including alternates, that may affect the flight), direction of flight, airplane performance charts, high speed winds aloft, and oxygen duration charts must also be considered. When possible, additional oxygen should be provided to allow for emergency situations. Breathing rates increase under stress and extra oxygen could be necessary.
(2) Flight planning should take into consideration factors associated with altitudes that will be transited while climbing to or descending from the high altitudes (e.g., airspeed limitations below 10,000 feet MSL, airspace, and minimum altitudes). Westward flights should generally be made away from the jet stream to avoid the strong headwind, and eastward flights should be made in the jet stream when possible to increase ground speed. Ground speed checks are particularly important in high-altitude flight. If fuel runs low because of headwinds or poor flight planning, a decision to fly to an alternate airport should be made as early as possible to allow time to re-plan descents and advise ATC.
b. Knowledge of Aircraft. Complete familiarity with the aircraft systems and limitations is extremely important. For example, many high-altitude airplanes feed from only one fuel tank at a time. If this is the case, it is important to know the fuel consumption rate to know when to change tanks. This knowledge should be made part of the preflight planning and its accuracy confirmed regularly during the flight.
c. Gradual Descents. Gradual descents from high altitudes should be planned in advance to prevent excessive engine cooling and provide passenger comfort. The manufacturer’s recommendations found in the AFM should be complied with, especially regarding descent power settings to avoid stress on the engines. Although most jets can descend rapidly at idle power, many turboprop and light twin airplanes require some power to avoid excessive engine cooling, cold shock, and metal fatigue. ATC does not always take aircraft type into consideration when issuing descent instructions. It is the pilot’s responsibility to fly the airplane in the safest manner possible.
Cabin rates of descent are particularly important and should generally not exceed 500 or 600 feet per minute. Before landing, cabin pressure should be equal to ambient pressure or inner ear injury can result. If delays occur en route, descents should be adjusted accordingly.
d. Weather Charts. Before beginning a high-altitude flight, all weather charts should be consulted, including those designed for low levels. Although high-altitude flight may allow a pilot to over-fly adverse weather, low altitudes must be transited on arrival, departure, and in an emergency situation that may require landing at any point en route.
e. Types of Weather Charts. Weather charts that provide information on high-altitude weather include Constant Pressure charts which provide information on pressure systems, temperature, winds, and temperature/dewpoint spread at the 850 millibar (mb), 700 mb, 500 mb, 300 mb, and 200 mb levels (5 charts are issued every 12 hours). Prognostic Charts forecast winds, temperature, and expected movement of weather over the 6-hour valid time of the chart. Observed Tropopause Charts provide jet stream, turbulence, and temperature-wind-pressure reports at the tropopause over each station. Tropopause Wind Prognostic Charts and Tropopause Height Vertical Wind Shear Charts are helpful in determining jet stream patterns and the presence of CAT and wind shear.
f. Wind Shear. Wind shear is indicated by dashed lines on Tropopause Height Vertical Wind shear Charts. Horizontal wind changes of 40 Kts within 150 NM, or vertical wind shear of 6 Kts or Par 105 Page 7
greater per 1,000 feet usually indicate moderate to severe turbulence and should be avoided. Pilot reports (PIREP) are one of the best methods of receiving timely and accurate reports on icing and turbulence at high altitudes.
g. Navigation. Specific charts have been designed for flight at FL 180 and above. En route high-altitude charts delineate the jet route system, which consists of routes established from FL 180 up to and including FL 450. The VOR airways established below FL 180 found on low-altitude charts must not be used at FL 180 and above. High-altitude jet routes are an independent matrix of airways, and pilots must possess the appropriate en route high-altitude charts before transitioning to the flight levels.
h. Jet Routes. Jet routes in the U.S. are predicated solely on VOR or VORTAC navigation facilities, except in Alaska where some are based on Low/Medium Frequency navigation aids. All jet routes are identified by the letter "J" followed by the airway number.
i. Reporting Points. Reporting points are designated for jet route systems and must be used by flights using the jet route unless otherwise advised by ATC. Flights above FL 450 may be conducted on a point-to-point basis, using the facilities depicted on the en route high-altitude chart as navigational guidance. Area Navigation (RNAV) routes, using either random or fixed way points, are also used for direct navigation at high altitudes. These routes are based on area navigation capability between way points defined in terms of latitude/longitude coordinates, degreedistance fixes, or offsets from established routes or airways at a specified distance and direction.
