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stages to slow the aircraft to normal taxi speed. During the lengthy taxi to the allocated stand, M
24, occasional gentle braking was required; the brakes worked correctly and all indications were
normal.
The aircraft was taxied onto the stand at a very slow speed, brought to a halt in the correct position
and the parking brake applied. However the commander then became aware of some movement on
the left side of the aircraft which he thought may have been movement of the jetway, but on
looking to his right he saw that the aircraft was slowly moving forward. He therefore applied
maximum pressure to the brake pedals, but to no avail. The ground crewman, who had plugged into
the ground intercom whilst the aircraft had been momentarily stationary, repeatedly instructed the
crew to stop but the aircraft continued moving until the No 1 engine struck the jetway. The crew
then shut down the No 2 engine and completed the associated checks. The passengers later vacated
the aircraft using steps which were positioned at the rear doors. When the aircraft had come to a
brief halt initially there had been no time for the chocks to be placed in position before the aircraft
had begun to move forward again.
The above account was compiled from crew statements, analysis of the Cockpit Voice Recorder
(CVR) and the Digital Flight Data Recorder (DFDR). However the DFDR data did not include a
parking brake discrete and thus it was not possible to identify the times at which the parking brake
had been applied and subsequently released.
Examination of the aircraft
The No 1 engine nose cowling had contacted a vertical steel post that formed part of the jetty
structure. A rotating beacon mounted on the post, together with its mounting bracket, became
embedded in the lip of the cowling at the three o'clock position (when viewed from the front). The
contact with the jetty had been sufficiently forceful to displace the jetty towards the terminal
building. The tyres on the steerable wheels that supported and positioned the end of the jetty had
left skidmarks over a metre in length from their initial position. The aircraft had finally halted some
11 metres forward of its correct parking position.
Subsequent inspections revealed that the No 1 engine and pylon had escaped damage in the
incident. Examination of the hydraulic components in the right hand main gear bay revealed fresh
hydraulic fluid around a sampling valve. The valve, which was attached to a manifold, was in the
YELLOW hydraulic system and reportedly sprayed fluid when the system was pressurised (by means
of an electric pump) in order to open the cargo doors after the incident. It was later found that an 'O'
ring seal in the valve had failed. After this had been replaced the system was charged, purged and
was then found to function normally.
Description of the hydraulic system
The aircraft has three independent hydraulic systems designated GREEN, BLUE and YELLOW. Each
engine has two variable displacement hydraulic pumps; engine No 1 has one GREEN and one BLUE
system pump, while engine No 2 has one GREEN and one YELLOW system pump. If the engine
pumps are unavailable, hydraulic power can be generated by:
- Two electric pumps in the GREEN system (primarily for ground
testing)
- One electric pump in the YELLOW system to pressurise the brake
accumulators, if required, or to operate the cargo doors
- A hand-pump in the YELLOW system to operate the cargo doors when
the YELLOW electric pump is not available
o An emergency ram air turbine (RAT) driven pump for the YELLOW system
In addition, two non-reversible power transfer units are installed to provide hydraulic power
transfer without fluid exchange from the GREEN system to the BLUE or YELLOW system. These
would be used in the event of an engine, or engine pump failure, or for ground testing. A diagram
of the YELLOW system is shown at Figure 1.
The loss of the YELLOW system has no effect on normal braking, which is supplied from the GREEN
hydraulic system. In the event of the loss of the latter however, alternate braking is available from
the YELLOW system. The brake accumulators are normally charged by the YELLOW system, and are
protected by non-return valves so that they maintain pressure when the engine driven and electric
pumps are not operating. The accumulators can be charged from the flight deck (when the YELLOW
system is otherwise unpressurised) by pressing a pedestal mounted switch for a few seconds. The
accumulator pressure is indicated on a gauge on the instrument panel. This gauge is combined with
left and right brake pressure indicators, which operate only when alternate braking has been
selected (ie YELLOW system), or when the parking brake is applied.
　
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