Subject: OPERATIONS OF AIRCRAFT AT Date: 1/2/03 AC No: 61-107A 

ALTITUDES ABOVE 25,000 FEET MSL AND/OR

MACH NUMBERS (MMO) GREATER THAN .75 Initiated by: AFS-820 Change: 

1. PURPOSE. This advisory circular (AC) is issued to alert pilots who are transitioning from 

aircraft with less performance capability to complex, high-performance aircraft that are capable of

operating at high altitudes and high airspeeds, of the need to be knowledgeable about the special 

physiological and aerodynamic considerations involved in these kinds of operations. 

2. CANCELLATION. AC 61-107, Operations of Aircraft at Altitudes Above 25,000 Feet MSL

and/or Mach Numbers (Mmo) Greater Than .75, dated January 23, 1991, is cancelled. 

3. DEFINITIONS. 

a. Aspect Ratio is the relationship between the wing chord and the wingspan. A short wingspan 

and wide wing chord equal a low aspect ratio. 

b. Aileron buzz is a very rapid oscillation of an aileron, at certain critical air speeds of some 

aircraft, which does not usually reach large magnitudes nor become dangerous. It is often caused by

shock-induced separation of the boundary layer. 

c. Drag Divergence is a phenomenon that occurs when an airfoil’s drag increases sharply and 

requires substantial increases in power (thrust) to produce further increases in speed. This is not to 

be confused with MACH crit. The drag increase is due to the unstable formation of shock waves 

that transform a large amount of energy into heat and into pressure pulses that act to consume a 

major portion of the available propulsive energy. Turbulent air may produce a resultant increase in 

the coefficient of drag. 

d. Force is generally defined as the cause for motion or of change or stoppage of motion. The 

ocean of air through which an aircraft must fly has both mass and inertia and, thus, is capable of 

exerting tremendous forces on an aircraft moving through the atmosphere. When all of the above 

forces are equal, the aircraft is said to be in a state of equilibrium. For instance, when an aircraft is 

in level unaccelerated 1 G flight, thrust and drag are equal, and lift and gravity (or weight plus 

aerodynamic downloads on the aircraft) are equal. Forces that act on any aircraft as the result of air 

resistance, friction, and other factors are: 
AC 61-107A 1/2/03 

 (1) Thrust. The force required to counteract the forces of drag in order to move an aircraft 

in forward flight. 

 (2) Drag. The force which acts in opposition to thrust. 

 (3) Lift. The force which sustains the aircraft during flight. 

 (4) Gravity. The force which acts in opposition to lift. 

e. MACH, named after Ernst Mach, a 19th century Austrian physicist, is the ratio of an 

aircraft’s true speed as compared to the local speed of sound at a given time or place. 

f. MACH Buffet is the airflow separation behind a shock-wave pressure barrier caused by

airflow over flight surfaces exceeding the speed of sound. 

g. MACH (or Aileron) Buzz is a term used to describe a shock-induced flow separation of the 

boundary layer air before reaching the ailerons. 

h. MACH Meter is an instrument designed to indicate MACH number. MACH indicating

capability is incorporated into the airspeed indicator(s) of current generation turbine-powered

aircraft capable of MACH range speeds. 

i. MACH number is a decimal number (M) representing the true airspeed (TAS) relationship to 

the local speed of sound (e.g., TAS 75 percent (.75M) of the speed of sound where 100 percent of 

the speed of sound is represented as MACH 1 (1.0M)). The local speed of sound varies with 

changes in temperature. 

j. MACH number (Critical) is the free stream MACH number at which local sonic flow such as 

buffet, airflow separation, and shock waves becomes evident. These phenomena occur above the 

critical MACH number, often referred to as MACH crit. These phenomena are listed as follows:

(1) SUBSONIC MACH Numbers below .75 

(2) TRANSONIC MACH Numbers from .75 to 1.20 

(3) SUPERSONIC MACH Numbers from 1.20 to 5.0 

(4) HYPERSONIC MACH Numbers above 5.0 

 k. MACH Speed is the ratio or percentage of the TAS to the speed of sound (e.g., 1,120 feet per 

second (660 Knots (Kts)) at mean sea level (MSL)). This may be represented by MACH number. 

 l. MACH Tuck is the result of an aftward shift in the center of lift causing a nose down pitching

moment. 

 m. MMO (MACH; maximum operation) is an airplane’s maximum certificated MACH number. 

Any excursion past MMO, whether intentional or accidental, may cause induced flow separation of 

boundary layer air over the ailerons and elevators of an airplane and result in a loss of control 

surface authority and/or control surface buzz or snatch. 

Page Par 3 ii
1/2/03 AC 61-107A 

 n. Q-Corner or Coffin Corner is a term used to describe operations at high altitudes where low 

indicated airspeeds yield high true airspeeds (MACH number) at high angles of attack. The high

angle of attack results in flow separation which causes buffet. Turning maneuvers at these altitudes 

increase the angle of attack and result in stability deterioration with a decrease in control 

effectiveness. The relationship of stall speed to MACH crit narrows to a point where sudden 

increases in angle of attack, roll rates, and/or disturbances (e.g., clear air turbulence) cause the limits 

of the airspeed envelope to be exceeded. Coffin Corner exists in the upper portion of the 

maneuvering envelope for a given gross weight and G-force. 

o. VMO (Velocity maximum operation) is an airplane’s indicated airspeed limit. Exceeding VMO

may cause aerodynamic flutter and G-load limitations to become critical during dive recovery. 

Structural design integrity is not predictable at velocities greater than VMO.

4. BACKGROUND. On September 17, 1982, the National Transportation Safety Board (NTSB)

issued a series of safety recommendations which included, among other things, that a minimum 

training curriculum be established for use at pilot schools covering pilots initial transition into 

general aviation turbojet airplanes. Aerodynamics and physiological aspects of high-performance 

aircraft operating at high altitudes were among the subjects recommended for inclusion in this 

training curriculum. These recommendations were the result of an NTSB review of a series of fatal 

accidents which were believed to involve a lack of flightcrew knowledge and proficiency in general 

aviation turbojet airplanes capable of operating in a high-altitude environment. Although the near 

total destruction of physical evidence and the absence of installed flight recorders have inhibited 

investigators' abilities to pinpoint the circumstances which led to these accidents, the NTSB is 

concerned that a lack of flightcrew knowledge and proficiency in the subject matter of this AC was 

involved in either the initial loss of control or the inability to regain control of the aircraft, or both. 

A requirement has been added to Title 14 of the Code of Federal Regulations (14 CFR) part 61 for

high-altitude training of pilots who transition to any pressurized airplane that has a service ceiling or

maximum operating altitude, whichever is lower, above 25,000 feet MSL. Recommended training

in high altitude operations that would meet the requirements of this regulation can be found in 

chapter 1, Recommendations for High-Altitude Training. On December 20, 2000, the NTSB issued 

another series of safety recommendations in response to a Learjet 35 accident on October 25, 1999, 

which occurred after a prolonged period of pilot incapacitation. This AC is updated to address the 

NTSB recommendation that the FAA revise existing guidance material concerning time of useful 

consciousness, hypoxia awareness training. 

5. DISCUSSION. 

a. Title 14 CFR part 61 prescribes the knowledge and skill requirements for the various airman 

certificates and ratings, including category, class, and type ratings authorized to be placed thereon. 

The civil aircraft fleet consists of numerous aircraft capable of high-altitude flight. Certain 

knowledge elements pertaining to high-altitude flight are essential for the pilots of these aircraft. 

Pilots who fly in this realm of flight must receive training in the critical factors relating to safe flight 

operations at high-altitudes. These critical factors include knowledge of the special physiological 

and/or aerodynamic considerations which should be given to high-performance aircraft operating in 

the high-altitude environment. High-altitude flight has different effects on the human body than 

those experienced in lower altitude flight. The aircraft's aerodynamic characteristics in high-altitude

flight may differ significantly from those in lower altitude flight. 

Par 3 Page iii
AC 61-107A 1/2/03 

b. Pilots who are not familiar with operations in the high altitude and high-speed environment 

are encouraged to obtain thorough and comprehensive training and a checkout in complex highperformance aircraft before engaging in extensive high-speed flight in such aircraft, particularly at 

high altitudes. The training should enable the pilot to become thoroughly familiar with aircraft 

performance charts and aircraft systems and procedures. The more critical elements of high-altitude

flight planning and operations should also be reviewed. The aircraft checkout should enable the 

pilot to demonstrate a comprehensive knowledge of the aircraft performance charts, systems, 

emergency procedures, and operating limitations, along with a high degree of proficiency in 

performing all flight maneuvers and in-flight emergency procedures. By attaining such knowledge 

and skill requirements of high-performance aircraft, the pilot’s preparedness to transition to the

operation of aircraft in the high-speed environment and high-altitude flight, should enhance their 

awareness on safe and efficient operation. 

6. SUMMARY. It is beyond the scope of this AC to provide a more definitive treatment of this 

subject. This AC serves its purpose if it aids pilots in becoming familiar with the basic phenomena

associated with high-altitude and high-speed flight. Pilots should recognize that greater knowledge 

and skills are needed for the safe and efficient operation of state-of-the-art turbine-powered aircraft

at high altitude. Pilots are strongly urged to pursue further study from the many excellent textbooks, 

charts, and other technical reference materials available through industry sources. From these 

sources pilots will obtain a detailed understanding of both physiological and aerodynamic factors 

which relate to the safe and efficient operation of the broad variety of high-altitude aircraft available

today and envisioned for the future. 

/s/ 

Louis C. Cusimano for 

James J. Ballough 

Director, Flight Standards Service 

Page Par 5 iv
1/2/03 AC 61-107A 

CONTENTS 

CHAPTER 1. RECOMMENDATIONS FOR HIGH-ALTITUDE TRAINING

Paragraph Page

100. Purpose .......................................................................................................... 1 

101. Outline ........................................................................................................... 1 

102. Ground Training............................................................................................. 3 

103. The High-Altitude Flight Environment.......................................................... 4 

104. Weather .......................................................................................................... 5 

105. Flight Planning and Navigation...................................................................... 6 

106. Physiological Training.................................................................................... 8 

107. Additional Physiological Training ............................................................... 12 

108. High-Altitude Systems and Equipment........................................................ 14 

109. Aerodynamics and Performance Factors...................................................... 17 

110. Emergencies and Irregularities at High Altitudes......................................... 18 

111. Flight Training.............................................................................................. 20 

112. - 199. Reserved.............................................................................................. 22 

CHAPTER 2. MACH FLIGHT AT HIGH ALTITUDES

200. Purpose ......................................................................................................... 23 

201. Critical Aspects of Mach Flight ................................................................... 23 

202. Aircraft Aerodynamics and Performance ..................................................... 25 

203. - 299. Reserved.............................................................................................. 29 

Page v (and vi)

1/2/03 AC 61-107A 

CHAPTER 1. RECOMMENDATIONS FOR HIGH-ALTITUDE TRAINING

100. PURPOSE. This chapter presents an outline for recommended high-altitude training that

meets the requirements of 14 CFR part 61, section 61.31(g). The actual training, which may be 

derived from this outline, should include both ground and flight training in high-altitude operations. 

