was used that aggregated data from both the USN and AF 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 248 of 260

flights, the model performance improves in a statistically significant way. Mask Pressure and 

Peak Flow can be reliably predicted with 93.4%, and 75.6% accuracy respectively for the high 

altitude Profile A, and at 86.3% and 87.4% accuracy respectively for the low altitude Profile D 

flights. These findings suggest that Mask Pressure is more consistently affected by each of the 

other aircraft variables. The table below summarizes model performance on each dataset.

Results: Single Pilot, Multiple Flight Profiles

Similar to analyzing the data for single flight profiles, model performance improves when both 

the USN and AF flights are included. Predicting Peak Flow when both high altitude and low 

altitude data are included resulted in a higher accuracy than the model performance from the 

high altitude data at 85.3%. Meanwhile Mask Pressure model performance was improved over 

both the high altitude and low altitude single datasets at 92.5% accuracy. These findings suggest 

that more data helps the model learn the latent features from the underlying data better. 

Additionally, the model is able to discern differences in breathing patterns in the high and low 

altitude data to therefore adjust the weights for the nodes taking in those inputs to therefore 

accurately predict what effect the altitude measurement has on the outputs for Mask Pressure and 

Peak Flow.

Results: Multiple Pilots, Single Flight Profile

Model performance is decreased when including all other pilots in each of the models, while the 

trend consists for Mask Pressure to be an easier target predictor compared to the Peak Flow. 

Accuracy for Peak Flow was 71.1% and 70.6% for the high altitude and low altitude flights 

respectively, while Mask Pressure was at 74.9% and 79.3% respectively. This indicates the there 

is some ability for the model to learn how to adjust the weights in the hidden layers for 

individual pilot’s breathing activity – as is necessary to model inputs that vary from person to 

person given variations in human physiological responses. It is therefore possible that with more 

data for training this model’s performance could be improved.

Results: Multiple Pilots, Multiple Flight Profiles

Aggregating all pilots who flew a flight for Profile A and/or D into a single dataset, to then use 

for training the neural network resulted in lower model performance for the targets. Likely due to 

data imbalances as Profile A had 5 pilots and Profile D had 4 pilots. Additionally, while the 

volume of data measured during these flights is adequate to train a model to learn one pilot’s 

breathing activity, to include many pilots likely requires more data than was obtained. However, 

accuracies at or slightly above 70% indicate the there was enough information in the data to 

make reliable predictions if given input values to achieve a hypothetical value for Peak Flow or 

Mask Pressure to generalize to all humans.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 249 of 260

 Model Accuracy

Flight Profile A Peak Flow Mask Pressure

FLT 50 (USN) 70.5% 73.4%

FLT 56 (AF) 77.1% 81.3%

FLT 50 + 56 75.6% 91.3%

Flight Profile D

FLT 76 (AF) 79.5% 84.6%

FLT 83 (USN) 86.8% 84.9%

FLT 76 + 83 87.4% 86.3%

Flight Profiles A + D

FLT 50 + 83 78.4% 83.3%

FLT 56 + 76 78.0% 83.0%

FLT 50 + 56 + 

76 + 83 85.3% 92.5%

All Pilots

Profile A 71.1% 74.9%

Profile D 70.6% 79.3%

Profile A + D 69.3% 73.8%
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 250 of 260

Appendix 11: Glossary of PBA Terms

1⁄2 Cuban 8: A Cuban 8 is an aerobatic maneuver in which the aircraft flight path looks like an 

‘8’ laying on its side. A 1⁄2 Cuban 8 is just the first half of the maneuver.

1-min data: Measurements collapsed into summaries of 1 flight-minute each

20-Hz data-streams: Continuous variables acquired at a rate of 20 points per second

AFRC: Armstrong Flight Research Center (AFRC) at Edwards Air Force Base, CA. All PBA 

flights were conducted from the NASA AFRC flight line. 

Aileron Roll: A low-G aerobatic maneuver in which the pilot pulls the aircraft flight path into a 

slight climb and then quickly rolls the aircraft around its longitudinal axis.

Aircraft Altitude: Vertical altitude of aircraft as derived from aircraft sensors in feet

Aircraft velocity: Airspeed of aircraft in miles/hour

Aircrew Flight Equipment (AFE): Life support breathing gear (mask, regulator), helmet, 

harness, parachute, survival kits, etc.

Aircrew Physiologic Monitoring Sensor Suite (AMPSS): System of pilot breathing sensors.

