High-Altitude Operations

High-altitude operations are flights at the levels where jet airplanes cruise, in the thin, cold air of the upper troposphere and the lower stratosphere. The FAA draws its line at 25,000 ft MSL: Advisory Circular 61-107B addresses operations above that altitude and at Mach numbers greater than .75, and 14 CFR 61.31(g) requires additional training before a pilot commands a pressurized airplane able to fly above it. From FL290 to FL410, in airspace where the reduced vertical separation minimum (RVSM) applies, airplanes are separated vertically by only 1,000 ft and must be approved for RVSM operations.

Flying high saves fuel and keeps the airplane above most weather, but three margins shrink together: if pressurization fails the crew have seconds, not minutes, to put on their masks; the wing works between low-speed and high-speed buffet boundaries that close in as altitude rises; and the engines have little thrust to spare. Hypoxia, decompression and maximum altitude each have their own article.

The high-altitude environment

Air pressure falls with height at a decreasing rate. In the ICAO standard atmosphere:

Oxygen stays at about 21 % of the air at every flight level. What falls is its partial pressure (Dalton's law), about 160 mmHg at sea level, and that pressure is what drives oxygen into the blood.

The ISA temperature falls by 1.98 °C per 1,000 ft up to the tropopause at 36,090 ft (11 km), where it reaches −56.5 °C, and stays constant above. The real tropopause is higher where the air beneath is warmer: about 16 to 18 km and −75 to −80 °C over the equator, about 8 km and −40 to −50 °C over the poles. A turbofan's thrust falls roughly with air density, so at FL370 it may produce only about a quarter of its sea-level thrust.

Cabin altitude is the pressure altitude inside the cabin: the altitude in the standard atmosphere whose pressure equals the cabin pressure. Oxygen rules are written in terms of it. Pressurization systems are designed to hold about 8,000 ft or less; ATPL texts give about 6,000 ft as typical at 30,000 ft. Normal cabin rates are about 500 ft/min up and 300 ft/min down, to spare the middle ear.

Physiology: hypoxia, decompression sickness and vision

Hypoxia and time of useful consciousness

Hypoxic hypoxia, too little oxygen reaching the blood, is painless and insidious: euphoria and the loss of self-criticism leave the victim unaware of being impaired. The FAA's Aeronautical Information Manual (AIM) notes that night vision deteriorates from a cabin altitude of 5,000 ft, that judgment, memory and coordination are impaired from 12,000 to 15,000 ft, and that above 15,000 ft tunnel vision and cyanosis develop.

The time of useful consciousness (TUC) runs from the loss of oxygen until the person can no longer take steps to help themselves, which comes before unconsciousness. FAA AC 61-107B gives these approximate values at rest:

The circular advises assuming roughly half these times after a rapid decompression to between 25,000 and 43,000 ft, and physical activity shortens them further. Sources differ, particularly at the lower levels: the AIM gives 5 to 12 minutes at 20,000 ft, the CAE Oxford ATPL text 30 minutes at rest at 20,000 ft and 30 to 90 seconds at 35,000 ft.

View full schematic

Oxygen equipment has altitude limits too. In the ATPL scheme, air suffices up to 10,000 ft and an air-oxygen mixture up to 33,700 ft, where 100 % oxygen is needed just to keep the sea-level alveolar oxygen pressure. Up to 40,000 ft only 100 % oxygen will do, and above it oxygen must be delivered under positive pressure (pressure breathing). Hyperventilation produces similar symptoms, but at altitude they are treated as hypoxia first.

Decompression sickness and trapped gas

Gas trapped in the ears, sinuses, gut and teeth expands as pressure falls (Boyle's law), doubling by about FL180 (see barotrauma and decompression sickness). Under Henry's law the nitrogen dissolved in the body is proportional to the surrounding pressure, and if the pressure falls far enough it forms bubbles: decompression sickness (DCS). ATPL texts describe it as unlikely below 14,000 ft and more likely the higher and longer the exposure above 18,000 ft, and any exposure above 25,000 ft is normally associated with a risk of DCS. Its forms are the bends (joints), the creeps (skin), the chokes (lungs) and the staggers (nervous system).

Diving greatly increases the risk. The AIM recommends waiting at least 12 hours after a dive that needed no controlled ascent, and 24 hours after one that did, before flying up to 8,000 ft, and at least 24 hours after any scuba dive before flying higher. The CAE Oxford ATPL text gives 12 hours, or 24 hours if a depth of 30 ft was exceeded.

Vision

Above the clouds, with nothing outside to focus on, the eyes relax to a focal distance of about 10 to 30 ft, a condition the AIM calls empty-field myopia: the pilot looks without seeing. Glare from cloud tops is intense, and the AIM recommends sunglasses that absorb at least 85 % of visible light, all colors equally.

Pressurization and oxygen equipment

A slow leak may go unnoticed while hypoxia erodes judgment. Certification rules (14 CFR 25.841 and CS 25.841) require a warning to the flight crew when the cabin altitude exceeds 10,000 ft, and passenger masks on many types drop automatically at about 14,000 ft. The crew respond by putting on their masks before anything else, establishing communication and making an emergency descent to 10,000 ft or the minimum safe altitude, whichever is higher.

