The Air Accidents Investigation Branch has issued an update (pdf) on its investigation into the 17 January crash-landing of a Boeing 777 at Heathrow, indicating that there "were no anomalies in the major aircraft systems" and that the "autopilot and the autothrottle systems behaved correctly and the engine control systems were providing the correct commands prior to, during, and after, the reduction in thrust".
Initial speculation into the cause of the accident, which saw BA038 (G-YMMM) suffer reduced thrust in both engines and fall short of the runway, centred around possible bird strike, fuel flow problems, or an autothrottle glitch. The AAIB explains:
The first officer took control for the landing at a height of approximately 780 ft, in accordance with the briefed procedure, and shortly afterwards the autothrottles commanded an increase in thrust from both engines. The engines initially responded but, at a height of about 720 ft, the thrust of the right engine reduced. Some seven seconds later, the thrust reduced on the left engine to a similar level. The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust. The engines failed to respond to further demands for increased thrust from the autothrottles, and subsequent movement of the thrust levers fully forward by the flight crew.
While analysis of the Digital Flight Data Recorder, a Cockpit Voice Recorder, and a Quick Access Recorder has now ruled out an engine control failure, and examination of the engines "indicated no evidence of a mechanical defect or ingestion of birds or ice", the AAIB did identify "some small items of debris" in the aircraft's fuel tanks. The AAIB notes: "The relevance of this debris is still being considered."
Tests on the fuel further revealed "no signs of contamination or unusual levels of water content", and while the fuel system was pretty well given a clean bill of health, the AAIB notes: "Detailed examination of both the left and right engine high pressure fuel pumps revealed signs of abnormal cavitation on the pressure-side bearings and the outlet ports. This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel. The manufacturer assessed both pumps as still being capable of delivering full fuel flow."
The AAIB concludes: "Investigations are now underway in an attempt to replicate the damage seen to the engine high pressure fuel pumps, and to match this to the data recorded on the accident flight. In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects." ®
Boeing has accepted a safety recommendation in relation to the procedure to cut off the fuel supply in the event of an emergency. After crashing, BA038 leaked some fuel, something which "was not causal to the accident but could have had serious consequences in the event of a fire".
The AAIB found: "On examination, both of the engine spar valves were found to be OPEN, allowing the fuel leak evident at the accident site."
The report elaborates:
The spar valves are designed to shut off the fuel supply to the engines following the operation of the fuel control switches or after operation of the fire handles in the cockpit. Their function is to cut off the fuel flow to the engine in the event of an engine fire or an accident. Each valve has two separate electrical wire paths which can be used to supply power to shut the valve; the first is via a run/cut-off relay, controlled by the fuel control switches, the other is directly from the fire handles.
The wiring on G-YMMM was as originally designed and manufactured, and such that when the fire handle was operated, it isolated the power supply to the run/cutoff relay. When tested, the run/cut-off relays for the left and right engines were still in the valve OPEN position, despite the fuel control switches being set to cut-off. The fire handles had also been pulled and the engine fire bottles had been fired. Therefore the fire handles had been operated prior to the fuel control switches.
The safety recommendation states: "Boeing should notify all Boeing 777 operators of the necessity to operate the fuel control switch to cut-off prior to operation of the fire handle, for both the fire drill and the evacuation drill, and ensure that all versions of its checklists, including electronic and placarded versions of the drill, are consistent with this procedure."