According to the developer, Cobham, serves to protect fighter pilots from the effects of 

unperceivable, debilitating hypoxia like symptoms before they occur.

Airline descent: a gradual descent maintaining a moderate airspeed and moderate power setting, 

such as might be accomplished by a commercial airliner.

Alignment of data: A crucial aspect of collating multiple data streams into a single larger data 

set. Usually refers to assigning a common and accurate time base for all data streams. 

ALSE: Aircrew Life Support Equipment

Alternobaric Vertigo: Arises from unequal pressure between the two middle ears, usually 

because the pressures are changing at different rates. Failure to equalize pressure 

symmetrically can cause the brain to erroneously perceive the difference as movement or 

spinning.

Anatomical deadspace: Volume of the upper respiratory system that does not participate in 

alveolar gas exchange

Anoxia: The state in which there is complete deprivation of oxygen supply 

Anthropomorphic mannequin: Life sized human figure used for wind blast testing and other 

gear tests.

Anti-G Straining Maneuver (AGSM): The AGSM involves a forced exhalation against a 

closed glottis with straining of limb and abdominal muscles just before and during high 

sustained G’s. The exhalation (increased intrathoracic pressure) is maintained for 3-4 sec 

and is interspersed with rapid inspirations less than 1 sec; the process is repeated 

cyclically. It is an effective anti-G procedure, increasing the mean-G-tolerance by as 

much as 4 G. (Bates et al., 1990) [Reference Bates et al., High G Physiological 

Protection Training, AGARDograph No. 322, AMP Working Group 14, AGARD-AG322, December 1990.]

Arterial Gas Embolization: Occurs when the dissolved gas (mainly nitrogen) comes out of 

solution and crosses directly into the arterial system

Arterial Spin Labeling (ASL): An analytical method using magnetic resonance imaging (MRI) 

to quantify cerebral blood perfusion of oxygen.
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Average flow volume: The total amount of air entering the pilot’s mask in a single flight 

minute, (liters/min) 

Bad actor jet: Pilot term for a particular aircraft that passes all systems tests yet still seems to 

exhibit more challenges to smooth breathing than other jets of the same type and 

configuration.

Barodontalgia: Commonly known as tooth squeeze and previously known as aerodontalgia, is a 

pain in the teeth or jaw caused by a change in ambient pressure

Barosinusitis: A condition manifested by inflammation of one or more of the paranasal sinuses. 

The inflammation is caused by a pressure gradient, usually negative, between the sinus 

cavity and the surrounding ambient environment. It can also be caused by large rapid 

changes in pressure.

Barrel Roll: A low-G aerobatic maneuver in which the aircraft flight path describes a 

corkscrew, as if rolling around the inside of a barrel.

Breath volume, mean: The average tidal volume within a single flight minute, (liters/breath)

Breath_Vol_mean: Calculated parameter: Average inhaled volume for each breath 

(liters/breath) calculated by dividing column “mean_FlowLPM_I” by column 

“Breaths_per_min” for that minute.

Breathing frequency: Number of complete breaths within a single flight minute, 

(breaths/minute)

Breathing Sequence Disorder: A condition that affects the ability for the pilot to get the proper 

amount of breathing air at the correct time

Breathing System Disorder: The class of system disorders that can contribute to a PE. Includes 

breathing sequence disorders, errors in cabin pressure scheduling, insufficient oxygen, 

rapidly changing levels on oxygen.

Breaths_per_min: Breathing frequency (breath/min) within each individual flight minute, as 

counted from ISB flow sensor peaks.

Breath-slicing Algorithm: A method developed by PBA for identifying the beginning and end 

of individual breaths from 20 Hz pressure and flow data streams.

BSD: The acronym of breathing sequence disorder (used many times in the report).

BuNo: Refers to aircraft “build number” indicating age and series of a particular aircraft type, 

sometimes referred to as CODEX. These are assigned when an aircraft is ordered, not 

when delivered, so there may be gaps due to contract cancellations. BuNo use is common 

in contemporary Navy aircraft, but there have been other numbering schemes over the 

years to differentiate among military aircraft.

Cabin altitude: This parameter is actually expressed as the equivalent cabin pressure as derived 

from VigilOX ISB sensor, in mmHg.

Cabin Pressure Surges: Rapid changes in cabin pressure control can interfere with proper 

mask/regulator response from on-demand breathing; these surges can be caused by a 

variety of factors including rapid ascents or descents coupled with over responsive cabin 

dump valve. 

