Iced-up fuel system brought down British Airways B777
The United Kingdom Air Accidents Investigation Branch (UK)/AAIB has released its final report on B777 loss of engine power and crash-landing in January 2008 at London-Heathrow.
The summary below is extracted from the AAIB report.
The left main fuel tank temperature at takeoff was -2°C, which was not unique and data mining revealed that a small percentage of B777 flights had a fuel temperature below 0°C at takeoff.
During the flight from the fuel temperature reached a minimum of -34°C and the minimum TAT reached was -45°C. These temperatures were unusual but were within the aircraft’s operating envelope and were not unique.
However data mining showed that the accident flight was unique amongst 175,000 flights as having a low cruise fuel flow and a high fuel flow during approach while at a low fuel temperature.
The co-pilot had taken control of the aircraft from the commander at 800 ft in accordance with the operator’s procedures.
At 720 ft agl (above ground level) the right engine suffered an uncommanded reduction in engine power to 1.03 EPR (engine pressure ratio) and seven seconds later the left engine suffered an uncommanded reduction to 1.02 EPR.
The right engine fuel flow reduced to 6,000 pph (pounds per hour) and the left engine fuel flow reduced to 5,000 pph, levels above those required by an engine at flight idle.
Both the left and right engine power levers moved to full open and the EECs with no effect on the fuel flow.
Data mining revealed only one other (non-accident flight, other than the G-YMMM accident flight and the N862DA incident flight, that had demonstrated similar symptoms.
The fuel temperature at the time of the engine rollback was -22°C., which was also the fuel temperature at which the rollback occurred on the other (incident) incident flight.
The flight crew became aware of a possible thrust problem with the only 48 seconds before touchdown. The co-pilot intended to disconnect the autopilot at 600 ft but became distracted by the engine rollback, so the autopilot remained engaged.
The loss of engine power led to a reduction in airspeed as the autopilot attempted to follow the ILS glideslope, leading to a nose-high pitch attitude.
Thirty-four seconds before touchdown the flight crew became concerned about the reduction in airspeed below the target approach speed and attempted manually to increase engine thrust to compensate; there was no response from the engines.
At 240 ft the commander retracted the flap from flap 30 to flap 25, which increased the distance to touchdown by about 50 metres; if left at flap 30 the touchdown would have still been within the airfield boundary.
At 200 ft the stick shaker [stall warning] activated and as a touchdown short of the runway was inevitable the commander transmitted a ‘MAYDAY’ call three seconds before touchdown.
At the operation of the stick-shaker, the co-pilot pushed forward on the control column and the autopilot disconnected.
The aircraft struck the ground within the airfield boundary at a recorded normal peak load of 2.9g, and a descent rate of about 1400 feet per minute, 330 m short of the runway and slid 372 m before coming to rest.
The AAIB determined that the 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 also 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.
