Par 2-7 Page 23
(d) Wind Blast. The loss of pressure in the aircraft may be felt by crew and passengers as a wind blast.
(e) Cooler Temperatures. When the pressure (air) departs the cabin, the temperature will drop.
(f) Gas Expansion. As the gas expands with decreasing pressure, it will be most noticeable in the ears and GI tract of the body.
(3) Time of Useful Consciousness (TUC) or Effective Performance Time (EPT).
This is the period of time from interruption of the oxygen supply, or exposure to an oxygen-poor environment, to the time when an individual is no longer capable of taking proper corrective and protective action. The faster the rate of ascent, the worse the impairment and the faster it happens. TUC also decreases with increasing altitude. Figure 2-3, Times of Useful Consciousness versus Altitude, shows the trend of TUC as a function of altitude. However, slow decompression is as dangerous as or more dangerous than a rapid decompression. By its nature, a rapid decompression commands attention. In contrast, a slow decompression may go unnoticed and the resultant hypoxia may be unrecognized by the pilot.
WARNING:
The TUC does not mean the onset of unconsciousness. Impaired performance may be immediate. Prompt use of 100 percent oxygen is critical.
WARNING:
Slow decompression is as dangerous as or more dangerous than a rapid or explosive decompression.
Page 24 Par 2-7 FIGURE 2-3. TIMES OF USEFUL CONSCIOUSNESS VERSUS ALTITUDE
ALTITUDE TUC/EPT Following Rapid Decompression 18,000 20–30 min 10–15 min 22,000 10 min 5–6 min 25,000 3–5 min 1.5–2.5 min 28,000 2.5–3 min 1–1.5 min 30,000 1–2 min 30 s–1 min 35,000 30 s–1 min 15–30 s 40,000 15–20 s Nominal 43,000 9–12 s Nominal 50,000 9–12 s Nominal
CAUTION:
TUCs are based on data that represent average values and reflect wide variation among individuals in time to incapacitation. This variation results from differences in an individual’s total surface area for gas exchange in the lungs, total amount of hemoglobin available in the blood to bind oxygen, and oxygen consumption rate at rest (related to body mass index). Other sources of variation are the extent to which hypoxia stimulates increases in depth and rate of breathing, and increases in the amount of blood the heart pumps (faster heart rate). Finally, individuals able to increase the amount of oxygen they can extract from the blood in muscle and brain tissue are more hypoxia tolerant. This is largely genetically determined, but can be enhanced by physical conditioning resulting from a regular aerobic exercise program. As many of these factors will be unknown by the aviator, the safest approach would be to assume the lower TUC value is limiting.
(a) Factors Affecting Hypoxia Recognition and TUC: Slow Decompression and Rapid or Explosive Decompression. Rapid loss of aircraft pressurization dramatically reduces TUC. As a general rule, it can be assumed that the TUC following decompression to altitudes between 25,000 ft and 43,000 ft will be reduced by 50 percent. Above 43,000 ft, the TUC is reduced to the time it takes for the blood to circulate from the lung to the brain, plus any reserve oxygen stored in the brain. This is approximately 9–12 s from the start of a rapid decompression to the loss of functional capability. In some cases, hypoxia exposure can lead to a shock type reaction that could further reduce the TUC. In any case, the potential for impairment begins
Par 2-7 Page 25 almost immediately. For this reason, the donning of the oxygen mask should be practiced from time to time.
(b) Other Factors that Determine TUC.
1. TUC decreases with:
(i) Rate of ascent. The faster you ascend to altitude, the shorter your EPT/TUC becomes.
(ii) Rate of decompression. Rapid decompression decreases the TUC by at least 50 percent.
(iii) Physical activity at altitude. Any physical activity will reduce your EPT/TUC. For example, if you did 10 deep knee bends at 25,000 ft with your oxygen mask off, your EPT/TUC would be reduced by 50 percent.
(iv) Fatigue. If you enter the cockpit in a fatigued state, you are less resistant to hypoxia.
(v) Diet. The brain feeds exclusively from glucose (blood sugar), so if your glucose is low, as in hypoglycemia, you are more prone to hypoxia.
(vi) Alcohol. Alcohol brings about its own form of hypoxia. When altitude is coupled with alcohol, you are a strong candidate for a hypoxic episode.
