DOT/FAA/AR-03/58 

Office of Aviation Research 

Washington, D.C. 20591 

Flight-Testing of the FAA Onboard 

Inert Gas Generation System on 

an Airbus A320 

Michael Burns 

William M. Cavage 

Richard Hill 

Robert Morrison 

June 2004 

Final Report 

This document is available to the U.S. public

Through the National Technical Information 

Service (NTIS), Springfield, Virginia 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
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This document is disseminated under the sponsorship of the U.S.

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because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA aircraft certification office as to its use. 

This report is available at the Federal Aviation Administration William J.

Hughes Technical Center's Full-Text Technical Reports page:

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-03/58

2. Government Accession No. 3. Recipient's Catalog No.

 4. Title and Subtitle 

FLIGHT-TESTING OF THE FAA ONBOARD INERT GAS GENERATION 

5. Report Date

June 2004 

SYSTEM ON AN AIRBUS A320 6. Performing Organization Code

ATO-P 

7. Author(s) 

Michael Burns, William M. Cavage, Richard Hill, and Robert Morrison

8. Performing Organization Report No. 

DOT/FAA/AR-03/58 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Federal Aviation Administration 

William J. Hughes Technical Center 

Airport and Aircraft Safety 

Research and Development Division 

Fire Safety Branch 

Atlantic City International Airport, NJ 08405

11. Contract or Grant No.

12. Sponsoring Agency Name and Address 

U.S. Department of Transportation 

Federal Aviation Administration

13. Type of Report and Period Covered

Final Report 

Office of Aviation Research 

Washington, DC 20591

14. Sponsoring Agency Code

ANM-112 

15. Supplementary Notes

16. Abstract

Extensive development and analysis has illustrated that fuel tank inerting could potentially be cost-effective if air separation 

modules (ASM), based on hollow-fiber membrane technology, could be used in an efficient way. To illustrate this, the Federal 

Aviation Administration has developed an onboard inert gas generation system that uses aircraft bleed air to generate nitrogenenriched air (NEA) at varying flow and purity (oxygen concentration) during a commercial airplane flight cycle. A series of 

ground and flight tests were performed, in conjunction with Airbus, designed to prove the simplified inerting concept. The 

system was mounted in the cargo bay of an A320 operated by Airbus for the purposes of research and development and used to 

inert the aircraft center wing fuel tank during testing. The system and center wing fuel tank were instrumented to allow for the 

analysis of the system performance and inerting capability. 

The results of the tests indicated that the concept of the simplified inerting system is valid and that the air separation module 

dynamic characteristics were as expected. ASM pressure had the expected effect on flow rate and purity; however, bleed air 

consumption was greater than expected during cruise. The fuel tank inerting results illustrated that no stratification or 

heterogeneous oxygen concentrations occurred in the tank. The measured average tank ullage oxygen concentration data agreed 

well with a simple analytical model applied to the flight test data. The measured effect of the high-flow mode was significant, 

allowing the single-membrane configuration to maintain an inert ullage during the entire flight cycle, even with the very high rate 

of descent employed for the flight tests. Fuel had virtually no effect on the resulting oxygen concentrations observed in all the 

tests. 

17. Key Words 

Nitrogen-enriched air, OBIGGS, Hollow-fiber membrane, 

Fuel tank inerting, Oxygen concentration, Air separation 

module 

18. Distribution Statement 

This Document is available to the public through the National 

Technical Information Service (NTIS), Springfield, Virginia 

22161

19. Security Classif. (of this report)

 Unclassified 

20. Security Classif. (of this page)

 Unclassified

21. No. of Pages

 39

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ACKNOWLEDGEMENT 

The Federal Aviation Administration (FAA) and AAR-440, Fire Safety Research, would like to 

acknowledge the contributions of the Airbus Company in accomplishing the very difficult task of

flight testing the FAA inerting system. Airbus provided all the aircraft modification and 

instrumentation as well as system airframe integration. The FAA was also supported by Airbus 

UK with additional instrumentation and systems safety and analysis. 

Further acknowledgement is extended to the many Airbus employees in Toulouse, France, that 

supported the joint FAA/Airbus flight test. The day-to-day dedication and commitment required 

to successfully complete the ground and flight test regiment was significant and their technical 

know-how and professionalism are the primary reasons for the success of the test program.

iii/iv
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY ix 

1. INTRODUCTION 1 

1.1 Background 1 

1.1.1 Air Separation Modules 1 

1.1.2 Previous Research 1 

1.1.3 Commercial Transport Fuel Tanks 2 

1.2 Scope 2 

2. TEST ARTICLE AND PROCEDURES 3 

2.1 Test Article 3 

2.1.1 A320 Test Article 3 

2.1.2 Instrumentation 3 

2.1.2.1 Center Wing Tank 4 

2.1.2.2 The FAA OBIGGS Instrumentation 4 

2.1.2.3 Additional Parameters Collected 5 

2.1.2.4 Aircraft Data Acquisition System 5 

2.2 Test Procedures 6 

3. THE FAA OBIGGS 7 

3.1 System Flow 7 

3.2 System Interfaces 8 

3.3 System Operation 9 

4. ANALYSIS 10 

4.1 Calculation of Bleed Air Consumption 10 

4.2 Tank Inerting Model 11 

5. DISCUSSION OF RESULTS 12 

5.1 System Peformance 12 

5.1.1 Air Separation Module Performance Characteristics 12 

5.1.2 Effect of Varying Number of Air Separation Modules 13 

v
5.1.3 Bleed Air Consumption 15 

5.1.4 Membrane Performance Degradation 17 

5.2 Fuel Tank Inerting 18 

5.2.1 Typical Mission Profile 18 

5.2.2 Effect of High-Flow Mode 20 

5.2.3 Effect of Fuel on Inerting 21 

6. SUMMARY OF FINDINGS 23 

7. REFERENCES 23 

APPENDICES 

A—The FAA OBIGGS Engineering Drawing

B—Four-Bolt Flange Engineering Specification 

C—The FAA OBIGGS Wiring Diagram

D—Calculation of Bleed Air Consumption

LIST OF FIGURES

Figure Page 

1 Airbus A320 Aircraft 3 

2 Side View of A320 Center Wing Tank With Sample Port Locations 4 

3 Engineering Drawing of Flow Meter 5 

4 System Block Diagram 7 

5 The FAA OBIGGS Installed in the Test Aircraft Cargo Bay 9 

6 Front Panel of the FAA OBIGGS Control Box 9 

7 System Performance Data for a Single-Membrane Test 12 

8 System Flow and Purity With ASM Pressure for Both Membrane Configurations 

During Ascent and Cruise 13 

9 System Flow and Purity With ASM Pressure for Both Membrane Configurations 

During Descent 14 

vi
10 Correlation of ASM Pressure With NEA Flow for Both System Membrane 

Configurations 15

11 Measured Membrane Characteristics for the Single-Membrane Configuration 

System 15

12 Calculated Bleed Airflow Into the Single-Membrane Configuration System 16 

13 Correlation of Bleed Airflow and Atmospheric Pressure for a Single-Membrane 

Test 17 

14 Oxygen Concentration Variation in the CWT During a Typical Flight Test 18 

15 Comparison of the Single-Membrane Inerting Flight Test Tank Ullage Oxygen 

Concentration With the Two-Membrane Configuration 19 

16 Comparison of the Single-Membrane Inerting Flight Test Average Tank Ullage 

Oxygen Concentration With the Model Data 20 

17 Comparison of the Single-Membrane Inerting Flight Test Average Tank Oxygen 

Concentration for Both High-Flow Descent and Low-Flow Descent 21 

18 Comparison of the Single-Membrane Inerting Flight Test Average Tank Oxygen 

Concentration for Both Empty Tank and a Constant Fuel Load 22 

19 Comparison of the Single-Membrane Inerting Flight Test Average Tank Oxygen 

Concentration for Both Empty Tank Tests and Consumed Fuel Load 22 

LIST OF TABLES 

Table Page 

1 Inerting Flight Tests on the Airbus A320 6 

2 Air Separation Module Performance Data Before and After 100 Hours of Aircraft 

Operation 17

vii
LIST OF ACRONYMS

ASM Air separation module 

CWT Center wing tank 

FAA Federal Aviation Administration 

HFM Hollow-fiber membrane

NEA Nitrogen-enriched air 

OBIGGS Onboard inert gas generation system

OBOAS Onboard oxygen analysis system

OEA Oxygen-enriched air 

SCFM Standard cubic feet per minute 

SOV Shutoff valve 

t Metric ton 

viii
EXECUTIVE SUMMARY

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in

July 1996. Extensive development and analysis has illustrated that fuel tank inerting could 

