Airbus A310-300, N835AB, 31 May 1998 at 1515 hrs 事故分析

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Airbus A310-300, N835AB, 31 May 1998 at 1515 hrs
AAIB Bulletin No: 4/99 Ref: EW/C98/5/9 Category: 1.1
Aircraft Type and Registration: Airbus A310-300, N835AB
No & Type of Engines: 2 Pratt & Whitney PW4152 turbofan engines
Year of Manufacture: 1992
Date & Time (UTC): 31 May 1998 at 1515 hrs
Location: Stand M24 at London Heathrow Airport
Type of Flight: Public Transport
Persons on Board: Crew - 12 - Passengers - 204
Injuries: Crew - Nil - Passengers - Nil
Nature of Damage: Damage to left engine cowl
Commander's Licence: Airline Transport Pilot's Licence
Commander's Age: 52 years
Commander's Flying Experience: 17,499 hours (of which 1,459 were on type)
Last 90 days - 195 hours
Last 28 days - 70 hours
Information Source: AAIB Field Investigation
History of the flight
Approximately one hour into a scheduled passenger flight from Kingston, Jamaica, to Heathrow
Airport the YELLOW hydraulic system failed as a result of a loss of fluid quantity. The appropriate
Electronic Centralised Aircraft Monitoring system (ECAM) drill was completed and this was
supplemented by reference to the Flight Crew Operations Manual (FCOM). Upon arrival at
Heathrow, where the meteorological conditions were CAVOK with a light westerly wind, a manual
landing with the flaps/slats set to 30/40 degrees was completed on Runway 27 where the surface
was dry. Reverse thrust was used during the landing run and gentle braking was applied in the later
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.
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
probable that the parking brake was not released after the aircraft started to move forward again,
which would have restored normal brake application via the foot brakes.
Safety action
The aircraft manufacturer, Airbus Industrie, responded to a draft of this Bulletin by stating that the
FCOM Standard Operating Procedures already require checking of the accumulator pressure when
applying the parking brake. However, the manufacturer additionally stated the following:
'We are developing a modification to avoid recurrence of such an incident. This
modification consists of changing the priority between the parking brake and the
brake pedals. In case of no accumulator pressure when the parking brake is set, the
normal braking through the brake pedals will remain available and the crew will be
able to stop the aircraft.'

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