The United Kingdom's Air Accidents Investigation Branch (AAIB) has released the final report on the investigation of the 2008 accident involving a British Airways Boeing 777-236ER (registration number G-YMMM) at London's Heathrow International Airport (LHR). On January 17, 2008, British Airways Flight BA 038 landed short of runway runway 27L at LHR after both of the aircraft's engines failed to respond to throttle inputs while the aircraft was on final approach. The aircraft was arriving at Heathrow on a scheduled flight from Beijing. There were no fatalities or serious injuries among the 136 passengers and 16 crew members on board.
The newly released report, which is lengthy and very complex, includes detailed analyses and 18 safety recommendations. Here is an excerpt from the AAIB's report synopsis, including statements regarding probable cause:
Whilst on approach to London (Heathrow) from Beijing, China, at 720 feet agl, the right engine of G-YMMM ceased responding to autothrottle commands for increased power and instead the power reduced to 1.03 Engine Pressure Ratio (EPR). Seven seconds later the left engine power reduced to 1.02 EPR. This reduction led to a loss of airspeed and the aircraft touching down some 330 m short of the paved surface of Runway 27L at London Heathrow. The investigation identified that the reduction in thrust was due to restricted fuel flow to both engines.Of particular interest to crew members is the section of the report that discusses the actions of the pilots in the final moments of the accident flight. Quoting from the AAIB report:
It was determined that this restriction occurred on the right engine at its Fuel Oil Heat Exchanger (FOHE). For the left engine, the investigation concluded that the restriction most likely occurred at its FOHE. However, due to limitations in available recorded data, it was not possible totally to eliminate the possibility of a restriction elsewhere in the fuel system, although the testing and data mining activity carried out for this investigation suggested that this was very unlikely. Further, the likelihood of a separate restriction mechanism occurring within seven seconds of that for the right engine was determined to be very low.
The investigation identified the following probable causal factors that led to the fuel flow restrictions:
- Accreted ice from within the fuel system1 released, causing a restriction to the engine fuel flow at the face of the FOHE, on both of the engines.
- Ice had formed within the fuel system, from water that occurred naturally in the fuel, whilst the aircraft operated with low fuel flows over a long period and the localised fuel temperatures were in an area described as the ‘sticky range’.
- The FOHE, although compliant with the applicable certification requirements, was shown to be susceptible to restriction when presented with soft ice in a high concentration, with a fuel temperature that is below ‑10°C and a fuel flow above flight idle.
- Certification requirements, with which the aircraft and engine fuel systems had to comply, did not take account of this phenomenon as the risk was unrecognised at that time.
Final approachInterested readers can download the entire report or any section by visiting this landing page on the AAIB Web site: Report on the accident to Boeing 777-236ER, G-YMMM, at London Heathrow Airport on 17 January 2008
The right engine ceased responding to autothrottle demands 57 seconds before the touchdown and within seconds the crew became aware that there was a problem with the engine thrust control of both engines. This presented the flight crew with a situation that was highly unusual, for which no specific training existed.
The rollback event occurred whilst the aircraft was close to the ground, leaving the crew very little time to react. The commander did make a ‘MAYDAY’ call during this period but he was not able, in the time available, to brief the cabin crew about the emergency or issue a ‘brace brace’ command.
Actions of the co-pilot
The co-pilot initially believed he had disconnected the autopilot at the intended height on the approach in order to carry out a manual landing on Runway 27L. However, as the aircraft descended through 600 ft he became aware of a problem with the engines, indicated by a split in the thrust lever positions. It is likely that, in attempting to understand the sudden and unprecedented problem with which he was presented he was distracted and thus omitted to disconnect the autopilot at this point.
Engine power had now reduced to a level at which the aircraft was losing airspeed and it started to descend below the glideslope. As the autopilot had remained engaged it attempted to maintain the ILS glide path by increasing the aircraft’s pitch attitude. This led to a further gradual reduction of airspeed, the initial ‘airspeed low’ master caution and the eventual triggering of the stall warning stick shaker. It was at this stage the co-pilot promptly pushed the control column forward, leading to the disconnect of the autopilot as overriding force was applied to the column to avoid the stall. However, the aircraft was now only 150 ft above the ground and a landing short of the runway surface was inevitable. At this point there was insufficient height available for the aircraft to develop the airspeed needed for a landing flare, to reduce the high rate of descent.
Actions of the commander
The commander, on realising that he was unable to obtain any additional thrust from the engines, attempted to reduce the drag of the aircraft by reducing the flap setting. However, the aircraft was now so close to the ground that there was little time for the beneficial affects of this action to take effect.
The action of reducing the flap setting was prompt and resulted in a reduction of the aerodynamic drag, with a minimal effect on the aircraft stall speed; it moved the point of initial ground contact about 50 m towards the runway threshold. Had the flaps remained at flap 30, the touchdown would have been just before the ILS antenna, but still within the airfield boundary. The effects of contact with the ILS antenna are unknown but such contact would probably have led to more substantial structural damage to the aircraft.
Assessment of flight crew actions - summary
From the available evidence, it is apparent that the flight crew’s preparation and conduct of the flight preceding the engine rollbacks was orderly, and in accordance with the operating company’s standard operating procedures.
On the final approach to land the flight crew were presented with an operational situation, a double-engine rollback at a low height, which was unprecedented. Most importantly at this point, when the stick shaker was alerting them to an impending stall, they kept the aircraft flying and under control so that, at impact, it was wings level and at a moderate pitch attitude. The reduction in flap setting did allow the aircraft to clear the ILS aerial array and, given more height, it would have been more effective.
In analysing the flight crew’s actions during final approach, the first indication of a problem was the thrust lever split. The flight crew did not, at this time, realise that this was associated with the ensuing engine rollback; slight splits in the thrust lever positions are common and manually moving the thrust levers back into alignment is a normal response. The flight crew became fully aware of the problem some 30 seconds before touchdown and at this point the subsequent high rate of descent at impact was inevitable.
The crew’s attention was on monitoring the approach and the external environment and, while the autopilot remained engaged, the crew’s focus was on the developing situation with falling engine thrust and reducing airspeed, and their subsequent attempts to restore power.
In the very limited time available after identification of the problem, the flight crew clearly prioritised their actions and thus did make the ‘MAYDAY MAYDAY MAYDAY’ call, although they were not able to make the ‘brace brace’ call. The initial use of the VHF radio rather than PA system for the cabin evacuation call had no effect on the accident outcome.
BBC News posted on their Web site an audio file of Air Traffic Control dialogue immediately before and after the crash of British Airways Flight 038.
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