potentially be cost-effective if air separation modules (ASM), based on hollow-fiber membrane 

technology, could be used in an efficient way. To illustrate this, the Federal Aviation 

Administration (FAA), with the assistance of several aviation-oriented companies, developed an 

onboard inert gas generation system with ASMs that uses aircraft bleed air to generate nitrogenenriched air at varying flow and purity (oxygen concentration) during a commercial airplane 

flight cycle. 

The FAA performed a series of ground and flight tests designed to prove the simplified inerting

concept that is being proposed by the FAA. The FAA-developed system was mounted in the 

cargo bay of an A320 operated by Airbus for the purposes of research and development and used 

to inert the aircraft center wing fuel tank during testing. The system and center wing fuel tank 

were instrumented to allow for the analysis of the system performance as well as inerting 

capability. The FAA onboard oxygen analysis system was used to measure the oxygen 

concentration in the center wing tank continuously during operation of the inerting system. 

The results of the tests indicated that the concept of the simplified inerting system is valid and 

that the air separation module dynamic characteristics were as expected for the limited test plan 

performed. Both one- and two-ASM configuration tests gave the expected performance with

ASM pressure having the expected effect on flow rate and the duel-flow performance being 

predictable. Bleed air consumption was greater than expected during the cruise phase of flight. 

Additional research is needed to determine what changes in system design or operational 

methodology would best reduce the bleed airflow and the associated cost. 

The fuel tank inerting results illustrated that, as expected, no stratification or heterogeneous 

oxygen concentrations occurred in the tank for the inerting tests performed, in part due to the 

essentially rectangular box configuration of the tank, allowing easy distribution of the inert gas. 

A simple analytical model of the inerting process illustrated good agreement with the measured 

data. The measured effect of the high-flow mode was significant, allowing the single-membrane

configuration to maintain an inert ullage (less then 12%) during the entire flight cycle, even with

the very high rate of descent employed for the flight-tests. When the high-flow mode was not 

used, the ullage reached a peak of 15% oxygen by volume. Fuel had virtually no effect on the 

resulting oxygen concentrations observed in all the tests. 

ix/x
1. INTRODUCTION.

1.1 BACKGROUND. 

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in

July 1996. Since the accident, the Federal Aviation Administration (FAA) has issued numerous 

Airworthiness Directives, enacted a comprehensive regulation to correct potential ignition

sources in fuel tanks, and conducted research into methods that could eliminate or significantly 

reduce the exposure of transport airplanes to flammable vapors. Extensive development and 

analysis has illustrated that fuel tank inerting during aircraft operation could potentially be costeffective if air separation modules (ASM), based on hollow-fiber membrane (HFM) technology, 

could be integrated into a system in an efficient manner. 

To illustrate this, the FAA, with the assistance of several aviation-oriented companies, developed

an onboard inert gas generation system (OBIGGS) with ASMs that used aircraft bleed air to 

generate nitrogen-enriched air (NEA) at varying flows and purities (NEA oxygen concentration) 

during a commercial airplane flight cycle. This system takes advantage of the operational

properties of the specified ASM as well as a two-flow methodology to keep the oxygen 

concentration of a fuel tank below 12% oxygen by volume. This oxygen concentration has been 

identified as the point at which a potential ignition source could create a reaction in a flammable 

tank with a measurable pressure rise [1]. 

1.1.1 Air Separation Modules.

Hollow-fiber membranes are fabricated into a vessel called an air separation module. When 

supplied with pressurized air, these modules will ventilate a waste stream of gas from a permeate 

vent that is rich in oxygen, carbon dioxide, and water vapor [2]. This allows the product gas 

passing through the ASM to be rich in nitrogen. As flow is limited through the ASM product 

port (orifice), more pure (lower oxygen concentration) NEA is generated at lower mass flows.

This operational property allows for a single-ASM system to generate different flows and 

purities of NEA to optimize the system’s ability to inert a center wing tank (CWT) throughout a 

given aircraft mission. 

In general, when an ASM is exposed to a specified feed pressure at a given pressure altitude, it 

will produce a mass flow of NEA at some purity, given a specific orifice setting (backpressure).

This statement assumes a constant operation temperature at which the ASM performs optimally. 

It is these performance characteristics that will drive the capabilities of a given ASM inerting

system to inert a given ullage space.

1.1.2 Previous Research.

Previous FAA research into using ASMs to inert fuel tanks has focused on the amount and purity 

of NEA required to reduce the oxygen concentration of a given ullage [2]. Other FAA research 

focused on the ability of a ground-based source of NEA to maintain an inert ullage of a Boeing 

737 during ground and initial flight operations [3]. The research validated the FAA’s in-flight 

oxygen measurement system and also documented the small, but measurable, effects the 

refueling process had on an inert aircraft CWT. 

1
Other previous studies include an extensive analysis of the performance of an aircraft-based 

OBIGGS based on permeable membranes (ASMs) versus one that uses a molecular sieve 

(pressure-swing absorption) [4]. This report represents the first time the OBIGGS research

attempted to quantify the ability of ASMs to inert an aircraft fuel tank. The study, which ended 

in 1984, concluded that the performance and reliability of the state-of-the-art membrane

materials was not adequate to incorporate into an aircraft environment. Follow-on research in 

the late 1980s illustrated that the introduction of so-called HFMs drastically decreased the size 

and weight of ASMs. This allowed for a relative performance increase of a factor of ten [5]. 

This study highlighted the favorable life cycle costs of a demand (inert gas generation) system 

over a stored inerting agent system and explosive suppressant foam. 

Improved manufacturing methods, fiber postprocessing, and permeate flow management has 

made HFM ASMs even more productive and efficient in the past 10 years. This makes it

possible to purchase ASMs off the shelf, already designed and optimized for use in aerospace

applications.

1.1.3 Commercial Transport Fuel Tanks.

Fuel tanks in commercial transport airplanes are generally sealed geometric structures in the 

wing or tail of the aircraft. This is usually the volume between the front and rear spar within the 

upper and lower wing surface. These tanks are vented to the aircraft exterior to ensure that 

ambient pressure is maintained within the tank. Venting is accomplished by a system of tubing 

and venting channels that terminate at a small overflow compartment at the end of the wing 

called the surge/vent tank. This tank vents to the outside through an aerodynamic opening in the 

underside of the wing. 

Many commercial transport aircraft have an extra wing fuel tank contained partially or fully 

within the aircraft fuselage. These tanks are generally referred to as center wing tanks. Most 

commercial transport aircraft have air cycle machines below these CWTs that condition hot, 

ducted air from the engines and reject heat during operation [6]. These tanks have been 

identified in the 1998 Aviation Rulemaking Advisory Committee report as potentially more

flammable and statistically more susceptible to explosions [7]. 