Radar monitoring by ATC is required on all random RNAV routes.
j. Navaids. VOR, DME, and TACAN depicted on high-altitude charts are designated as class H navaids, signifying that their standard service volume is from 1,000 feet AGL up to and including 14,500 feet AGL at radial distances out to 40 NM; from 14,500 feet AGL up to and including 60,000 feet AGL at radial distances out to 100 NM; and from 18,000 feet AGL up to and including 45,000 feet AGL at radial distances out to 130 NM. Ranges of NDB service volumes are the same at all altitudes.
106. PHYSIOLOGICAL TRAINING. To ensure safe flights at high altitudes, pilots of highaltitude aircraft must understand the physiological effects of high-altitude flight and the effect of hypoxia on an individual’s ability to perform complex tasks in a changing environment. Additional physiological training information, including locations and application procedures for attending an altitude chamber, can be found in paragraph 107, Additional Physiological Training. Although not required, altitude chamber training is highly recommended for all pilots.
a. Respiration is the exchange of gases between the organism and its environment. In humans, external respiration is the intake of oxygen from the atmosphere by the lungs and the elimination of some carbon dioxide from the body into the surrounding atmosphere. Each breath intake is comprised of approximately 21 percent oxygen, which is absorbed into the bloodstream and carried by the blood throughout the body to burn food material and to produce heat and kinetic energy. The partial pressure of oxygen forces oxygen through air sacs (alveoli), located at the end of each of the smaller tubes that branches out from the bronchial tubes and lungs, into the bloodstream. Other gases contained in the lungs reduce the partial pressure of oxygen entering the air sacs to about 102 mm Hg at ground level, which is approximately 21 percent of the total atmospheric pressure.
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b. The human body functions normally in the atmospheric area extending from sea level to 12,000 feet MSL. In this range, brain oxygen saturation is at a level that allows for normal functioning. (Optimal functioning is 96 percent saturation. At 12,000 feet, brain oxygen saturation is approximately 87 percent which begins to approach a level that could affect human performance.
Although oxygen is not required below 12,500 feet MSL, its use is recommended when flying above 10,000 feet MSL during the day and above 5,000 feet MSL at night when the eyes become more sensitive to oxygen deprivation.)
c. Although minor physiological problems, such as middle ear and sinus trapped gas difficulties, can occur when flying below 12,000 feet, shortness of breath, dizziness, and headaches will result when an individual ascends to an altitude higher than that to which his or her body is acclimated. From 12,000 to 50,000 feet MSL, atmospheric pressure drops by 396 mm Hg. This area contains less partial pressure of oxygen which can result in problems such as trapped or evolved gases within the body. Flight at and above 50,000 feet MSL requires sealed cabins or pressure suits.
d. Hypoxia is a lack of sufficient oxygen in the body cells or tissues caused by an inadequate supply of oxygen, inadequate transportation of oxygen, or inability of the body tissues to use oxygen. A common misconception among many pilots who are inexperienced in high-altitude flight operations and who have not been exposed to physiological training is that it is possible to recognize the symptoms of hypoxia and to take corrective action before becoming seriously impaired. While this concept may be appealing in theory, it is both misleading and dangerous for an untrained crewmember. Symptoms of hypoxia vary from pilot to pilot, but one of the earliest effects of hypoxia is impairment of judgment. Other symptoms can include one or more of the following:
(1) Behavioral changes (e.g., a sense of euphoria).
(2) Poor coordination.
(3) Discoloration at the fingernail beds (cyanosis).
(4) Sweating.
(5) Increased breathing rate, headache, sleepiness, or fatigue.
(6) Loss or deterioration of vision.
(7) Light-headedness or dizzy sensations and listlessness.
(8) Tingling or warm sensations.
e. While other significant effects of hypoxia usually do not occur in a healthy pilot in an un-pressurized aircraft below 12,000 feet, there is no assurance that this will always be the case.
The onset of hypoxic symptoms may seriously affect the safety of flight and may well occur even in short periods of exposure to altitudes from 12,000 to 15,000 feet. The ability to take corrective measures may be totally lost in 5 minutes at 22,000 feet. However, that time would be reduced to only 7 to 10 seconds at 40,000 feet and the crewmember may suffer total loss of consciousness soon thereafter. A description of the four major hypoxia groups and the recommended methods to combat each follows.
