Cabin Decompression

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Editor : Copyright Ó Smartcockpit.com / Ludovic ANDRE PAGE 1 version 01 
 
 
 
 
 
Original idea from Fred George 
 
In the "Highly unlikely event" your aircraft loses pressurization, how well 
would you and your passengers fair ? 
 
Bang! "Explosive" or rapid decompression 
makes quite an impression Bon your senses, 
even if it's a simulated event in a hypobaric 
altitude chamber. Your ears pop, your eyes 
water, dust flies in the cockpit and the 
temperature plunges below freezing. Water 
vapor in the "cabin" may instantly condense as 
fog. If you were in a transport category airplane, 
warning lights would glare and/or warning horns 
would blare. 
All those sensations are unmistakable signs of 
explosive decompression. At typical business 
aircraft cruise altitudes, you have only a few 
seconds of useful consciousness time to don 
your oxygen mask before hypoxia claims you as 
a victim, as shown by the accompanying Time of 
Useful Consciousness graph. Yet, eight out of 
10 pilots who haven't rehearsed the event take as long as 15 seconds to respond with corrective 
action when they experience loss of cabin pressurization, according to U.S. Air Force research. 
Notably, USAF decompression experiments involved young, fit pilots who passed rigorous military 
flight physicals. 
 
The classic aviation physiology model is based upon putting "normal" people into an abnormal 
environment. However, in reality, chances are some of your passengers will have abnormal 
physiology, such as advanced age or other aggravating cardiovascular issues that may exacerbate 
the effects of hypoxia. 
If you were to lose pressurization suddenly, your priorities would be clear. In a business aircraft, you 
have a small fraction of the time to respond to the incident compared to pilots of an airliner, because 
of the relatively small cabin air volume escaping from the pressure vessel, according to data 
compiled by the FAA's Civil Aeromedical Institute (CAMI). Moreover, you're probably cruising at a 
considerably higher flight level than most airliners, increasing the severity of the problem. 
Immediately, you and your passengers would need supplemental oxygen. If you get your mask on 
properly and start the flow of oxygen, you will recover in as little as 15 seconds, even if you are on 
the verge of unconsciousness, according to USAF research. 
 
You would have to start an emergency descent. Most diluter demand and pressure demand 
emergency oxygen masks only are rated for a maximum altitude of 40,000 to 43,000 feet. Above 
45,000 feet, even pressure breathing masks can't supply enough oxygen because the gas pressure 
in the lungs is too low. 
Passengers and cabin attendants don't have the same level of protection as the flightcrew. Their 
continuous-flow masks typically are rated by the manufacturer to 30,000 feet. Most business aircraft 
manufacturers and the USAF recommend not using continuous-flow passenger oxygen systems 
above 25,000 feet cabin altitude. 
During the descent, you also would have to avoid other air traffic, terrain and hazardous weather. 
And you have to communicate your problems with ATC. 
 
 
CABIN DECOMPRESSION 
 
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In the chamber, or in a flight simulator, the entire compound emergency event is over and you're 
safely back at ground elevation within a few moments. Another training event to log and forget, 
some might say. 
Over a cup of coffee, if you bring up the possibility of cabin depressurization with any group of 
experienced business aircraft pilots, more than a few may roll their eyes if as to say, "Yeah, sure! It 
couldn't happen in my airplane. That only happens in altitude chambers and onboard aging 
airliners." 
 
Statistically, that's true. ln the late 1960s, the FAA conducted a study of depressurization events in 
business, airliner and military jet transport aircraft. The FAA concluded that the odds of 
experiencing cabin depressurization were one in 54,300 flight hours, according to research 
conducted by a team headed by Stanley R. Mohler, M.D., then chief of the FAA's Aeromedical 
Applications Division. 
Today, cabin depressurization incidents in business aircraft are few and far between. But every few 
years they still occur, according to the FANs Service Difficulty Reports. And they happen frequently 
enough to merit consideration, especially if one happens when you're hundreds of miles away from 
the closest suitable divert field. 
 
Imagine how you would handle a cabin pressurization loss if you were midway between San 
Francisco and Honolulu, stretching the legs of a midsize jet at FL 390. Or, put yourself 1,500 miles 
northeast of White Plains, N.Y., when you lose cabin pressurization and then find out that Gander, 
Goose and Halifax are below minimums. 
 
Then, consider your options if you experience depressurization over Alaska or the Yukon, halfway 
between the United States and West Asia in mid-winter in an ultra-long-range business aircraft. 
During any of these scenarios, the need to conserve fuel or avoid high terrain enroute to a suitable 
divert field might force you to fly for an extended period well above a maximum safe cabin altitude. 
 