1.2 SCOPE. 

The FAA and Airbus performed a series of ground and flight tests designed to prove the 

simplified inerting concept that is being proposed by the FAA. The FAA developed an inerting 

system, based on ASM technology, designed to maintain an inert ullage in a commercial 

transport CWT during normal ground and flight operations. The system was mounted on a cargo 

pallet and installed in the cargo bay of an A320 operated by Airbus for the purposes of research 

and development. The system was interfaced with the aircraft systems, and instrumentation was 

installed to study the system performance during flight-tests. The CWT of the aircraft was 

instrumented with gas sample tubing to allow for continuous monitoring of the tank oxygen 

concentration during each test flight cycle with the system performance. 

The purpose of the tests was to validate the simplified inerting concept as well as the expected 

system performance. The tests will also give insight into the effect of aircraft operations on the 

2
system performance and operational considerations. The performance degradation of one 

membrane during the test will also be examined to allow for a better understanding of the effects

of aircraft bleed air on ASM performance. 

2. TEST ARTICLE AND PROCEDURES. 

2.1 TEST ARTICLE. 

The test article consisted of an Airbus A320 (figure 1) with associated instrumentation and data 

acquisition system. The FAA OBIGGS with associated instrumentation was installed in the

cargo bay. The CWT was instrumented with eight gas sample tubes that were routed to the 

onboard oxygen analysis system (OBOAS). The OBOAS was developed by the FAA to 

continuously measure the oxygen concentration in a fuel tank environment during a typical 

commercial transport airplane flight cycle using conventional oxygen analyzers. 

2.1.1 A320 Test Article. 

Airbus provided an A320 airplane for the series of tests. The aircraft has a basic operating 

weight of 93,079 lb with a gross takeoff weight of 162,040 lb. It has a 112-foot wing span and is 

123 feet long with a wing area of 1320 square feet. The maximum cruise speed of the aircraft is 

487 knots at an altitude of 28,000 feet. 

FIGURE 1. AIRBUS A320 AIRCRAFT 

The CWT has a capacity of 8200 liters of fuel (2165 gal) and is located below the floor and in 

between the wings, within the fuselage of the aircraft. The empty CWT ullage volume is 289 

normal cubic feet for the purpose of inerting the tank with NEA. 

2.1.2 Instrumentation.

The primary instrumentation employed in the test centered around the gas sample tubing in the 

CWT that allowed for continuous oxygen concentration measurements in the tank during each 

flight test using the OBOAS. Additionally, the FAA OBIGGS was instrumented with extensive

pressure and temperature sensors as well as oxygen concentration measurements and flow to 

allow analysis of system performance and health monitoring for each test. Additional test 

parameters were also collected to allow for analysis of the inerting process during the flight tests. 

3
2.1.2.1 Center Wing Tank. 

The CWT of the A320 is a nearly rectangular box with associated structural reinforcement in the

interior. The approximate inside dimensions are 9 ft long by 11 ft wide with the maximum 

height of approximately 3 feet at the highest point. The internal volume is estimated at 289 cubic 

feet. Eight sample ports are located in the tank at the eight locations identified in figure 2.

Locations 1, 4, 5, and 6 are near the centerline of the aircraft (laterally), while locations 2, 3, 7, 

and 8 are located away from the aircraft centerline. 

Vent Location NEA Deposit

(units in millimeters)

FIGURE 2. SIDE VIEW OF A320 CENTER WING TANK WITH SAMPLE 

PORT LOCATIONS 

The OBOAS was used to measure the oxygen concentrations at the eight specified locations 

within the center wing fuel tank. This system consists of a regulated sample train with flowthrough, in-line oxygen sensors and ancillary equipment. Two identical four-channel systems

were developed. Each four-channel system was self-contained in a flight test 19-inch rack. Each 

system has four independent sample trains that can draw an ullage sample at four different 

locations in the fuel tank, regulate the sample pressure, expose the sample to the oxygen sensor, 

and redeposit the sample back into the fuel tank. Each oxygen sensor has a companion analyzer 

mounted on the same 19-inch rack. Also mounted on each rack is a four-channel inlet pressure 

controller and a single-outlet pressure controller electronic unit. These electronic units support 

the five pressure regulators and controllers in each four-channel system. Additional information 

on the construction and operation of the OBOAS is contained in reference 8. 

2.1.2.2 The FAA OBIGGS Instrumentation. 

The FAA OBIGGS was mounted in the cargo bay of the aircraft and was equipped with 

instrumentation to monitor and set system parameters. The system had six thermocouples and 

four pressure transducers (Airbus part no. GG691) installed to monitor system health and 

performance. The specific location and names of the pressure and temperature sensors are given 

in the engineering drawing in appendix A. 

The system was also equipped with gas sample ports to measure both NEA and oxygen-enriched 

air (OEA) permeate oxygen concentration. A laboratory-based altitude oxygen analyzer

developed by the FAA was used to measure the oxygen concentration of both the NEA and OEA 

4
during the test. This analyzer employed the same oxygen analyzer/sensor combination as the 

OBOAS and also used the same sample train pressure control methodology. The two-channel 

analyzer did not employ the same liquid traps and flash arrestors necessary when sampling from 

the tank nor was the sample train contained in a ventilated box. 

Additionally, a totalizing mass flow meter was installed at the NEA output to measure the NEA 

flow rate. The flow meter employs two thermopiles within the flow stream, allowing one to shed 

vortices about the other. The difference in power required to maintain the thermopiles at a

constant temperature is directly related to flow. The meter measures static pressure and total 

temperature and has a digital controller to acquire the meter signals and give a linear voltage 

output calibrated against flow (figure 3). 

FIGURE 3. ENGINEERING DRAWING OF FLOW METER

2.1.2.3 Additional Parameters Collected. 

Additional instrumentation was employed to determine the validity of certain data and to better

understand the fuel tank inerting process in general as it relates to the FAA concept. Flight 

altitude and total air temperature were measured to examine the effect flight conditions have on 

the inerting process. The cargo bay temperature and the backpressure on the OEA line were also

acquired. Additional NEA and OEA line temperatures were also taken.

2.1.2.4 Aircraft Data Acquisition System. 

All signals were recorded by the aircraft data acquisition system, which accepted all analog and 

digital signals required for the test in a multiplexed method and stored the data on the flight test 

data computer. Some data were presented on a master display, in a block diagram format, for

real-time monitoring of many signals, making it easy to examine significant parameters during 

the test. 

5
2.2 TEST PROCEDURES.

All tests were performed at the Airbus test facility in Toulouse, France. Nine flight tests were

performed after a requisite 40 hours of ground tests were accomplished, though only six flight

tests were used by the FAA to evaluate the inerting system capabilities. The primary purpose of 

the ground tests was to familiarize personnel with the inerting system operation and to adjust the

system flow valves to ensure the system operated as designed during the two flow modes. 

Additionally, the aircraft systems’ compatibility was also validated to ensure the inerting system

had no adverse effect on the existing aircraft systems or instrumentation. 

The six relevant tests performed were used to validate the system’s capability to inert the tank

under conditions deemed challenging (short flights with high descent rates) for an OBIGGS in 

addition to allowing study of the dynamics of center tank inerting and fuel/air evolution. System 

performance was also studied from the specified test plan. Some tests used two membranes to 

inert the tank, while most used only one. This allowed for a gauging of system size and 

validation of correct system operation. Table 1 shows a list of relevant tests with the test 

descriptions. 

TABLE 1. INERTING FLIGHT TESTS ON THE AIRBUS A320 

No. 

