The
Department for Transport's Air Accidents Investigation Branch (AAIB) investigated the accident, with the US
National Transportation Safety Board,
Boeing, and
Rolls-Royce also participating. The
flight data recorder (FDR) and the
cockpit voice recorder (CVR), along with the
quick access recorder (QAR), were recovered from the aircraft within hours of the accident, and they were transported to the AAIB's Farnborough headquarters, some from Heathrow. The information downloaded from these devices confirmed what the crew had already told the investigators, that the engines had not responded when the throttles were advanced during final approach.
Fuel system similar to that used in the Boeing 777. In its Special Bulletin of 18 February 2008, the AAIB noted evidence that
cavitation had taken place in both high-pressure fuel pumps, which could be indicative of a restriction in the fuel supply or excessive aeration of the fuel, although the manufacturer assessed both pumps as still being able to deliver full fuel flow. The report noted the aircraft had flown through air that was unusually cold (but not exceptionally so), and concluded that the temperature had not been low enough to freeze the fuel. Tests were continuing in an attempt to replicate the damage seen in the fuel pumps and to match this to the data recorded on the flight. A comprehensive examination and analysis was to be conducted on the entire aircraft and engine fuel system, including modelling fuel flows, taking account of environmental and aerodynamic effects. The AAIB issued a further bulletin on 12 May 2008, which confirmed that the investigation continued to focus on fuel delivery. It stated, "The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this." The report confirmed that the fuel was of good quality and had a freezing point below the coldest temperatures encountered, appearing to rule out fuel freezing as a cause. As in the aforementioned February bulletin, the report noted cavitation damage to the high-pressure fuel pumps of both engines, indicative of abnormally low pressure at the pump inlets. After ruling out fuel freezing or contamination, the investigation then focused on what caused the low pressure at the pump inlets. "Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected." The fuel delivery system was being investigated at Boeing, and the engines at manufacturer
Rolls-Royce in
Derby. The AAIB issued an interim report on 4 September. Offering a tentative conclusion, it stated: The report summarised the extensive testing performed in an effort to replicate the problem suffered by G-YMMM. This included creating a mock-up of G-YMMM's fuel delivery system, to which water was added to study its freezing properties. After a battery of tests, the AAIB had not yet succeeded in reproducing the suspected icing behaviour and was undertaking further investigation. Nevertheless, the AAIB believed its testing showed that fuel flow was restricted on G-YMMM and that frozen water in the jet fuel could have caused the restriction, ruling out alternative hypotheses such as a failure of the aircraft's
FADEC (computerised engine control system). The hypothesis favoured in the report was that ice had accreted somewhere downstream of the boost pumps in the wing fuel tanks and upstream of the engine-mounted fuel pumps. Either enough ice had accumulated to cause a blockage at a single point, or ice throughout the fuel lines had become dislodged as fuel flow increased during the landing approach, and the dislodged ice had then formed a blockage somewhere downstream. As temperatures in flight had not dropped below the 777's designed operating parameters, the AAIB recommended Boeing and Rolls-Royce take interim measures on
Trent 800-powered 777s to reduce the risk of ice restricting fuel delivery. Boeing did so by revising the 777 operating procedures so as to reduce the opportunities for such blockages to occur, and by changing the procedure to be followed in the event of power loss to take into account the possibility that ice accumulation was the cause. The report acknowledged that a redesign of the fuel system would not be practical in the near term, and suggested two ways to lower the risk of recurrence. One was to use a fuel additive (
FSII) that prevents water ice from forming down to . Western air forces have used FSII for decades, and although it is not widely used in commercial aviation, it is nonetheless approved for the 777.
Rejected theories The Special Bulletin of 18 February, stated "no evidence of a mechanical defect or ingestion of birds or ice" was found, "no evidence of fuel contamination or unusual levels of water content" was seen within the fuel, and the recorded data indicated "no anomalies in the major aircraft systems". Some small foreign bodies, however, were detected in the fuel tanks, although these were later concluded to have had no bearing on the accident.--> The Special Bulletin of 12 May 2008 specifically ruled out certain other possible causes, stating: "There is no evidence of a wake vortex encounter, a bird strike, or core engine icing. There is no evidence of any anomalous behaviour of any of the aircraft or engine systems that suggests electromagnetic interference."
Probable cause The AAIB issued a full report on 9 February 2010. It concluded:
Other findings The AAIB also studied the
crashworthiness of the aircraft during the accident sequence. It observed that the main attachment point for the main
landing gear was the rear spar of the aircraft's wing; because this spar also formed the rear wall of the main fuel tanks, the crash landing caused the tanks to rupture. The report recommended that Boeing redesign the landing gear attachment to reduce the likelihood of fuel loss in similar circumstances. The report went on to note that the fire extinguisher handles had been manually deployed by the crew before the fuel shut-off switches. The fire extinguisher handles also have the effect of cutting off power to the fuel switches, meaning that the fuel may continue to flow – a potentially dangerous situation. The report restated a previous Boeing Service Bulletin giving procedural advice that fuel switches should be operated before fire handles. It went on: "This was not causal to the accident, but could have had serious consequences in the event of a fire during the evacuation." Indeed, the need to issue
Safety Recommendation 2008–2009, affecting all 777 airframes, which had yet to incorporate the Boeing Service Bulletin (SB 777-28-0025) – as was the case with G-YMMM – was given as the main reason for issuing the first special bulletin, well before the accident investigation itself was complete. ==Similar incidents==