Cabin Pressurization: Aircraft cabins are pressurized according to a specific schedule related to 

altitude. At altitude below 8,000 feet, no additional pressurization is applied; in the 

isobaric region, from 8,000 to 23,000 feet altitude, cabin pressure is maintained at the 
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equivalent of 8,000 feet altitude; above 23,000 feet, cabin pressure is maintained 5 psi 

above the corresponding external pressure.

Cerebral blood flow (CBF): A measure of the blood supply to the brain in a given period of 

time. In an adult, CBF is typically 750 ml per minute. 

Change Altitude: Change in altitude for each individual flight minute as calculated as max-min 

value contained within that minute, in feet.

Change cabin alt.: This parameter is actually expressed as the change in equivalent cabin 

pressure for each individual flight minute as calculated as max-min value contained 

within that minute, in mmHg.

Check 6 assessment: An event designed for PBA to assess any impact on breathing dynamics 

when the pilot turns in the cockpit to check for enemy aircraft approaching from behind 

(as in the 6:00 position on a clock).

Combat Descent: A very rapid descent, such as a tactical aircraft might use to escape or evade 

enemy aircraft or defenses by quickly descending to low altitude where it would be 

harder to detect.

Combat Descent: A flight maneuver characterized by a 45-degree descent, dropping 17,000 

feet/minute. Has implications on cabin pressure changes.

CRU-103 Breathing Regulator: A particular chest mounted breathing gas regulator used in 

Navy jets; manufactured by Cobham, Orchard Park, NY. Provides pressure regulated, ondemand breathing gas to the pilot’s mask.

CRU-73 Panel-Mounted Breathing Regulator: A particular panel mounted breathing gas 

regulator used in Air Force jets; manufactured by Cobham, Orchard Park, NY. Provides 

pressure regulated, on-demand breathing gas to the pilot’s mask.

Curation of data: A method for evaluating complex data streams; purpose is to detect metadata 

errors, measurement errors, detector noise, data dropouts, and other disruptions, with the 

goal of correcting (cleaning) the data based on interpolation or other techniques, or 

eliminating certain data streams from further consideration. 

Dance Card: A specific reference list for the pilot prescribing an order of flight maneuvers 

representing a particular scripted flight profile

Data Tile Visualization: A data visualization tool developed by PBA in standardized graphs 

arranged in a 7-tile array. These show a series of pressure and flow breathing interactions 

with aircraft maneuvers and are used to investigate single minutes of a flight during 

especially difficult aerobatics flight segments.

Data Visualization: General term for graphical representations of measurements to illustrate 

patterns and behaviors.

Defog: A feature of the aircraft ECS which blows heated air on the inside of the canopy to 

eliminate condensation.

Delayed Regulator Response: The regulator requires a pressure signal form the mask to react to 

on-demand gas flow; for various reasons this signal can be delayed resulting in difficulty 

in initializing a new breath.

Delta Line Pressure (DLP): A parameter derived from the VigilOX ISB inlet pressure channel 

calculating the first derivative (slope) of adjacent data points at 20 Hz resolution. Used as 

a time base alignment marker.
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Dependent variables: continuous measurements and calculations of pilot physiological response 

parameters, including breathing rates, breath volumes, breath flows, breathing pressure, 

oxygen usage, etc.

DFRL bit: A “self-diagnosis” output from the VigilOX sensor suite data indicating unexpected 

reverse flow when the differential sensor records a pressure magnitude larger than -10 

Pascals.

Differential mask pressure, DMP: A calculated variable based on the difference between the 

highest and lowest value of mask pressure within a single flight minute, considered to be 

an indicator of breathing effort, (mmHg)

Diluter demand: An oxygen schedule used in some Air Force configurations wherein cabin air 

is mixed with mask supply. Represents an effort to reduce oxidative stress at lower 

altitudes, but 100% oxygen simplifies the implementation of the regulator mechanics.

DMP: Differential mask pressure (mmHg) calculated by subtracting “Min_maskpress” column 

from “Max_maskpress” columns of original data within each minute.

Effort of breathing: The perceived effort needed to get sufficient ventilation to move gas in and 

out of the lungs. 

Electromagnetic Interference: New flight gear is tested to determine if external 

electromagnetic fields or pulses could disrupt function.

Event Marker: A tracking system, electronic, audio, or written indicating the start of a new 

flight maneuver. 

