Accident Boeing 777-236ER G-YMMM,

ASN Wikibase Occurrence # 321801

Date:	Thursday 17 January 2008
Time:	12:42
Type:	Boeing 777-236ER
Owner/operator:	British Airways
Registration:	G-YMMM
MSN:	30314/342
Year of manufacture:	2001
Total airframe hrs:	28675 hours
Cycles:	3957 flights
Engine model:	Rolls-Royce Trent 895-17
Fatalities:	Fatalities: 0 / Occupants: 152
Aircraft damage:	Substantial, written off
Category:	Accident
Location:	London-Heathrow Airport (LHR) - United Kingdom
Phase:	Landing
Nature:	Passenger - Scheduled
Departure airport:	Beijing-Capital International Airport (PEK/ZBAA)
Destination airport:	London-Heathrow Airport (LHR/EGLL)
Investigating agency:	AAIB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
Following an uneventful flight (BA38) from Beijing, China, the aircraft was established on an ILS approach to runway 27L at London Heathrow. Initially the approach progressed normally, with the Autopilot and Autothrottle engaged, until the aircraft was at a height of approximately 600 ft and 2 miles from touch down. The aircraft then descended rapidly and struck the ground, some 1,000 ft short of the paved runway surface, just inside the airfield boundary fence. The aircraft stopped on the very beginning of the paved surface of runway 27L. During the short ground roll the right main landing gear separated from the wing and the left main landing gear was pushed up through the wing root. A significant amount of fuel leaked from the aircraft but there was no fire. An emergency evacuation via the slides was supervised by the cabin crew and all occupants left the aircraft, some receiving minor injuries.
Initial indications from the interviews and Flight Recorder analyses show the flight and approach to have progressed normally until the aircraft was established on late finals for runway 27L. At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond. The aircraft speed reduced and the aircraft descended onto the grass short of the paved runway surface.

CAUSAL FACTORS:
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.
It was determined that this restriction occurred on the right engine at its 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:
1) Accreted ice from within the fuel system released, causing a restriction to the engine fuel flow at the face of the FOHE, on both of the engines.
2) 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.
3) 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.
4) 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.

Accident investigation: