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At around the same time, the cabin pressure controller (CPC), which controls
cabin pressurization, commanded the outflow valve (OFV) to close in order to keep
cabin pressure from falling.
At 11:45:50 the said aircraft started to descent. The recorded cabin altitude
peaked at 21,600ft at 11:48:20, and then decreased with the descent of the said
aircraft.
At around 11:52, the CPC commanded the OFV to open in order to maintain the
rate of change of cabin altitude at –750ft/min (the minus sign indicates cabin
pressure increasing). As a result of this control, as the aircraft descent rate
exceeded the cabin altitude descent rate for a time, the cabin altitude exceeded the
flight altitude for a time.
At around 11:53, the said aircraft leveled off. Consequently, the CPC ceased
control of a rate of change of the cabin altitude and commanded the OFV to close.
After 11:56 the cabin altitude of said aircraft maintained a constant value rather
lower than the flight altitude, and the rates of change of flight altitude and cabin
altitude were virtually unchanging.
(See Figure 2.)
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(2) Analysis of the CVR Recordings
The result of analyzing the CVR recordings was as follows:
No abnormal alert sounds etc. related to cabin pressure were recorded before
11:45:38. At 11:45:38, the Captain and First Officer recognized from the ECAM
display that the cabin pressure was dropping because the gate of a safety valve had
opened. Five seconds later, the Captain decided to make an emergency descent and
he and the First Officer donned oxygen masks. Thereafter, the CVR recorded the
voices of the Captain and First Officer mixed with sounds of oxygen supply from the
oxygen masks. The sound of the cabin altitude warning was recorded on the CVR
recordings twenty-five seconds after the Captain and First Officer had recognized
the abnormal cabin pressure.
After around ten seconds, the Captain had recognized the abnormal cabin
pressure; he made a request to make an emergency descent to ATC. Forty seconds
after that, the prerecorded cabin address regarding putting on oxygen masks,
fastening seat belts etc. played automatically.
At around 11:50, the Captain judged that the abnormal cabin pressure would not recover,
and decided to land at Osaka International Airport. At around 11:54, the Captain
confirmed that the cabin altitude had become lower than 13,000ft and cancelled the
passenger indication to use oxygen masks. At around 11:56 the Captain confirmed the
cabin altitude had become lower than 10,000ft, and he and the First Officer released their
oxygen masks. At around 12:25 the said aircraft landed at Osaka International Airport.
2.9 Other Relevant Information
2.9.1 Airplane Operating Manual of the Said Aircraft
The manual specifies that in the event of sudden decrease in cabin pressure, oxygen
masks should be put on immediately, and if decompression occurs, speed brakes should be
fully extended and an emergency descent made to 10,000ft, with the aircraft flown either at
maximum airspeed or at an appropriate airspeed (if there are concerns about structural
failure, airspeed should be decreased to a suitable value while paying attention to
controlling the aircraft). ATC should be notified beforehand, with the intentions of the pilot
etc. reported clearly.
Further, it is specified that at the time the cabin altitude reaches 13,000ft or below, the
cabin crew should be notified that oxygen masks may be released.
2.9.2 Effect of Decompression in a Cabin on the Human Body
According to “Aerospace Medicine: Flight and the Human Body” (by Haruo Ikeda,
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
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