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(1) Hypoxic (Altitude) Hypoxia. Altitude hypoxia poses the greatest potential physiological hazard to a flight crewmember while flying in the high-altitude environment. This type of hypoxia is caused by an insufficient partial pressure of oxygen in the inhaled air resulting from reduced oxygen pressure in the atmosphere at altitude. If a person is able to recognize the onset of hypoxic symptoms, immediate use of supplemental oxygen will combat hypoxic hypoxia within seconds. Oxygen systems should be checked periodically to ensure that there is an adequate supply of oxygen and that the system is functioning properly. This check should be performed frequently with increasing altitude. If supplemental oxygen is not available, an emergency descent to an altitude below 10,000 feet should be initiated.
(2) Histotoxic Hypoxia. This is the inability of the body cells to use oxygen because of impaired cellular respiration. This type of hypoxia, caused by alcohol or drug use, cannot be corrected by using supplemental oxygen because the uptake of oxygen is impaired at the tissue level. The only method of avoiding this type of hypoxia is to abstain, before flight, from alcohol or drugs that are not approved by a flight surgeon or an aviation medical examiner.
(3) Hypemic (Anemic) Hypoxia. This type of hypoxia is defined as a reduction in the oxygen-carrying capacity of the blood. Hypemic hypoxia is caused by a reduction in circulating red blood cells (hemoglobin) or contamination of blood with gases other than oxygen as a result of anemia, carbon monoxide poisoning, or excessive smoking. Pilots should take into consideration the effect of smoking on altitude tolerance when determining appropriate cabin pressures. If heavy smokers are among the crew or passengers, a lower cabin altitude should be set because apparent altitudes for smokers are generally much higher than actual altitudes. For example, a smoker’s apparent altitude at sea level is approximately 7,000 feet. Twenty thousand feet actual altitude for a nonsmoker would be equivalent to an apparent altitude of 22,000 feet for a smoker. The smoker is thus more susceptible to hypoxia at lower altitudes than the nonsmoker. Hypemic hypoxia is corrected by locating and eliminating the source of the contaminating gases. A careful preflight of heating systems and exhaust manifold equipment is mandatory. Also, cutting down on smoking would minimize the onset of this type of hypoxia. If symptoms are recognized, initiate use of supplemental oxygen and/or descend to an altitude below 10,000 feet. If symptoms persist, ventilate the cabin and land as soon as possible because the symptoms may be indicative of carbon monoxide poisoning and medical attention should be sought.
(4) Stagnant Hypoxia. This is an oxygen deficiency in the body resulting from poor circulation of the blood because of a failure of the circulatory system to pump blood (and oxygen) to the tissues. Evidence of coronary artery disease is grounds for immediate denial or revocation of a medical certificate. In flight, this type of hypoxia can sometimes be caused by positive pressure breathing for long periods of time or excessive G-forces.
f. Effective Performance Time (EPT) or Time of Useful Consciousness (TUC) is the amount of time in which a person is able to effectively or adequately perform flight duties with an insufficient supply of oxygen. EPT decreases with altitude, until eventually coinciding with the time it takes for blood to circulate from the lungs to the head usually at an altitude above 35,000 feet. Table 1-1 shows the TUC (shown as average TUC) at various altitudes. The rate of ascent directly affects TUC. Faster rates of ascent result in shorter TUC.
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TABLE 1-1. TIMES OF USEFUL CONSCIOUSNESS AT VARIOUS ALTITUDES Standard Ascent Rate After Rapid Decompression Altitude (Feet) Time Time 18,000 20 to 30 minutes 10 to 15 minutes 22,000 10 minutes 5 minutes 25,000 3 to 5 minutes 1.5 to 3.5 minutes 28,000 2.5 to 3 minutes 1.25 to 1.5 minutes 30,000 1 to 2 minutes 30 to 60 seconds 35,000 30 to 60 seconds 15 to 30 seconds 40,000 15 to 20 seconds 7 to 10 seconds 43,000 9 to 12 seconds 5 seconds 50,000 9 to 12 seconds 5 seconds
g. Other factors that determine EPT are physical activities (exercise decreases EPTs), and dayto-day factors such as physical fitness, diet, rest, prescription drugs, smoking, and illness. Altitude chamber experiments found a significantly longer TUC for nonsmoker pilots who exercise and watch their diet than for pilots who smoke and are not physically fit.
h. Prolonged oxygen use can also be harmful to human health. One hundred percent aviation oxygen can produce toxic symptoms if used for extended periods of time. The symptoms can consist of bronchial cough, fever, vomiting, nervousness, irregular heart beat, and lowered energy.