Upon completion of the ground and flight training, the flight instructor who conducted the training

should provide an endorsement in the pilot’s logbook or training record, certifying that training in 

high-altitude operations was given. A sample high-altitude endorsement is available in the most 

recent version of AC 61-65, Certification: Pilots and Flight and Ground Instructors. 

a. Although 14 CFR section 61.31(g) applies only to pilots who fly pressurized airplanes with a 

service ceiling or maximum operating altitude above 25,000 feet MSL, whichever is lower, this 

training is recommended for all pilots who fly at altitudes above 10,000 feet MSL.

(1) A service ceiling is the maximum height above MSL at which an airplane can maintain a 

rate of climb of 100 feet per minute under normal conditions. 

(2) All pressurized aircraft have a specified maximum operating altitude above which 

operation is not permitted. This maximum operating altitude is determined by flight, structural, 

powerplant, functional, or equipment characteristics. An airplane’s maximum operating altitude is 

limited to 25,000 feet or lower, unless certain airworthiness standards are met. 

(3) Maximum operating altitudes and service ceilings are specified in the Aircraft Flight 

Manual (AFM). 

b. The training outlined in this chapter is designed primarily for single engine and light twinengine airplanes that fly at high altitudes but do not require type ratings. The training should, 

however, be incorporated into type rating courses for aircraft that fly above 25,000 feet MSL if the 

pilot has not already received training in high-altitude flight. The training in this chapter does not 

encompass high-speed flight factors such as acceleration, G-forces, MACH, and turbine systems 

that do not apply to reciprocating engine and turboprop aircraft. Information on high-speed flight 

can be found in chapter 2, MACH Flight at High Altitudes. 

101. OUTLINE. Additional information should be used to complement the training provided 

herein. The training, which is outlined below and explained in further detail throughout the 

remainder of this chapter, covers the minimum information needed by pilots to operate safely at

high altitudes. 

a. Ground Training.

(1) The High Altitude Flight Environment. 

 (a) Airspace. 

(b) Title 14 CFR section 91.211, requirements for use of supplemental oxygen, and the 

donning of oxygen masks. 

 (2) Weather. 

 (a) The atmosphere. 

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AC 61-107A 1/2/03 

(b) Winds and clear air turbulence. 

(c) Clouds and thunderstorms. 

 (d) Icing.

(3) Flight Planning and Navigation. 

 (a) Flight planning. 

 (b) Weather charts. 

 (c) Navigation. 

 (d) Navaids. 

(4) Physiological Training. 

 (a) Respiration. 

 (b) Hypoxia. 

(c) Effects of prolonged oxygen use. 

 (d) Decompression sickness. 

 (e) Vision. 

(f) Altitude chamber (optional). 

(5) High Altitude Systems and Components. 

 (a) Turbochargers. 

(b) Oxygen and oxygen equipment. 

 (c) Pressurization systems. 

 (d) High-altitude components. 

(6) Aerodynamics and Performance Factors. 

 (a) Acceleration. 

 (b) G-Forces. 

(c) MACH Tuck and MACH Critical. 

 (7) Emergencies. 

 (a) Decompressions. 

(b) Donning of oxygen masks. 

(c) Failure of a mask, or complete loss of oxygen supply/system. 

Page Par 101 2
1/2/03 AC 61-107A 

 (d) Turbocharger malfunction. 

 (e) In-flight fire. 

(f) Flight into severe turbulence or thunderstorms. 

b. Flight Training.

(1) Preflight Briefing. 

(2) Preflight Planning. 

(a) Weather briefing and considerations. 

 (b) Course plotting.

(c) Aircraft Flight Manual review. 

 (d) Flight plan. 

(3) Preflight Inspection. 

(a) Functional test of the oxygen system, including the verification of supply and 

pressure, regulator operation, oxygen flow, mask fit, and cockpit and air traffic control (ATC) 

communication using mask microphones. 

(4) Runup, Takeoff, and Initial Climb. 

(5) Climb to High Altitude and Normal Cruise Operations While Operating Above 

25,000 Feet MSL.

 (6) Emergencies. 

(a) Simulated rapid decompression, including the immediate donning of oxygen masks. 

 (b) Emergency descent. 

(7) Planned Descents. 

(8) Shutdown Procedures. 

(9) Postflight Discussion. 

102. GROUND TRAINING. Thorough ground training should cover all aspects of high-altitude 

flight, including the flight environment, weather, flight planning and navigation, physiological 

aspects of high-altitude flight, the need for the immediate donning of oxygen masks following

activation of cabin altitude warning, systems and equipment, aerodynamics and performance, and 

high-altitude emergencies. The ground training should include the history and causes of some past 

accidents and incidents involving the topics included in paragraph 101a. Accident reports are 

available from the NTSB and some aviation organizations. 

Par 101 Page 3
AC 61-107A 1/2/03 

103. THE HIGH-ALTITUDE FLIGHT ENVIRONMENT. For the purposes of 14 CFR 

section 61.31(g), flight operations conducted above 25,000 feet are considered high altitude. 

However, the high-altitude environment itself begins below 25,000 feet. For example, flight levels 

(FL) are used at and above 18,000 feet (e.g., FL 180) to indicate levels of constant atmospheric 

pressure in relation to a reference datum of 29.92" Hg. Certain airspace designations and Federal 

Aviation Administration (FAA) requirements become effective at different altitudes. Pilots must be

familiar with these elements before operating in each realm of flight. 

a. Airspace. Pilots of high-altitude aircraft are subject to two principle types of airspace at 

altitudes above 10,000 feet MSL. These are the Class E Airspace which extends from the surface 

up to FL 180, and the Class A Airspace, which extends from FL 180 to FL 600. 

b. Federal Aviation Regulations. In addition to the training required by 14 CFR 

section 61.31(g), pilots of high-altitude aircraft should be familiar with 14 CFR section 91.211 that 

applies specifically to flight at high altitudes. 

(1) Title 14 CFR section 91.215 requires that all aircraft operating within the continental 

U.S. at and above 10,000 feet MSL be equipped with an operable transponder with Mode C 

capability (unless operating at or below 2,500 feet above ground level (AGL). 

(2) Title 14 CFR section 91.211(a) requires that the minimum flightcrew on U.S. registered 

civil aircraft be provided with, and use supplemental oxygen at cabin pressure altitudes above 

12,500 feet MSL up to and including 14,000 feet MSL for that portion of the flight that is at those 

altitudes for more than 30 minutes. The required minimum flightcrew must be provided with and 

use supplemental oxygen at all times when operating an aircraft above 14,000 feet MSL. At cabin 

pressure altitudes above 15,000 feet MSL, all occupants of the aircraft must be provided with 

supplemental oxygen. 

(3) Title 14 CFR section 91.211(b) requires pressurized aircraft to have at least a 10-minute 

additional supply of supplemental oxygen for each occupant at flight altitudes above FL 250 in the 

event of a decompression. At flight altitudes above FL 350, one pilot at the controls of the airplane 

must wear and use an oxygen mask that is secured and sealed. The oxygen mask must supply

oxygen at all times or must automatically supply oxygen when the cabin pressure altitude of the 

airplane exceeds 14,000 feet MSL. An exception to this regulation exists for two-pilot crews that 

operate at or below FL 410. One pilot does not need to wear and use an oxygen mask if both pilots 

are at the controls and each pilot has a quick donning type of oxygen mask that can be placed on the 

face with one hand from the ready position and be properly secured, sealed, and operational within 

5 seconds. If one pilot of a two-pilot crew is away from the controls, then the pilot that is at the 

controls must wear and use an oxygen mask that is secured and sealed. 

(4) Title 14 CFR section 91.121 requires that aircraft use an altimeter setting of 29.92" Hg

at all times when operating at or above FL 180. 

(5) Title 14 CFR section 91.135 requires that all flights operating within Class A Airspace 

be conducted under instrument flight rules (IFR) in an aircraft equipped for IFR and flown by a 

pilot, who is rated for instrument flight. 

Page Par 103 4
1/2/03 AC 61-107A 

(6) Title 14 CFR section 91.159 specifies cruising altitudes and flight levels for visual flight 

rules (VFR) and IFR flights, respectively. For VFR flights between FL 180 to 290, except within 

the Class A Airspace where VFR flight is prohibited, odd flight levels plus 500 feet should be flown 

if the magnetic course is 0° to 179°, and even flight levels plus 500 feet should be flown if the 

magnetic course is 180° to 359°. VFR flights above FL 290 should be flown at 4,000 foot intervals 

beginning at FL 300 if the magnetic course is 0° to 179° and FL 320 if the magnetic course is 180° 

to 359°. 

(7) Title 14 CFR section 91.179 specifies IFR flights in uncontrolled airspace between 

FL 180 and FL 290, odd flight levels should be flown if the magnetic course is 0° to 179°, and even 

flight levels should be flown if the magnetic course is 180° to 359°. IFR flights in uncontrolled 

airspace at or above FL 290 should be flown at 4,000 foot intervals beginning at FL 290 if the 

magnetic course is 0° to 179° and FL 310 if the magnetic course is 180° to 359°. When flying in the 

Class A Airspace, flight levels assigned by air traffic control (ATC) should be maintained. 

104. WEATHER. Pilots should be aware of and recognize the meteorological phenomena 

associated with high altitudes and the effects of these phenomena on flight. 

a. The Atmosphere. The atmosphere is a mixture of gases in constant motion. It is composed 

of approximately 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Water vapor is 

constantly being absorbed and released in the atmosphere which causes changes in weather. The 

three levels of the atmosphere where high-altitude flight may occur are the troposphere, which can 

extend from sea level to approximately FL 350 around the poles and up to FL 650 around the 

equator; tropopause, a thin layer at the top of the troposphere that traps water vapor in the lower 

level; and stratosphere, which extends from the tropopause to approximately 22 miles. The 

stratosphere is characterized by lack of moisture and a constant temperature of -55° C, while the 

temperature in the troposphere decreases at a rate of 2° C per 1,000 feet. Condensation trails, or 

contrails, are common in the upper levels of the troposphere and in the stratosphere. These cloudlike streamers that are generated in the wake of aircraft flying in clear, cold, humid air, form by

water vapor from aircraft exhaust gases being added to the atmosphere causing saturation or 

supersaturation of the air. Contrails can also form aerodynamically by the pressure reduction around 

airfoils, engine nacelles, and propellers cooling the air to saturation. 

b. Atmospheric density in the troposphere decreases 50 percent at 18,000 feet. This means that 

at FL 180, the air contains only one-half the oxygen molecules as at sea level. Because the human 

body requires a certain amount of oxygen for survival, aircraft that fly at high altitudes must be 

equipped with some means of creating an artificial atmosphere, such as cabin pressurization. 

c. Winds. 

