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.
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e. Navigation. Specific charts are available 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 Very high frequency Omnidirectional Range (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 FLs.
(1) Jet Routes. Jet routes in the United States are predicated solely on VOR or VOR Collocated Tactical Air Navigation (VORTAC) facilities, except in Alaska where some are based on low/medium-frequency NAVAIDs. All jet routes are identified by the letter “J” followed by the airway number.
(2) Q-Routes. The United States and Canada use “Q” as a designator for Area Navigation (RNAV) routes. Q-routes routes are designed for RNAV- or Required Navigation Performance (RNP)-capable aircraft. One benefit of this system is that aircraft with RNAV or RNP capability can fly safely along closely spaced parallel flight paths on high-density routes, which eases airspace congestion.
(3) Reporting Points. Designated reporting points must be used by flights using the jet route unless otherwise advised by ATC. Pilots may conduct flights above FL 450 on a point-to-point basis, using the facilities depicted on the en route high-altitude chart as navigational guidance. RNAV routes, using either random or fixed waypoints, are also used for direct navigation at high altitudes. These routes are based on RNAV capability between waypoints defined in terms of latitude/longitude coordinates, degree-distance 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.
f. NAVAIDs. VOR, distance measuring equipment (DME), and Tactical Air Navigation Aids (TACAN) depicted on high-altitude charts are designated as Class H NAVAIDs, signifying that their standard service volume is from 1,000 ft AGL up to and including 14,500 ft AGL at radial distances out to 40 NM; from 14,500 ft AGL up to and including 60,000 ft AGL at radial distances out to 100 NM; and from 18,000 ft AGL up to and including 45,000 ft AGL at radial distances out to 130 NM. See Figure 2-1, Standard High-Altitude Service Volume, for an illustration of the high-altitude service volume. Ranges of Non-Directional Radio Homing Beacon (NDB) service volumes are the same at all altitudes.
Page 12 Par 2-6 FIGURE 2-1. STANDARD HIGH-ALTITUDE SERVICE VOLUME
2-7. PHYSIOLOGICAL TRAINING. To ensure safe flights at high altitudes, pilots of high-altitude 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 2-8. Although not required, altitude chamber training is highly recommended for all pilots.
a. Physics of the Atmosphere. The combined weight, or force, of all gases in the atmosphere at any given point gives us our atmospheric pressure. As you ascend from sea level, the atmospheric pressure will correspondingly drop. As atmospheric pressure drops, the air becomes less dense. The primary reason for this phenomenon lies in the kinetic nature of atoms and molecules. Molecules, especially those of a gas, are in a constant state of motion. As pressure around the molecules is reduced, the molecules will travel further apart. This explains why air becomes less dense as altitude increases, and accounts for gas expansion. Since air is a mixture of gases, it is subject to the laws that govern all gases. Table 2-1, Gas Laws and Their Effect on the Human Body, explains the effects of reduced barometric pressure and its interplay on the human body.
TABLE 2-1. GAS LAWS AND THEIR EFFECT ON THE HUMAN BODY
GAS LAW EXPLANATION AVIATION APPLICATION Graham’s Law Law of gaseous diffusion A gas will diffuse from an area of high concentration to an area of low concentration.
TRANSFER OF GAS IN BODY This law explains the transfer of gases between the atmosphere and the lungs, the lungs and the blood, and the blood and the cell.
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GAS LAW EXPLANATION AVIATION APPLICATION Boyle’s Law P1 = V2 P2 V1
A volume of a gas is inversely proportional to the pressure to which it is subjected with the temperature remaining constant.
TRAPPED GAS This law explains how pressure change allows the gas to expand and contract in body cavities (ears, sinuses, and gastrointestinal (GI) tract) with increasing and decreasing altitude.
Dalton’s Law PT=P1+P2+…PN The total pressure of a mixture of gas is equal to the sum of the partial pressure of each gas in the mixture.
HYPOXIA This law explains how ascent to altitude reduces the total atmospheric pressure, thus reducing each of the partial pressures associated with the total atmospheric pressure.
Henry’s Law P1 = A1 P2 A2
The amount of gas dissolved in solution varies directly with the pressure of that gas over the solution.
DECOMPRESSION SICKNESS This law explains why nitrogen in the body comes out of solution forming bubbles that cause decompression sickness. As altitude increases, pressure decreases and nitrogen will attempt to leave the body and equalize with the surrounding environment. If pressure change is too rapid, the excess nitrogen may form a bubble(s).
Charles’ Law V1 = T1 V2 T2 The pressure of a gas is directly proportional to its temperature.
This gas law has no physiological bearing since body temperature is a constant 98.6 °F.
b. Respiration. When we are exposed to a high-altitude environment, our ability to obtain and utilize oxygen is dramatically affected. External respiration is the exchange of gases between the lungs and the surrounding atmosphere. With each breath (about half a liter of air), we inhale
2.79 x 1021 oxygen molecules.