Airbus 

Designator Date Description

1 1969 8/18/03 Started with CWT not inerted (20% O2) and empty. Two ASMs, 

OBIGGS started 10 mins before takeoff. Low-flow climb and 

cruise (39,000 ft), high flow in descent (normal descent). 

2 1970 8/19/03 Started with CWT inerted (9% O2) and empty. Two ASMs, 

OBIGGS started 10 mins before takeoff. Low-flow climb and 

cruise (39,000 ft), high flow in descent (rapid descent). 

3 1972 8/20/03 Started with CWT not inerted (20% O2) and empty. One ASM, 

OBIGGS started 10 mins before takeoff. Low-flow climb and 

cruise (39,000 ft), high flow in descent (rapid descent at 

beginning). 

4 1973 8/21/03 Started with CWT not inerted (19% O2) and with 1.5t (23%) fuel 

all flight. One ASM, OBIGGS started 10 mins before takeoff. 

Low-flow climb and cruise (39,000 ft), high flow in descent 

(normal descent). 

5 1974 8/22/03 Started with CWT not inerted (20% O2) and with 3t fuel (46%) 

normal usage. One ASM, OBIGGS started 10 mins before 

takeoff. Low-flow climb and cruise (39,000 ft), high flow in 

descent (normal descent). 

6 1976 8/27/03 Started with CWT inerted (10% O2) and with 3t fuel (46%) 

normal usage. One ASM, OBIGGS started 10 mins before 

takeoff. Low-flow for entire flight (rapid descent). 

6
3. THE FAA OBIGGS.

The system consists of a single unregulated flow path that is plumbed to a manifold of three 

ASMs. The nitrogen-rich gas that passes through the modules is then plumbed to the fuel tank to 

reduce the oxygen concentration, a process known as ullage washing. The flow path has a heat

exchanger that controls the ASM inlet temperature and a filter. The flow passes through the 

ASMs and the flow control valves, which allows the system to flow in both high- and low-flow 

modes. For the Airbus tests, one or two of the ASMs were blocked to allow the system to better

fit the needs of the A320 CWT. Figure 4 shows a block diagram illustrating the primary 

components of the FAA OBIGG system as well as points for instrumentation. 

FIGURE 4. SYSTEM BLOCK DIAGRAM 

3.1 SYSTEM FLOW.

The system accepts bleed air from the aircraft through a 2-inch-diameter fitting, which contained 

the system shutoff valve (SOV). When energized, the SOV valve opens, allowing flow into the 

system through the heat exchanger bypass Y into the heat exchanger. The heat exchanger 

accepts the hot bleed air and uses a 4-inch-diameter cooling bypass loop to cool the system flow 

to a temperature of 180° ±10°F. The cooling bypass uses a fan to flow conditioned cabin air 

through the heat exchanger cooling loop and uses a 4-inch-diameter, motor-operated modulating 

valve to throttle the airflow. The modulating valve has six positions and is operated by the 

system temperature controller by using the manual-modulating valve position controller, often

referred to as the Parker box. The heat exchanger bypass Y allows a 1-inch-diameter line to 

bypass a portion of the system flow into the heat exchanger to decrease the effectiveness of the 

cooling loop, giving better control of the air temperature into the ASMs. 

7
After passing through the temperature control portion of the system, the cooled bleed air passes 

through a desiccating filter, past a temperature sensor used by the operator in the temperature 

control loop, and through a section of pipe with a 187-watt clamp heater before entering the 

ASMs via a manifold. This clamp heater is designed to increase the temperature of the ASM 

inlet air significantly if the temperature should drop well below the target control temperature

(due to system heat rejection). However, it provided very little additional temperature where the

air was maintained close to the temperature obtained in the temperature control portion of the 

system. 

After the air passes into the ASMs through the manifold pipe, the air is separated in the ASMs

with the NEA portion passing through and the oxygen-enriched portion passing out of the

permeate vent. The ASM waste flow is eliminated from the system through the OEA manifold 

with a port in the side of the aircraft. It is important that this manifold be exposed directly to 

ambient pressure with little or no obstructions. This will prevent large pressure differentials 

between the OEA vent port and the ambient air pressure, which can compromise system 

performance. 

After the air is separated, the NEA passes through the flow control portion of the system, which 

allows the system to flow at either low- or high-flow conditions. The conditions are set by two 

changeable orifices (needle valves). With the high-flow valve closed, the system delivers NEA 

in low-flow mode, and when opened, it delivers NEA in high-flow mode. The high-flow mode 

allows the system to produce higher volumes of NEA at higher oxygen concentrations. This

system feature allows greater flow into the tank during aircraft descent to minimize air entry into

the ullage due to increasing static air pressure. This should allow for a resulting ullage oxygen 

concentration less than if the low-flow mode was simply employed the entire flight cycle. The 

flow control needle valves are set, given the system configuration, prior to the system flight test. 

Note that changing the system ASM configuration (1 or 2 ASMs) requires the FAA OBIGGS 

flow control needle valves to be adjusted. 

3.2 SYSTEM INTERFACES.

To allow a relatively simple interface with the aircraft, the system was installed in the aft cargo

bay of the test aircraft on an LD3 pallet. The primary mechanical interfaces for the system, as 

installed in the A320, was the bleed air inlet interface, the NEA deposit interface, the OEA 

discharge, the heat exchanger cooling air inlet, and the heat exchanger cooling air exit. In this

test program, the heat exchanger cooling air interfaces were not integrated with the aircraft 

exterior. The cooling air was simply drafted out of the cargo bay and deposited in the rear of the 

bay near the outflow valve by using a piece of 4-inch-diameter flexible ducting. A 2-inch 

stainless steel pipe was mated to the SOV four-bolt flange (appendix B) that connected to the 

system bleed air. The system NEA outflow was connected by a 1-inch flange with seal assembly 

and clamp fitting (AS1650 compatible). The OEA was vented to a 1.6-inch-diameter drain mast

that exited the aft part of the aircraft. It was attached to the system aluminum vent pipe with a 2-

inch flexible coupling. All system interfaces and their locations are specified on the mechanical 

drawing shown in appendix A. Figure 5 shows the FAA OBIGGS installation in the cargo bay 

of the A320 test aircraft. 

8
FIGURE 5. THE FAA OBIGGS INSTALLED IN THE TEST AIRCRAFT CARGO BAY 

Each system part has a specified electrical connector to communicate power and signals to the 

individual component. They are wired with aviation-grade wire to a system connector, allowing 

the system to easily interface to the FAA OBIGGS control box. This box allows the system and 

its components to be turned on or off and to switch the system from one flow mode to the other 

(see figure 6). It also provides the system operator with an ASM inlet temperature and manual 

temperature control box. A complete wiring diagram of the system and control box is shown in 

appendix C. 

FIGURE 6. FRONT PANEL OF THE FAA OBIGGS CONTROL BOX 

3.3 SYSTEM OPERATION.

The system is designed to operate during normal aircraft flight and ground operations, provided 

bleed air is available. Since the system temperature controller did not work to the specifications

set by the ASM manufacturer (180° ±10°F), the ASM inlet temperature was controlled manually 

by a system operator. The operator used a system thermocouple attached to a reader in the FAA 

OBIGGS control box to adjust several potentiometers that controlled the position of the heat 

exchanger cooling air-modulating valve. The heat exchanger bypass valve can be opened to 

decrease the effectivity of the heat exchanger. This was accomplished by bypassing a small 

9
amount of flow (10%-20%) around the heat exchanger, which makes controlling the air 

temperature to the ASMs easier with the heat exchanger cooling air-modulating valve. 

