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was removed and subjected to functional and other tests at the ARS. The results of these
tests were as follows:
(1) Functional test of S/N129
􁶃 A visual check revealed no abnormalities.
􁶄 In functional test for positive differential pressure, the valve could not support a
pressure differential of more than 0.39psi, a worse condition than when it had been
installed in the aircraft.
􁶅 The stem assembly (an internal part of the control valve) was assembled into a good
safety valve (S/N127) and the positive differential pressure functional test was repeated.
As a result, the gate opened at 0.44psi. The focus of the search for the abnormal
operation therefore shifted elsewhere.
􁶆 The parts of the control valve apart from the stem assembly were substituted with
parts from a good safety valve (S/N127) and the positive differential pressure functional
test repeated. As a result, the gate has opened at approximately 9.01psi, nearly the
specified limit, and the anomalous behavior was not exhibited.
(2) Disassembly Investigation
􁶃 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.
􁶄 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.
􁶃 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
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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.
􁶄 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
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