Excel: A part of the Microsoft software suite family used for handling and calculating data sets; 

from Microsoft Corporation, Redmond CA.

Exhalation Valve Lag: A phenomenon sometime referred to as “sticky valve” in that there is a 

delay for breaking open the exhalation pathway.

Expiration: Alternative word for exhalation

Flight Health Check quad tile: A data visualization tool developed by PBA to show a series of 

pressure and flow behaviors in standardized graphs arranged in a 4-tile array.

Flight minute: A consecutive designator for the individual minutes constituting an individual 

flight; a 60 min long flight is broken up into numbered 1, 2, 3... 59, 60 individual flight 

minutes.

Flight Segment: A subset of a real-world sortie typically defined by a particular maneuver 

(ascent, descent, high-G turn, etc.), or time spent at a particular altitude or velocity.

FPA: Flight Path Angle – The angle of the aircraft flight path relative to the horizon (positive for 

a climb and negative for a descent).

G-Breathing: This report uses the term ‘G-breathing’ to refer to the respiratory aspect of the 

Anti-G Straining Maneuver (AGSM). The AGSM involves a forced exhalation against a 

closed glottis with straining of limb and abdominal muscles just before and during high 

sustained G’s. The exhalation (increased intrathoracic pressure) is maintained for 3-4 sec 

and is interspersed with rapid inspirations less than 1 sec; the process is repeated 

cyclically. It is an effective anti-G procedure, increasing the mean-G-tolerance by as 

much as 4 G. (Bates et al., 1990)
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G-Exercise: A warm up maneuver used at the beginning of a tactical training sortie to ensure the 

pilot’s G-suit is functioning correctly and to make sure the pilot feels physically ready for 

elevated-G maneuvering.

G-force vector (G3): Composite G-force vector calculated from VigilOX ISB 3-directional 

accelerometer in units of G.

G-LOC: G Loss of Consciousness - High load maneuvers cause blood in the body to shift, 

reducing blood flow to the brain, causing a PE. Less severe cases affect vision and higher 

order thinking. More severe cases can result in loss of consciousness.

Graphpad Prism: A commercially available statistical software platform that combines 

scientific graphing, comprehensive curve fitting (nonlinear regression), univariate 

statistics, and data organization; from GraphPad Software, San Diego, CA.

Group: j

th minute within an individual flight

Heat map: Visualization graphics tool using color coded fields indicating numeric values within 

a rectangular array; for PBA, x-axis (column) is flight minute and the y-axis (row) 

represents individual flights.

Histogram: A standard statistical visualization graphic showing the frequency distribution of a 

data set; uses bars of different heights where each bar groups numbers into ranges. Taller 

bars show that more data falls in that range.

Hyperoxia Priming: Hyperoxia may be defined as the inspiration of supra-physiologic levels of 

inhaled oxygen such as concentrations of oxygen in excess of ninety per cent. The overall 

state of hypoxia in aviators experiencing physiologic episodes is paradoxically initiated 

by the pre-condition of hyperoxia that is established from the moment the pilot begins 

breathing oxygen concentrations that are in excess of 90%.

Hypoxemia and anoxemia: Refers specifically to states that have low or zero arterial oxygen 

supply.

Hypoxia Recognition Training: Used to train pilots to recognize early hypoxia symptoms

Hypoxia: Hypoxia is medically defined as deficiency in the amount of oxygen reaching the 

tissues. Involves four steps: 1) breathing gas delivery system, 2) respiratory system, 

3) pulmonic subsystem, and 4) circulatory system

Hysteresis (breathing): Measures the delay in on-demand breathing response due to regulator 

and mask valve reactions to pressure changes. Early during inhalation demand (mask 

pressure) exceeds supply (flow);

ILS: Instrument Landing System – A ground-based precision approach capability which 

provides the pilot lateral and vertical guidance to landing.

Immelman: An aerobatic maneuver in which the pilot executes the first half of a loop, but at the 

top when the aircraft is inverted, the pilot rolls the aircraft upright and the maneuver is 

complete.

Independent variables: flight meta-data such as pilot i.d, flight #, aircraft type, tail number, 

mask configuration, flight profile, etc. plus continuous measurements of aircraft 

parameters including altitude, speed, acceleration (G-force), cabin pressure, etc.

Inductive Monitoring System (IMS): A NASA machine learning program that detects Mask 

Pressure-No-Flow (PNF) situations related to disruptions in normal breathing.