(1) The jet stream is a narrow band of high-altitude winds, near or in the tropopause, that 

results from large temperature contrasts over a short distance (typically along fronts) creating large 

pressure gradients aloft. The jet stream usually travels in an easterly direction between 50 and 

200 Kts. The speed of the jet stream is greater in the winter than in the summer months because of 

greater temperature differences. It generally drops more rapidly on the polar side than on the 

equatorial side. In the mid-latitudes, the Polar Front Jet Stream is found in association with the 

Polar Front. This jet stream has a variable path, sometimes flowing almost due North and South. 

Par 103 Page 5
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(2) Because of its meandering path, the Polar Front Jet Stream is not found on most 

circulation charts. One almost permanent jet is a westerly jet found over the subtropics at 

25° latitude about 8 miles (42,200 ft) above the surface. Low pressure systems usually form to the 

south of the jet stream and move northward until they become occluded lows which move north of

the jet stream. Horizontal windshear and turbulence are frequently found on the northern side of the 

jet stream. 

d. Clear Air Turbulence (CAT). CAT is a meteorological phenomenon associated with highaltitude winds. This high-level turbulence occurs where no clouds are present and can take place at 

any altitude (normally above 15,000 feet AGL), although it usually develops in or near the jet 

stream where there is a rapid change in temperature. CAT is generally stronger on the polar side of 

the jet and is greatest during the winter months. CAT can be caused by wind shear, convection 

currents, mountain waves, strong low pressures aloft, or other obstructions to normal wind flow. 

CAT is difficult to forecast because it gives no visual warning of its presence and winds can carry it

far from its point of origin. 

e. Clouds and Thunderstorms.

(1) Cirrus and cirriform clouds are high-altitude clouds that are composed of ice crystals. 

Cirrus clouds are found in stable air above 30,000 feet in patches or narrow bands. Cirriform 

clouds, such as the white clouds in long bands against a blue background known as cirrostratus 

clouds, generally indicate some type of system below. Cirrostratus clouds form in stable air as a 

result of shallow convection currents and also may produce light turbulence. Clouds with extensive 

vertical development (e.g., towering cumulus and cumulonimbus clouds) indicate a deep layer of 

unstable air and contain moderate to heavy turbulence with icing. The bases of these clouds are 

found at altitudes associated with low to middle clouds but their tops can extend up to 60,000 feet or 

more. 

(2) Cumulonimbus clouds are thunderstorm clouds that present a particularly severe hazard 

to pilots and should be circumnavigated if possible. Hazards associated with cumulonimbus clouds 

include embedded thunderstorms, severe or extreme turbulence, lightning, icing, and dangerously

strong winds and updrafts. 

f. Icing. Icing at high altitudes is not as common or extreme as it can be at low altitudes. When

it does occur, the rate of accumulation at high altitudes is generally slower than at low altitudes. 

Rime ice is generally more common at high altitudes than clear ice, although clear ice is possible. 

Despite the composition of cirrus clouds, severe icing is generally not a problem although it can 

occur in some detached cirrus. It is more common in tops of tall cumulus buildups, anvils, and over 

mountainous regions. Many airplanes that operate above 25,000 feet are equipped with deice or 

anti-ice systems, reducing even further the dangers of icing.

105. FLIGHT PLANNING AND NAVIGATION. 

a. Flight Planning. 

(1) Careful flight planning is critical to safe high-altitude flight. Consideration must be

given to power settings, particularly on takeoff, climb, and descent to assure operation in accordance 

with the manufacturer’s recommendations. Fuel management, reporting points, weather briefings 

(not only thunderstorms, the freezing level, and icing at altitude but at all levels and destinations,

Page Par 104 6
1/2/03 AC 61-107A 

including alternates, that may affect the flight), direction of flight, airplane performance charts, high

speed winds aloft, and oxygen duration charts must also be considered. When possible, additional 

oxygen should be provided to allow for emergency situations. Breathing rates increase under stress 

and extra oxygen could be necessary. 

(2) Flight planning should take into consideration factors associated with altitudes that will 

be transited while climbing to or descending from the high altitudes (e.g., airspeed limitations below 

10,000 feet MSL, airspace, and minimum altitudes). Westward flights should generally be made 

away from the jet stream to avoid the strong headwind, and eastward flights should be made in the 

jet stream when possible to increase ground speed. Ground speed checks are particularly important 

in high-altitude flight. If fuel runs low because of headwinds or poor flight planning, a decision to 

fly to an alternate airport should be made as early as possible to allow time to re-plan descents and 

advise ATC. 

b. Knowledge of Aircraft. Complete familiarity with the aircraft systems and limitations is 

extremely important. For example, many high-altitude airplanes feed from only one fuel tank at a

time. If this is the case, it is important to know the fuel consumption rate to know when to change

tanks. This knowledge should be made part of the preflight planning and its accuracy confirmed 

regularly during the flight. 

c. Gradual Descents. Gradual descents from high altitudes should be planned in advance to 

prevent excessive engine cooling and provide passenger comfort. The manufacturer’s 

recommendations found in the AFM should be complied with, especially regarding descent power 

settings to avoid stress on the engines. Although most jets can descend rapidly at idle power, many

turboprop and light twin airplanes require some power to avoid excessive engine cooling, cold 

shock, and metal fatigue. ATC does not always take aircraft type into consideration when issuing

descent instructions. It is the pilot’s responsibility to fly the airplane in the safest manner possible. 

Cabin rates of descent are particularly important and should generally not exceed 500 or 600 feet per 

minute. Before landing, cabin pressure should be equal to ambient pressure or inner ear injury can 

result. If delays occur en route, descents should be adjusted accordingly.

d. Weather Charts. Before beginning a high-altitude flight, all weather charts should be 

consulted, including those designed for low levels. Although high-altitude flight may allow a pilot 

to over-fly adverse weather, low altitudes must be transited on arrival, departure, and in an 

emergency situation that may require landing at any point en route. 

e. Types of Weather Charts. Weather charts that provide information on high-altitude

weather include Constant Pressure charts which provide information on pressure systems, 

temperature, winds, and temperature/dewpoint spread at the 850 millibar (mb), 700 mb, 500 mb, 

300 mb, and 200 mb levels (5 charts are issued every 12 hours). Prognostic Charts forecast winds, 

temperature, and expected movement of weather over the 6-hour valid time of the chart. Observed 

Tropopause Charts provide jet stream, turbulence, and temperature-wind-pressure reports at the 

tropopause over each station. Tropopause Wind Prognostic Charts and Tropopause Height Vertical 

Wind Shear Charts are helpful in determining jet stream patterns and the presence of CAT and wind 

shear. 

f. Wind Shear. Wind shear is indicated by dashed lines on Tropopause Height Vertical Wind 

shear Charts. Horizontal wind changes of 40 Kts within 150 NM, or vertical wind shear of 6 Kts or 

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greater per 1,000 feet usually indicate moderate to severe turbulence and should be avoided. Pilot 

reports (PIREP) are one of the best methods of receiving timely and accurate reports on icing and 

turbulence at high altitudes. 

g. Navigation. Specific charts have been designed for flight at FL 180 and above. En route 

high-altitude charts delineate the jet route system, which consists of routes established from FL 180 

up to and including FL 450. The VOR airways established below FL 180 found on low-altitude 

charts must not be used at FL 180 and above. High-altitude jet routes are an independent matrix of 

airways, and pilots must possess the appropriate en route high-altitude charts before transitioning to 

the flight levels. 

h. Jet Routes. Jet routes in the U.S. are predicated solely on VOR or VORTAC navigation 

facilities, except in Alaska where some are based on Low/Medium Frequency navigation aids. All 

jet routes are identified by the letter "J" followed by the airway number.

i. Reporting Points. Reporting points are designated for jet route systems and must be used by

flights using the jet route unless otherwise advised by ATC. Flights above FL 450 may be 

conducted on a point-to-point basis, using the facilities depicted on the en route high-altitude chart 

as navigational guidance. Area Navigation (RNAV) routes, using either random or fixed way

points, are also used for direct navigation at high altitudes. These routes are based on area

navigation capability between way points defined in terms of latitude/longitude coordinates, degreedistance fixes, or offsets from established routes or airways at a specified distance and direction. 

Radar monitoring by ATC is required on all random RNAV routes. 

j. Navaids. VOR, DME, and TACAN depicted on high-altitude charts are designated as

class H navaids, signifying that their standard service volume is from 1,000 feet AGL up to and 

including 14,500 feet AGL at radial distances out to 40 NM; from 14,500 feet AGL up to and 

including 60,000 feet AGL at radial distances out to 100 NM; and from 18,000 feet AGL up to and 

including 45,000 feet AGL at radial distances out to 130 NM. Ranges of NDB service volumes are 

the same at all altitudes. 

106. PHYSIOLOGICAL TRAINING. To ensure safe flights at high altitudes, pilots of highaltitude aircraft must understand the physiological effects of high-altitude flight and the effect of 

hypoxia on an individual’s ability to perform complex tasks in a changing environment. Additional 

physiological training information, including locations and application procedures for attending an 

altitude chamber, can be found in paragraph 107, Additional Physiological Training. Although not 

required, altitude chamber training is highly recommended for all pilots. 

a. Respiration is the exchange of gases between the organism and its environment. In humans, 

external respiration is the intake of oxygen from the atmosphere by the lungs and the elimination of

some carbon dioxide from the body into the surrounding atmosphere. Each breath intake is 

comprised of approximately 21 percent oxygen, which is absorbed into the bloodstream and carried 

by the blood throughout the body to burn food material and to produce heat and kinetic energy. The 

partial pressure of oxygen forces oxygen through air sacs (alveoli), located at the end of each of the 

smaller tubes that branches out from the bronchial tubes and lungs, into the bloodstream. Other 

gases contained in the lungs reduce the partial pressure of oxygen entering the air sacs to about 

102 mm Hg at ground level, which is approximately 21 percent of the total atmospheric pressure. 

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b. The human body functions normally in the atmospheric area extending from sea level to 

12,000 feet MSL. In this range, brain oxygen saturation is at a level that allows for normal 

functioning. (Optimal functioning is 96 percent saturation. At 12,000 feet, brain oxygen saturation 

is approximately 87 percent which begins to approach a level that could affect human performance. 

Although oxygen is not required below 12,500 feet MSL, its use is recommended when flying above 

10,000 feet MSL during the day and above 5,000 feet MSL at night when the eyes become more 

sensitive to oxygen deprivation.) 

c. Although minor physiological problems, such as middle ear and sinus trapped gas 

difficulties, can occur when flying below 12,000 feet, shortness of breath, dizziness, and headaches 

will result when an individual ascends to an altitude higher than that to which his or her body is 

acclimated. From 12,000 to 50,000 feet MSL, atmospheric pressure drops by 396 mm Hg. This 

area contains less partial pressure of oxygen which can result in problems such as trapped or 

evolved gases within the body. Flight at and above 50,000 feet MSL requires sealed cabins or 

pressure suits. 

d. Hypoxia is a lack of sufficient oxygen in the body cells or tissues caused by an inadequate 

supply of oxygen, inadequate transportation of oxygen, or inability of the body tissues to use 

oxygen. A common misconception among many pilots who are inexperienced in high-altitude flight 

operations and who have not been exposed to physiological training is that it is possible to recognize

the symptoms of hypoxia and to take corrective action before becoming seriously impaired. While

this concept may be appealing in theory, it is both misleading and dangerous for an untrained 

crewmember. Symptoms of hypoxia vary from pilot to pilot, but one of the earliest effects of 

hypoxia is impairment of judgment. Other symptoms can include one or more of the following: 

(1) Behavioral changes (e.g., a sense of euphoria). 

