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published by Houbun-Shorin, November 1971), the effects of cabin decompression on the
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human body are as follows.
(1) Hypoxia
The symptoms of lack of sufficient oxygen to human body cells or tissues are called
“hypoxia”, and there are following relations:
Altitude (ft) Stage Symptoms
0–10,000 Indifferent None, but visual sensitivity reduced at
night.
10,000–15,000 Compensatory Major symptoms may not appear due to
compensatory mechanisms.
15,000–20,000 Disturbance Hazards of visual disturbance and
intellectual impairment, etc.
above 20,000 Critical Danger of rapid loss of consciousness with
little or no warning and loss of life.
The Time of Useful Consciousness (TUC) is defined as the amount of the time between the
start of oxygen deficiency and the appearance of slight indications of loss of consciousness.
The following table shows the TUC at various altitudes; however, TUC varies according to
the individual.
Altitude (ft) Time of Useful Consciousness
22,000 5 minutes
25,000 2–3 minutes
28,000 1 minute 30 seconds
30,000 1 minute 15 seconds
35,000 45 seconds
(2) Decompression Sickness
Decompression sickness results from the expansion and contraction of gases
trapped in the ears, nasal passages etc. and from nitrogen and other gases dissolved
in the blood, tissues etc. coming out of solution.
Trapped gases cause symptoms of earache, nose ache, stomach ache etc. Bubbles formed by
gases coming out of solution cause symptoms of arthralgia of the shoulders, elbow, hands
etc. and also difficulty in breathing, aching lungs etc. due to the restriction of blood vessels.
Although rare, restriction of blood vessels in the brain by evolved gas bubbles can cause
loss of consciousness and visual impairment.
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2.9.3 Classification of “Abnormal drop of cabin pressure in an aircraft” specified by the
United States Federal Aviation Administration
Advisory Circular (AC) 61-107 issued by Federal Aviation Authority (FAA) of the United
States contains the following classification of degrees of decompression rate.
(1) Explosive Decompression
This is a phenomenon in which cabin pressure equalizes with ambient pressure in
less than 0.5 seconds. There is a high probability of damage to the human body by
decompression sickness etc.
Because it is considered that unsecured objects will fly around, all loose items
such as baggage should be properly secured before flight. Also, aircraft with smaller
pressurized cabin volumes are more prone to this type of decompression.
(2) Rapid Decompression
This drop of cabin pressure is not as abrupt as in the case of explosive
decompression, and the likelihood of damage to the human body by decompression
sickness etc. is significantly lower.
(3) Gradual Decompression or Slow Decompression
This decompression is difficult to perceive by bodily sensations as opposed to cases
(1) and (2) above. The consequent possibility that recognition will be late makes this
type of decompression dangerous.
Generally, automatic visual and aural warning systems provide indication of
decompression so that it may be detected even if the pilot does not recognize it by
bodily sensations.
A Safety Recommendation dated December 20, 2000 issued by the United States National
Transportation Safety Board (NTSB) describes in a Reference Note that Rapid Decompression
is a phenomenon in which pressure reduces to ambient within a period of between 0.5 second
and 10 seconds, whereas Gradual Decompression is a phenomenon in which pressure
decreases to ambient over a long period of the time.
2.9.4 Concerning the replacement of the Safety Valves at the “4C” Check of the Said
Aircraft
In the event of replacement of a safety valve, there is no specification in the AMM of the
said aircraft that calls for a subsequent test to confirm that the gate opens and closes at the
appropriate differential pressures. However, it is specified that when the opening and
closing of the said valves is verified, it must be confirmed that this is displayed on the
cockpit instruments.
According to the maintenance records, the replacement of the safety valves was accomplished
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in accordance with the specification of the AMM.
2.9.5 Concerning the Maintenance for S/N127 and S/N129
The only one maintenance service carried out on S/N127 and S/N129 as described in
section 2.8.3, that was the adjustment of the setting at which the gate opened, was
accomplished on November 16, 2001 at an ARS.
According to the maintenance records of the ARS, the adjustment was performed
　
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