Occurrence of PNF are indicators of regulator/mask dysfunction. 
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In-Mask CO2 and Water Vapor Sensor (IMCWS): Development effort with NASA’s Jet 

Propulsion Laboratory for in mask sensor suites. Prototype has been successfully flown.

Inspiration: Alternative word for inhalation

instantaneous Delta Mask Pressure (iDMP): A parameter derived from the VigilOX ESB 

mask pressure channel calculating the first derivative (slope) of adjacent data points at 20 

Hz resolution. Used as a time base alignment marker.

Interview Responses: Pilots were interviewed in a casual soon after flights to get more specific 

information about their physiological and mental status with respect to different flight 

segments.

Isobaric region: Defined as 8,000 to 23,000 feet altitude; here, cabin pressure is maintained at 

the equivalent of 8,000 feet altitude.

JPL: NASA’s Jet Propulsion Laboratory is a research and development center in Pasadena CA; 

collaborating with NESC on in mask sensors.

KCAS: Knots Calibrated AirSpeed – the airspeed displayed to the pilot later during inhalation 

supply (flow) exceeds demand (mask pressure).

Life Support Systems (LSS) team: Aircrew Flight Equipment (AFE) inspectors, technicians, 

maintainers, medical officers, engineers, and scientists.

Lognormal distribution: Sometimes referred to as a “multiplicative” distribution, the lognormal 

frequency distribution is characterized by the geometric mean (GM), and the geometric 

standard deviation (GSD). It is asymmetric being limited on the left by zero, but may 

include larger numbers. The defining feature of a lognormally distributed dataset is that it 

becomes normal (Gaussian) upon log-transformation.

Lognormal transformation: A standard mathematical calculation wherein each individual point 

within a dataset is replaced by its own natural logarithm. If the original data were 

lognormally distributed, then the log-transformed data are normally distributed. This a 

tool for calculating confidence limits.

Low Boom Dive: a maneuver unique to NASA AFRC supersonic research, flown in an F-18 to 

put a quiet sonic boom on a desired location on the ground. The pilot starts at 49,000’ PA 

and pulls the aircraft down into a dive to achieve 1.10 Mach in a 53 degree dive at 

40,000’. The pilot then pulls 3.5 G to bring the nose back above the horizon and the 

maneuver is complete.

LOX: Liquid oxygen (LOX) based system. LOX is an earlier technology for providing enriched 

oxygen for pilot breathing needs; depends literally on the vapor pressure above a liquid 

oxygen reservoir for breathing. Disadvantage is that it stops working when the LOX 

reservoir is depleted.

Machine learning: Computer tools used to improve automatically through experience; 

considered a subset of artificial intelligence programming. Machine learning algorithms 

build a mathematical model based on sample data, known as "training data", in order to 

make predictions or decisions without being explicitly programmed to do so when faced 

with new data.

MadgeTech: An onboard monitoring system for data-logging cabin pressure. Manufactured by 

MadgeTech, Inc., Warner, NH.

MatLab: A commercially available statistical software suite from MathWorks, Natick, MA.
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max_FlowLPM_I: Maximum value measurement from ISB flow sensor for each individual 

flight minute (liters/min); same as “maximum flow volume”. 

Maximum flow volume: The highest instantaneous rate of inhalation within a single flight 

minute, (liters/min)

Maximum power: Full power, including afterburner, on an aircraft engine.

MBU-20/23P oxygen masks: A particular non-rebreathing oxygen mask type manufactured by 

GenTex, Carbondale PA; used for all PBA flights.

mean_FlowLPM_I: Average measurement from ISB flow sensor values within each individual 

flight minute (liters/min); same as “Average flow volume”.

Metadata: Information gathered beyond empirical parameter measurements; refers to fixed 

flight parameters such as date, time, pilot id, flight number, mask/regulator configuration, 

aircraft type, tail number, weather, location, etc.

MIL-C-85521 Breathing Standards: Requirements contain a figure of 28.3 cubic feet per hour 

[=13.35 LPM] of 100% O2 at Sea Level (NTPD), as the baseline to provide each member 

of the aircrew.

MIL-D-8683C Breathing Standards: Minor change in maximum flow rate per aircrew from 

13.12 LPM to 13.35 LPM

Military power: Full, non-afterburning power on an aircraft engine.