The system did not have a flow mode controller. This required the operator to change from lowto high-flow mode manually. The low-flow mode was used for ground taxi, takeoff, climb, and 

cruise phases of flight, while the high-flow mode was used for the descent phase of flight. For

this test, if descent was halted for an extended period of time, the system was switched back to 

low-flow mode. One test used low-flow mode during the complete flight cycle. To set the flow 

control needle valves to the proper setting, prior to flight test, the bleed air system was charged 

by the aircraft auxiliary power unit (APU). This bleed air pressure was used to run the system at 

sea level with the low-flow needle valve set to generate 5% oxygen NEA, while the high-flow

needle valve was set to generate 11% oxygen NEA. 

4. ANALYSIS.

Calculations performed on the test data determined the quantity of the bleed air consumed by the 

system. Additionally, a simple inerting model was developed to calculate the oxygen 

concentration in a tank volume, given a flight profile and performance schedule of the system. 

4.1 CALCULATION OF BLEED AIR CONSUMPTION. 

To determine the amount of bleed air the system was consuming, the dynamics of an ASM were 

examined. Intuitively, the bleed airflow into the system is equal to the sum of the NEA flow and

the permeate flow. 

(1) QBleed QNEA QPerm & & & = +

Also, a balance of oxygen in and out of the ASM yields the following equation. 

 (2) QBleed NEA QNEA Perm QPerm & & & (0.21) = [O ] ⋅ + [O ] ⋅ 2 2

With: = NEA Oxygen Concentration NEA [O ] 2

 = OEA Oxygen Concentration Perm [O ] 2

Combining equations 1 and 2 gives the following equation for bleed airflow. 

(0.21 [O ] )

([O ] [O ] )

2

2 2

Perm

NEA Perm QBleed QNEA −

− = ⋅ & & (3)

A more complete derivation of the equation is given in appendix D. 

10
4.2 TANK INERTING MODEL.

To allow for better understanding of the inerting process in a wider range of flight scenarios and 

system methodologies, an analytical model was developed to calculate the average tank ullage 

oxygen concentration, given a specific tank volume and system performance schedule. This 

schedule gives a mission time and altitude with the system volume flow and purity. The tank 

ullage temperature is also included, and the model makes the assumption that the pressure inside 

the tank is equal to ambient (altitude) pressure outside the aircraft. 

The model calculates the mass of oxygen in the tank at the start of the mission, given a starting 

tank oxygen concentration. The model then converts the volume flow and purity at each time

step to a mass of oxygen deposited in the tank, given the altitude and temperature. The model 

also tracks the change in mass of oxygen in the tank due to changing pressure and temperature. 

The model tracks the mass of oxygen in and out of the tank, given the changing system 

performance and flight conditions, including the appropriate net oxygen entering the tank as a 

result of descent. The mass of oxygen calculated in the tank at each time step is divided by the 

tank gas mass to obtain oxygen concentration at that time. The following equation governs the 

model process. 

m ( ) ( 1) ( 1) ( * ) ( 1) ( * )*0.21 O2 O2 Tank VTank t = m t − + m& ∗IGOF − m& ∗UGOFt − − ∆ρ V ∗UGOFt − + ∆ρ (4) 

In this equation, UGOF(t-1) is the fraction of oxygen in the ullage gas. It is calculated by 

dividing the mass of oxygen in the tank at t-1 by the mass of gas in the tank ullage or: 

( 1) ( 1)/ ( 1) (5) O2UGOF t − = m t − mTank t −

Where: ( ) = Mass of oxygen in tank at time t O2

m t

 m& = Mass flow rate of inerting gas (in terms of t) 

 IGOF = Fraction of oxygen in inerting gas 

 ∆ρ = Change in ullage density due to altitude change 

 VTank = Volume of tank ullage

 mTank = Mass of gas in tank 

 mair = Mass of air entering tank 

Equation 4 can be simplified by term as the mass of oxygen in the tank at time t is equal to the 

mass of oxygen in the tank at time (t-1), plus the mass of oxygen being deposited by the inert 

gas, minus the mass of oxygen being vented due to depositing inert gas, minus the mass of 

oxygen being vented due to changing altitude (ascent), plus the mass of oxygen being deposited 

due to changing altitude (descent). 

To compare the model with the data acquired, the average oxygen concentration in the fuel tank 

was calculated by taking the normal average of the eight sample ports in the tank. When one or 

more sample ports were not available, the average was simply calculated without that sample 

location. 

11
5. DISCUSSION OF RESULTS. 

The results of the flight tests are presented for two areas of analysis: system performance and 

fuel tank inerting. 

5.1 SYSTEM PEFORMANCE. 

The system performance data presented here focuses on the volume and purity (oxygen 

concentration) of inert gas generated by the system, given the aircraft flight profile and bleed air 

pressure (ASM feed pressure). ASM pressure was correlated with NEA flow for tests using both 

the one- and two-membrane configurations in the inerting system. Membrane performance 

characteristics were also examined, illustrating bleed air used that was also correlated with 

pressure altitude. Additionally, membrane performance degradation was calculated for one 

ASM before and after the flight tests. 

5.1.1 Air Separation Module Performance Characteristics.

The primary factors influencing the performance of an ASM is feed pressure, permeate pressure 

(pressure altitude), and membrane backpressure (orifice setting). Given the fixed orifice settings 

used for the flight test for each system configuration, ASM inlet pressure and pressure altitude 

are the parameters most affecting system performance. Figure 7 shows the system flow and 

purity with ASM pressure and flight altitude for a flight test using a single membrane to inert the 

aircraft CWT over a given flight profile. This illustrates the effect of ASM inlet pressure on flow

for the given flight cycle. 

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100 120

Time (min)

[O2] (% vol)

0

10

20

30

40

50

60

Alt (kft) /

Pressure (psi) / Flow (scfm) 

NEA Line O2 (%)

ASM Inlet Pressure (Psig)

NEA Flow (SCFM)

Alt (kft)

Single Membrane Test

FIGURE 7. SYSTEM PERFORMANCE DATA FOR A SINGLE-MEMBRANE TEST 

12
In general, increasing ASM inlet pressure will increase NEA flow and purity (decreasing oxygen 

concentration) simultaneously, given a fixed outflow orifice setting. Decreasing atmospheric 

pressure during ascent will increase the separating efficiency of the membrane module, having

the effect of decreasing the flow and increasing the purity. During the descent, a high-flow mode 

is selected, giving an instantaneous increase in flow and decrease in purity. As the atmospheric 

pressure rises, flow increases and purity decreases. 

5.1.2 Effect of Varying Number of Air Separation Modules.

Similar tests were performed with the system using both one- and two-ASM configurations. 

These results were compared for consistency. The high- and low-flow orifices were fixed 

differently for the two-system configuration to give essentially the same system purity, given the 

flight profile. This should allow the two-ASM system to give twice the flow as the one-ASM

configuration. Figure 8 compares the ASM flow and purity for the two different system

configurations for the ascent to cruise portion of the tests. The ASM inlet pressure is also 

shown. 

