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CONTENTS Paragraph Page CHAPTER 1. GENERAL INFORMATION
1-1. Purpose1
1-2. Cancellation1
1-3. Related Regulations1
1-4. Definitions1
1-5. Related Reading Material3
1-6. Warning, Caution, and Note3
1-7. Background4
1-8. Discussion4
CHAPTER 2. RECOMMENDATIONS FOR HIGH-ALTITUDE TRAINING
2-1. Purpose5
2-2. Outline5
2-3. Ground Training7
2-4. The High-Altitude Flight Environment7
2-5. Weather8
2-6. Flight Planning and Navigation9
2-7. Physiological Training12
2-8. FAA Physiological Training30
2-9. High-Altitude Systems and Equipment31
2-10. Aerodynamics and Performance Factors35
2-11. Emergencies and Irregularities at High Altitudes35
2-12. Flight Training38
CHAPTER 3. MACH FLIGHT AT HIGH ALTITUDES
3-1. Purpose41
3-2. Critical Aspects of Mach Flight41
3-3. Aircraft Aerodynamics and Performance43
LIST OF TABLES
Table 2-1. Gas Laws and Their Effect on the Human Body12
Table 2-2. Trapped Gas Issues16
Table 2-3. In-Flight Treatment of Trapped Gas Emergencies17
Table 2-4. Hypoxia Types18
Table 2-5. Common Hypoxia Signs and Symptoms20
Table 2-6. Common Emergency Procedures for Hypoxia22
Table 2-7. Types of Evolved Gas Decompression Sickness27
Table 2-8. Aircraft Cabin Volume Ratios36
LIST OF FIGURES
Figure 2-1. Standard High-Altitude Service Volume12
Figure 2-2. Blood Cells and Alveoli14
Figure 2-3. Times of Useful Consciousness Versus Altitude24
Par 1-1 Page 1 CHAPTER 1. GENERAL INFORMATION
1-1. PURPOSE. This advisory circular (AC) alerts pilots transitioning from aircraft with less performance capability to complex, high-performance aircraft capable of operating at high altitudes and high airspeeds of the special physiological, equipment, and aerodynamic considerations involved in these kinds of operations. It also provides information to aid pilots in becoming familiar with the basic phenomena associated with high-altitude and high-speed flight.
1-2. CANCELLATION. This AC cancels AC 61-107A, Operations of Aircraft at Altitudes Above 25,000 Feet MSL and/or Mach Numbers (MMO) Greater Than .75, dated January 2, 2003.
1-3. RELATED REGULATIONS. In addition to the training required by Title 14 of the Code of Federal Regulations (14 CFR) part 61, § 61.31(g), pilots of high-altitude aircraft should be familiar with the following regulations:
• Title 14 CFR Part 91, § 91.121, Altimeter Settings.
• Section 91.135, Operations in Class A Airspace.
• Section 91.159, VFR Cruising Altitude or Flight Level.
• Section 91.179, IFR Cruising Altitude or Flight Level.
• Section 91.180, Operations Within Airspace Designated as Reduced Vertical Separation Minimum Airspace.
• Section 91.211, Supplemental Oxygen.
• Section 91.215, ATC Transponder and Altitude Reporting Equipment and Use.
1-4. DEFINITIONS.
a. Aspect Ratio. The relationship between the wing chord and the wingspan. A short wingspan and wide wing chord equal a low aspect ratio.
b. Aileron Buzz. A very rapid oscillation of an aileron, at high airspeeds on some aircraft, that does not usually reach large magnitudes or become dangerous. It is often caused by shock-induced separation of the boundary layer.
c. Aileron Snatch. A violent back and forth movement of the aileron control as airflow changes over the surface.
d. Cabin Altitude. Cabin pressure in terms of equivalent altitude above sea level.
e. Critical Mach Number (Mcr). The free stream Mach number (M) at which local sonic flow such as buffet, airflow separation, and shock waves become evident. These phenomena occur above Mcr, and are as follows:
(1) Subsonic. M below .75.
(2) Transonic. M from .75 to 1.20.
(3) Supersonic. M from 1.20 to 5.0.
(4) Hypersonic. M above 5.0.
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f. Drag Divergence. A phenomenon that occurs when an airfoil’s drag increases sharply and requires substantial increases in power (thrust) to produce further increases in speed. Do not confuse drag divergence with Mcr. The drag increase is due to the unstable formation of shock waves that transform energy into heat and into pressure pulses that act to consume a major portion of the available propulsive energy. Turbulent air may produce an increase in the coefficient of drag.
g. Force. The four forces of lift, drag, thrust, and weight, which act on an aircraft in flight.