MIL-STD-3050: A document with design criteria standards for Aircraft Crew Breathing 

Systems (published 2015, under revision in 2019). Establishes the minimum design 

criteria for an aircraft crew breathing system using an On-Board Oxygen Generating 

System (OBOGS). Prescribes flow rates, volumes, and mask pressure minimums, 

maximums and swings for given peak flow rates.

Mixed Effects Models: Calculational multivariate models to determine influence of independent 

variables on pilot breathing response; main feature is the ability to incorporate both 

continuous parameters (e.g., altitude, cabin pressure, velocity, G-force, etc.) as well as 

metadata (e.g., pilot i.d., aircraft type, mask configuration, etc.) into a single model. The 

outcomes are interpreted to assess the influence of independent variables on dependent 

pilot breathing response variables.

mmHg: Millimeters of mercury; A unit of gas pressure referenced to the pressure exerted by a 

height of a column of mercury (Hg) in millimeters (mm) at earth normal (1-G) gravity.

Neurologic signs: Due to vascular profusion, reduced brain function due to oxygen reduction 

can exhibit as a “soft” neurologic sign, such as confusion or a missing or radio calls, 

rather than a “hard” neurologic deficit such as unilateral paralysis or loss of 

consciousness. 

Normal distribution: Sometimes referred to as the “Gaussian” curve or the “Bell-shaped” 

curve, the normal frequency distribution is characterized by the mean and the standard 

deviation of the constituent numbers; it is symmetric about the mean and can include 

negative numbers.

OBOGS descent: a long, slow, idle power descent. These conditions have reportedly stressed 

OBOGS breathing systems due to reduced ECS supply air to the OBOGS.

OBOGS: On board oxygen generation system (OBOGS) takes pressurized bleed-air from the jet 

engine and processes it across adsorbent beds that pass oxygen preferentially. Beds are 
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 257 of 260

periodically backflushed to remove residual nitrogen; system is capable of providing 

close to pure oxygen into a plenum from which pilots breathe. Advantage is that it is 

unlimited as long as there is fuel for the jet. Disadvantage is that it relies on a stable and 

clean bleed air supply.

Oxidative Stress: Reactive oxygen species (ROS), commonly called “free radicals”, are 

produced by the human body in the course of normal metabolism by a variety of 

mechanisms. The term “oxidative stress” describes an imbalance between oxygen free 

radical formation, and oxygen free radical dissolution through antioxidant repair 

mechanisms. Hyperoxia is associated with an increased level of reactive oxygen species 

(ROS).

Oxygen Transport model: Theoretical description of oxygen transport and losses from mask 

supply to lung to blood to organs (brain). Used to explain different paths along which 

oxygen could be depleted before reaching vital organs.

Peak Inspiratory Pressure (PIP): A data marker indicating the initial slope of a new breath 

used for data alignment

Phase Shift: Concept developed for PBA to measure driving pressure and resulting flow 

temporal disharmony for masked breathing. Describes the mismatch of expected flow and 

pressure relationship occurring in on-demand breathing.

Physiological episode (PE): An adverse pilot effect from the complex pilot-aircraft interaction 

that is serious enough to results in an aborted mission; often related to hypoxia or 

barotrauma.

Physiological Episode Action Team (PEAT): Internal team within US Navy and US Air Force 

investigating physiological episodes in fighter aircraft; as of Sept. 2018, led by Rear 

Adm. Fredrick R. “Lucky” Luchtman and Air Force Brig. Gen. Edward L. “Hertz” 

Vaughan.

Physiological response: Pilot parameters treated as dependent variables within 1-min blocks, 

such as breaths/min, liters/breath, etc. 

Pop Pattern: A training maneuver in which the pilot practices a Pop Up Attack. A Pop Up 

Attack is used to approach a target area at low altitude to avoid detection by the enemy.

The pilot then climbs rapidly to visually locate the target and pulls down into a dive to 

drop bombs on it before descending back to low altitude to egress the target area.

Positive pressure: Similar to the broader mask “safety pressure” definition, but defined more 

specifically when comparing the flows, pressures, and timing of on-demand breathing 

systems that maintain a mask pressure that is greater than cabin pressure.

ppCO2: Partial pressure of carbon dioxide (CO2); parameter indicating the pressure of carbon 

dioxide in a mixture of gases as if it were alone within that volume.

ppO2: Partial pressure of oxygen (O2); parameter indicating the pressure of oxygen in a mixture 

of gases as if it were alone within that volume.

Pressure oscillations: Cabin pressure control is not perfect, especially for rapid transition 

through the isobaric region limits. The pressure system may over- or under-shoot 

resulting in oscillations. 