(2) Poor coordination. 

(3) Discoloration at the fingernail beds (cyanosis). 

 (4) Sweating.

(5) Increased breathing rate, headache, sleepiness, or fatigue. 

(6) Loss or deterioration of vision. 

(7) Light-headedness or dizzy sensations and listlessness. 

(8) Tingling or warm sensations. 

e. While other significant effects of hypoxia usually do not occur in a healthy pilot in an 

un-pressurized aircraft below 12,000 feet, there is no assurance that this will always be the case. 

The onset of hypoxic symptoms may seriously affect the safety of flight and may well occur even in 

short periods of exposure to altitudes from 12,000 to 15,000 feet. The ability to take corrective 

measures may be totally lost in 5 minutes at 22,000 feet. However, that time would be reduced to 

only 7 to 10 seconds at 40,000 feet and the crewmember may suffer total loss of consciousness soon 

thereafter. A description of the four major hypoxia groups and the recommended methods to 

combat each follows. 

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 (1) Hypoxic (Altitude) Hypoxia. Altitude hypoxia poses the greatest potential 

physiological hazard to a flight crewmember while flying in the high-altitude environment. This 

type of hypoxia is caused by an insufficient partial pressure of oxygen in the inhaled air resulting

from reduced oxygen pressure in the atmosphere at altitude. If a person is able to recognize the

onset of hypoxic symptoms, immediate use of supplemental oxygen will combat hypoxic hypoxia 

within seconds. Oxygen systems should be checked periodically to ensure that there is an adequate 

supply of oxygen and that the system is functioning properly. This check should be performed 

frequently with increasing altitude. If supplemental oxygen is not available, an emergency descent 

to an altitude below 10,000 feet should be initiated. 

 (2) Histotoxic Hypoxia. This is the inability of the body cells to use oxygen because of 

impaired cellular respiration. This type of hypoxia, caused by alcohol or drug use, cannot be 

corrected by using supplemental oxygen because the uptake of oxygen is impaired at the tissue 

level. The only method of avoiding this type of hypoxia is to abstain, before flight, from alcohol or 

drugs that are not approved by a flight surgeon or an aviation medical examiner. 

 (3) Hypemic (Anemic) Hypoxia. This type of hypoxia is defined as a reduction in the 

oxygen-carrying capacity of the blood. Hypemic hypoxia is caused by a reduction in circulating red 

blood cells (hemoglobin) or contamination of blood with gases other than oxygen as a result of 

anemia, carbon monoxide poisoning, or excessive smoking. Pilots should take into consideration 

the effect of smoking on altitude tolerance when determining appropriate cabin pressures. If heavy

smokers are among the crew or passengers, a lower cabin altitude should be set because apparent 

altitudes for smokers are generally much higher than actual altitudes. For example, a smoker’s

apparent altitude at sea level is approximately 7,000 feet. Twenty thousand feet actual altitude for a 

nonsmoker would be equivalent to an apparent altitude of 22,000 feet for a smoker. The smoker is 

thus more susceptible to hypoxia at lower altitudes than the nonsmoker. Hypemic hypoxia is 

corrected by locating and eliminating the source of the contaminating gases. A careful preflight of

heating systems and exhaust manifold equipment is mandatory. Also, cutting down on smoking

would minimize the onset of this type of hypoxia. If symptoms are recognized, initiate use of

supplemental oxygen and/or descend to an altitude below 10,000 feet. If symptoms persist, ventilate 

the cabin and land as soon as possible because the symptoms may be indicative of carbon monoxide 

poisoning and medical attention should be sought. 

 (4) Stagnant Hypoxia. This is an oxygen deficiency in the body resulting from poor 

circulation of the blood because of a failure of the circulatory system to pump blood (and oxygen) to 

the tissues. Evidence of coronary artery disease is grounds for immediate denial or revocation of a 

medical certificate. In flight, this type of hypoxia can sometimes be caused by positive pressure

breathing for long periods of time or excessive G-forces. 

f. Effective Performance Time (EPT) or Time of Useful Consciousness (TUC) is the amount of 

time in which a person is able to effectively or adequately perform flight duties with an insufficient 

supply of oxygen. EPT decreases with altitude, until eventually coinciding with the time it takes for 

blood to circulate from the lungs to the head usually at an altitude above 35,000 feet. Table 1-1 

shows the TUC (shown as average TUC) at various altitudes. The rate of ascent directly affects 

TUC. Faster rates of ascent result in shorter TUC. 

Page Par 106 10
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TABLE 1-1. TIMES OF USEFUL CONSCIOUSNESS AT VARIOUS ALTITUDES

Standard Ascent Rate After Rapid Decompression 

Altitude (Feet) Time Time

18,000 20 to 30 minutes 10 to 15 minutes 

22,000 10 minutes 5 minutes 

25,000 3 to 5 minutes 1.5 to 3.5 minutes 

28,000 2.5 to 3 minutes 1.25 to 1.5 minutes 

30,000 1 to 2 minutes 30 to 60 seconds 

35,000 30 to 60 seconds 15 to 30 seconds 

40,000 15 to 20 seconds 7 to 10 seconds 

43,000 9 to 12 seconds 5 seconds 

50,000 9 to 12 seconds 5 seconds 

g. Other factors that determine EPT are physical activities (exercise decreases EPTs), and dayto-day factors such as physical fitness, diet, rest, prescription drugs, smoking, and illness. Altitude

chamber experiments found a significantly longer TUC for nonsmoker pilots who exercise and 

watch their diet than for pilots who smoke and are not physically fit. 

h. Prolonged oxygen use can also be harmful to human health. One hundred percent aviation 

oxygen can produce toxic symptoms if used for extended periods of time. The symptoms can 

consist of bronchial cough, fever, vomiting, nervousness, irregular heart beat, and lowered energy. 

These symptoms appeared on the second day of breathing 90 percent oxygen during controlled 

experiments. It is unlikely that oxygen would be used long enough to produce the most severe of 

these symptoms in any aviation incidence. However, prolonged flights at high altitudes using a high

concentration of oxygen can produce some symptoms of oxygen poisoning such as infection or 

bronchial irritation. The sudden supply of pure oxygen following a decompression can often 

aggravate the symptoms of hypoxia. Therefore, oxygen should be taken gradually, particularly

when the body is already suffering from lack of oxygen, to build up the supply in small doses. If

symptoms of oxygen poisoning develop, high concentrations of oxygen should be avoided until the 

symptoms completely disappear. 

i. When nitrogen is inhaled, it dilutes the air we breathe. While most nitrogen is exhaled from 

the lungs along with carbon dioxide, some nitrogen is absorbed by the body. The nitrogen absorbed 

into the body tissues does not normally present any problem because it is carried in a liquid state. If

the ambient surrounding atmospheric pressure lowers drastically, this nitrogen could change from a 

liquid and return to its gaseous state in the form of bubbles. These evolving and expanding gases in 

the body are known as decompression sickness and are divided into two groups. 

 (1) Trapped Gas. Expanding or contracting gas in certain body cavities during altitude 

changes can result in abdominal pain, toothache, or pain in ears and sinuses if the person is unable 

to equalize the pressure changes. Above 25,000 feet, distention can produce particularly severe 

gastrointestinal pain. 

 (2) Evolved Gas. When the pressure on the body drops sufficiently, nitrogen comes out of 

solution and forms bubbles which can have adverse effects on some body tissues. Fatty tissue 

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contains more nitrogen than other tissue; thus making overweight people more susceptible to 

evolved gas decompression sicknesses. 

(a) SCUBA diving will compound this problem because of the compressed air used in 

the breathing tanks. After SCUBA diving, a person who flies in an aircraft to a pressure altitude of

8,000 feet would experience the same effects as a non-diver flying at 40,000 feet un-pressurized. 

The recommended waiting period before going to flight altitudes of 8,000 feet is at least 12 hours 

after non-decompression stop diving (diving which does not require a controlled ascent), and 

24 hours after decompression stop diving (diving which requires a controlled ascent). For flight 

altitudes above 8,000 feet, the recommended waiting time is at least 24 hours after any SCUBA 

diving. (See Airman's Information Manual (AIM) paragraph 8-1-2 (d) (2).) 

(b) The bends, also known as caisson disease, is one type of evolved gas decompression 

sickness and is characterized by pain in and around the joints. The pain gradually becomes more 

severe, can eventually become temporarily incapacitating, and can result in collapse. The chokes 

refers to a decompression sickness that manifests itself through chest pains and burning sensations, 

a desire to cough, possible cyanosis, a sensation of suffocation, progressively shallower breathing

and, if a descent is not made immediately, collapse and unconsciousness. Paresthesia is a third type 

of decompression sickness, characterized by tingling, itching, a red rash, and cold and warm 

sensations, probably resulting from bubbles in the central nervous system (CNS). CNS disturbances 

can result in visual deficiencies such as illusionary lines or spots, or a blurred field of vision. Some 

other effects of CNS disturbances are temporary partial paralysis, sensory disorders, slurred speech, 

and seizures. 

j. Shock can often result from decompression sicknesses as a form of body protest to disrupted 

circulation. Shock can cause nausea, fainting, dizziness, sweating, and/or loss of consciousness. 

The best treatment for decompression sickness is descent to a lower altitude and landing. If

conditions persist after landing, recompression chambers can be located through an aviation medical 

examiner. 

k. Vision has a tendency to deteriorate with altitude. A reversal of light distribution at high

altitudes (bright clouds below the airplane and darker, blue sky above) can cause a glare inside the 

cockpit. Glare effects and deteriorated vision are enhanced at night when the body becomes more 

susceptible to hypoxia and can occur at altitudes as low as 5,000 feet. In addition, the empty visual 

field caused by cloudless, blue skies during the day can cause inaccuracies when judging the speed, 

size, and distance of other aircraft. Sunglasses are recommended to minimize the intensity of the

sun’s ultraviolet rays at high altitudes. 

107. ADDITIONAL PHYSIOLOGICAL TRAINING. There are no specific requirements in

14 CFR part 91 or 125 for physiological training. However, in addition to the high-altitude training

required by 14 CFR section 61.31(g), which should include the physiological training outlined in 

this chapter, 14 CFR parts 121 and 135 require flight crewmembers that serve in operations above 

25,000 feet to receive training in specified subjects of aviation physiology. None of the 

requirements includes altitude chamber training. The U.S. military services require its flight 

crewmembers to complete both initial and refresher physiological training, including instruction in 

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basic aviation physiology and altitude chamber training. Other U.S. Government agencies, such as

the National Aeronautics and Space Administration and FAA, also require their flight personnel 

who operate pressurized aircraft in the high-altitude flight environment to complete similar training. 

