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flown in the other three flights since the 4C Check in which the safety valves had
been replaced. It is considered possible that during cruise, changes in the
environment due to vibrations during flight etc. resulted in the control valve
operating (due to motion of the stem poppet, which was stuck to the seat housing) at
a lower pressure differential than the limit that had been set, which caused the gate
to open. It is further estimated that as a result of the abnormalities of the stem
assembly described in (3) above, the motion of the stem poppet was restricted and as
a result the open condition remained until the time of landing.
(5) The gate of the safety valve first opened at FL350 (differential pressure 8.03psi), it
opened at 5.16psi at the time of investigation of the cabin pressurization system of
the aircraft, and at the time of the detailed investigation of the safety valves a
differential pressure above 0.39psi could not be supported. The function was thus
getting clearly worse. Furthermore, when the differential pressure functional test
was carried out after swapping the safety valve of S/N129 with that of S/N127, a
differential pressure above 0.44psi could not be supported.
It is estimated that the reason for this deterioration in performance was the final
jamming of the stem poppet, the movement of which had already had a tendency to
be restricted.
(6) When the adjusting screw was driven inwards during the differential pressure adjustment
maintenance work as described in the section 2.9.5, it is estimated that the result of the
adjustment was within specified limits for the following reasons: anomalies that existed
inside the stem assembly at the manufacturing stage (the diameter of bush hole being
smaller than the specified value and the existence of foreign particles inside the stem
assembly) caused increased friction between the bush and the stem poppet which resulted
in the stem poppet not sliding smoothly; and a shim had been set inappropriately at the
time of manufacture.
3.1.8 Based on the statements of the Captain and First Officer and the analysis of the CVR
recordings, the Captain and First Officer donned oxygen masks five seconds after they had
confirmed the abnormal cabin altitude on the ECAM display. It is recognized that their
subsequent actions were carried out speedily and appropriately; five seconds after donning
oxygen masks, they requested an emergency descent to ATC while almost simultaneously
deploying full speed-brake, and commenced a descent to an altitude of approximately
12,000ft at the 260kt airspeed that had been maintained until that time, etc. In addition, it
is considered that as oxygen masks were used appropriately, there was practically no affect
on the bodies of the passengers and crew.
Although the Airplane Operating Manual of the said aircraft specifies that an emergency
descent be made to an altitude of 10,000ft as described in the section 2.9.1, since according
to the statements of the Captain and First Officer there was much cloud at around 10,000ft
and some turbulence had been forecasted, it is recognized that the judgment to descend to
and maintain an altitude of up to 12,000ft was appropriate.
The said Operating Manual further prescribes that an emergency descent should be
made at maximum airspeed. While the maximum operating airspeed at FL350 was 280kt,
the descent was initiated at the airspeed of 260kt that had been maintained until that time.
Thereafter the airspeed increased and did not exceed 300kt during the descent. Comparing
to the table below, the emergency descent was not conducted at maximum airspeed
operating limits. It is estimated that this was because the Captain could not have dispel
suspicions that there might have been structural failure besides the malfunction of the
safety valves, and so in accordance with the said Operating Manual he made the emergency
descent selecting an airspeed within normal operating airspeed ranges specified in OM.
Altitude (FL)
Maximum Operating
Airspeed Limit (kt)
350 280
320 300
246 and below 350
4. PROBABLE CAUSE
In this serious incident, it is estimated that while the aircraft was operating at an
altitude of 35,000ft for the first time since the replacement of the safety valves, the gate of a
safety valve opened at a differential pressure lower than the specified limit and this open
condition persisted. As a result there was a decrease in cabin pressure.
It is estimated that the opening of the gate of the safety valve below the specified
pressure limit and the persistence of its open condition was caused mutually by the
existence of anomalies in the stem assembly and an inappropriate shim adjustment, both at
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