Pressure-Flow Disharmony: Measured mismatch between the pressure profile and the flow 

profile, including start/stop and time it takes to reach the peak. Similar to breathing 

hysteresis.
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Pressure-no-flow (PNF): A condition observed during on-demand breathing wherein the subject 

exerts pulmonary pressure (positive or negative) but the breathing system does not 

respond with flow. Typically this is very brief, but the occurrences indicate a problem 

with the regulator/mask configuration.

Proc Mixed: A specific statistical subsystem procedure within the SAS suite designed for mixed 

effects modeling that incorporates metadata and continuous variables into a single 

statistical analysis.

Pulmonary function tests (PFT): A systematic set of measurements of the flows and volumes 

of maximum breath maneuvers designed to assess current human lung performance using 

medical spirometry equipment.

QQ-plot: Visualization graphics tool using least-squares linear regression to indicate overall 

data distribution, and to show the position of each measurement within the distribution; 

for PBA, the x-axis is the calculated z-score for the particular data set, and the y-axis 

represents the measured value of the measurement. Data are typically log-transformed to 

achieve coherence with the statistical model as described by a linear plot.

Questionnaires: 6PBA developed detailed questionnaires using numerical scales to assess pilot 

subjective feelings/attitudes, as well as any experiences with physiological episodes. The 

purpose was to complement the empirical data and to correlate the subjective with the 

objective information.

Rad-97: A brand name for a handheld pulse oximetry, CO, and CO2 monitor from Masimo 

Corp., Irvine, CA, USA. Used for measuring pre- and post-flight pilot breathing 

parameters.

Rapid Decompression Compliance: New flight gear is tested to determine if closed and sealed 

elements might rupture during rapid decompression as in an ejection event.

RCCA: Root Cause Corrective Analysis (RCCA) is a deductive safety engineering method used

to analyze a problem, identify its causes and the measures that could be taken to prevent 

it from occurring again.

Repeat Measures Design: Experimental plan wherein activities are repeated so that “withinsubject” variability can be assessed. In PBA, pilots repeated the same flight profiles on 

different days. 

ROBD: Acronym for reduced oxygen breathing device (ROBD) used to train pilots to recognize 

early hypoxia symptoms

Royal Australian Air Force (RAAF): Aerial warfare branch of the Australian Defence Force 

(ADF); operates the majority of the ADF's fixed wing aircraft, although both the 

Australian Army and Royal Australian Navy also operate aircraft in various roles.

RTB: Return To Base

Safety pressure: A general term used to describe the regulator and mask systems that provide 

breathing gas at a slightly higher pressure than the surrounding environment. Used by 

Navy to define aid to inhalation during high-G maneuvers and as a protective measure in 

case of ejection; increases the effort to crack open the exhalation valve in the mask 

during normal operation. Sometimes used interchangeably with “positive pressure”.

SAS: A commercially available statistical software platform from SAS Institute, Cary, NC; 

provides a wide suite of statistical analyses; capable of dealing with large data sets.
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Scripted Flight Profiles: Predesigned flight maneuvers to test pilot physiological response to 

specific aircraft parameters including climbs, altitude, g-force, velocity, etc.

SME: Subject matter expert.

SpiroDoc: A brand name for a handheld pulmonary function and testing (PFT) instrument; 

manufactured by Medical International Research (MIR), New Berlin, WI. Used for 

measuring pre- and post-flight pilot breathing parameters.

Split-S: an aerobatic maneuver in which the pilot rolls the aircraft inverted and pulls the nose 

down as in the second half of a loop.

Squirrel Cage: a continuous series of aerobatic maneuvers accomplished without pausing 

consisting of a loop, a 1⁄2 Cuban 8, an Immelman, and a Split-S.

Squirrel Cage: A flight maneuver during aerobatic flight containing three consecutive 10,000 ft 

span vertical loops. Has implications on cabin pressure changes and g-force.

st. dev. MP: Standard deviation of mask pressure (mmHg) of original data within each minute.

Standard deviation mask pressure, St. dev. MP: A calculated variable based on the standard 

deviation of the 20-hz data stream captured within each individual flight minute, 

considered to be an indicator of rapid breathing gas fluctuations, (mmHg)

Standard temperature and pressure (dry) conditions (STPD): Abbreviation indicating that a 

gas volume has been expressed as if it were at standard temperature (0°C), standard 

pressure (760 mm Hg absolute), and dry; under these conditions a mole of gas occupies 

22.4 L.

Summary Statistics: Tabulated entries for any given data set represented by central tendency 

(mean, median, geometric mean), spread (standard deviation, range, geometric standard 

deviation), and confidence limits.