Although most of the subject material normally covered in physiological training concerns problems 

associated with reduced atmospheric pressure at high-flight altitudes, other equally important 

subjects are covered as well. Such subjects of aviation physiology as vision, disorientation, physical 

fitness, stress, and survival affect flight safety and are normally presented in a good training

program. The FAA Civil Aerospace Medical Institute (CAMI) offers a one-day aviation physiology

course for FAA flightcrews, civil aviation pilots, and FAA aviation medical examiners (AME). In

addition to the basic academic contents, this course offers practical demonstrations of rapid 

decompression (8 to 18,000 feet) and hypoxia (25,000 feet) in a hypobaric chamber, as well as a 

practical demonstration of spatial disorientation in a Vertigon or the new General Aviation Spatial 

Disorientation Demonstrator (Gyro-1). 

a. Physiological training programs are offered at locations across the U.S. (table 1-2) for civil 

pilots at U.S. Air Force physiology training units under the USAF/FAA physiological Training

Agreement. Trainees who attend these programs will receive classroom lectures and learn to 

recognize and overcome vertigo, hypoxia, hyperventilation, etc., during flight, and a high-altitude 

flight in an altitude chamber. The U.S. Army and the FAA are currently working on the

development of a similar joint training program. 

b. Persons who wish to take this training must be at least 18 years of age, hold a current FAA 

Airman Medical Certificate, and must not have a cold or any other significant health problem when 

enrolling for the course. 

NOTE: Anyone can attend the training regardless of whether they are a pilot or 

not. However, they are still required to obtain a minimum of a class III medical 

certificate in order to participate in the altitude chamber training. 

c. Scheduling. CAMI’s Airman Education Programs obtains a list of training dates from each 

base that are available to anyone interested in the training. These dates can be accessed by calling

405-954-4837. To schedule a training slot the following information will be required for each 

student: 

(1) Pilot certificate number or social security number 

(2) Last name 

(3) First name

(4) Middle initial 

(5) Date of birth 

(6) Mailing address 

(7) Daytime phone number 

(8) Date of FAA medical 

(9) The class of the medical the day it was issued. 

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d. Submitting an Application. Once you have been assigned a training date, CAMI will mail 

an application and a notification letter to you. The application must be completed and mailed to the 

address provided within 30 days of the scheduled training, along with the fee, made payable to the

FAA. The applicant should take the notification letter, along with a current medical certificate, to 

the base, the day of the training. Effective August 13, 2001, the course fee of $50.00 is nonrefundable and not transferable. 

TABLE 1-2. TRAINING LOCATIONS

Andrews AFB, MD Fort Rucker (Army), AL Offut AFB, NE 

Beale AFB, CA Holloman AFB, NM Peterson AFB, CO 

Brooks AFB, TX Langley AFB, VA Randolph AFB, TX 

Columbus AFB, MS Laughlin AFB, TX Shaw AFB, SC 

Fairchild AFB, WA Little Rock AFB, AR Vance AFB, OK 

108. HIGH-ALTITUDE SYSTEMS AND EQUIPMENT. Several systems and equipment are 

unique to aircraft that fly at high altitudes, and pilots should be familiar with their operation before 

using them. Before any flight, a pilot should be familiar with all the systems on the aircraft to be 

flown. 

a. Turbochargers. Most light piston engine airplanes that fly above 25,000 feet MSL are 

turbocharged. Turbochargers compress air in the carburetor or cylinder intake by using exhaust 

gases from an engine-driven turbine wheel. The increased air density provides greater power and 

improved performance. Light aircraft use one of two types of turbocharge systems. 

(1) The first is the normalized system, which allows the engine to develop sea level pressure 

from approximately 29 inches of manifold pressure up to a critical altitude (generally between

14,000 to 16,000 feet MSL). 

(2) The second is the supercharger system, which is a more powerful system that allows the 

engine to develop higher than sea level pressure (up to 60 inches of manifold pressure) up to a 

critical altitude. To prevent overboosting at altitudes below the critical altitude, a waste gate is 

installed in the turbocompressor system to release unnecessary gases. The waste gate is a damperlike device that controls the amount of exhaust that strikes the turbine rotor. As the waste gate 

closes with altitude, it sends more gases through the turbine compressor causing the rotor to spin 

faster. This allows the engine to function as if it were maintaining sea level or, in the case of a

supercharger, above sea level manifold pressure. The three principle types of waste gate operations 

are manual, fixed, and automatic. 

 (a) Manual Waste Gate. Manual waste gate systems are common in older aircraft but 

have been discontinued due to the additional burden on the pilot. Waste gates were often left closed 

on takeoff, resulting in an overboost that could harm the engine. 

 (b) Fixed Waste Gate. Fixed waste gates pose less of a burden on the pilot, but the 

pilot must still be careful not to overboost the engine, especially on takeoff, initial climb, and on 

cold days when the air is especially dense. This type of waste gate remains in the same position 

during all engine operations, but it splits the exhaust flow allowing only partial exhaust access to 

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the turbine. The pilot simply controls manifold pressure with smooth, slow application of the 

throttle to control against overboost. If overboost does occur, a relief valve on the intake manifold 

protects the engine from damage. This is not a favorable system due to fluctuations in manifold 

pressure and limited additional power from the restricted control over the exhaust flow. In addition, 

the compressor can produce excessive pressure and cause overheating.

 (c) Automatic Waste Gate. Automatic waste gates operate on internal pressure. When 

internal pressure builds towards an overboost, the waste gate opens to relieve pressure, keeping the 

engine within normal operating limits regardless of the air density. 

 1 The pressure-reference automatic waste gate system maintains the manifold 

pressure set by the throttle. Engine oil pressure moves the waste gate to maintain the appropriate

manifold pressure, thus reducing the pilot’s workload and eliminating the possibility of overboost. 

If the airplane engine is started up and followed by an immediate takeoff, cold oil may cause a

higher than intended manifold pressure. Allow the oil to warm up and circulate throughout the 

system before takeoff. 

 2 The density-reference waste gate system is controlled by compressor discharge air. 

A density controller holds a given density of air by automatically adjusting manifold pressure as

airspeed, ambient pressure, temperature, altitude, and other variables change. 

b. Turbocharged engines are particularly temperature sensitive. Manufacturers often 

recommend increasing the fuel flow during climbs to prevent overheating. It is also important to 

cool the engine after landing. Allowing the engine to idle for approximately one minute before 

shutting it down permits engine oil to flow through the system, cooling the engine while

simultaneously cooling and lubricating the turbocharger. 

c. Most high-altitude airplanes come equipped with some type of fixed oxygen installation. If

the airplane does not have a fixed installation, portable oxygen equipment must be readily accessible 

during flight. The portable equipment usually consists of a container, regulator, mask outlet, and 

pressure gauge. A typical 22 cubic-foot portable container will allow four people enough oxygen to 

last approximately 1.5 hours at 18,000 feet MSL. Aircraft oxygen is usually stored in high-pressure 

system containers of 1,800-2,200 pounds per square inch (PSI). The container should be fastened 

securely in the aircraft before flight. When the ambient temperature surrounding an oxygen cylinder 

decreases, pressure within that cylinder will decrease because pressure varies directly with 

temperature if the volume of a gas remains constant. Therefore, if a drop in indicated pressure on a 

supplemental oxygen cylinder is noted, there is no reason to suspect depletion of the oxygen supply,

which has simply been compacted due to storage of the containers in an unheated area of the 

aircraft. High pressure oxygen containers should be marked with the PSI tolerance (i.e., 1,800 PSI) 

before filling the container to that pressure. The containers should be supplied with aviation oxygen 

only, which is 100 percent pure oxygen. Industrial oxygen is not intended for breathing and may

contain impurities, and medical oxygen contains water vapor that can freeze in the regulator when

exposed to cold temperatures. To assure safety, oxygen system periodic inspection and servicing

should be done at FAA certificated stations found at some fixed base operations and terminal 

complexes. 

d. Regulators and masks work on continuous flow, diluter demand, or on pressure demand 

systems. The continuous flow system supplies oxygen at a rate that may either be controlled by the 

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user or controlled automatically on some regulators. The mask is designed so the oxygen can be 

diluted with ambient air by allowing the user to exhale around the face piece, and comes with a 

rebreather bag which allows the individual to reuse part of the exhaled oxygen. Pilot masks 

sometimes allow greater oxygen flow than passengers’ masks, so it is important that pilots use the

masks that are indicated for them. Although certificated up to 41,000 feet, very careful attention to 

system capabilities is required when using continuous flow oxygen systems above 25,000 feet. 

e. Diluter demand and pressure demand systems supply oxygen only when the user inhales 

through the mask. An automix lever allows the regulators to automatically mix cabin air and 

oxygen or supply 100 percent oxygen, depending on the altitude. The demand mask provides a tight 

seal over the face to prevent dilution with outside air and can be used safely up to 40,000 feet. 

Pilots who fly at those altitudes should not have beards and moustaches because air can easily seep 

in through the border of the mask. Pressure demand regulators also create airtight and oxygen-tight

seals but they also provide a positive pressure application of oxygen to the mask face-piece which 

allows the user’s lungs to be pressurized with oxygen. This feature makes pressure demand 

regulators safe at altitudes above 40,000 feet. 

f. Pilots should be aware of the danger of fire when using oxygen. Materials that are nearly

fireproof in ordinary air may be susceptible to burning in oxygen. Oils and greases may catch fire if

exposed to oxygen and, therefore, cannot be used for sealing the valves and fittings of oxygen 

equipment. Smoking during any kind of oxygen equipment use must also be strictly forbidden. 

g. Surplus oxygen equipment must be inspected and approved by a certified FAA inspection 

station before use. Before each flight, the pilot should thoroughly inspect and test all oxygen 

equipment. The inspection should be accomplished with clean hands and should include a visual 

inspection of the mask and tubing for tears, cracks, or deterioration; the regulator for valve and lever

condition and positions; oxygen quantity; and the location and functioning of oxygen pressure

gauges, flow indicators, and connections. The mask should be donned and the system should be 

tested. After any oxygen use, verify that all components and valves are shut off. 

h. Cabin pressurization is the compression of air in the aircraft cabin in order to maintain a

cabin altitude lower than the actual flight altitude. Because of the ever-present possibility of 

decompression, supplemental oxygen is still required. Pressurized aircraft meeting specific 

requirements of 14 CFR part 23 or 25 have cabin altitude warning systems which are activated at

10,000 feet. Pressurized aircraft meeting the still more stringent requirements of 14 CFR part 25 

have automatic passenger oxygen mask dispensing devices which activate before exceeding

15,000 feet cabin altitude. 

i. Pressurization in most light aircraft is sent to the cabin from the turbocharger’s compressor or

from an engine-driven pneumatic pump. The flow of compressed air into the cabin is regulated by

an outflow valve which keeps the pressure constant by releasing excess pressure into the

atmosphere. The cabin altitude can be manually selected and is monitored by a gauge which 

indicates the pressure difference between the cabin and ambient altitudes. The rate of change

between these two pressures is automatically controlled with a manual backup control. 