 
 
 
 
 
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RECOGNIZING LOSS OF PRESSURIZATION 
 
Most depressurization events aren't as obvious as the sudden loss of pressure in the hypobaric 
chamber. If they occur, they usually are more insidious, according to Service Difficulty Reports. For 
example, after takeoff, a faulty landing-gear squat switch or electrical short might keep the solenoid 
of a cabin outflow valve in the fully open, or ground, depressurized position. En route, a fissure or 
break in a pressurization or bleed air duct might cause a partial loss of pressurization. A door or 
emergency exit seal unseats, causing a substantial air pressure leak. 
 
Transport category aircraft systems provide three types of warnings if the cabin altitude climbs 
above die 8,000-foot maximum limit specified by FAR Part 25.841. First, the rule requires that an 
aural alarm or visual warning alert the crew upon reaching 10,000 feet cabin altitude. Second, 
passenger oxygen masks must deploy automatically prior to reaching 15,000 feet cabin altitude. On 
the Learjet 45, for example, the passenger masks automatically deploy at 14,500 feet cabin altitude. 
Automatic mask deployment is triggered at 13,000 feet cabin altitude in the Gulfstream V 
 
Third, in some turbofan aircraft, an emergency pressurization system automatically will activate, 
routing hot, high pressure, high velocity (read unmistakably "noisy") engine bleed air directly to the 
pressure vessel for maximum possible pressurization. For example, the Learjet 35's emergency 
pressurization system automatically is activated at 9,500 feet cabin altitude on serial number 113 
and subsequent aircraft. 
 
In addition to aircraft systems activation, your passengers alsoprovide warnings of excessively high 
cabin altitude in the form of hypoxia symptoms. There are four types of hypoxia, as shown in the 
accompanying table. Hot and cold flashes, a feeling of ants crawling on the skin and dizziness, 
along with nausea, blurred vision, slurred speech and mental confusion are typical signs. Reaction 
times are slowed. Senses of touch and pain are diminished. Skin under the finger nails may turn 
bluish. Hearing, though, is one of the last senses to go. Behavior shifts, also may be signs. Once 
you confront the loss of cabin pressure and have to deal with the effects of hypoxia by using 
supplemental oxygen, your oxygen allocation priorities are clear. Flightcrew members are first, 
passengers and cabin attendants are second. None of the occupants will survive the event if the 
crew loses consciousness. 
 
The crew first, passengers second principle is part of the design of the supplemental oxygen 
system. Flightcrew members are provided with quick-donning masks that can provide as much 
diluted or 100-percent oxygen as the crew demands by breathing. Diluter demand masks, if 
properly fitted to the face, can provide a sea-level partial pressure of oxygen up to an altitude of 
33,700 feet, thus ensuring full cognitive and motor skills, plus normal visual acuity. 
Passengers and cabin attendants typically are provided with a low volume, continuous flow of 
oxygen to their masks. The intent is to keep the cabin occupants physically safe from the dangers 
of hypoxia, but not necessarily mentally sharp at excessive cabin altitudes. Continuous flow masks 
for the passengers only have to deliver 68 to 82 percent as much oxygen as diluter demand 
flightcrew masks, according to the Part 25 certification rules. 
Ideally, supplemental oxygen only will be needed until you can descend, stabilize the cabin altitude 
below 10,000 feet, and proceed to the closest suitable divert field. 
 
 
SUPPLEMENTAL OXYGEN DURATION 
 
For extended-range operations, fitting the aircraft with the largest capacity emergency oxygen 
system increases your options should your aircraft suffer a partial or total loss of pressurization. 
The actual duration of available oxygen depends upon the number of flightcrew members, the 
pressure altitude of the cabin and the number of cabin occupants. 
 
 
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The amount of oxygen supplied by the diluter demand masks in the cockpit is dependent upon the 
pressure altitude of the cabin, coupled with the crew's respiration rate and depth. Momentarily, this 
can be as much as 19 liters per minute above 35,000 feet cabin altitude, according to Part 25 
certification requirements. Typically, though, oxygen duration charts are based on each 
crewmember's consuming two to five liters per minute at 10,000- to 25,000-foot cabin altitudes. 
Four liters per minute for each passenger is a good rule of thumb, although some continuous-flow 
 
 
FOUR TYPES OF HYPOXIA 
 
„ Hypoxic Hypoxia - Respiration fundamentally involves the supply of oxygen to the air sacs of the lungs 
and the exhaust of carbon dioxide. Oxygen compromises 20.9 percent of the volume of the atmosphere 
up to 80,000 feet. Assuming a sea level pressure of 760 mm Hg, the partial pressure of oxygen is 159 mm 
Hg in ambient air. However, the lungs not only exhaust carbon dioxide, they also transpire water vapor at 
a relatively constant pressure of 47 mm Hg. This "hydrostatic pressure" effectively reduces the overall air 
pressure to 713 mm Hg; thus, the partial pressure of oxygen in the trachea is reduced to 149 mm Hg at 
sea level. 
 