Symptomatology and Sensations: Pilots were asked about short- and long-term symptoms (e.g.,

disorientation, headache, nausea, cough, and about other sensations e.g., tingling in 

hands, blurred vision, drowsiness, etc.)).

T-45 Goshawk: A tandem-seat, carrier capable, jet trainer whose mission is to train Navy and 

Marine Corps pilots.

TACAN: TACtical Air Navigation – a ground based navigation aid used by the US military. Can 

be used as the source of guidance for an instrument approach to landing.

Tail number: Serves as a “license plate” identifier visible externally that identifies a specific 

aircraft.

Talking scripts: Standardized cripts normally used in formal testing of communications 

systems. Adapted for PBA to assess the impact of speaking on pilot breathing dynamics.

Technology Readiness Level (TRL): A numeric designator method for estimating the maturity 

of technologies during the acquisition phase of a program, developed at NASA during the 

1970s. The use of TRLs enables consistent, uniform discussions of technical maturity 

across different types of technology.

TM-93-59 SY Breathing Standards: Pilots in fighters may consume considerably more oxygen 

in excess of 200 LPM per crew member

Trumpet Curve: A data visualization tool relating pressure and flow rate during inhalation and 

exhalation.
NESC Document #: NESC-RP-18-01320, Vol. 2, V.1.2 Page 260 of 260

TTC recorders: Flight data instrumentation system from Teletronics Technology Corp, 

Davidson, North Carolina, USA. Used for datalogging a variety of aircraft parameters 

including altitude, velocity, and acceleration.

UniqueID: Numbered from 1 to n, representing the nth minute of all PBA flights in sequence for 

all flight minutes.

USN QIK System: Flight data instrumentation system designed by NAVAIR to record all MilSTD-1553 Channels; used for datalogging a variety of aircraft parameters including 

altitude, velocity, and acceleration, plus various communications channels.

Variance components: Estimates of complex datasets that indicate partitioning of variability 

attributed to individual parameters

Vascular perfusion: The passage of fluid through blood vessels to an organ or a tissue. 

Perfusion reflects the delivery of essential nutrients (including oxygen) to tissues, and so 

is directly related to its status.

VigilOX ESB: Exhalation sensor block: Instrumentation for collecting multiple 20-Hz data 

streams from the interior or from the exhalation port of the pilot’s mask. Includes mask 

pressure, gas flow, temperature, CO2 concentration, 3-D accelerometer.

VigilOX ISB: Inhalation sensor block: Instrumentation for collecting multiple 20-Hz data 

between the regulator and the inhalation port of the pilot’s mask. Includes inlet flow, inlet 

pressure, O2 concentration, 3-D accelerometer.

VigilOX: An onboard monitoring system for measuring pilot related parameters including 

inhalation flows and pressures, oxygen concentration, cockpit pressure, G-force, as well 

as exhalation flows and pressures, carbon dioxide and oxygen concentration. 

Manufactured by Cobham, Orchard Park, NY.

Weight-on-Wheels (WoW): A parameter indicating the ground vs. flight portions of a total 

sortie; although important for data alignment, this is not routinely logged in F-18 and F15 aircraft.

Windblast Compliance: Items worn by the pilot need to pass a windblast test before they can be 

declared airworthy. If there is an ejection seat event, items attached to the pilot’s flight 

suit must maintain integrity. This is a specific test performed by Navy.

Wingover: A low-G aerobatic maneuver in which the pilot slowly pulls the nose of the aircraft 

up and then slowly rolls to 90 degrees of bank and lets the flight path fall back below the 

horizon before slowly rolling and pulling back to level flight.

Work-of-breathing: A defined physiological parameter; calculated in terms of the pulmonary 

pressure exerted multiplied by the change in pulmonary volume, or similarly in terms of 

the just the oxygen fraction consumption attributable to breathing.

Zoom climb: A rapid climb maneuver in which the pilot does not attempt to maintain a constant 

airspeed, but instead trades excess airspeed for an increased rate of climb.

z-score: In a normal distribution, z-score is the number of standard deviations an individual 

point is from the mean. For example, the area in a standard normal distribution curve 

between z-score = -1.96 to z-score = 1.96 represents 95% of the data.