These symptoms appeared on the second day of breathing 90 percent oxygen during controlled experiments. It is unlikely that oxygen would be used long enough to produce the most severe of these symptoms in any aviation incidence. However, prolonged flights at high altitudes using a high concentration of oxygen can produce some symptoms of oxygen poisoning such as infection or bronchial irritation. The sudden supply of pure oxygen following a decompression can often aggravate the symptoms of hypoxia. Therefore, oxygen should be taken gradually, particularly when the body is already suffering from lack of oxygen, to build up the supply in small doses. If symptoms of oxygen poisoning develop, high concentrations of oxygen should be avoided until the symptoms completely disappear.
i. When nitrogen is inhaled, it dilutes the air we breathe. While most nitrogen is exhaled from the lungs along with carbon dioxide, some nitrogen is absorbed by the body. The nitrogen absorbed into the body tissues does not normally present any problem because it is carried in a liquid state. If the ambient surrounding atmospheric pressure lowers drastically, this nitrogen could change from a liquid and return to its gaseous state in the form of bubbles. These evolving and expanding gases in the body are known as decompression sickness and are divided into two groups.
(1) Trapped Gas. Expanding or contracting gas in certain body cavities during altitude changes can result in abdominal pain, toothache, or pain in ears and sinuses if the person is unable to equalize the pressure changes. Above 25,000 feet, distention can produce particularly severe gastrointestinal pain.
(2) Evolved Gas. When the pressure on the body drops sufficiently, nitrogen comes out of solution and forms bubbles which can have adverse effects on some body tissues. Fatty tissue Par 106 Page 11
contains more nitrogen than other tissue; thus making overweight people more susceptible to evolved gas decompression sicknesses.
(a) SCUBA diving will compound this problem because of the compressed air used in the breathing tanks. After SCUBA diving, a person who flies in an aircraft to a pressure altitude of 8,000 feet would experience the same effects as a non-diver flying at 40,000 feet un-pressurized.
The recommended waiting period before going to flight altitudes of 8,000 feet is at least 12 hours after non-decompression stop diving (diving which does not require a controlled ascent), and 24 hours after decompression stop diving (diving which requires a controlled ascent). For flight altitudes above 8,000 feet, the recommended waiting time is at least 24 hours after any SCUBA diving. (See Airman's Information Manual (AIM) paragraph 8-1-2 (d) (2).)
(b) The bends, also known as caisson disease, is one type of evolved gas decompression sickness and is characterized by pain in and around the joints. The pain gradually becomes more severe, can eventually become temporarily incapacitating, and can result in collapse. The chokes refers to a decompression sickness that manifests itself through chest pains and burning sensations, a desire to cough, possible cyanosis, a sensation of suffocation, progressively shallower breathing and, if a descent is not made immediately, collapse and unconsciousness. Paresthesia is a third type of decompression sickness, characterized by tingling, itching, a red rash, and cold and warm sensations, probably resulting from bubbles in the central nervous system (CNS). CNS disturbances can result in visual deficiencies such as illusionary lines or spots, or a blurred field of vision. Some other effects of CNS disturbances are temporary partial paralysis, sensory disorders, slurred speech, and seizures.
j. Shock can often result from decompression sicknesses as a form of body protest to disrupted circulation. Shock can cause nausea, fainting, dizziness, sweating, and/or loss of consciousness.
The best treatment for decompression sickness is descent to a lower altitude and landing. If conditions persist after landing, recompression chambers can be located through an aviation medical examiner.
k. Vision has a tendency to deteriorate with altitude. A reversal of light distribution at high altitudes (bright clouds below the airplane and darker, blue sky above) can cause a glare inside the cockpit. Glare effects and deteriorated vision are enhanced at night when the body becomes more susceptible to hypoxia and can occur at altitudes as low as 5,000 feet. In addition, the empty visual field caused by cloudless, blue skies during the day can cause inaccuracies when judging the speed, size, and distance of other aircraft. Sunglasses are recommended to minimize the intensity of the sun’s ultraviolet rays at high altitudes.