(vii) Medications. Some drugs, including over-the-counter medications, will cause cells not to utilize oxygen properly and therefore will make you less altitude resistant.
(viii) Other factors. Smoking, poor physical condition, and illness also decrease the TUC.
2. Impairment increases with:
• Increasing altitude.
• Increasing difficulty and complexity of the task.
• New tasks compared to learned, practiced tasks.
(4) Altitude-Induced Decompression Sickness (DCS). According to Henry’s Law, when the partial pressure of a gas over a liquid is decreased, the amount of gas dissolved in that liquid will also decrease. One of the best practical demonstrations of this law is offered by opening a soft drink. When the cap is removed from the bottle, gas is heard escaping and bubbles can be seen forming in the soda. This is carbon dioxide gas coming out of solution as a result of sudden exposure to a lower partial pressure of carbon dioxide in the environment.
(a) Nitrogen Absorption. One of the more dangerous problems an aviator may face is the threat of DCS (nitrogen bubbles in body fluids and tissues) at high altitudes. Nitrogen we breathe is taken into the lungs at a pressure of 608 mmHg (80 percent of the total
Page 26 Par 2-7 atmospheric pressure 760 mmHg at sea level). The nitrogen is then distributed throughout the body by the circulatory system and stored at a pressure of about 608 mmHg. At sea level, the nitrogen pressure inside the body and outside of the body is in equilibrium. When atmospheric pressure is reduced as a result of ascent, the equilibrium is upset. This results in nitrogen leaving the body by passing from the cells, to the blood, and then out through the respiratory system. If the nitrogen is forced to leave the solution too rapidly because of a large partial pressure difference, bubbles may form, causing a variety of signs and symptoms. The symptoms result from the location of the bubbles. These evolving and expanding gases in the body are known as DCS. There are 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 ft MSL, distention can produce particularly severe GI 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.
See Table 2-7, Types of Evolved Gas Decompression Sickness. Fatty tissue contains more nitrogen than other tissue, thus making overweight people more susceptible to evolved gas DCS.
Par 2-7 Page 27 TABLE 2-7. TYPES OF EVOLVED GAS DECOMPRESSION SICKNESS DCS Type Bubble Location Signs & Symptoms (Clinical Manifestations) Bends Mostly large joints of the body (shoulders, elbows, wrists, hips, knees, ankles) Localized deep pain, ranging from mild (a “niggle”) to excruciating. Sometimes a dull ache, but rarely a sharp pain. Active or passive motion of the joint aggravates the pain. Pain can occur at altitude, during the descent, or many hours later. The pain gradually becomes more severe, can become incapacitating, and can even result in collapse.
Neurologic Brain Confusion or memory loss. Headache. Spots in visual field (scotoma), tunnel vision, double vision (diplopia), or blurry vision. Unexplained extreme fatigue or behavior changes. Seizures, dizziness, vertigo, nausea, vomiting, and unconsciousness may occur.
Spinal Cord Abnormal sensations such as numbness, burning, stinging, and tingling around the lower chest and back.
Symptoms may spread from the feet up and may be accompanied by ascending weakness or paralysis.
Girdling abdominal or chest pain.
Peripheral Nerves Urinary and rectal incontinence. Abnormal sensations such as numbness, burning, stinging, and tingling (paresthesia). Muscle weakness or twitching.
Chokes Lungs Burning deep chest pain (under the sternum). Pain is aggravated by breathing. Dry constant cough. Possible cyanosis (a bluish color of the skin, nail beds, and the mucous membranes due to insufficient oxygen in the blood). Shortness of breath (dyspnea), sensation of suffocation, progressively shallower breathing, and, if a descent is not made immediately, collapse and unconsciousness.
Skin Manifestations Skin Itching usually around the ears, face, neck, arms, and upper torso. Sensation of tiny insects crawling over skin (formication). Mottled or marbled skin usually around the shoulders, upper chest, and abdomen, accompanied by itching. Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema).
(b) Risk Trends. Most cases of altitude DCS occur among individuals exposed to altitudes of 25,000 ft or higher. A United States Air Force (USAF) study of altitude DCS cases reported that only 13 percent occurred below 25,000 ft. The risk of developing altitude DCS increases with altitude and time of exposure.
Page 28 Par 2-7
(c) Predisposing Factors.