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j. Each pressurized aircraft has a determined maximum pressure differential, which is the

maximum differential between cabin and ambient altitudes that the pressurized section of the

aircraft can support. The pilot must be familiar with these limitations, as well as the manifold 

pressure settings recommended for various pressure differentials. Some aircraft have a negative

pressure relief valve to equalize pressure in the event of a sudden decompression or rapid descent to 

prevent the cabin pressure from becoming higher than the ambient pressure. 

k. Reducing exposure to low barometric pressure lowers the occurrence of decompression 

sickness and the need for an oxygen mask is eliminated as a full-time oxygen source above certain 

altitudes. Many airplanes are equipped with automatic visual and aural warning systems that

indicate an unintentional loss of pressure. 

l. Technology is continuously improving flight at high altitudes through the development of 

new devices and the improvement of existing systems. One such example is the pressurized 

magneto. Thin air at high altitudes makes the unpressurized magneto susceptible to crossfiring. 

The high tension pressurized system is composed of sealed caps and plugs that keep the electrodes 

contained within the body. A pressure line extends directly from the turbodischarger to the 

magneto. Pressurized magnetos perform better at high altitudes where low pressure and cold 

atmosphere have a detrimental effect on electrical conductivity. Flight above 14,000 feet with an 

unpressurized magneto should be avoided because of its higher susceptibility to arcing. 

m. Another airplane component recommended for flight at high altitudes is the dry vacuum 

pump. Engine-driven wet vacuum pumps cannot create a sufficient vacuum to drive the gyros in the 

low air density found at high altitudes. Furthermore, gyros and rubber deicing boots can be ruined 

by oil contamination from the wet pump system, which uses engine oil for lubrication and cooling. 

Dry vacuum pumps are lightweight, self-lubricating systems that eliminate oil contamination and 

cooling problems. These pumps can power either a vacuum or pressure pneumatic system, allowing

them to drive the gyros, deice boots, and pressurize the door seals. 

109. AERODYNAMICS AND PERFORMANCE FACTORS. Thinner air at high altitudes has 

a significant impact on an airplane’s flying characteristics because surface control effects, lift,

thrust, drag, and horsepower are all functions of air density. 

a. The reduced weight of air moving over control surfaces at high altitudes decreases their 

effectiveness. As the airplane approaches its absolute altitude, the controls become sluggish, 

making altitude and heading difficult to maintain. For this reason, most airplanes that fly at above 

25,000 feet are equipped with an autopilot. 

b. A determined weight of air is used by the engine for producing an identified amount of 

horsepower through internal combustion. For a given decrease of air density, horsepower decreases 

at a higher rate which is approximately 1.3 times that of the corresponding decrease in air density. 

c. For an airplane to maintain level flight, drag and thrust must be equal. Because density is

always greatest at sea level, the velocity at altitude given the same angle of attack will be greater

than at sea level, although the indicated air speed (IAS) will not change. Therefore, an airplane’s 

TAS increases with altitude while its IAS remains constant. In addition, an airplane’s rate of climb 

will decrease with altitude. 

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110. EMERGENCIES AND IRREGULARITIES AT HIGH ALTITUDES. All emergency

procedures in the AFM should be reviewed before flying any airplane, and be readily accessible 

during every flight. A description of some of the most significant high-altitude emergencies and 

remedial action for each follows. 

a. Decompression is defined as the inability of the aircraft’s pressurization system to maintain 

its designed pressure schedule. Decompression can be caused by a malfunction of the system itself

or by structural damage to the aircraft. A decompression will often result in cabin fog because of 

the rapid drop in temperature and the change in relative humidity. A decompression will also affect

the human body. Air will escape from the lungs through the nose and mouth because of a sudden 

lower pressure outside of the lungs. Differential air pressure on either side of the eardrum should 

clear automatically. Exposure to wind blast and extremely cold temperatures are other hazards the 

human body may face with decompression. 

b. Decompression of a small cabin volume pressurized aircraft is more critical than a large one, 

given the same size hole or conditions, primarily because of the difference in cabin volumes. 

Table 1-3 is a comparison of cabin volume ratios between several large transport airplanes and some

of the more popular general aviation turbojet airplanes in current use. Table 1-3 shows that, under 

the same conditions, a typical small pressurized aircraft can be expected to decompress on the order 

of 10 to 200 times faster than large aircraft. The B-747/Learjet comparison is an extreme example 

in that the human response, TUC, and the protective equipment necessary are the same. Actual 

decompression times are difficult to calculate due to many variables involved (e.g., the type of 

failure, differential pressure, cabin volume, etc.). However, it is more probable that the crew of the 

small aircraft will have less time in which to take lifesaving actions. 

TABLE 1-3. AIRCRAFT CABIN VOLUME RATIOS

Aircraft Type Cabin Volumes in Cubic Feet Ratio 

DC-9 vs CE-650 5,840 vs 576 10:1 

B-737 vs LR-55 8,010 vs 502 16:1 

B-727 vs NA-265 9,045 vs 430 21:1 

L-1011 vs G-1159 35,000 vs 1,850 19:1 

B-747 vs Learjet 59,000 vs 265 223:1 

Data Source: Physiological Considerations and Limitations in the High-altitude Operation of SmallVolume Pressurized Aircraft. E. 

B. McFadden and D. de Steigner, Federal Aviation Administration (FAA) Civil Aeromedical Institute (CAMI). 

(1) An explosive decompression is a change in cabin pressure faster than the lungs can 

decompress. Most authorities consider any decompression which occurs in less than 0.5 seconds as 

explosive and potentially dangerous. This type of decompression is more likely to occur in small 

volume pressurized aircraft than in large pressurized aircraft and often results in lung damage. To 

avoid potentially dangerous flying debris in the event of an explosive decompression, all loose items 

such as baggage and oxygen cylinders should be properly secured. 

(2) A rapid decompression is a change in cabin pressure where the lungs can decompress 

faster than the cabin. The risk of lung damage is significantly reduced in this decompression as

compared with an explosive decompression.

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(3) Gradual or slow decompression is dangerous because it may not be detected. Automatic

visual and aural warning systems generally provide an indication of a slow decompression. 

(4) Recovery from all types of decompression is similar. Oxygen masks should be donned, 

and a rapid descent initiated as soon as possible to avoid the onset of hypoxia. Although top priority

in such a situation is reaching a safe altitude, pilots should be aware that cold-shock in piston 

engines can result from a high-altitude rapid descent, causing cracked cylinders or other engine

damage. The time allowed to make a recovery to a safe altitude before loss of useful consciousness 

is, of course, much less with an explosive decompression than with a gradual decompression. 

c. Increased oil temperature, decreased oil pressure, and a drop in manifold pressure could 

indicate a turbocharger malfunction or a partial or complete turbocharger failure. The consequences 

of such a malfunction or failure are twofold. The airplane would not be capable of sustaining

altitude without the additional power supplied by the turbocharged system. The loss in altitude in 

itself would not create a significant problem, weather and terrain permitting, but ATC must be

notified of the descent. A more serious problem associated with a failed turbocharger would be loss 

of cabin pressurization if the pressurization system is dependent on the turbocharger compressor. 

Careful monitoring of pressurization levels is essential during the descent to avoid the onset of 

hypoxia from a slow decompression. 

d. Another potential problem associated with turbochargers is fuel vaporization. Engine-driven 

pumps that pull fuel into the intake manifold are susceptible to vapor lock at high altitudes. Most 

high-altitude aircraft are equipped with tank-mounted boost pumps to feed fuel to the engine-driven 

pump under positive pressure. These pumps should be turned on if fuel starvation occurs as a result 

of vapor lock. 

e. Because of the highly combustible composition of oxygen, an immediate descent to an 

altitude where oxygen is not required should be initiated if a fire breaks out during a flight at high

altitude. The procedures in the AFM should be followed. 

f. Flight through thunderstorm activity or known severe turbulence should be avoided, if 

possible. When flight through severe turbulence is anticipated and/or unavoidable, the following

procedures are highly recommended: 

(1) Airspeed is critical for any type of turbulent air penetration. Use the AFM 

recommended turbulence penetration target speed or, if unknown, an airspeed below maneuvering

speed. Use of high airspeeds can result in structural damage and injury to passengers and 

crewmembers. Severe gusts may cause large and rapid variations in indicated airspeed. Do not 

chase airspeed. 

(2) Penetration should be at an altitude that provides adequate maneuvering margins in case 

severe turbulence is encountered to avoid the potential for catastrophic upset. 

(3) If severe turbulence is penetrated with the autopilot on, the altitude hold mode should be 

off. If the autopilot has an attitude hold mode, it should be engaged. The autopilot attitude hold 

mode can usually maintain attitude more successfully than a pilot under stress. With the autopilot 

off, the yaw damper should be engaged. Controllability of the aircraft in turbulence becomes more 

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difficult with the yaw damper off. Rudder controls should be centered before engaging the yaw 

damper. 

(4) When flight through a thunderstorm cannot be avoided, turn up the intensity of panel 

and cabin lights so lightening does not cause temporary blindness. White lighting in the cockpit is 

better than red lighting during thunderstorms. 

(5) Keep wings level and maintain the desired pitch attitude and approximate heading. Do 

not attempt to turn around and fly out of the storm because the speed associated with thunderstorms 

usually makes such attempts unsuccessful. Use smooth, moderate control movements to resist 

changes in attitude. If large attitude changes occur, avoid abrupt or large control inputs. Avoid, as 

much as possible, use of the stabilizer trim in controlling pitch attitudes. Do not chase altitude. 

111. FLIGHT TRAINING. Flight training required to comply with 14 CFR section 61.31(f) may

be conducted in a high-altitude airplane or a simulator that meets the requirements of 14 CFR 

section 121.407. The simulator should be representative of an airplane that has a service ceiling or 

maximum operating altitude, whichever is lower, above 25,000 feet MSL. The training should 

consist of as many flights as necessary to cover the following procedures and maneuvers. If an 

airplane is being used, each flight should consist of a preflight briefing, flight planning, a preflight 

inspection, demonstrations by the instructor of certain maneuvers or procedures when necessary, 

and a postflight briefing and discussion. 

a. Preflight Briefing. The instructor should verbally cover the material that will be introduced 

during the flight. If more than one flight is required, previous flights should be reviewed at this 

time. The preflight briefing is a good time to go over any questions the trainee may have regarding

operations at high altitudes or about the aircraft. Questions by the trainee should be encouraged 

during all portions of the flight training. 

b. Preflight Planning. A thorough flight plan should be completed for a predetermined route. 

The flight plan should include a complete weather briefing. If possible, a trip to a Flight Service 

Station (FSS) is encouraged rather than a telephone briefing so the trainee can use actual weather 

charts. Winds, pilot reports, the freezing level and other meteorological information obtained from 

the briefing should be used to determine the best altitude for the flight. The information should be 

retained for future calculations. 