107. ADDITIONAL PHYSIOLOGICAL TRAINING. There are no specific requirements in 14 CFR part 91 or 125 for physiological training. However, in addition to the high-altitude training required by 14 CFR section 61.31(g), which should include the physiological training outlined in this chapter, 14 CFR parts 121 and 135 require flight crewmembers that serve in operations above 25,000 feet to receive training in specified subjects of aviation physiology. None of the requirements includes altitude chamber training. The U.S. military services require its flight crewmembers to complete both initial and refresher physiological training, including instruction in Par 106 Page 12
basic aviation physiology and altitude chamber training. Other U.S. Government agencies, such as the National Aeronautics and Space Administration and FAA, also require their flight personnel who operate pressurized aircraft in the high-altitude flight environment to complete similar training.
Although most of the subject material normally covered in physiological training concerns problems associated with reduced atmospheric pressure at high-flight altitudes, other equally important subjects are covered as well. Such subjects of aviation physiology as vision, disorientation, physical fitness, stress, and survival affect flight safety and are normally presented in a good training program. The FAA Civil Aerospace Medical Institute (CAMI) offers a one-day aviation physiology course for FAA flightcrews, civil aviation pilots, and FAA aviation medical examiners (AME). In addition to the basic academic contents, this course offers practical demonstrations of rapid decompression (8 to 18,000 feet) and hypoxia (25,000 feet) in a hypobaric chamber, as well as a practical demonstration of spatial disorientation in a Vertigon or the new General Aviation Spatial Disorientation Demonstrator (Gyro-1).
a. Physiological training programs are offered at locations across the U.S. (table 1-2) for civil pilots at U.S. Air Force physiology training units under the USAF/FAA physiological Training Agreement. Trainees who attend these programs will receive classroom lectures and learn to recognize and overcome vertigo, hypoxia, hyperventilation, etc., during flight, and a high-altitude flight in an altitude chamber. The U.S. Army and the FAA are currently working on the development of a similar joint training program.
b. Persons who wish to take this training must be at least 18 years of age, hold a current FAA Airman Medical Certificate, and must not have a cold or any other significant health problem when enrolling for the course.
NOTE: Anyone can attend the training regardless of whether they are a pilot or not. However, they are still required to obtain a minimum of a class III medical certificate in order to participate in the altitude chamber training.
c. Scheduling. CAMI’s Airman Education Programs obtains a list of training dates from each base that are available to anyone interested in the training. These dates can be accessed by calling 405-954-4837. To schedule a training slot the following information will be required for each student:
(1) Pilot certificate number or social security number
(2) Last name
(3) First name
(4) Middle initial
(5) Date of birth
(6) Mailing address
(7) Daytime phone number
(8) Date of FAA medical
(9) The class of the medical the day it was issued.
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d. Submitting an Application. Once you have been assigned a training date, CAMI will mail an application and a notification letter to you. The application must be completed and mailed to the address provided within 30 days of the scheduled training, along with the fee, made payable to the FAA. The applicant should take the notification letter, along with a current medical certificate, to the base, the day of the training. Effective August 13, 2001, the course fee of $50.00 is nonrefundable and not transferable.
TABLE 1-2. TRAINING LOCATIONS Andrews AFB, MD Fort Rucker (Army), AL Offut AFB, NE Beale AFB, CA Holloman AFB, NM Peterson AFB, CO Brooks AFB, TX Langley AFB, VA Randolph AFB, TX Columbus AFB, MS Laughlin AFB, TX Shaw AFB, SC Fairchild AFB, WA Little Rock AFB, AR Vance AFB, OK
108. HIGH-ALTITUDE SYSTEMS AND EQUIPMENT. Several systems and equipment are unique to aircraft that fly at high altitudes, and pilots should be familiar with their operation before using them. Before any flight, a pilot should be familiar with all the systems on the aircraft to be flown.
a. Turbochargers. Most light piston engine airplanes that fly above 25,000 feet MSL are turbocharged. Turbochargers compress air in the carburetor or cylinder intake by using exhaust gases from an engine-driven turbine wheel. The increased air density provides greater power and improved performance. Light aircraft use one of two types of turbocharge systems.
(1) The first is the normalized system, which allows the engine to develop sea level pressure from approximately 29 inches of manifold pressure up to a critical altitude (generally between 14,000 to 16,000 feet MSL).