1. Repetitive Exposures. Repetitive exposures to altitudes above 18,000 ft within a short period of time (few hours) also increase the risk of developing altitude DCS.
2. Rate of Ascent. The faster the rate of ascent to altitude, the greater the risk of developing altitude DCS. An individual exposed to a rapid decompression (high rate of ascent) above 18,000 ft has a greater risk of altitude DCS than being exposed to the same altitude at a lower rate of ascent.
3. Time at Altitude. The longer the duration of the exposure to altitudes of 18,000 ft and above, the greater the risk of altitude DCS. Time at altitude has been shown to be the strongest predictor of DCS.
4. Age. There are some reports indicating a higher risk of DCS with increasing age.
5. Previous Injury. There is some indication that recent joint or limb injuries may predispose individuals to developing “the bends.”
6. Ambient Temperature. Exposure to very cold ambient temperatures may increase the risk of altitude DCS.
7. Body Type. A person who has a high body fat content may be at greater risk of altitude DCS, as nitrogen is stored in greater amounts in fat tissues.
8. Exercise. When a person is physically active while flying at altitudes above 18,000 ft, there is greater risk of altitude DCS.
9. Alcohol Consumption. The after-effects of alcohol consumption, such as dehydration, increase susceptibility to DCS.
(d) Self-Contained Underwater Breathing Apparatus (SCUBA) Diving Before Flying. SCUBA diving requires breathing air under high pressure. Under these conditions, there is a significant increase in the amount of nitrogen dissolved in the body (body nitrogen saturation). The deeper the SCUBA dive, the greater the body nitrogen saturation. Following SCUBA diving, if not enough time is allowed to eliminate the excess nitrogen stored in the body, altitude DCS can occur during exposure to altitudes as low as 5,000 ft or less. After SCUBA diving, a person who flies in an aircraft to a pressure altitude of 8,000 ft MSL would experience the same effects as a nondiver flying unpressurized at 40,000 ft MSL.
1. The recommended waiting period before going to flight altitudes up to 8,000 ft MSL is at least:
• Twelve hours after nondecompression stop diving (diving that does not require a controlled ascent), and
• Twenty-four hours after decompression stop diving (diving that requires a controlled ascent).
Par 2-7 Page 29
2. For flight altitudes above 8,000 ft MSL, the recommended wait time is at least 24 hours after any SCUBA diving. (See AIM paragraph 8-1-2, subparagraph d2.)
(5) What to Do If You Suspect DCS in Flight.
(a) Put on your oxygen mask immediately and switch the regulator to 100 percent oxygen.
(b) Begin an emergency descent and land as soon as possible. Even if the symptoms disappear during descent, you should still land and seek medical evaluation while continuing to breathe oxygen.
(c) If one of your symptoms is joint pain, keep the affected area still; do not try to work pain out by moving the joint around.
(6) Medical Treatment. A mild case of the bends may disappear during descent from high altitude. However, it still requires medical evaluation. If the signs and symptoms persist during descent or reappear at ground level, it may be necessary to receive hyperbaric oxygen treatment immediately. This involves 100 percent oxygen delivered in a high-pressure chamber.
WARNING:
Neurological DCS, “the chokes,” and skin manifestations (mottled or marbled skin lesions) are very serious and potentially fatal if untreated.
(7) Things to Remember about DCS.
(a) Altitude DCS is a potential risk every time you fly in an unpressurized aircraft above 18,000 ft, and at lower altitudes if you SCUBA dive prior to the flight.
(b) Be familiar with the signs and symptoms of altitude DCS and monitor all aircraft occupants, including yourself, any time you fly an unpressurized aircraft above 18,000 ft.
(c) Avoid unnecessary strenuous physical activity prior to flying an unpressurized aircraft above 18,000 ft and for 24 hours after the flight.
(d) Even if you are flying a pressurized aircraft, altitude DCS can occur as a result of sudden loss of cabin pressure (in-flight rapid decompression).
(e) Following exposure to an in-flight rapid decompression, do not fly for at least 24 hours. In the meantime, remain vigilant for the possible onset of delayed symptoms or signs of altitude DCS. If you experience delayed symptoms or signs of altitude DCS, seek medical attention immediately.
Page 30 Par 2-7
(f) Keep in mind that breathing 100 percent oxygen during flight (ascent, en route, descent) without oxygen prebreathing prior to take off, does not prevent the occurrence of altitude DCS.
