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chamber in which the temperature could be controlled. The valve was
supplied with hydraulic fluid from an external reservoir, the temperature
of which was also controllable. Left and right brake pedal input
pressures were supplied from two master cylinders. The BDDV input
and output pressures were monitored and recorded. Before installation
on the test rig, hydraulic fluid samples were taken from the valve ports
and attaching pipelines. At this point it was observed that a bead of
sealant was missing from the join between the main body of the valve
and a cover at its base. However, there was a residue that suggested
that sealant had been present at some stage. The reason for the loss of
the sealant was not apparent, but was most probably associated with
smooth surfaces of the valve and cover providing an inadequate key for
the sealant.
The initial test roughly followed the total air temperature profile
for the accident flight, as derived from the FDR. Thus the valve was
cooled at a near-constant rate to the lowest value of -28°C before being
warmed up again to +17°C, as shown below:
27
Temperature Profile
Time (mins)
Te
m
p
°C
-30
-20
-10
0
10
20
0 30 60 90 120 150
The BDDV function was tested after one hour, at which point the
temperature was -11°C, with the hydraulic fluid temperature at -3°C. A
satisfactory pressure output was obtained in response to brake pedal
input. However, when the test was repeated after 1 hr 25 min elapsed
time, with the test chamber and fluid temperatures at -20°C and -12°C
respectively, no output was obtained, thus reproducing the failure
condition. This condition persisted at the lowest temperature of -28°C.
The ensuing warming-up process was intended to simulate the descent
and landing on the accident flight. After reaching +17°C the fluid
reservoir temperature was still at -16.8°C, although its temperature at
the input to the BDDV was +12.9°C. The left and right brake functions
were tested in turn by applying the maximum input pressure of 12 bar,
and maintaining it for one minute. No brake output pressure was
obtained on either side. After waiting an additional minute, the tests
were repeated, and it was found that normal operation had been
restored.
It was then decided to find the lowest temperature at which the
BDDV ceased to function. Following a satisfactory function test at
ambient conditions, the chamber was cooled to -5°C and stabilised for
35 minutes, by which time the fluid input temperature was still at
+4.5°C. The left and right inputs were applied at half the maximum
pressure, ie 6 bar, which resulted in brake output pressure. It was noted
that a small container of water, which had been placed in the chamber,
28
was not completely frozen at this stage. The act of functioning the
BDDV introduced colder fluid from the reservoir into the valve body,
which would not be representative of the aircraft installation, where the
Alternate brake components are essentially in a stagnant part of the
hydraulic system, and where generally warmer fluid would be
introduced as a result of brake operation. After an additional 30 minutes
at -5°C, with the fluid input temperature close to 0°C, the valve still
operated normally. The chamber was then cooled to -10°C and
maintained at this value for 30 minutes before testing the BDDV once
more. This time there was no output, ie the valve was in the failed
condition.
After the chamber had been force-warmed to ambient temperature,
the BDDV was subjected to a further test to confirm that it was fully
functional again, before being removed from the test rig. The cupshaped
cover at the base of the valve body was removed and found to
contain a quantity of fluid. This consisted of 30 ml of dirty water and
3.5 ml of what appeared to be a mixture of hydraulic fluid and grease.
This was later analysed by the Fuels and Lubricants laboratories at
DERA, Pyestock, United Kingdom, along with the samples of hydraulic
fluid taken from the BDDV prior to its installation in the environmental
test rig.
The quantity of water/grease was sufficient to fill the cup to within
25 mm from its brim. The lowest part of the rocker assembly extended
into the cup by 29 mm. It was thus clear that the rocker assembly would
have been immersed in the water to the extent that when it had frozen it
could not be moved under the action of brake pedal input pressure.
The moving parts of the valve had been assembled with silicone
grease and it was apparent that emulsification had occurred as result of
　
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