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50

Time (min)

Flo

w (scfm) / [O2] (% vol)

0

10

20

30

40

50

60

Pressure (psi

g)

NEA Flow (SCFM) NEA Flow (SCFM)

NEA Line O2 (%) NEA Line O2 (%)

ASM Inlet Pressure (Psig) ASM Inlet Pressure (Psig)

1 Membrane 2 Membranes

FIGURE 8. SYSTEM FLOW AND PURITY WITH ASM PRESSURE FOR BOTH 

MEMBRANE CONFIGURATIONS DURING ASCENT AND CRUISE 

Figure 9 gives the same comparison for the descent portion of the tests, where the system is 

switched to high-flow mode. This segment required time to be adjusted for the two-membrane 

test to allow for a side-by-side comparison of the data. Both graphs illustrate that the ASM inlet 

pressure and resulting purity were very similar for the relevant segments of the flight, while the 

NEA flow was approximately double for the two-membrane configuration, as expected. It is

unclear why the two-membrane system flow at times is considerably less than double 

(approximately 50%) the single-membrane configuration. This could be due to the inefficiencies

13
of the permeate venting, because high OEA venting backpressure was measured on the twomembrane test. The result from this backpressure would be to reduce the ASMs ability to flow 

permeate, decreasing the overall efficiency and total flow out of the ASM. As the system is 

switched to high-flow mode in descent for both configurations, the NEA flow tends to be more

representative of the expected relationship. This is consistent with the analysis of the problem in 

that the effect of decreasing the ASM inlet pressure would be less permeate flow and, thus, less 

permeate backpressure. The problem is observed at cruise and higher altitudes, which is also the 

point at which permeate backpressure was the greatest. This conclusion is also supported by the 

measured permeate backpressure of the two-membrane configuration, which was about three

times the backpressure of the single-membrane configuration. 

0

2

4

6

8

10

12

14

16

18

20

70 72 74 76 78 80 82 84 86 88 90

Time (min)

Oxygen Concentration (%vol)

0

5

10

15

20

25

30

35

40

45

50

Flow (scfm) / Pressure (Psig)

NEA Line O2 (%) NEA Line O2 (%)

ASM Inlet Pressure (Psig) ASM Inlet Pressure (Psig)

NEA Flow (SCFM) NEA Flow (SCFM)

1 Membrane 2 Membranes

FIGURE 9. SYSTEM FLOW AND PURITY WITH ASM PRESSURE FOR BOTH 

MEMBRANE CONFIGURATIONS DURING DESCENT 

The relationship between ASM pressure and NEA flow shown in figures 8 and 9 are correlated 

in figure 10. Again, this correlation is only valid for a similar flight profile with bleed air 

schedule, as NEA flow is a strong function of pressure altitude. This is why the cruise curves 

indicate a much lesser slope than the ascent and descent curves. The changing altitude causes 

vast changes in membrane permeability and a wider range in NEA flows, while the cruise 

portion of the flight exhibits no altitude change effects and, thus, gives a true correlation between 

pressure and flow with the given low-flow fixed orifice at 39,000 feet. The ascent and descent

portions of the graph illustrates the large increase in normalized NEA flow from the system, 

given a relatively small change in ASM pressure, exhibiting a benefit of using HFM technology 

to generate inert gas at altitude. Only selected data for each flight phase were plotted because

widely varying throttle settings produce rapid changes in the system causing uncorrelated data 

for brief periods due to delays in some instrumentation. 

14
0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45 50

ASM Pressure (psig)

NEA Flow (SCFM)

 Accent - Two Membranes

 Cruise - Two Membranes

 Descent - Two Membranes

 Ascent - One Membrane

 Cruise - One Membrane

 Descent - One Membrane

Correlation of ASM Pressure and Flow

FIGURE 10. CORRELATION OF ASM PRESSURE WITH NEA FLOW FOR BOTH 

SYSTEM MEMBRANE CONFIGURATIONS 

5.1.3 Bleed Air Consumption.

To help determine the feasibility and practicality of an ASM inerting system for a commercial 

transport airplane, the bleed air consumed by the system during the flight profile was calculated 

for a test with the system configured with a single membrane. Figure 11 shows the measured 

NEA flow, NEA oxygen concentration, and OEA oxygen concentration used to calculate system 

bleed airflow. Figure 11 illustrates that as NEA oxygen concentration decreases, OEA oxygen 

concentration decreases. This nonintuitive relationship is the result of the characteristics of the 

ASM that generates more pure NEA by essentially venting greater quantities of OEA, more of 

which is nitrogen as compared to the ASM operating at lower NEA concentrations. 

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100 120

Time (min)

Flow Rate (SCFM)

0

5

10

15

20

25

30

35

40

Oxygen Concentration (% vol)

NEA Flow (SCFM)

NEA Line O2 (%)

OEA Line O2 (%)

Single Membrane Test Data

FIGURE 11. MEASURED MEMBRANE CHARACTERISTICS FOR THE SINGLEMEMBRANE CONFIGURATION SYSTEM 

15
Figure 12 gives the calculated bleed airflow and permeate flow of the system for the same singlemembrane inerting flight test. The graph illustrates the effect of increased recovery of the ASM 

with altitude and the effect of venting more nitrogen, as stated in the previous paragraph. In the 

beginning of the test, at sea level, the ASM generated 5 standard cubic feet per minute (SCFM) 

of NEA at 5% oxygen concentration and was consuming 14 SCFM of bleed air. At cruise, the 

system is generating 4 SCFM of less than 1% NEA but was consuming almost 50 SCFM of

bleed air. The increased performance of the ASM at altitude allowed excellent production of 

NEA, especially when considering the effect of NEA on inerting the tank is five-fold greater at

39,000 ft than at sea level. However, the system discarded more than three times the amount of 

bleed air. 

0

10

20

30

40

50

60

0 20 40 60 80 100

Time (min)

Flow Rate (SCFM)

120

NEA Flow (SCFM)

Bleedair Flow (SCFM)

Permeate Flow (SCFM)

Single Membrane Data

FIGURE 12. CALCULATED BLEED AIRFLOW INTO THE SINGLE-MEMBRANE 

CONFIGURATION SYSTEM 

The effect of altitude on bleed air was examined by correlating bleed airflow with atmospheric

pressure during the accent portion of the test. Figure 13 illustrates that the bleed airflow more 

than doubles as altitude increases, although the grouping of data points in a lateral manner 

illustrates dependence on another parameter. This is most likely due to changing throttle settings

(ASM pressure) during the aircraft climb to cruise. 

16
0

5

10

15

20

25

30

35

40

45

50

0 100 200 300 400 500 600 700 800 900 1000

Atmo Pressure (mBar)

Bleedair Flow (S

CFM)

 Ascent

FIGURE 13. CORRELATION OF BLEED AIRFLOW AND ATMOSPHERIC PRESSURE 

FOR A SINGLE-MEMBRANE TEST 

5.1.4 Membrane Performance Degradation.

An ASM can be sensitive to contaminents in the air source. These contaminents can decrease

the effectiveness or even damage the fibers in the ASM, degrading the ASM performance over 

time exposure to these contaminents. As previously stated, the FAA OBIGGS has a filter to 

remove particulate, but how effective this type of filter is at protecting the ASM is unknown. 

Most hollow fiber is intollerant of hydrocarbon vapor in general and the amount of hydrocarbon 

vapor in the bleed air and the filter’s ability to remove it is in question. 

To determine the effect of aircraft bleed air systems on an ASM, a single-ASM used during 

ground and flight tests was subjected to a single test point before and after the tests. The NEA 

and ASM inlet (bleed) flow were measured for a given NEA purity at a specific inlet pressure

and temperature at sea level. Table 2 shows the difference in ASM performance before and after 

the test. The 14 percent drop in NEA produced is more than the expected decrease in

productivity due to normal break-in of the ASM over the first hours of service. 

TABLE 2. AIR SEPARATION MODULE PERFORMANCE DATA BEFORE AND AFTER 

100 HOURS OF AIRCRAFT OPERATION 

Membrane 

State 

ASM Inlet 

Temperature 

ASM Inlet 

Pressure Altitude

NEA 

Purity NEA Flow

ASM Inlet 

Flow 

Before 172°F 25 psig Sea Level 5% 0.36 lbs/min 1.48 lbs/min

After 169°F 25 psig Sea Level 5% 0.31 lbs/min 1.25 lbs/min

Difference 0.05 lbs/min 0.23 lbs/min

17
5.2 FUEL TANK INERTING.