(g) Do not ignore any symptoms or signs that go away during the descent. In fact, this could confirm that you are actually suffering altitude DCS.
(h) Any case of altitude DCS should be medically evaluated as soon as possible, even if symptoms are mild or disappear on descent.
(i) If there is any indication that you may have experienced altitude DCS, do not fly again until you are cleared to do so by a physician familiar with and experienced with treatment of DCS. For questions about your continued eligibility for your airman medical certificate contact an FAA medical officer or your AME.
(j) Allow at least 24 hours to elapse between SCUBA diving or hypobaric (altitude) chamber exposure and flying.
(k) Be prepared for a future emergency by familiarizing yourself with the availability of hyperbaric chambers in your area of operations. However, keep in mind that not all of the available hyperbaric treatment facilities have personnel qualified to handle altitude DCS emergencies. To obtain information on location of hyperbaric treatment facilities capable of handling altitude DCS emergencies, call the Divers Alert Network at 919-684-9111.
f. Vision Deterioration. Vision has a tendency to deteriorate with altitude; smoking worsens this effect. 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 ft MSL. 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. We recommend sunglasses to minimize the intensity of the sun’s ultraviolet rays at high altitudes.
2-8. FAA PHYSIOLOGICAL TRAINING. There are no specific requirements in 14 CFR part 91 or 125 for physiological training. However, § 61.31(g) requires the high-altitude training outlined in this AC. Also, 14 CFR parts 121 and 135 require flightcrew members that serve in operations above 25,000 ft MSL to receive training in specified subjects of aviation physiology.
None of the regulations include altitude chamber training. Although most of the subject material normally covered in physiological training concerns problems associated with reduced atmospheric pressure at high-flight altitudes, it covers other equally important subjects as well.
Such subjects of aviation physiology as vision, disorientation, physical fitness, stress, and survival affect flight safety and are normally present in a good training program.
a. FAA Civil Aerospace Medical Institute (CAMI) Aviation Physiology Course.
CAMI offers a 1-day aviation physiology course for FAA flightcrews, civil aviation pilots, and FAA AMEs. In addition to the basic academic contents, this course offers practical demonstrations of rapid decompression (8,000 to 18,000 ft MSL) and hypoxia (25,000 ft MSL)
Par 2-8 Page 31 in a hypobaric chamber. Additionally, the course features a practical demonstration of spatial disorientation in a spatial disorientation demonstrator.
(1) Requirements. Persons who wish to take this training must be at least 18 years of age, hold a current FAA Airman Medical Certificate, and 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, trainees still must obtain a minimum of a Class III medical certificate in order to participate in the altitude chamber training.
(2) Scheduling. CAMI’s Airman Education Programs obtains a list of training dates from each base that are available to anyone interested in the training. Call 405-954-4837 to access these dates. Scheduling a training slot requires the following information:
• Full name,
• Date of birth,
• Name of organization (if applicable),
• Mailing address,
• Daytime phone number,
• Aircrew position or non-aircrew position, and
• All applicants that are not U.S. citizens must provide additional information (Security Clearance Request) found at http://www.faa.gov/pilots/training/airman_education/media/ SecurityClearanceRequest.doc.
(3) Submitting an Application. CAMI will mail a notification letter to you. The applicant should take the notification letter, along with a current medical certificate, to the training facility on the day of the training. For more information see the CAMI Web site at www.faa.gov.
b. Physiology of Flight Video Library. The FAA has developed a series of educational programs developed to address the human factors hazards of high-altitude flight. You may view these videos at http://www.faa.gov/pilots/training/airman_education/.
c. Airplane Upset Recovery. The FAA has developed a training aid on upset recovery and high-altitude operations. You can obtain the training aid at http://www.faa.gov/other_visit/ aviation_industry/airline_operators/training/media/Appendix_3-E_HighAltOperations.pdf.
2-9. 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 ft 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. Turbocharged engines are particularly temperature-sensitive.
Page 32 Par 2-9 Manufacturers often recommend increasing the fuel flow during climbs to prevent overheating. It is also important to cool the engine after landing. Allow the engine to idle for approximately 1 minute before shutting it down. This permits engine oil to flow through the system, cooling the engine while simultaneously cooling and lubricating the turbocharger. Light aircraft use one of two types of turbocharger systems.
