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(2) Disassembly Investigation
① Servo Chamber of S/N129
a. Visual inspection of the inside of the servo chamber revealed no abnormalities. Also,
no foreign particles were found in the orifice of the servo chamber through which
cabin air flows.
b. No foreign particles were found in the filter of the servo chamber.
② Comparison of Control Valves of S/N127 and S/N129
a. The internal state of the stem assemblies was confirmed by X-ray inspection
photographs. As far as visual inspection of the X-ray photographs could show, the
shapes of the internal parts of both valves were the same, and no foreign objects etc.
were found to have entered. However, the amount of deformation of the stem spring
of S/N129 was slightly larger than the other.
b. Comparing the two stem valves, the total length etc. of the stem poppet, the
external dimensions of each part, and the spring loads were found to be virtually
the same. Further, visual inspection revealed no bending of the stem poppet of
either valve.
(3) Visual Inspection of the Stem Assembly using a Microscope
Microscopic inspection of the stem poppet and of the seat assembly, which forms part of
the stem assembly, revealed the following.
① On S/N129, several foreign particles, scratches and contamination were confirmed
around the contacting surface of the stem poppet of the seat assembly, and the bush
1 1
hole in which the stem poppet slides was found to be partially worn. Also, several
burrs were found on the edge of a drill hole in the seat assembly.
② On S/N127, a few burrs were found on the edge of a drill hole in the seat assembly,
but no other abnormality was found.
(See Photographs 3–7.)
(4) Adjusting Ranges of Adjusting Screw
The adjusting screw, which sets the valve’s positive differential pressure cracking
point (indicated as “set screw” in Figures 4 and 5), was driven in to an abnormal
depth on S/N129. The length from the control cover to the tip of adjusting screw
head was measured as 1.63mm, but is nominally around 5.6mm.
It was found to be normal on S/N127.
(5) Dimensional Measurement of the Stem Bush Hole of S/N129
From outside the seat assembly, a pin gage with 0.9mm diameter could be inserted
into the stem bush hole in which the stem poppet slides, but a pin gage with
1.14mm diameter could not be inserted.
The minimum internal diameter specified for the said bush hole is 1.155mm. It
was thus confirmed that the said bush hole had been drilled to a smaller diameter
than the specified minimum.
(See Figures 4–6.)
2.8.5 Additional Investigations of the Safety Valve
Following drop test, and functional tests followed by reassembly of the stem assembly
were performed to further investigate safety valve S/N129 that had been installed in the
said aircraft. The results of these tests, which were conducted at the safety valve
manufacturer’s facilities, are summarized as follows.
(1) Drop test
In the drop test, the stem poppet was dropped vertically into the stem bush hole
with the seat assembly in an upright position. Normally, the stem poppet dropped
into the seat assembly smoothly without resistance. However, in the test, the stem
poppet did not pass smoothly through the stem bush hole.
(2) Reassembly of the stem assembly
During the reassembly of the stem assembly, burrs were pushed out from the stem
bush.
(3) Confirmation of stem poppet movement after reassembly
The compression force against the spring load of a spring in the stem assembly was
1 2
found to be within specified limits.
(4) Air leak test after reassembly
No air leak was found beyond specified limits.
(5) As the result of the investigation in subsection 2.8.4 (4), it was confirmed that the
adjustment range of adjusting screw was abnormal. This was because the thickness of
a shim used when the stem assembly was assembled into the control valve was 2.16mm
instead of the nominal 0.9mm. The shim was therefore adjusted to normal thickness.
(6) Functional test for positive differential pressure after reassembly
The anomalous operation of the valve described in sections 2.8.3 and 2.8.4 was not
reproduced.
2.8.6 Analysis of DFDR Data and CVR Recordings
(1) Analysis of the DFDR data
The result of analyzing the recordings of the DFDR etc. was as follows:
The said aircraft took off from Niigata Airport on 11:20 and reached its cruising
altitude of FL350 at around 11:37. At this time the differential pressure between
the inside of the cabin and the outside indicated 8.03psi, and the cabin altitude
indicated around 6,600ft.
From 11:45:38, the cabin altitude started to increase suddenly.
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AIRCRAFT SERIOUS INCIDENT INVESTIGATION REPORT ALL NIPPON AI(5)