(2) The second is the supercharger system, which is a more powerful system that allows the engine to develop higher than sea level pressure (up to 60 inches of manifold pressure) up to a critical altitude. To prevent overboosting at altitudes below the critical altitude, a waste gate is installed in the turbocompressor system to release unnecessary gases. The waste gate is a damperlike device that controls the amount of exhaust that strikes the turbine rotor. As the waste gate closes with altitude, it sends more gases through the turbine compressor causing the rotor to spin faster. This allows the engine to function as if it were maintaining sea level or, in the case of a supercharger, above sea level manifold pressure. The three principle types of waste gate operations are manual, fixed, and automatic.
(a) Manual Waste Gate. Manual waste gate systems are common in older aircraft but have been discontinued due to the additional burden on the pilot. Waste gates were often left closed on takeoff, resulting in an overboost that could harm the engine.
(b) Fixed Waste Gate. Fixed waste gates pose less of a burden on the pilot, but the pilot must still be careful not to overboost the engine, especially on takeoff, initial climb, and on cold days when the air is especially dense. This type of waste gate remains in the same position during all engine operations, but it splits the exhaust flow allowing only partial exhaust access to Par 107 Page 14
the turbine. The pilot simply controls manifold pressure with smooth, slow application of the throttle to control against overboost. If overboost does occur, a relief valve on the intake manifold protects the engine from damage. This is not a favorable system due to fluctuations in manifold pressure and limited additional power from the restricted control over the exhaust flow. In addition, the compressor can produce excessive pressure and cause overheating.
(c) Automatic Waste Gate. Automatic waste gates operate on internal pressure. When internal pressure builds towards an overboost, the waste gate opens to relieve pressure, keeping the engine within normal operating limits regardless of the air density.
1 The pressure-reference automatic waste gate system maintains the manifold pressure set by the throttle. Engine oil pressure moves the waste gate to maintain the appropriate manifold pressure, thus reducing the pilot’s workload and eliminating the possibility of overboost.
If the airplane engine is started up and followed by an immediate takeoff, cold oil may cause a higher than intended manifold pressure. Allow the oil to warm up and circulate throughout the system before takeoff.
2 The density-reference waste gate system is controlled by compressor discharge air.
A density controller holds a given density of air by automatically adjusting manifold pressure as airspeed, ambient pressure, temperature, altitude, and other variables change.
b. Turbocharged engines are particularly temperature sensitive. Manufacturers often recommend increasing the fuel flow during climbs to prevent overheating. It is also important to cool the engine after landing. Allowing the engine to idle for approximately one minute before shutting it down permits engine oil to flow through the system, cooling the engine while simultaneously cooling and lubricating the turbocharger.
c. Most high-altitude airplanes come equipped with some type of fixed oxygen installation. If the airplane does not have a fixed installation, portable oxygen equipment must be readily accessible during flight. The portable equipment usually consists of a container, regulator, mask outlet, and pressure gauge. A typical 22 cubic-foot portable container will allow four people enough oxygen to last approximately 1.5 hours at 18,000 feet MSL. Aircraft oxygen is usually stored in high-pressure system containers of 1,800-2,200 pounds per square inch (PSI). The container should be fastened securely in the aircraft before flight. When the ambient temperature surrounding an oxygen cylinder decreases, pressure within that cylinder will decrease because pressure varies directly with temperature if the volume of a gas remains constant. Therefore, if a drop in indicated pressure on a supplemental oxygen cylinder is noted, there is no reason to suspect depletion of the oxygen supply, which has simply been compacted due to storage of the containers in an unheated area of the aircraft. High pressure oxygen containers should be marked with the PSI tolerance (i.e., 1,800 PSI) before filling the container to that pressure. The containers should be supplied with aviation oxygen only, which is 100 percent pure oxygen. Industrial oxygen is not intended for breathing and may contain impurities, and medical oxygen contains water vapor that can freeze in the regulator when exposed to cold temperatures. To assure safety, oxygen system periodic inspection and servicing should be done at FAA certificated stations found at some fixed base operations and terminal complexes.