The fuel tank inerting results illustrates the effect that system performance has on the fuel tank in 

terms of oxygen concentration achieved during the flight test profile. This data will be compared

to a model that calculates the fuel tank oxygen concentration, given the system performance and 

flight cycle. The effect of high-flow mode and fuel tank fuel load on the ability of the system to 

inert the tank was also examined. 

5.2.1 Typical Mission Profile.

To determine the variation of oxygen concentration within the tank during a typical flight, the 

results of the eight gas sample ports in the CWT were plotted with the flight profile for the total 

test (figure 14). This test (see table 1—Airbus 1972) employed the inerting system with the 

dual-mode operation and the single ASM configuration. The results show a small variation in

the oxygen concentration in the tank for most of the mission with some noticeable, but small,

variation among the measurements during the ascent and descent phase of the flight. As 

previously stated, the Airbus A320 tank design is a simple rectangular box, with no web 

structures to compartmentalize the tank, as is the case with most large commercial transport 

aircraft manufactured in the United States. The only appreciable difference any oxygen 

concentration measurement has from the collective average is sample port 1 during the descent. 

This sample port is very close to the vent port, which is depositing air into the tank during 

descent. This sample port gives an excellent visual indication of what altitude the tank stops 

“breathing” air in as a result of the inerting system flow into the tank. Virtually every flight test

exhibits this same characteristic appearance, illustrating no observed conditions that would give 

localized heterogeneous oxygen concentrations. 

0

5

10

15

20

25

0 20 40 60 80 100 120

Time (min)

[O2] (% vol)

0

5

10

15

20

25

30

35

40

45

Altitude (kft)

O2 Sample 1 (%)

O2 Sample 2 (%)

O2 Sample 3 (%)

O2 Sample 4 (%)

O2 Sample 5 (%)

O2 Sample 6 (%)

O2 Sample 7 (%)

O2 Sample 8 (%)

Alt (kft)

Single Membrane Test

FIGURE 14. OXYGEN CONCENTRATION VARIATION IN THE CWT DURING A 

TYPICAL FLIGHT TEST 

18
The average oxygen concentration of the ullage was plotted for the baseline configuration test

(figure 14) as well as for the two-membrane configuration test, which is compared in figure 15. 

This illustrates the effect of increased system size on the ability of the system to maintain an

inert ullage in the CWT of the flight test aircraft compared to the single-membrane test for a 

comparable flight profile. This flight profile had the aircraft descend from 39,000 feet to 3,000 

feet in 9 minutes (a constant 4,000 ft/min descent), which represents the high limit of 

commercial airplane descents not executed for emergency purposes. Section 5.1.2 stated that 

system flow rate performance was more than 50% greater in low-flow mode and double in highflow mode. This drastic increase in system performance has the effect of decreasing the 

resulting average ullage oxygen concentration approximately 1%, while decreasing the 

maximum average oxygen concentration achieved during the descent by 2%, from 12 to 10 

percent oxygen by volume. This illustrates the diminishing returns of increasing the system size 

drastically for this inerting methodology. 

0

5

10

15

20

25

0 20 40 60 80 100

Time (min)

Oxygen Concentration (% vol)

120

 One Membrane

 Two Membrane

Average Tank [O2]

FIGURE 15. COMPARISON OF THE SINGLE-MEMBRANE INERTING FLIGHT 

TEST TANK ULLAGE OXYGEN CONCENTRATION WITH THE 

TWO-MEMBRANE CONFIGURATION 

Generally, the system scaling (size and amount of ASMs) should be reflected in the tuning of the 

system. In the above-stated case with two ASMs, the high-flow mode was so overwhelming that

very little air entered the tank, and the increase in oxygen concentration is essentially due to the 

increase oxygen concentration of the NEA “deinerting” the tank during descent. For both 

membrane configurations, the system flow control valves were set to make as much 12% NEA 

at the bottom of the descent as possible. Given the increased capability of the two-membrane 

configuration, it is likely that setting the flow control valves to make a lower oxygen 

concentration NEA at the bottom of the descent would have a positive effect on resulting ullage 

oxygen concentration. Although more air would enter the tank during the initial part of descent, 

19
the ullage would benefit from a net average lower oxygen concentration being deposited during 

the descent. This is an example of how a different tuning (flow control valve settings) may help 

increase the effectiveness of this particular system methodology (two flow modes) with the 

greater system performance of the two-membrane configuration. More research is required to 

evaluate the flexibility of this configuration to be applied to a variety of inerting situations. 

Figure 16 illustrates the calculated and measured average ullage oxygen concentration for the 

single-membrane inerting flight test with a zero fuel load using the duel-mode inerting 

methodology and the single-membrane system configuration. Even with the relative simplicity 

of the analytical model, it has no trouble illustrating both the trend of the ullage oxygen 

concentration, given the dynamic conditions, as well as the peak and resulting values. This 

model method can be used to analyze the ability of a different system performance or the

effectiveness of another system’s operational methodology in inerting a given ullage space. 

0

5

10

15

20

25

0 20 40 60 80 100 120

Time (min)

Oxygen Concentration (% vol)

0

10

20

30

40

50

Altitude (kft)

 Flight Test Data

 Model Data

 Altitude

Single Membrane Test

FIGURE 16. COMPARISON OF THE SINGLE-MEMBRANE INERTING FLIGHT TEST 

AVERAGE TANK ULLAGE OXYGEN CONCENTRATION WITH THE MODEL DATA 

5.2.2 Effect of High-Flow Mode.

To illustrate the effect of the dual-flow configuration on the inerting capability of the FAA

OBIGGS in general, a flight test was performed that did not employ the high-flow mode on 

descent. Figure 17 illustrates the CWT average oxygen concentration during descent as a result 

of not employing the high-flow mode compared to the single ASM test data given in figure 14. 

The resulting ullage oxygen concentration is almost 2 percent higher than when employing the

high-flow mode on descent. Additionally, the maximum average oxygen concentration obtained 

in the ullage during descent was 3 percent higher than when employing the high-flow mode on 

descent, reaching a peak of 15 percent oxygen by volume. This illustrates the improved 

capability of a system using a dual-mode configuration, given the same flight profile and

baseline (low flow) system performance. 

20
0

5

10

15

20

25

60 65 70 75 80 85 90 95 100 105

Time (min)

Oxygen Concentration (% vol)

 High Flow Descent

 Low Flow Descent

Average Tank [O2]

Single Membrane

FIGURE 17. COMPARISON OF THE SINGLE-MEMBRANE INERTING FLIGHT 

TEST AVERAGE TANK OXYGEN CONCENTRATION FOR BOTH HIGH-FLOW 

DESCENT AND LOW-FLOW DESCENT 

5.2.3 Effect of Fuel on Inerting.

To examine the effect of CWT fuel on the inerting process for an operational aircraft, two flight 

tests were performed with fuel in the tank. Both tests used the FAA OBIGGS in the duel-flow

mode with the single-membrane configuration, allowing comparison with the baseline singlemembrane, zero fuel test (figure 14). To examine the effect of air potentially evolving from a

fuel load, a test was performed that replicated the baseline, zero fuel flight profile, but with a 

static (not used) 1.5-metric ton (t) fuel load in the CWT, making the tank approximately 23% 

full. Figure 18 shows the results of this flight test compared to the empty tank baseline data. 

This illustrates that the tank is slightly easier to inert and has a slightly lower resulting oxygen 

concentration. This is due to the decreased ullage size, as a result of the fuel load. 