(1) The course should be plotted on high-altitude navigation chart noting the appropriate jet 

routes and required reporting points on a navigation log. Low-altitude charts should be available for 

planning departures and arrivals to comply with airspace and airspeed requirements. Alternate 

airports should also be identified and noted. 

(2) The AFM should be reviewed with particular attention to weight and balance, 

performance charts, and emergency procedures. Oxygen requirements, airspeeds, groundspeeds, 

time en route, and fuel burn should be calculated using the AFM and weather data, when applicable. 

Fuel management and descents should also be planned at this time. The AFM should be readily

accessible in the cabin in the event of an emergency.

(3) A flight plan should be completed using appropriate jet routes from the en route highaltitude chart. The flight plan should be filed with the nearest Flight Service Station or ATC facility

as appropriate. 

Page Par 110 20
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c. Preflight Inspection. The aircraft checklist should be followed carefully. Particular 

attention should be given to the aircraft’s fuselage, windshields, window panels, and canopies to 

identify any cracks or damage that could rupture under the stress of cabin pressurization. The 

inspection should also include a thorough examination of the aircraft oxygen equipment, including

available supply, an operational check of the system, and assurance that supplemental oxygen is in a 

readily accessible location. 

(1) Preflighting the oxygen equipment so that the system is ready for use as soon as the 

mask is donned. 

(2) Including a requirement to demonstrate that the crew can establish communications 

using the oxygen equipment, both between crewmembers, if more than one required pilot, and with 

ATC.

d. Runup, Takeoff, and Initial Climb. Procedures in the AFM should be followed, 

particularly the manufacturer’s recommended power settings and airspeeds to avoid overboosting

the engine. Standard call-out procedures are highly recommended and should be used for each 

phase of flight where the airplane crew consists of more than one crewmember. 

e. Climb to High Altitude and Normal Cruise Operations While Operating Above 

25,000 feet MSL. The transition from low to high altitude should be performed repeatedly to assure 

familiarity with appropriate procedures. Specific oxygen requirements should be met when 

climbing above 12,500 feet and pressurization should be adjusted with altitude. When passing

through FL 180, the altimeter should be set to 29.92" Hg and left untouched until descending below 

that altitude. Reporting points should be complied with, as should appropriate altitude selection for 

direction of flight. Throughout the entire climb and cruise above 25,000 feet, emphasis should be 

given to monitoring cabin pressurization. 

f. Simulated Emergencies. Training should include at least one simulated rapid 

decompression and emergency descent. Do not actually depressurize the airplane for this or any

other training. Actual decompression of an airplane can be extremely dangerous and should never 

be done intentionally for training purposes. The decompression should be simulated by donning the 

oxygen masks, turning on the oxygen controls, configuring the airplane for an emergency descent,

and performing the emergency descent as soon as possible. This maneuver can be practiced at any

altitude. 

g. Descents. Gradual descents from altitude should be practiced to provide passenger comfort 

and compliance with procedures for transitioning out of the high-altitude realm of flight. The

airplane manufacturer’s recommendations should be followed with regard to descent power settings

to avoid stress on the engine and excessive cooling. Particular emphasis should be given to cabin 

pressurization and procedures for equalizing cabin and ambient pressures before landing. Emphasis 

should also be given to changing to low-altitude charts when transitioning through FL 180, 

obtaining altimeter settings below FL 180, and complying with airspace and airspeed restrictions at 

appropriate altitudes. 

h. Engine Shutdown. Allow the turbocharged engine to cool for at least one minute and assure 

that all shutdown procedures in the AFM are followed. Before exiting the airplane, always check 

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that all oxygen equipment and oxygen valves have been turned off or placed in the position 

recommended by the manufacturer. 

i. Postflight Discussion. The instructor should review the flight and answer any questions the 

trainee may have. If additional flights are necessary to ensure thorough understanding of highaltitude operations, the material for the next flight should be previewed during the postflight 

discussion. 

112. - 199. RESERVED. 

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CHAPTER 2. MACH FLIGHT AT HIGH ALTITUDES

200. PURPOSE. To present certain factors involved in the high-speed flight environment at high

altitudes. It is the lack of understanding of many of these factors involving the laws of 

aerodynamics, performance, and MACH speeds that has produced a somewhat higher accident rate

in some types of turbojet aircraft 

201. CRITICAL ASPECTS OF MACH FLIGHT. In recent years, a number of corporate jet 

airplanes have been involved in catastrophic loss of control during high-altitude/high-speed flight. 

A significant causal factor in these accidents may well have been a lack of knowledge by the pilot 

regarding critical aspects of high-altitude, MACH flight. 

a. Maximum operating altitudes of general aviation turbojet airplanes now reach 51,000 feet. It

is, therefore, logical to expect these types of accidents to continue unless pilots learn to respect the 

more critical aspects of high-altitude, high-speed flight and gain as much knowledge as possible 

about the specific make and model of aircraft to be flown and its unique limitations. 

b. From the pilot’s viewpoint, MACH is the ratio of the aircraft’s true airspeed to the local

speed of sound. At sea level, on a standard day (59° F/15° C) the speed of sound equals 

approximately 660 Kts or 1,120 feet per second. MACH 0.75 at sea level is equivalent to a TAS of 

approximately 498 Kts (0.75 x 660 Kts) or 840 feet per second. The temperature of the atmosphere 

normally decreases with an increase in altitude. The speed of sound is directly related only to 

temperature. The result is a decrease in the speed of sound up to about 36,000 feet. 

c. The sleek design of some turbojet airplanes has caused some operators to ignore critical

airspeed and MACH limitations. There are known cases in which corporate turbojet airplanes have 

been modified by disabling the airspeed and MACH warning systems to permit intentional 

excursions beyond the FAA certificated VMO/MMO limit for the specific airplane. Such action may

critically jeopardize the safety of the airplane by setting the stage for potentially hazardous 

occurrences. 

d. The compulsion to go faster may result in the onset of aerodynamic flutter, which in itself 

can be disastrous, excessive G-loading in maneuvering, and induced flow separation over the 

ailerons and elevators. This may be closely followed by the physical loss of a control surface, an 

aileron buzz or snatch, coupled with yet another dangerous phenomenon called MACH tuck, leading

to catastrophic loss of the airplane and the persons onboard. 

e. MACH-tuck is caused principally by two basic factors: 

(1) Shock wave-induced flow separation, which normally begins near the wing root, causes 

a decrease in the down wash velocity over the elevator and produces a tendency for the aircraft to 

nose down. 

(2) Aftward movement of the center of pressure, which tends to unbalance the equilibrium 

of the aircraft in relation to its center of gravity (CG) in subsonic flight. 

f. The airplane’s CG is now farther ahead of the aircraft’s aerodynamic center than it was in

slower flight. This dramatically increases the tendency of the airplane to pitch more nose down. 

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g. Pressure disturbances in the air caused by an airfoil in high-altitude/high-speed flight result 

from molecular collisions. These molecular collisions are the result of air that moves over an airfoil 

faster than the air it is overtaking can dissipate. When the disturbance reaches a point at which its

propagation achieves the local speed of sound, MACH 1 is attained. One hundred percent (100%) 

of the speed of sound at MSL with a temperature of 15° C is 760 statute or 660 NM per hour. This 

speed is affected by temperature of the atmosphere at altitude. Thus, optimum thrust fuel, and range

considerations are significant factors in the design of most general aviation turbine powered 

airplanes which cruise at some percentage of MACH 1. 

h. Because of the critical aspects of high-altitude/high-MACH flight, most turbojet airplanes 

capable of operating in the MACH speed ranges are designed with some form of trim and autopilot 

MACH compensating device (stick puller) to alert the pilot to inadvertent excursions beyond its 

certificated MMO. This stick puller should never be disabled during normal flight operations in the 

aircraft. 

i. If for any reason there is a malfunction that requires disabling the stick puller, the aircraft

must be operated at speeds well below MMO as prescribed in the applicable AFM procedures for the 

aircraft. 

j. An airplane’s IAS decreases in relation to TAS as altitude increases. As the IAS decreases

with altitude, it progressively merges with the low-speed buffet boundary where prestall buffet 

occurs for the airplane at a load factor of 1.0 G. The point where high-speed MACH, IAS, and lowspeed buffet boundary IAS merge is the airplane’s absolute or aerodynamic ceiling. Once an aircraft 

has reached its aerodynamic ceiling, which is higher than the altitude limit stipulated in the AFM, 

the aircraft can neither be made to go faster without activating the design stick puller at MACH 

limit nor can it be made to go slower without activating the stick shaker or pusher. This critical area 

of the aircraft’s flight envelope is known as coffin corner. 

k. MACH buffet occurs as a result of supersonic airflow on the wing. Stall buffet occurs at 

angles of attack that produce airflow disturbances (burbling) over the upper surface of the wing

which decreases lift. As density altitude increases, the angle of attack that is required to produce an 

airflow disturbance over the top of the wing is reduced until a density altitude is reached where

MACH buffet and stall buffet converge (described in paragraph 5n as coffin corner). When this 

phenomenon is encountered, serious consequences may result causing loss of control of the aircraft. 

l. Increasing either gross weight or load factor (G factor) will increase the low-speed buffet and 

decrease MACH buffet speeds. A typical turbojet airplane flying at 51,000 feet altitude at 1.0 G 

may encounter MACH buffet slightly above the airplane’s MMO (0.82 MACH) and low speed buffet 

at 0.60 MACH. However, only 1.4 G (an increase of only 0.4 G) may bring on buffet at the 

optimum speed of 0.73 MACH and any change in airspeed, bank angle, or gust loading may reduce 

this straight and level flight 1.4 G protection to no protection. Consequently, a maximum cruising

flight altitude must be selected which will allow sufficient buffet margin for the maneuvering

necessary and for gust conditions likely to be encountered. Therefore, it is important for pilots to be 

familiar with the use of charts showing cruise maneuvering and buffet limits. Flightcrews operating

airplanes at high speeds must be adequately trained to operate them safely. This training cannot be 

complete until pilots are thoroughly educated in the critical aspect of aerodynamic factors described 

herein pertinent to MACH flight at high altitudes.

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202. AIRCRAFT AERODYNAMICS AND PERFORMANCE. Pilots who operate aircraft at 

high speeds and high altitudes are concerned with the forces affecting aircraft performance caused

by the interaction of air on the aircraft. With an understanding of these forces, the pilot will have a 

sound basis for predicting how the aircraft will respond to control inputs. The importance of these 

aerodynamic forces and their direct application to performance and execution of aircraft maneuvers 

and procedures at altitude will be evident. The basic aerodynamics definitions that apply to highaltitude flight are contained in paragraph 3, Definitions. 

a. Wing Design. 