(1) Normalized System. 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 and 16,000 ft MSL).
(2) Supercharger System. The second is the supercharger system, which is driven by the engine through a gear train at one, two, or variable speeds. It is a more powerful system that allows the engine to develop higher than sea level pressure (up to 60 in 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 damper-like 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 principal 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 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 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.
Par 2-9 Page 33
2. Compressor discharge air controls the density-reference waste gate system.
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. Oxygen and Oxygen Equipment. 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 ft portable container will contain enough oxygen for approximately 1.5 hours at 18,000 ft MSL for four people. Aircraft oxygen is usually stored in high-pressure system containers of 1,800 to 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. A drop in indicated pressure on a supplemental oxygen cylinder can be an indication that the oxygen supply has merely compacted (not necessarily a leak).
High-pressure oxygen containers should be marked with the psi tolerance (i.e., 1,800 psi) before filling the container to that pressure. Only oxygen that meets or exceeds the Society of Automotive Engineers (SAE) International Aerospace Standard AS8010 (as revised), Aviator’s Breathing Oxygen Purity Standard, should be used. To assure safety, periodic inspection and servicing should take place by a certified maintenance provide found at some fixed base operations and terminal complexes.
(1) Regulator and Mask Systems. Regulators and masks work on continuous flow, diluter demand, or pressure demand systems.
(a) Continuous Flow System. The continuous flow system supplies oxygen at a rate that may be controlled automatically or by the user. The mask is designed so the oxygen can be diluted with ambient air by allowing the user to exhale around the facepiece, 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 indicated for them. Although certificated up to 41,000 ft MSL, system capabilities require very careful attention when using continuous flow oxygen systems above 25,000 ft MSL.
(b) Diluter Demand and Pressure Demand Systems. 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 ft MSL. Pilots who fly at those altitudes should not have beards and mustaches 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 facepiece, which allows the user’s lungs to be pressurized with oxygen. This feature makes pressure demand regulators safe at altitudes above 40,000 ft MSL.
(2) Increased Fire Risk. Pilots should be aware of the increased danger of fire when using oxygen. Materials that are nearly fireproof in ordinary air may be susceptible to burning in
Page 34 Par 2-9 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 the use of any kind of oxygen equipment must also be strictly forbidden.
(3) Equipment Inspection. A certified maintenance provider must inspect the aircraft system, and a Department of Transportation (DOT)-approved facility must hydrostatic test the oxygen bottles, 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 should also be checked for valve and lever condition and positions. Also be sure to check the oxygen quantity as well as the location and function of oxygen pressure gauges, flow indicators, and connections. The mask should be donned and the system tested. After any oxygen use, verify that all components and valves are shut off.
c. Pressurization Systems. 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, the aircraft still requires supplemental oxygen.
Pressurized aircraft meeting specific requirements of 14 CFR part 23 or 25 have cabin altitude warning systems, which activate at a cabin altitude of 10,000 ft. Pressurized aircraft meeting the more stringent requirements of part 25 have automatic passenger oxygen mask-dispensing devices that activate before exceeding 15,000 ft cabin altitude. It should be noted that some aircraft require that the flightcrew disable the automatic passenger oxygen mask-dispensing devices prior to landing at airports over 10,000 ft MSL to prevent inadvertent deployment when the cabin depressurizes at landing. The system is then re-armed after departure.
(1) Pressure Control. Turbine aircraft use a steady supply of engine bleed air for cabin pressurization. In the case of most pressurized light aircraft, the air supply is sent to the cabin from the turbocharger’s compressor or from an engine-driven pneumatic pump. In any case, an outflow valve regulates the flow of compressed air out of the cabin, which keeps the pressure constant by releasing excess pressure into the atmosphere. The cabin altitude selection can be manual, or if available, electronic. A gauge that indicates the pressure difference between the cabin and ambient altitudes helps the pilot to monitor the cabin altitude. A manually set backup control automatically controls the rate of change between these two pressures.
(2) Pressure Differentials. 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 lower than the ambient pressure. Reducing exposure to low barometric pressure lowers the occurrence of decompression sickness and eliminates the need for an oxygen mask 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.
d. High-Altitude Components. Thin air at high altitudes has created a need for specialized aircraft components that are adapted to this environment.