The same flight profile was also repeated with a 3-t fuel load (46% full) that is used in the 

conventional manner (burned first). Figure 19 illustrates the average ullage oxygen 

concentration compared with the empty tank baseline data. Again, the two tests are very similar, 

illustrating no potential problems for the FAA OBIGGS concept with using CWT fuel. Some

small anomalies exist in the average oxygen concentration for the consumed fuel test that could 

be the result of air evolving from the fuel. Also, the tank should inert slightly differently because

burned fuel increases the size of the ullage during the inerting process. 

21
0

5

10

15

20

25

0 20 40 60 80 100 120

Time (min)

Oxygen Concentration (% vol)

 Empty Tank

 Constant Fuel Load

Average Tank [O2]

Single Membrane

FIGURE 18. COMPARISON OF THE SINGLE-MEMBRANE INERTING FLIGHT 

TEST AVERAGE TANK OXYGEN CONCENTRATION FOR BOTH EMPTY 

TANK AND A CONSTANT FUEL LOAD 

0

5

10

15

20

25

0 20 40 60 80 100 120

Time (min)

Oxygen Concentration (% vol)

 Empty Tank

 Consumed Fuel Load

Average Tank [O2]

Single Membrane

FIGURE 19. COMPARISON OF THE SINGLE-MEMBRANE INERTING FLIGHT 

TEST AVERAGE TANK OXYGEN CONCENTRATION FOR BOTH EMPTY 

TANK TESTS AND CONSUMED FUEL LOAD 

22
6. SUMMARY OF FINDINGS. 

The results of the tests indicated that the concept of the simplified inerting system is valid and 

that the air separation module (ASM) dynamic characteristics were as expected for the limited 

discussed test plan. Both one- and two-ASM configuration tests gave the expected performance

with ASM pressure having the expected affect on flow rate, and the duel-flow performance 

having the predicted effect on the ullage oxygen concentration. Bleed air consumption was 

greater than expected. Additional research is needed to determine what changes in system 

design or operational methodology would best reduce the bleed airflow and the associated cost. 

The effect of aircraft bleed air on the performance degradation of an ASM needs to be studied 

further. 

The fuel tank inerting results illustrated that no stratification or heterogeneous oxygen 

concentrations occurred in the tank for the inerting tests performed. The essentially rectangular 

box configuration of the tank allowed easy distribution of the inert gas. The measured effect of 

the high-flow mode was significant, allowing the single-membrane configuration to maintain an 

inert ullage during the entire flight cycle, even with the abnormally high rate of descent 

employed for the flight test. When the high-flow mode was not used, the ullage reached a peak 

of 15% oxygen by volume. The fuel quantity in the fuel tank had virtually no effect on the 

resulting oxygen concentrations observed during the tests. 

7. REFERENCES. 

1. Summer, Steven M., “Limiting Oxygen Concentration Required to Inert Jet Fuel Vapors

Existing at Reduced Fuel Tank Pressures,” FAA Report DOT/FAA/AR-TN02/79, August 

2003. 

2. Burns, Michael and Cavage, William M., “Inerting of a Vented Aircraft Fuel Tank Test 

Article With Nitrogen-Enriched Air,” FAA Report DOT/FAA/AR-01/6, April 2001. 

3. Burns, Michael and Cavage, William M., “Ground and Flight Testing of a Boeing 737

Center Wing Fuel Tank Inerted With Nitrogen-Enriched Air,” DOT/FAA/AR-01/63, 

August 2001. 

4. Anderson, C. L., “Volume III—On-Board Inert Gas Generation System (OBIGGS) 

Studies, Part 1—OBIGGS Ground Performance Tests,” Aero Propulsion Laboratory 

Report AFWAL-TR-85-2060, January 1986. 

5. Clodfelter, R. G., Anderson, C. L., and Vannice, W. L., “OBIGGS for Fighter Aircraft,” 

SAE Technical Paper 871903, Aerospace Technology Conference and Exposition, Long 

Beach, California, October 5-8, 1987. 

23
6. “A Review of the Flammability Hazard of Jet A Fuel Vapor in Civil Transport Aircraft

Fuel Tanks,” Fuel Flammability Task Group, FAA Report DOT/FAA/AR-98/26, June 

1998. 

7. “Fuel Tank Harmonization Working Group Final Report,” Aviation Rulemaking 

Advisory Committee, July 1998. 

8. Burns, Michael and Cavage, William M., “A Description and Analysis of the FAA 

Onboard Oxygen Analysis System,” FAA Report DOT/FAA/AR-TN03/52, June 2003. 

24
APPENDIX A—THE FAA OBIGGS ENGINEERING DRAWING

A-1/A-2
Static Pressure (282) 

Note:

Instrumentation Identified with Parenthetic Numbers Cited in Airbus Z-Heat Exchanger Exhaust Deposited in Rear of Cargo Bay via 4” Flexible Flexible Copuling

NEA [O2] (28-73)Temperature (28-15)

Static Pressure (28-14)

Spare Port

Temperature (28-9)

Static Pressure (28-8) 

Interface Description:Interface 1: 2” Flexible Coupling Interface 2: 2” Dia SS Pipe with 4-Bolt flange with Face Mating Seal Interface 3: 1” Dia Al tube with Hydra-flow 14T02 Sleeve, Coupling, Seal, and Clamp Assy 

Interface 3: System NEA Output

Heat Exchanger Cooling Air Exhaust(No Interface)

Interface 2: System Bleed Air Connection

Temperatu (28-4)

Interface 1: OEA Exhaust

OEA [O2] (28-74)

Temperature  (28-5)

Temperature (28-1)

Heat Exchanger Cooling Air Inlet(No Interface)
APPENDIX B—FOUR-BOLT FLANGE ENGINEERING SPECIFICATION

B-1/B-2
APPENDIX C—THE FAA OBIGGS WIRING DIAGRAM

C-1/C-2

APPENDIX D—CALCULATION OF BLEED AIR CONSUMPTION

Given bleed airflow QBleed

, NEA flow , and Permeate (OEA) flow Q for a given ASM,

intuitively the following equation applies:

& QNEA

& Perm

&

QBleed QNEA QPerm & & & = +

or, 

 (D-1) QPerm QBleed QNEA & & & = −

Also, a mole balance of oxygen for the same ASM yields the following equation. 

 Moles of O2 in Air = Moles of O2 in NEA + Moles of O2 in Permeate 

. QBleed NEA QNEA Perm QPerm (D-2) & & & (0 21) = [O ] ⋅ + [O ] ⋅ 2 2

Where: [O ] NEA = NEA Oxygen Concentration 2

 = OEA Oxygen Concentration Perm [O ] 2

Combining equations 1 and 2 and solving for QBleed gives: &

(0.21) [O ] [O ] ( ) QBleed 2 NEA QNEA 2 Perm QBleed QNEA & & & & = ⋅ + ⋅ −

QBleed NEA QNEA Perm QBleed Perm QNEA & & & & (0.21) = [O ] ⋅ + [O ] ⋅ − [O ] ⋅ 2 2 2

QBleed Perm QBleed NEA QNEA Perm QNEA & & & & (0.21) − [O ] ⋅ = [O ] ⋅ − [O ] ⋅ 2 2 2

(0.21 [O ] ) ([O ] [O ] ) QBleed 2 Perm QNEA 2 NEA − 2 Perm ⋅ − = ⋅ & &

(0.21 [O ] )

([O ] [O ] )

2

2 2

Perm

NEA Perm QBleed QNEA −

− = ⋅ & & (D-3)

This equation allows for the calculation of the bleed air consumed by an ASM based inerting 

system given the NEA flow and purity as well as the oxygen concentration of the permeate. 

D-1/D-2