An aircraft is in a state of equilibrium when the thrust and drag are equal and opposite. It will continue to move forward at the same uniform speed. If thrust or drag becomes greater than the opposite force, the aircraft loses its state of equilibrium. If thrust is greater than drag, the aircraft will accelerate. If drag is greater than thrust, the aircraft will lose speed and eventually descend.
When lift and weight are equal and opposite, the airplane is in a state of equilibrium. If lift is greater than weight, the aircraft will climb. If weight is greater than lift, the airplane will descend.
h. Mach. Named after Ernst Mach, a 19th century Austrian physicist, Mach is the ratio of an aircraft’s true speed compared to the local speed of sound at a given time or place.
i. Mach Buffet. The airflow separation behind a shock wave pressure barrier caused by airflow over flight surfaces exceeding the speed of sound.
j. Mach (or Aileron) Buzz. A term used to describe a shock-induced flow separation of the boundary layer air before reaching the ailerons.
k. Mach Meter. 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.
l. Mach Number (M). A decimal number (M) representing the true airspeed (TAS) relationship to the local speed of sound (e.g., TAS 75 percent (.75 M) of the speed of sound where 100 percent of the speed of sound is represented as Mach 1 (1.0 M)). The local speed of sound varies with changes in temperature.
m. Mach Speed. The ratio or percentage of the TAS to the speed of sound
(e.g., 1,120 feet (ft) per second (ft/s) (660 knots (kts)) at mean sea level (MSL)), represented by a Mach number.
n. Mach Tuck. The result of an aft shift in the center of pressure causing a nosedown pitching moment.
o. Maximum Operating Limit Speed (MMO) in Mach. 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.
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p. Maximum Operating Limit Speed (VMO) Expressed in Knots Calibrated Airspeed (KCAS). An airplane’s maximum operating limit speed. 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.
q. Q-Corner or Coffin Corner. A term used to describe operations at high altitudes where low indicated airspeeds (IAS) yield high TAS (as indicated by Mach number) at high angles of attack (AOA). The high AOA results in flow separation, which causes buffet. Turning maneuvers at these altitudes increase the AOA and result in stability deterioration with a decrease in control effectiveness. The relationship of stall speed to the Mcr narrows to a point where sudden increases in AOA, roll rates, and/or disturbances (e.g., Clear Air Turbulence (CAT)) 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.
r. Service Ceiling. The maximum height above MSL at which an airplane can maintain a rate of climb of 100 ft per minute (ft/min) under normal conditions.
1-5. RELATED READING MATERIAL. The following documents provide additional information related to the subject of this AC:
• Airplane Flying Handbook (FAA-H-8083-3).
• Glider Flying Handbook (FAA-H-8083-13).
• Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25).
• Aeronautical Information Manual (AIM).
• Airplane Upset Recovery Training Aid.
• Roy L. DeHart & Jeffrey R. Davis, Fundamentals of Aerospace Medicine (3rd ed.).
(Philadelphia, PA: Lippincott Williams & Wilkins, 2002).
• David Gradwell & David J. Rainford (Eds.), Ernsting’s Aviation Medicine (4th ed.).
(London: CRC Press, 2006).
• https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/physiologc.pdf.
• http://www.faa.gov/data_research/research/med_humanfacs/oamtechreports/2010s/med ia/201322.pdf.
1-6. WARNING, CAUTION, AND NOTE. This document uses “Warning,” “Caution,” and “Note” throughout to highlight different levels of importance of information provided. The list below defines them in order of decreasing severity or importance.
a. Warning. Explanatory information about an operating procedure, practice, condition, etc., that may result in injury or death if not carefully observed or followed.
b. Caution. Explanatory information about an operating procedure, practice, or condition, etc., that may result in damage to equipment if not carefully observed or followed.
c. Note. Explanatory information about an operating procedure, practice, or condition, etc.
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1-7. BACKGROUND. On September 17, 1982, the National Transportation Safety Board (NTSB) issued a series of safety recommendations that included, among other things, the establishment of a minimum training curriculum for use at pilot schools covering pilots’ initial transition into General Aviation (GA) 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 that the NTSB believed to involve a lack of flightcrew knowledge and proficiency in GA turbojet airplanes 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 that led to these accidents, the NTSB is concerned that a lack of flightcrew knowledge and proficiency in the subject matter of this AC contributed to the initial loss of control, or the inability to regain control of the aircraft, or both. A requirement has been added to 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 ft MSL.
Recommended training in high-altitude operations that would meet the requirements of this regulation can be found in Chapter 2. We are updating this AC to include strong emphasis on hypoxia awareness, time of useful consciousness (TUC), and recovery.
