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The parking brake is operated by the YELLOW system, with the accumulators maintaining brake
pressure after the aircraft is shut down. The description of the system in the FCOM notes that:
'Operating the Parking Brake control handle deactivates the other braking modes and the antiskid
system and supplies the brakes with yellow high pressure or accumulator pressure limited at 2100
psi. The return lines are shut off to allow an autonomy of at least 12 hours.' An additional note in
the 'Controls' section of the manual states that: 'Application of the parking brake deactivates
normal and alternate modes.'
Tests on the braking system
Following an initial inspection, the aircraft was towed to a maintenance area where the 'O' ring in
the sampling valve was replaced and further investigation was conducted. Although chocks were
used, it was found that the parking brake operated normally, with pressure being available in the
accumulators when the switch on the pedestal was operated. Moreover, it was observed that the
accumulator (and brake) pressures had remained steady after the aircraft had been left overnight. It
was additionally noted that when the parking brake was selected OFF, and the accumulators were
charged to their maximum regulated pressure, the gauge indicated a noticeable pressure drop over a
period of around 10 to 15 minutes. However the rate of decay subsequently appeared to reduce.
These observations were at variance with a later test conducted by maintenance engineers which
monitored accumulator pressure, with the parking brake OFF, over a period of two hours. In this
case, the engineers reported negligible pressure loss.
Despite the contradictory nature of these two tests, it was concluded that the pressure loss noted on
the first occasion was in accordance with an expanded description of the parking brake system
contained in the Maintenance Manual. This noted that the parking brake operated valve (located
between the brake automatic selector and the distribution dual valve) shuts off the supply to the
alternate brake system to prevent leakage through the dual valve. This implied that the 12 hour
period of accumulator pressure is maintained by closing off the leakage path noted above. Thus
some leakage would be expected to occur with the parking brake set to OFF.
Each brake assembly contained nine pistons operated by the normal (GREEN) system, and six
operated by the alternate (YELLOW) system. Starting with all the brakes OFF, the accumulator was
charged by operating the pedestal mounted switch, followed by the application of the parking
brake. The YELLOW system pistons were observed to operate. The GREEN hydraulic system was
then pressurised by means of the electric pump, and the parking brake was set to OFF. The foot
brakes were then operated and the movement of the GREEN system brake pistons was noted. The
parking brake was then set to ON again (which operated the appropriate brake pistons) and the foot
brakes operated. This produced no movement of the GREEN pistons. The accumulators were then
discharged (by selecting alternate braking and repeatedly applying the foot brakes), which enabled
the 'pre-charge' accumulator pressures to be noted, which were approximately 1,000 psi in each
case. The subsequent application of the parking brake did not of course produce any brake piston
movement; neither did operating the foot brakes as long as the parking brake remained ON.
The foregoing simply proved that the braking system functioned in the way described in the
FCOM, with no defects being found. The final check was to conduct a taxi trial with the aircraft
during which no problems were identified.
Discussion
The loss of the YELLOW hydraulic system early in the flight presented no problems other than an
extended landing distance due to the loss of some of the spoilers. However there would have been a
period of around 6 hours when the YELLOW system was unpressurised, apart from the
accumulators. In the event that the leak had occurred 'downstream' of the non-return valves that
protected the accumulators, then these also would have depressurised as a result. However,
although the leak in this case due to the failed 'O' ring seal was 'upstream' of the non-return valves,
it was possible that the accumulators lost a significant amount of pressure through the alternate
brake system over the 6 hour period due to the parking brake having been OFF in flight. The
Checklist contained no warning of this possibility, or advice on taking the precaution of charging
the accumulators before parking the aircraft at a stand.
It seems probable that there was insufficient pressure remaining in the accumulators to hold the
aircraft against the idle thrust from the engines after the parking brake had been applied. It is also
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