Date: 1/2/03 AC No: 61-107A Subject: OPERATIONS OF AIRCRAFT AT 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:
(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.
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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.
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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 Par 5 Page iv
CONTENTS CHAPTER 1. RECOMMENDATIONS FOR HIGH-ALTITUDE TRAINING Paragraph Pa ge
100. Purpose1
101. Outline1
102. Ground Training3
103. The High-Altitude Flight Environment4
104. Weather5
105. Flight Planning and Navigation6
106. Physiological Training8
107. Additional Physiological Training12
108. High-Altitude Systems and Equipment14
109. Aerodynamics and Performance Factors17
110. Emergencies and Irregularities at High Altitudes18
111. Flight Training20
112. - 199. Reserved22
CHAPTER 2. MACH FLIGHT AT HIGH ALTITUDES
200. Purpose23
201. Critical Aspects of Mach Flight23
202. Aircraft Aerodynamics and Performance25
203. - 299. Reserved29
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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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(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.
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(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.
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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.
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(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.
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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, Par 104 Page 6