1-8. DISCUSSION.
a. Knowledge and Skill Requirements. 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 of 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 elements 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.
b. Training and Review. Pilots who are not familiar with operations in the high-altitude and high-speed environment are encouraged to obtain thorough and comprehensive training. This should include a checkout in complex high-performance aircraft before engaging in extensive high-speed flight, particularly at high altitudes. The training should enable the pilot to become thoroughly familiar with aircraft performance charts, aircraft systems, and procedures. The pilot should also review the more critical elements of high-altitude flight planning and operations. 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. Knowledge of and skill in high-performance aircraft will enhance the pilot’s ability to transition to operating aircraft in the high-speed and high-altitude flight environment.
Par 2-1 Page 5 CHAPTER 2. RECOMMENDATIONS FOR HIGH-ALTITUDE TRAINING
2-1. PURPOSE. This chapter presents an outline for recommended high-altitude training that meets the requirements of § 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 certify that he or she gave training in high-altitude operations by providing an endorsement in the pilot’s logbook or training record. A sample high-altitude endorsement is available in the current edition of Advisory Circular (AC) 61-65, Certification: Pilots and Flight and Ground Instructors.
a. Training Applicability. Note that § 61.31(g) applies only to pilots who fly pressurized airplanes with a service ceiling or maximum operating altitude above 25,000 ft mean sea level (MSL), whichever is lower. But we highly recommend it for all pilots who fly at altitudes above 10,000 ft MSL.
NOTE: All pressurized aircraft have a specified maximum operating altitude above which they must not operate. 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 ft MSL or lower, unless certain airworthiness standards are met. Maximum operating altitudes and service ceilings are specified in the Airplane Flight Manual (AFM).
b. Aircraft Applicability. The training outlined in this chapter is designed primarily for single-engine and light twin-engine airplanes that fly at high altitudes, but that do not require type ratings. The training should be incorporated into all training courses for aircraft that fly above 25,000 ft MSL if the pilot has not already received training in high-altitude flight. The training in this chapter is intended mainly for reciprocating engine and turboprop aircraft.
Therefore, it does not encompass high-speed flight factors such as acceleration, G-forces, Mach, and turbine systems. You can find information on high-speed flight in Chapter 3.
2-2. OUTLINE. This training covers the minimum information needed by pilots to operate safely at high altitudes. We encourage pilots to seek additional information specific to their operation.
a. Ground Training.
(1) The High-Altitude Flight Environment.
(2) Weather.
• The atmosphere.
• Winds and wind shear.
• Clear Air Turbulence (CAT).
• Clouds and thunderstorms.
• Icing.
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(3) Flight Planning and Navigation.
• Flight planning.
• Weather charts.
• Navigation.
• Navigational Aids (NAVAID).
(4) Physiological Training.
• Physics of the atmosphere.
• Respiration.
• Trapped gas.
• Physiological problems at increased altitudes.
• Decompression in pressurized aircraft.
• Altitude chamber (optional).
• Physiology of flight video library.
(5) High-Altitude Systems and Components.
• Turbochargers.
• Oxygen and oxygen equipment.
• Pressurization systems.
• High-altitude components.
(6) Aerodynamics and Performance Factors.
• Air density.
• Knots true airspeed (KTAS).
(7) Emergencies.
• Decompression.
• Turbocharger failure or malfunction.
• Vapor lock.
• In-flight fire.
• Flight through thunderstorm activity or severe turbulence.
b. Flight Training.
(1) Preflight Briefing.
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(2) Preflight Planning.
• Weather briefing and considerations.
• Course plotting.
• AFM/pilot’s operating handbook (POH) review.
• Flight plan.
(3) Preflight Inspection.
• Inspection/setup of oxygen system—verify quantity and proper switch position.
• Functional test of the oxygen system to include communications.
(4) Run-Up, Takeoff, and Initial Climb.
(5) Climb to High Altitude and Normal Cruise Operations While Operating Above 25,000 ft MSL.
(6) Simulated Emergencies.
• Simulated rapid decompression.
• Emergency descent.
(7) Planned Descents.
(8) Shutdown Procedures.
(9) Postflight Discussion.
2-3. GROUND TRAINING. Ground training should cover all aspects of high-altitude flight, including the flight environment, physiological aspects of high-altitude flight, and the need for the immediate donning of oxygen masks following activation of cabin altitude warning. Training should also address high-altitude 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 subparagraph 2-2a. Accident reports are available from the National Transportation Safety Board (NTSB) and some aviation organizations.
2-4. THE HIGH-ALTITUDE FLIGHT ENVIRONMENT. Section 61.31(g) considers all flight operations conducted above 25,000 ft MSL to be high altitude. However, the high-altitude environment itself begins below 25,000 ft MSL. For example, flight levels (FL) are used at and above 18,000 ft MSL (e.g., FL 180) to indicate levels of constant atmospheric pressure in relation to a reference datum of 29.92 inches of mercury (inHg). 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. Pilots of high-altitude aircraft are subject to two principal types of airspace at altitudes above 10,000 ft MSL. These are Class E airspace, which extends from the surface up to FL 180, and Class A airspace, which extends from FL 180 to FL 600.