d. Regulators and masks work on continuous flow, diluter demand, or on pressure demand systems. The continuous flow system supplies oxygen at a rate that may either be controlled by the Par 108 Page 15
user or controlled automatically on some regulators. The mask is designed so the oxygen can be diluted with ambient air by allowing the user to exhale around the face piece, and comes with a rebreather bag which allows the individual to reuse part of the exhaled oxygen. Pilot masks sometimes allow greater oxygen flow than passengers’ masks, so it is important that pilots use the masks that are indicated for them. Although certificated up to 41,000 feet, very careful attention to system capabilities is required when using continuous flow oxygen systems above 25,000 feet.
e. Diluter demand and pressure demand systems supply oxygen only when the user inhales through the mask. An automix lever allows the regulators to automatically mix cabin air and oxygen or supply 100 percent oxygen, depending on the altitude. The demand mask provides a tight seal over the face to prevent dilution with outside air and can be used safely up to 40,000 feet.
Pilots who fly at those altitudes should not have beards and moustaches because air can easily seep in through the border of the mask. Pressure demand regulators also create airtight and oxygen-tight seals but they also provide a positive pressure application of oxygen to the mask face-piece which allows the user’s lungs to be pressurized with oxygen. This feature makes pressure demand regulators safe at altitudes above 40,000 feet.
f. Pilots should be aware of the danger of fire when using oxygen. Materials that are nearly fireproof in ordinary air may be susceptible to burning in oxygen. Oils and greases may catch fire if exposed to oxygen and, therefore, cannot be used for sealing the valves and fittings of oxygen equipment. Smoking during any kind of oxygen equipment use must also be strictly forbidden.
g. Surplus oxygen equipment must be inspected and approved by a certified FAA inspection station before use. Before each flight, the pilot should thoroughly inspect and test all oxygen equipment. The inspection should be accomplished with clean hands and should include a visual inspection of the mask and tubing for tears, cracks, or deterioration; the regulator for valve and lever condition and positions; oxygen quantity; and the location and functioning of oxygen pressure gauges, flow indicators, and connections. The mask should be donned and the system should be tested. After any oxygen use, verify that all components and valves are shut off.
h. Cabin pressurization is the compression of air in the aircraft cabin in order to maintain a cabin altitude lower than the actual flight altitude. Because of the ever-present possibility of decompression, supplemental oxygen is still required. Pressurized aircraft meeting specific requirements of 14 CFR part 23 or 25 have cabin altitude warning systems which are activated at 10,000 feet. Pressurized aircraft meeting the still more stringent requirements of 14 CFR part 25 have automatic passenger oxygen mask dispensing devices which activate before exceeding 15,000 feet cabin altitude.
i. Pressurization in most light aircraft is sent to the cabin from the turbocharger’s compressor or from an engine-driven pneumatic pump. The flow of compressed air into the cabin is regulated by an outflow valve which keeps the pressure constant by releasing excess pressure into the atmosphere. The cabin altitude can be manually selected and is monitored by a gauge which indicates the pressure difference between the cabin and ambient altitudes. The rate of change between these two pressures is automatically controlled with a manual backup control.
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j. Each pressurized aircraft has a determined maximum pressure differential, which is the maximum differential between cabin and ambient altitudes that the pressurized section of the aircraft can support. The pilot must be familiar with these limitations, as well as the manifold pressure settings recommended for various pressure differentials. Some aircraft have a negative pressure relief valve to equalize pressure in the event of a sudden decompression or rapid descent to prevent the cabin pressure from becoming higher than the ambient pressure.
k. Reducing exposure to low barometric pressure lowers the occurrence of decompression sickness and the need for an oxygen mask is eliminated as a full-time oxygen source above certain altitudes. Many airplanes are equipped with automatic visual and aural warning systems that indicate an unintentional loss of pressure.
l. Technology is continuously improving flight at high altitudes through the development of new devices and the improvement of existing systems. One such example is the pressurized magneto. Thin air at high altitudes makes the unpressurized magneto susceptible to crossfiring.
The high tension pressurized system is composed of sealed caps and plugs that keep the electrodes contained within the body. A pressure line extends directly from the turbodischarger to the magneto. Pressurized magnetos perform better at high altitudes where low pressure and cold atmosphere have a detrimental effect on electrical conductivity. Flight above 14,000 feet with an unpressurized magneto should be avoided because of its higher susceptibility to arcing.
m. Another airplane component recommended for flight at high altitudes is the dry vacuum pump. Engine-driven wet vacuum pumps cannot create a sufficient vacuum to drive the gyros in the low air density found at high altitudes. Furthermore, gyros and rubber deicing boots can be ruined by oil contamination from the wet pump system, which uses engine oil for lubrication and cooling.