(1) Partial Pressure of Oxygen. The partial pressure of oxygen is approximately 20 percent of the total atmospheric pressure. At sea level, this would be about 152 millimeters of mercury (mmHg) of pressure (20 percent of the total atmospheric pressure of 760 mmHg). When a breath is drawn into the lungs, one would expect the partial pressure of oxygen to remain at Ground Level 17,000 MSL
Page 14 Par 2-7 152 mmHg. However, the lungs contain other gases that exert a constant pressure (water vapor at 47 mmHg and carbon dioxide at 40 mmHg). These gases tend to displace part of the oxygen, reducing the partial pressure of oxygen at the air sac (alveoli) level to 102 mmHg.
(2) Oxygen and Carbon Dioxide Diffusion in the Blood. Oxygen and carbon dioxide move into and out of the blood in the lungs in accordance with Graham’s Law, which states:
“An area of high gaseous pressure will exert force towards an area of low gaseous pressure.”
This will cause gases to move back and forth across a gas-permeable membrane (such as the air sacs). At sea level, the high partial pressure of oxygen (102 mmHg) diffuses through the air sac wall and into the blood (see Figure 2-2, Blood Cells and Alveoli). This in turn raises the partial pressure of oxygen in venous blood (blood that has left the cells and therefore is low in oxygen) from 40 mmHg to 102 mmHg. At the same time this is happening, the high pressure of carbon dioxide (approximately 47 mmHg) in the blood will cause some of the carbon dioxide to diffuse into the air sacs where carbon dioxide pressure is a constant 40 mmHg. This principle also applies to internal respiration (the exchange of gases from the blood to the cells). The high partial pressure of oxygen in arterial blood causes the oxygen to move from the blood into the cells. Due to cellular metabolism, the high partial pressure of carbon dioxide in the cell will cause it to diffuse into the blood for transport to the lungs.
(3) Atmospheric Areas and Normal Body Function. The human body functions normally in the atmospheric area extending from sea level to 12,000 ft 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 ft MSL, brain oxygen saturation is approximately 87 percent, which begins to approach a level that could affect human performance. Although oxygen is not required below on 12,500 ft MSL when operating in accordance with part 91, we recommend its use when flying unpressurized above 10,000 ft MSL during the day and above 5,000 ft MSL at night, when the eyes become more sensitive to oxygen deprivation.
FIGURE 2-2. BLOOD CELLS AND ALVEOLI
c. Trapped Gas. Gases readily expand with any decrease in pressure. Gas expands in accordance with Boyle’s Law, which states: “A volume of a gas is inversely proportional to the pressure to which it is subjected, temperature remaining constant.” From this law it is apparent that if you reduce the pressure, as you ascend to altitude, gases increase in volume and then decrease in volume on descent.
Red Blood Cells Alveoli
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(1) The human body has several cavities that contain varying amounts of gas. Most of these cavities have an opening that will allow the gas to enter and escape. If the opening is reduced in size or closed, then the gas is trapped. Once trapped, it is still subject to gas expansion and compression in accordance with Boyle’s Law. The result of having changes in gas volume within these cavities without equalization will usually be pain. Table 2-2, Trapped Gas Issues, gives examples of body areas that may be affected by trapped gas during the specified phases of flight.
(2) Table 2-3, In-Flight Treatment of Trapped Gas Emergencies, gives the recommended procedures for in-flight treatment of trapped gas emergencies.
Page 16 Par 2-7 TABLE 2-2. TRAPPED GAS ISSUES
BODY AREA PROBLEM PHASE
OF FLIGHT
PHYSIOLOGY
MIDDLE EAR
DESCENT On ascent, the air and pressure of expanding gases will escape via the eustachian tube. The base of each eustachian tube is collapsed, which acts as a one-way valve to allow gases and liquids to escape and not travel up to the middle ear. On descent, the gas is naturally trapped. Because of the increasing pressure on descent, this pressure will need to be equalized or an ear block will result.
SINUSES
DESCENT or ASCENT (rare) The maxillary sinuses that sit under the cheekbones and the frontals that lie under the eyebrows have an unobstructed opening that will allow gas to enter and escape. In the event of an upper respiratory infection (URI), the openings will be swollen and possibly closed, allowing no route for the gas and pressure to equalize, resulting in a sinus block.
TEETH
ASCENT A tooth block is very rare. They can occur if you have had a recent cavity filling. If there is any airspace trapped between the filling and the pulp of the tooth, it will expand on ascent and cause pain.
GI
ASCENT The GI tract will always contain a varying amount of gas. This gas is usually a result of the digestion process and can escape by either flatulation or belching. If the gas expands, as in unpressurized flight to altitude, and is not allowed to escape, it could result in pain and possible syncope (fainting).
LUNGS
DECOMPRESSION Gas in the lungs will normally enter and escape via the trachea. If the breath is held during a rapid increase in altitude, the gas within the lungs will expand in accordance with Boyle’s Law, but have no pathway to escape. This could cause possible lung damage.