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as unable to be controlled by the outflow valve may occur. These safety valves are
pneumatic valves consisting of two chambers and control part.
As shown in Figure 4, if the cabin pressure rises such that the differential between the cabin
pressure and the ambient pressure exceeds a specified limit (positive pressure), the
diaphragm (􁶊 in Figure 4) in the control valve moves to the right, the gate 􁶅 of the safety
valve opens and the cabin pressure is relieved. On the other hand, if the cabin pressure
becomes less than the ambient pressure (negative pressure), the gate 􁶅 of the safety valve
opens by the opening of a poppet valve 􁶍 in the cabin chamber, and the differential pressure
between the inside and outside of the cabin is regulated within the specified limits.
(See Figures 4 and 6 and Pictures 1 and 2.)
Reference: The relationship between Altitude and Standard Atmospheric Pressure
At Sea Level: 1 atmosphere (approx. 14.70psi, 1013hPa)
At an altitude of 8,000ft: approx. 3/4 atmosphere (approx. 11.03psi, 760hPa)
At an altitude of 18,000ft: approx. 1/2 atmosphere (approx. 7.35psi, 507hPa)
At an altitude of 34,000ft: approx. 1/4 atmosphere (approx. 3.68psi, 253hPa)
2.8.2 Investigation of the communications of the said aircraft’s air to ground data link
system
The said company’s ground facility automatically received and recorded downlink data
from the said aircraft’s ACARS (Aircraft Communications Addressing and Reporting
System) air to ground data link system. The following information on warnings had been
recorded:
(1) 11:45: SAFETY VALVE OPEN
(Safety valve not fully closed)
(2) 11:46: EXCESS CAB ALTX
(Cabin altitude exceeded 9,550 ft􀊶350 ft)
(3) 11:48: LO DIFF PRXX
(Loss of cabin pressure)
􀀚
2.8.3 Investigation of the cabin pressure system of the said aircraft
On December 28, 2001, the cabin pressurization system and structure of the said aircraft
were investigated at the hanger of the said company at Osaka International Airport. The
set of two safety valves was installed in the aircraft, serial numbers 9632127 (S/N127) and
9632129 (S/N129). It was found by a positive differential pressure test (Positive Relief Test)
that the gate of the safety valve (S/N129) opened at 5.16psi. The Aircraft Maintenance
Manual (AMM) specifies that the gate opens in the range 8.52–8.67psi. No other anomalies
were found except for this safety valve.
The history of this safety valve is as follows.
The set of two safety valves in question, S/N127 and S/N129, had been installed in
another aircraft of the same model belonging to the said company since manufacture in
1997.
The maintenance program manual of the said company specifies a functional test of the
safety valves as a required inspection item to be accomplished during the “4C” aircraft
maintenance check (carried out at 60-month intervals). On November 8, 2001, during the
aircraft’s first 4C Check, these valves were found to be outside operating tolerances (the
gate valve should not open at the specified maximum positive differential pressure), and
they were removed from the said aircraft.
Thereafter, the differential pressure at which the safety valves should operate (the actuation
differential pressure) was adjusted at an Approved Repair Station (ARS) certified by the Civil
Aviation Bureau of Japan. The Component Maintenace Manual (CMM) prescribes the limit at
which the gate should open as 8.55–8.60psi. At that time, S/N127 and S/N129 were adjusted
to approximately 8.59psi and approximately 8.57psi respectively. Subsequently, they were
tagged with “Authorized Release Certificate” in accordance with Civil Aviation Regulation
Operating Standard Article 41, and were stored as serviceable spare parts of the said
company.
During the first 4C Check of the said aircraft, it was found that the safety valves
fitted at that time were out of tolerance (they should not have opened when the differential
pressure reached the positive maximum pressure specified). They were removed and
replaced with S/N127 and S/N129, which had been stored as spare parts, on December 25,
2001. After that, the said aircraft made three flights before the occurrence of this serious
incident, and the maximum cruising altitude reached during that period was 25,000ft
(maximum differential pressure: 7.37psi).
There were no maintenance records on S/N127 and S/N129, apart from the above-mentioned
adjustment of the differential pressure.
2.8.4 Detailed Investigation of the Safety Valves
The set of two of safety valves installed in the said aircraft at the time of the incident
　
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