Dry vacuum pumps are lightweight, self-lubricating systems that eliminate oil contamination and cooling problems. These pumps can power either a vacuum or pressure pneumatic system, allowing them to drive the gyros, deice boots, and pressurize the door seals.
109. AERODYNAMICS AND PERFORMANCE FACTORS. Thinner air at high altitudes has a significant impact on an airplane’s flying characteristics because surface control effects, lift, thrust, drag, and horsepower are all functions of air density.
a. The reduced weight of air moving over control surfaces at high altitudes decreases their effectiveness. As the airplane approaches its absolute altitude, the controls become sluggish, making altitude and heading difficult to maintain. For this reason, most airplanes that fly at above 25,000 feet are equipped with an autopilot.
b. A determined weight of air is used by the engine for producing an identified amount of horsepower through internal combustion. For a given decrease of air density, horsepower decreases at a higher rate which is approximately 1.3 times that of the corresponding decrease in air density.
c. For an airplane to maintain level flight, drag and thrust must be equal. Because density is always greatest at sea level, the velocity at altitude given the same angle of attack will be greater than at sea level, although the indicated air speed (IAS) will not change. Therefore, an airplane’s TAS increases with altitude while its IAS remains constant. In addition, an airplane’s rate of climb will decrease with altitude.
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110. EMERGENCIES AND IRREGULARITIES AT HIGH ALTITUDES. All emergency procedures in the AFM should be reviewed before flying any airplane, and be readily accessible during every flight. A description of some of the most significant high-altitude emergencies and remedial action for each follows.
a. Decompression is defined as the inability of the aircraft’s pressurization system to maintain its designed pressure schedule. Decompression can be caused by a malfunction of the system itself or by structural damage to the aircraft. A decompression will often result in cabin fog because of the rapid drop in temperature and the change in relative humidity. A decompression will also affect the human body. Air will escape from the lungs through the nose and mouth because of a sudden lower pressure outside of the lungs. Differential air pressure on either side of the eardrum should clear automatically. Exposure to wind blast and extremely cold temperatures are other hazards the human body may face with decompression.
b. Decompression of a small cabin volume pressurized aircraft is more critical than a large one, given the same size hole or conditions, primarily because of the difference in cabin volumes.
Table 1-3 is a comparison of cabin volume ratios between several large transport airplanes and some of the more popular general aviation turbojet airplanes in current use. Table 1-3 shows that, under the same conditions, a typical small pressurized aircraft can be expected to decompress on the order of 10 to 200 times faster than large aircraft. The B-747/Learjet comparison is an extreme example in that the human response, TUC, and the protective equipment necessary are the same. Actual decompression times are difficult to calculate due to many variables involved (e.g., the type of failure, differential pressure, cabin volume, etc.). However, it is more probable that the crew of the small aircraft will have less time in which to take lifesaving actions.
TABLE 1-3. AIRCRAFT CABIN VOLUME RATIOS Aircraft Type Cabin Volumes in Cubic Feet Ratio DC-9 vs CE-650 5,840 vs 576 10:1 B-737 vs LR-55 8,010 vs 502 16:1 B-727 vs NA-265 9,045 vs 430 21:1 L-1011 vs G-1159 35,000 vs 1,850 19:1 B-747 vs Learjet 59,000 vs 265 223:1 Data Source: Physiological Considerations and Limitations in the High-altitude Operation of SmallVolume Pressurized Aircraft. E.
B. McFadden and D. de Steigner, Federal Aviation Administration (FAA) Civil Aeromedical Institute (CAMI).
(1) An explosive decompression is a change in cabin pressure faster than the lungs can decompress. Most authorities consider any decompression which occurs in less than 0.5 seconds as explosive and potentially dangerous. This type of decompression is more likely to occur in small volume pressurized aircraft than in large pressurized aircraft and often results in lung damage. To avoid potentially dangerous flying debris in the event of an explosive decompression, all loose items such as baggage and oxygen cylinders should be properly secured.
(2) A rapid decompression is a change in cabin pressure where the lungs can decompress faster than the cabin. The risk of lung damage is significantly reduced in this decompression as compared with an explosive decompression.
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