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medical attention. The cabin crew commented that there was a significant delay
before the ambulance arrived at the scene.
24
1.16. Tests and research.
1.16.1. Tests with the complete aircraft.
Following temporary repairs the aircraft was flown on 17 June,
with the landing gear extended and at a maximum altitude of 9,000 ft,
from Ibiza to Toulouse. Except the brakes, the braking system
components were not changed for this flight.
During the flight, the crew attempted to reproduce the Autobrake
defect that occurred prior to the accident. Numerous selections were
made on the LO, MED and MAX push-buttons, without result.
A successful Autobrake 'LO' landing was carried out, although the
push-button did not illuminate. During the landing roll, the DECEL
caption also failed to illuminate, and an ECAM message, to the effect
that BSCU Channel 2 was inoperative, did belatedly appear. LO
Autobrake effectively selects a longitudinal deceleration of 0.17g, and
the DECEL light illuminates at 80% of this value. Later evaluation of
the recorded data for this flight showed that the peak retardation was
0.16g; thus the light should have been on.
On the ground, the aircraft was prepared for a series of simulated
flights during which the Alternate brake system was functioned during
each "landing". This involved interposing a breakout box between the
BSCU and the aircraft such that the BSCU could be fed with signals
which simulated the inputs from, for example, the inertial reference
system (giving groundspeed), main landing gear wheel tacho-generators
(giving wheel speed), as well as discretes such as ground spoiler
deployment. In addition the LO Autobrake button-push was simulated
by a pulse of sufficiently short duration to precipitate the
command/monitor disagree on both BSCU channels, and which
produced the ‘Autobrake fail’ condition at "touchdown" that occurred
on the accident flight. Ground power units were used to pressurise the
25
Green (Normal brakes) and Yellow (Alternate brakes) hydraulic
systems.
During each simulated landing roll, an operator on the flight deck
applied the brake pedals which, given the BSCU fault, functioned the
Alternate brakes via the Yellow hydraulic system. All parameters were
recorded, such that it was possible to monitor the hydraulic pressure at
the brakes, together with pedal deflection, as well as the function test of
the Normal system that was conducted by the BSCU at each simulated
landing gear extension. The test procedure was later modified to include
the setting of the park brake at the end of each simulation, which
functioned the park brake valve and the operated valve, the integrity of
which is fundamental to the operation of the Alternate brake system.
None of these tests showed any abnormality with Alternate brake
operation, and so they were repeated with dry ice packed around the
brake system hydraulic components in the right hand landing gear bay.
This time the accident flight condition was reproduced in that there was
no brake pressure in response to brake pedal deflection. Additional
pressure transducers that had been installed in the hydraulic system
showed that the automatic selector, the Park Brake valve and the
operated valve had been functioning as expected, and that Yellow
system pressure was available to the BDDV. However no BDDV output
pressure was apparent, which thus indicated a problem with this
component.
The tests were repeated in order to demonstrate the consistency of
the failure condition, and it was found that after the BDDV had been
cooled to -40°C and the ice pack removed, then brake function was not
recovered until the temperature had risen to an indicated +15°C.
However, it must be appreciated that these temperature values cannot be
regarded as accurate, as measurement was by means of a probe attached
to the valve body, which was therefore not capable of assessing the
internal temperature.
26
The operator on the flight deck reported that during the failure
condition, the brake pedals required a higher force than usual to deflect
them.
It was concluded that the most probable explanation for the
BDDV's behaviour was water trapped inside which turned to ice at subzero
temperatures, thus jamming the internal mechanism. It was then
decided to conduct additional investigations at Messier-Bugatti, the
valve's manufacturers. Accordingly, the BDDV was removed from the
aircraft and hand-carried to the manufacturer's facility at Velizy, near
Paris.
1.16.2. Tests on the BDDV.
The BDDV was tested in Messier-Bugatti's environmental test
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