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2-5. 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.
(1) Composition. The atmosphere is a mixture of gases in constant motion, composed of approximately 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. The atmosphere constantly absorbs and releases water vapor, which causes changes in weather. There are three levels of the atmosphere where high-altitude flight may occur. The first is the troposphere, which can extend from sea level to approximately FL 350 around the poles and up to FL 650 around the equator. The next is the tropopause, which is a thin layer at the top of the troposphere that traps water vapor in the lower level. The highest is the stratosphere, which extends from the tropopause to approximately 22 miles (mi). 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 ft. Condensation trails, or contrails, are common in the upper levels of the troposphere and in the stratosphere. Aircraft flying in clear, cold, humid air generate these cloud-like streamers formed by the addition of water vapor from aircraft exhaust gases 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.
(2) Atmospheric Density. Atmospheric density in the troposphere decreases gradually with altitude, decreasing to 50 percent of its sea-level value at 18,000 ft MSL. This means that at FL 180 the same volume of 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.
b. Winds and Wind Shear.
(1) Jet Stream. 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.
(2) Polar Front Jet Stream. In the midlatitudes, 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. 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 latitude 25°, about 8 mi (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 that move north of the jet stream.
c. Wind Shear. Wind shear is a sudden change of wind velocity and/or direction.
Horizontal wind shear and turbulence are frequently found on the northern side of the jet stream.
Par 2-5 Page 9 Horizontal wind changes of 40 kts within 150 nautical miles (NM) or vertical wind shear of 6 kts or greater per 1,000 ft usually indicate moderate to severe turbulence and should be avoided.
d. Clear Air Turbulence (CAT). CAT is a meteorological phenomenon associated with high-altitude winds. This high-level turbulence occurs where no clouds are present and can take place at any altitude (normally higher than 15,000 ft above ground level (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 greatest during the winter months. Causes of CAT are 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. Pilot Weather Reports (PIREP) are one of the best methods of receiving timely and accurate reports on icing and turbulence at high altitudes.
e. Clouds and Thunderstorms. Cirrus and cirriform clouds are high-altitude clouds composed of ice crystals. Cirrus clouds are found in stable air above 30,000 ft MSL 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 ft MSL or more.
NOTE: Cumulonimbus clouds are thunderstorm clouds that present a particularly severe hazard that pilots should circumnavigate. 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.
2-6. FLIGHT PLANNING AND NAVIGATION.
a. Flight Planning.
(1) Operational Considerations. Careful flight planning is critical to safe high-altitude flight. Pilots must consider pressurization system settings, particularly on takeoff, climb, and descent to assure operation in accordance with the manufacturer’s recommendations. They must also consider fuel management, reporting points, weather briefings, direction of flight, airplane performance charts, aircraft systems and procedures, high-speed winds aloft, and oxygen
Page 10 Par 2-6 duration charts. When possible, additional oxygen should be provided to allow for emergency situations. Breathing rates increase under stress and extra oxygen could be necessary.
(2) Altitude Considerations. Flight planning should take into consideration factors associated with altitudes that pilots will transit while climbing to or descending from the high altitudes (e.g., airspeed limitations below 10,000 ft 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 replan descents and advise air traffic control (ATC).
b. Gradual Descents. Pilots should plan gradual descents from high altitudes in advance to prevent excessive engine cooling and provide passenger comfort. They should comply with the manufacturer’s recommendations found in the AFM, especially regarding descent power settings, to avoid stress on the engines. Although most jets can descend rapidly at idle power, 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 ft/min.
Before landing, cabin pressure should be equal to ambient pressure or inner ear injury can result.
If delays occur en route, pilots should adjust descents.
c. Weather Charts. Before beginning a high-altitude flight, pilots should consult all weather charts, including those designed for low levels. High-altitude flight allows a pilot to overfly most adverse weather, but pilots must consider low-altitude weather for arrival, departure, and en route emergencies that require an immediate diversion.
d. Types of Weather Charts. Weather charts that provide information on high-altitude weather include:
• Constant pressure charts 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 (the five charts are issued every 12 hours).
• Prognostic charts, which provide forecast winds, temperature, and expected movement of weather over the 6-hour valid time of the chart.
• Observed tropopause charts, which provide jet stream, turbulence, and temperature/wind/pressure reports at the tropopause over each station.
• Tropopause wind prognostic charts, which are helpful in determining jet stream patterns and the presence of CAT and wind shear.
• Tropopause height vertical wind shear charts, which are helpful in determining jet stream patterns and the presence of CAT and wind shear. Dashed lines indicate wind shear.