(1) The wing of an airplane is an airfoil or aircraft surface designed to obtain the desired 

reaction from the air through which it moves. The profile of an aircraft wing is an excellent 

example of an efficient airfoil. The difference in curvature between the upper and lower surfaces of 

the wing generates a lifting force. Air passing over the upper wing surface moves at a higher 

velocity than the air passing beneath the wing because of the greater distance it must travel over the 

upper surface. This increased velocity results in a decrease in pressure on the upper surface. The 

pressure differential created between the upper and lower surfaces of the wing lifts the wing upward 

in the direction of the lowered pressure. This lifting force is known as induced lift. Induced lift 

may be increased, within limits, by: 

(a) Increasing the angle of attack of the wing or changing the shape of the airfoil,

changing the geometry, e.g., aspect ratio. 

(b) Increasing the wing area. 

(c) Increasing the free-stream velocity. 

(d) A change in air density. 

(2) The pilot may have only varying degrees of control over these factors. Thus, the pilot 

must keep firmly in mind that an aircraft will obey the laws of physics just as precisely at its highspeed limits as it does during a slower routine flight, and that regardless of wing shape or design, 

MACH range flight requires precise control of a high volume of potential energy without exceeding

the critical MACH number or MACH crit. 

(3) MACH crit is important to high-speed aerodynamics because it is the speed at which the 

flow of air over a specific airfoil design reaches MACH 1, but the most important effect is

formation of a shock wave and drag divergence. 

(4) Sweeping the wings of an airplane is one method used by aircraft designers to delay the 

adverse effects of high MACH flight and bring about economical cruise with an increase in the 

critical MACH number. Sweep allows a faster airfoil speed before critical MACH is reached when 

compared to an equal straight wing. This occurs because the airflow now travels over a different

cross section (camber) of the airfoil. This new cross section has less effective camber which results 

in a reduced acceleration of airflow over the wing, thus allowing a higher speed before critical 

MACH is reached. Sweep may be designed either forward or rearward; the overall effect is the 

same. However, rearward sweep appears to be somewhat more desirable, since it has rapidly

presented fewer problems to manufacturers of models of general aviation aircraft in terms of

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AC 61-107A 1/2/03 

unwanted design side effects. In effect, the wing is flying slower than the airspeed indicator 

indicates and, similarly, it is developing less drag than the airspeed indicator would suggest. Since 

less drag is being developed for a given indicated airspeed, less thrust is required to sustain the 

aircraft at cruise flight. 

(5) There is a penalty, however, on the low-speed end of the spectrum. Sweeping the wings 

of an aircraft increases the landing/stall speed which, in turn, means higher touchdown speed, with 

proportionally longer runway requirements and more tire and brake wear as opposed to a straightwing design. A well-stabilized approach with precise control of critical "V" speeds is necessary. In

other words, to achieve a safe margin airspeed on the wing that will not result in a stalled condition 

with the wingtips stalling prior to the rest of the wing and possibly rolling uncontrollably to the right 

or left, the swept-wing aircraft must be flown at a higher actual airspeed than a straight wing

aircraft. 

(6) Drag curves are approximately the reverse of the lift curves, in that a rapid increase in

drag component may be expected with an increase of angle of attack with the swept wing; the 

amount being directly related to the degree of sweep or reduction of aspect ratio. 

(7) The extension of trailing edge flaps and leading edge devices may, in effect, further

reduce the aspect ratio of the swept wing by increasing the wing chord. This interplay of forces 

should be well understood by the pilot of the swept-wing aircraft, since raising the nose of the 

aircraft to compensate for a mild undershoot during a landing approach at normal approach speeds 

will produce little lift, but may instead lead to a rapid decay in airspeed, thus critically

compromising the margin of safety

(8) Another method of increasing the critical MACH number of an aircraft wing is through 

the use of a high-speed laminar airflow airfoil in which a small leading edge radius is combined

with a reduced thickness ratio. This type of wing design is more tapered with its maximum 

thickness further aft, thus distributing pressures and boundary layer air more evenly along the chord 

of the wing. This tends to reduce the local flow velocities at high MACH numbers and improve

aircraft control qualities. 

(9) Several modern straight-wing, turbojet aircraft make use of the design method described 

in subparagraph (4). To delay the onset of MACH buzz and obtain a higher MMO, these aircraft

designs may incorporate the use of both vortex generators and small triangular upper wing strips as 

boundary layer energizers. Both systems seem to work equally well, although the boundary layer 

energizers generally produce less drag. Vortex generators are small vanes affixed to the upper wing

surface, extending approximately 1 to 2 inches in height. This arrangement permits these vanes to 

protrude through the boundary layer air. The vortex generators deflect the higher energy airstream 

downward over the trailing edge of the wing and accelerate the boundary layer aft of the shock 

wave. This tends to delay shock-induced flow separation of the boundary layer air which causes 

aileron buzz, and thus permits a higher MMO. The lift characteristics of straight-wing and sweptwing airplanes related to changes in angle of attack are more favorable for swept-wing airplanes. 

An increase in the angle of attack of the straight-wing airplane produces a substantial and constantly

increasing lift vector up to its maximum coefficient of lift and, soon thereafter, flow separation 

(stall) occurs with a rapid deterioration of lift. 

Page Par 202 26
1/2/03 AC 61-107A 

(10) By contrast, the swept wing produces a much more gradual buildup of lift with no welldefined maximum coefficient, the ability to fly well beyond this point, and no pronounced stall 

break. The lift curve of the short, low-aspect ratio (short span, long chord) wing used on presentday military fighter aircraft compares favorably with that of the swept wing, and that of other wing

designs which may be even more shallow and gentle in profile. 

(11) Regardless of the method used to increase the critical MACH number, airflow over the 

wing is normally smooth. However, as airspeed increases, the smooth flow becomes disturbed. The 

speed at which this disturbance is usually encountered is determined by the shape of the wing and 

the degree of sweep. 

(12) When the aircraft accelerates, the airflow over the surface of the wing also accelerates 

until, at some point on the wing, it becomes sonic. The indicated airspeed at which this occurs is 

the critical MACH number (MACH crit) for that wing. 

b. Jet Engine Efficiency. 

(1) The efficiency of the jet engine at high altitudes is the primary reason for operating in 

the high-altitude environment. The specific fuel consumption of jet engines decreases as the outside

air temperature decreases for constant revolutions per minute (RPM) and TAS. Thus, by flying at a 

high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the 

most advantageous cruise speed. For efficiency, jet aircraft are typically operated at high altitudes 

where cruise is usually very close to RPM or exhaust gas temperature limits. At high altitudes, little

excess thrust may be available for maneuvering. Therefore, it is often impossible for the jet aircraft

to climb and turn simultaneously, and all maneuvering must be accomplished within the limits of 

available thrust and without sacrificing stability and controllability. 

(2) Compressibility also is a significant factor in high-altitude flight. The low temperatures

that make jet engines more efficient at high altitudes also decrease the speed of sound. Thus, for a 

given TAS, the MACH number will be significantly higher at high altitude than at sea level. This 

compressibility effect due to supersonic airflow will be encountered at slower speeds at high altitude 

than at low altitude. 

c. Controllability Factors. 

(1) Static stability is the inherent flight characteristic of an aircraft to return to equilibrium 

after being disturbed by an unbalanced force or movement. 

(2) Controllability is the ability of an aircraft to respond positively to control surface 

displacement, and to achieve the desired condition of flight. 

(3) At high-flight altitudes, aircraft stability and control may be greatly reduced. Thus, 

while high-altitude flight may result in high TAS and high MACH numbers, calibrated airspeed is 

much slower because of reduced air density. This reduction in density means that the angle of 

attack must be increased to maintain the same coefficient of lift with increased altitude. 

Consequently, jet aircraft operating at high altitudes and high MACH numbers may simultaneously

experience problems associated with slow-speed flight such as Dutch roll, adverse yaw, and stall. In

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addition, the reduced air density reduces aerodynamic damping, overall stability, and control of the 

aircraft in flight. 

(a) Dutch roll is a coupled oscillation in roll and yaw that becomes objectionable when 

roll, or lateral stability is reduced in comparison with yaw or directional stability. A stability

augmentation system is required to be installed on the aircraft to dampen the Dutch roll tendency

when it is determined to be objectionable, or when it adversely affects control stability requirements 

for certification. The yaw damper is a gyro-operated autocontrol system installed to provide rudder 

input and aid in canceling out yaw tendencies such as those in Dutch roll. 

(b) Adverse yaw is a phenomenon in which the airplane heading changes in a direction 

opposite to that commanded by a roll control input. It is the result of unequal lift and drag

characteristics of the down-going and up-going wings. The phenomena are alleviated by tailoring

the control design by use of spoilers, yaw dampers, and interconnected rudder and aileron systems. 

(4) Supersonic flow over the wing is responsible for: 

(a) The formation of shock waves on the wing which result in drag rise. 

(b) An aft shift in the center of lift resulting in a nosedown pitching moment called 

MACH tuck. 

(c) Airflow separation behind the shock waves resulting in MACH buffet. 

(5) Swept wing and airfoil design alone, with boundary layer energizers such as the vortex

generators described earlier, has reduced the hazardous effect of the problems described above. 

However, these problems are still encountered to some extent by the modern turbojet airplane in 

high-altitude flight.

(6) In general, this discussion has been confined to normal level, unaccelerated 1.0 G-flight, 

when turning or maneuvering about the pitch axis; however, acceleration of G-forces can occur 

while maintaining a constant airspeed. As G-forces increase, both the aircraft’s aerodynamic weight

and angle of attack increase. The margin over low-speed stall buffet decreases, as well as the 

margin below MACH buffet, because of the increased velocity of the air over the wing resulting

from the higher angle of attack. This, in effect, could lower the aerodynamic ceiling for a given 

gross-weight. Increased G-loading can also occur in non-maneuvering flight because of 

atmospheric turbulence or the lack of fine-touch skill by the pilot. Pilots flying at high altitudes in 

areas where turbulence may be expected must carefully consider acceptable safety margins 

necessary to accommodate the sudden and unexpected vertical accelerations which may be 

encountered with little or no warning. How wide is the safety margin between low-speed and highspeed buffet boundaries for an altitude and weight in a 30° bank? The answer may be easily

determined by reference to the Cruise Maneuver/Buffet Limit Chart for a particular aircraft. For

example, in a typical jet aircraft, the 1.0 G buffet-free margin at FL 350 is 135 Kts; at FL 450 this 

speed is reduced to a mere 26 Kts. Thus, the safety margin in airspeed spread diminishes rapidly as

the aircraft climbs and leaves little room for safety in the event of an air turbulence encounter or 

accidental thunderstorm penetration. 

Page Par 202 28
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Par 202 Page 29 (and 30)

 (7) If a thunderstorm cannot be avoided, follow high-altitude thunderstorm penetration 

procedures and avoid over-action of thrust levers. When excessive airspeed buildup occurs, pilots 

may wish to use speed brakes. The use of aerodynamic speed brakes, when they are part of the 

lateral control system, may change the roll rate any time there is a lateral control input. 

 (8) For detailed information concerning the operation of specific turbojet aircraft, refer to 

the aircraft’s AFM. 

203. - 299. RESERVED.