By donning a diluter demand crew mask and using supplemental oxygen, the 149 mm Hg sea level partial 
pressure of oxygen in the lungs can be maintained up to 33,700 feet. Above that altitude, there isn't 
enough ambient pressure to provide 100 percent oxygen saturation in the lungs, even when using pure 
oxygen. At 36500 feet, for example, 100 percent oxygen provides the same oxygen partial pressure as 
ambient air at 51000 feet. Breathing pure oxygen at 39,500 feet is equivalent to breathing ambient air at 
10,000 feet. Above that altitude, the partial pressure of oxygen in the trachea is insufficient to provide 
adequate oxygen in the blood. 
As the partial oxygen pressure is reduced, breathing rate and depth increase to help compensate for the 
effects of hypoxia. 
Pneumonia can cause scarring and abscesses in the lungs. Emphysema causes irreversible lung tissue 
degeneration. One of the most-common lung maladies is bronchiectasis, or thickening of the alveoli, 
commonly characterized by violent coughing and heavy expectoration of sputum. Such diseases can 
decrease lung capacity by as much as 50 percent, essentially raising the density altitude in the lungs from 
sea level to 18,000 feet for the affected individual. 
Alcohol, certain prescription painkillers containing opiates and antihistamines can suppress respiration rate 
and depth, thereby increasing the severity of hypoxic hypoxia. 
 
„ Hypemic or Anemic Hypoxia - Blood flowing through the lungs transports oxygen diffused through air 
sacs or alveoli. The blbbd's hemoglobin is remarkably efficient at binding with oxygen, even at reduced 
partial oxygen pressures in the lungs associated with increased attitudes At 5,000 feet pressure altitude, 
for example, hemoglobin still is 94-percent saturated with oxygen, only three percent less than at sea 
level. At 1,000 feet, hemoglobin is 90-percent saturated, and at 15,000 feet it's 80-percent saturated 
Anemia results when there is a reduction in blood hemoglobin content. This reduces the blood's ability to 
carry sufficient oxygen to the tissues. Anemic hypoxia can be caused by blood loss. In addition, carbon 
monoxide nitrates and certain prescription drugs can bind with hemoglobin, thereby preventing the 
hemoglobin from binding with oxygen. 
 
„Stagnant Hypoxia - If cardiovascular circulation is impaired, then even the most-oxygen-rich blood can't 
transport oxygen to the tissues where it's needed. Arterio- and atherosclerosis, right to left cardiac shunts 
(leaks) that mix oxygen-rich venous blood with oxygen-rich arterial blood, or a weakened left heart 
ventricle can cause stagnant circulation. 
 
„ Histotoxic Hypoxia - Alcohol, certain narcotics and cyanide compounds can prevent the tissue cells from 
making full use of the oxygen available to them in the blood supply. If histotoxic hypoxia occurs, blood 
oxygen saturation levels typically are high because the cells cannot remove the oxygen from the 
hemoglobin. What's the single, most-important thing you can do to prevent multiple causes of hypoxia? 
"Don't smoke. There are 4,000 compounds in cigarette smoke. Numerous cardiovascular diseases and 
various forms of hypoxia are linked directly to smoking," emphasized Pat 0. Daily, M.D., director of cardiac 
surgery at Sharp Hospital in San Diego. 
 
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systems may reduce 
oxygen consumption to 
as little as one liter per 
minute at a cabin altitude 
of 12,500 feet. 
Computing liters per 
minute is the easy part. 
Most aircraft, except for 
the latest models, don't 
have an oxygen level 
gauge calibrated inliters. 
Instead, the gauge reads 
psi. The oxygen duration 
in liters must be 
calculated as a function 
of bottle pressure, 
according to charts 
provided by the aircraft 
manufacturer. 
If oxygen reserves 
become tight, at 15,000 
feet cabin altitude, you 
legally can turn off the 
supplemental oxygen 
supply to the passengers, 
according to Part 91.211, 
Part 121.329 and Part 
135.157. This typically will reduce oxygen consumption to about 3.9 liters per minute for each 
flightcrew member. For example, 600 liters of oxygen will supply two crewmembers for 77 minutes, 
according to the Learjet 45 Approved Flight Manual. 
 