Flight crews of pressurized airplanes operated above 25,000 ft must have quick-donning masks, which can be put on with one hand within five seconds (EASA CAT.IDE.A.235 and its guidance material; the FAA rules use the same definition). These diluter-demand masks deliver diluted oxygen, 100 % oxygen or oxygen under pressure. The supplemental oxygen rules differ:

High-altitude aerodynamics: Mach, buffet and coffin corner

Low down, an airliner's speed limit is VMO, set by dynamic pressure; high up it is MMO, set by compressibility, and FAA ATP material places the changeover at about 25,000 ft. Climbing at constant Mach, the indicated airspeed falls with density, bringing the airplane closer to the low-speed buffet boundary.

Air accelerates over the wing, so locally it reaches Mach 1 while the airplane is still subsonic. The free-stream Mach number at which that first happens is the critical Mach number (Mcrit); it falls as the angle of attack rises, so weight, altitude and turns all lower it. Above Mcrit a shock wave forms, and as it strengthens the boundary layer separates behind it and the airframe shakes: Mach buffet, also called high-speed buffet or shock stall. The accompanying nose-down Mach tuck is countered by a Mach trim system or, on fly-by-wire types, by the control laws.

At the other end, the thin air demands a high lift coefficient just to hold 1 g, and above about M 0.4 compressibility lowers the angle of attack at which the flow separates. The low-speed buffet speed therefore rises with altitude while the high-speed buffet Mach number falls. Where the two meet is the coffin corner, or aerodynamic ceiling: only one speed can be flown without buffet, and the margin is zero.

Airliners stay well below it. The flight manual's buffet onset chart gives the load factor available before buffet for a weight, center of gravity and altitude, and the conventional cruise margin is 1.3 g, a level turn at about 40° of bank. On the A320, the MAX REC flight level on the MCDU PROG page keeps at least a 0.3 g margin, and the FMGS accepts a higher cruise level only while the margin stays above 0.2 g. The day's maximum altitude is the lowest of the certified maximum operating altitude (39,100 ft for an unmodified A320), the thrust-limited altitude and the buffet-limited altitude; weight and warm air lower it.

With little thrust in reserve, a turn or a speed loss at maximum altitude may be recoverable only by descending. Swept-wing jets are also more prone to Dutch roll in thin air, and with the yaw damper inoperative the minimum equipment list usually imposes altitude and speed restrictions. The A320 procedure in alternate or direct law at high altitude is to descend: about 4,000 ft below REC MAX significantly reduces stall warnings in turbulence.

Weather at altitude

The tropopause marks the upper limit of most weather and the level of maximum wind, jet streams and clear air turbulence. Jet streams are upper winds of more than 60 kt, lying close to the tropopause breaks near 40° and 60° latitude; the subtropical jet's core is near 200 hPa, about FL390. Clear air turbulence (CAT) is strongest on the cold, low-pressure side of a jet, near or just below the axis, and worse around upper troughs, whose sharp changes in wind direction produce strong shear. Mountain waves can reach high levels too, with lenticular clouds sometimes above the tropopause (see jet streams).

Convective tops are the other hazard. In the intertropical convergence zone they can exceed 50,000 ft, and the anvils of such storms hold high-altitude ice crystals, which do not stick to a cold airframe but can accrete inside engines and air data probes, causing power loss and airspeed errors without the usual icing cues. The loss of Air France Flight 447 in 2009 began with ice crystals obstructing its pitot probes. Guidance is to keep clear of the air above and downwind of strong cells, using the radar tilt to look at the reflective cores below the cruise level (see weather radar). Volcanic ash gives no useful radar echo and can melt in the hot section of a turbine, causing flameout.

Temperature matters too. A deviation of ISA +15 °C can cost several thousand feet of maximum altitude and increase fuel burn, while on long, cold flights the fuel cools toward the total air temperature: Jet A freezes near −40 °C and Jet A-1 near −47 °C.

The FAA high-altitude endorsement

Under 14 CFR 61.31(g), no person may act as pilot in command of a pressurized airplane whose service ceiling or maximum operating altitude, whichever is lower, is above 25,000 ft MSL without training in high-altitude operations and an instructor's logbook endorsement:

• Ground training covering high-altitude aerodynamics and meteorology, respiration, hypoxia and its effects, the time of useful consciousness, gas expansion and bubble formation, decompression, and the other physiological aspects of high-altitude flight.
• Flight training in a pressurized airplane, or in a simulator representing one, covering normal cruise above 25,000 ft, a simulated rapid decompression and an emergency descent.

The endorsement is separate from the complex and high-performance endorsements, so a pressurized piston twin able to fly above 25,000 ft may need all three. An unpressurized airplane flown high needs oxygen under 91.211 but no endorsement. AC 61-107B gives a recommended ground and flight training outline that meets the rule, and notes that none of the regulations require altitude chamber training.

EASA licensing has no equivalent endorsement: high-altitude physiology is part of the Human Performance theory syllabus, meteorology and high-speed aerodynamics are taught in their own subjects, and altitude chamber training is not required for license issue.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.