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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
appropriately according to the updated CMM issued by the safety valve manufacturer. The
adjustment screw of the control valve was driven inwards, and the setting brought to within
specified limits by increasing the preload on spring-“A” shown in Figure 6.
Further, according to the CMM used at the time, one adjustment suffices to set the positive
differential pressure.
3. ANALYSIS
3.1 Analysis
3.1.1 The Captain and First Officer had valid airman proficiency certificates and valid
airman medical certificates in accordance with applicable regulations.
3.1.2 The aircraft had a valid certificate of airworthiness and had been maintained in
accordance with applicable regulations.
3.1.3 It is estimated that the weather conditions at the time of the serious incident were
not contributed with the serious incident.
3.1.4 Based on the statements of the Captain, cabin crew and passengers, the recordings of
ACARS down link data and DFDR data, and the investigation of the safety valves, it was
recognized that an abnormal loss of cabin pressure on the said aircraft had actually
occurred.
　
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