Above 15,000 feet, your passengers are at risk, according to the FAA and other sources. However, 
if you had to stretch your fuel supply, you might be able to fly at up to FL 200 for one to two hours, 
according to Pat 0. Daily, M.D., director of cardiac surgery at Sharp Hospital in San Diego. Daily 
also is a commercial instrument pilot and CFII who is type rated in the Cessna Citation. 
Daily's comments were echoed by Stanley R. Mohler, M.D., director of aerospace medicine at 
Wright State University in Dayton, Ohio. Daily, Mohler and Russ Rayman, executive director of the 
Aerospace Medical Association in Washington, D.C., all caution that there is a high degree of 
variability in hypoxia tolerance caused by abnormal physiology and aging. Cardiovascular disease, 
prescription painkillers and alcohol can reduce the critical altitude for time of useful consciousness 
and hypoxia-induced unconsciousness by several thousand feet. just as importantly, cardiovascular 
disease, which is not symptomatic at sea level, becomes acutely apparent at high altitude. 
 
 
RANGE PERFORMANCE VS. SUPPLEMENTAL OXYGEN ENDURANCE 
 
Descending from FL 450 to FL 250 can reduce specific range performance by as pass-ratio 
turbofans. Older, lower efficiency turbofans or much as 30 percent in a turbofan aircraft, as shown 
in the accompanying Specific Range Performance chart. Simple math indicates that if you have 
two-thirds of your estimated fuel burn remaining at the equal time point on a long trip and are forced 
to descend from optimum cruise altitude to FL 250, you may have the option of pressing on to the 
destination airport or returning to the origin airport if there are no closer suitable divert landing 
facilities. However, sustained cruise at FL 250 requires having supplemental oxygen for both pilots 
 
 
Note: Range performance is for high-efficiency, high-bypass-ratio turbofans. Older, 
lower efficiency turbofans or turbojets may have less range at lower altitudes. 
 
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and passengers. If you have to conserve both oxygen and fuel to fly to the closest suitable divert 
field, cruising FL 200 with supplemental oxygen available for crews but not the passengers poses 
serious risks. If your passengers all are in their early 20s and are in excellent health, the odds of 
long-term injury from sustained hypoxia may be minimal, according to Daily and Mohler. At FL 200, 
though, a high-efficiency turbofan aircraft retains 63 percent of its optimum specific range 
performance, which might prevent a "feet wet" landing short of the divert field during an extended-
range mission after loss of pressurization. 
 
Down at 15,000 feet, the crew should still be on oxygen, but passengers with normal physiology who 
are not using medications or alcohol should be safe from long-term injury from hypoxia, according to 
FAA regulations. At 15,000 feet, a turbofan business aircraft may retain up to 56 percent of its 
optimum specific range performance. If you are out of oxygen completely down at 10,000 feet, don't 
count on more than one-half of the range performance shown on your flight plan, as illustrated by 
the Specific Range Performance chart. 
The odds are against your experiencing depressurization in your flying career. But loss of cabin 
pressurization remains a statistical possibility four decades after the first business jets started flying 
extended range missions over water. Pilots who frequently make such trips compute the equal-time 
point between origin and destination, along with OEI range performance to the closest suitable divert 
fields. If you include loss of cabin pressurization among your list of contingency preparations for 
extended range missions, you'll afford yourself an extra measure of protection against this 
statistically rare, but potentially serious malfunction. 
 
 
 
 
 
 
 
 
 
Hypobaric Chamber Training Facilities 
 
The FAA and, the U.S. Air Force have a joint training agreement to offer high-altitude hypobaric chamber training to 
civilians for a nominal $35.00 fee. Applicants should contact FAA Aeromedical Education Division (AAM-400), 
Airman Education Programs, Civil Aeromedical Institute, Oklahoma City Okla. at (405) 954-4837 to schedule 
training sessions at any of the following facilities. 
 
· Beale AFB, Marysville, Calif. 
· Brooks AFB, san Antonio., Texas 
· Columbus AFB, Columbus, Mich. 
· Edwards AFB, Mojave, Calif. 
· Fairchild AFB Spokane, Wash. 
· Holloman AFB, Alamogordo, N.M. 
· Langley AFB, Norfolk, Va. 
· Laughlin AFB Del Rio, Texas 
· Little Rock AFB, Little Rock, Ark. 
· Mike Monroney Aeronautical Ctr., 
· Offutt AFB, Omaha, Neb. 
· Peterson AFB Colorado Springs, Colo. 
· Randolph AFB, San Antonio, Texas 
· Shaw AFB, Columbus, S.C. 
· Sheppard AFB, Wichita Falls, Texas 
· Tyndall AFB, Panama City, Fla; 
· Vance AFB, Enid, Okla. 
· Wright-Patterson AFB, Dayton Ohio.