Heathrow 777 crash: Siberian cold to blame?

The Air Accidents Investigation Branch (AAIB) yesterday issued a further update (pdf) on its investigation into the 17 January crash-landing of a Boeing 777 at Heathrow following an otherwise uneventful long-haul flight from Beijing.

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 explained earlier this year:

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.

The latest update stresses:

Extensive examination of the aircraft and detailed analysis of the recorded data have revealed no evidence of an aircraft or engine control system malfunction. 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.

The fuel has been tested extensively; it is of good quality, in many respects exceeding the appropriate specification, and shows no evidence of contamination or excessive water. Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages. The spar valves and the aircraft fuel boost pumps were serviceable and operated correctly during the flight.

However, the AAIB adds: "The high pressure (HP) fuel pumps from both engines have unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure.

"The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected."

Quite what may have caused these "reduced fuel flows" remains to be seen, and while the AAIB notes that during the flight "there was a region of particularly cold air, with ambient temperatures as low as -76°C, in the area between the Urals and Eastern Scandinavia", offering a possible fuel-freeze scenario, it explains:

The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’. The lowest total air temperature recorded during the flight was -45°C, and the minimum recorded fuel temperature was -34°C. The specified fuel freezing temperature for Jet A-1 is not above -47°C; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft complied with the Jet A-1 specification and had a measured fuel freezing temperature of -57°C. The aircraft was operated within its certified flight envelope throughout the flight.

The AAIB concludes:

The focus of the investigation continues to be the fuel system of both the aircraft and the engines, in order to understand why neither engine responded to the demanded increase in power when all of the engine control functions operated normally. Under the direction of the AAIB, extensive full scale engine testing has been conducted at Rolls-Royce, Derby, and fuel system testing is ongoing at Boeing, Seattle.

The engine test cell at Rolls-Royce was altered to enable the introduction of calibrated restrictions at various locations in the engine and aircraft fuel feed systems to replicate the engine fuel and control system response.

The primary challenge at Boeing is to create the environmental conditions experienced on the flight over Siberia, at altitudes up to 40,000 ft, in which to test a representation of the aircraft fuel system.

These tests are collectively aimed at understanding and, if possible, replicating the fuel system performance experienced on the day and the potential for formation of restrictions. In addition, work has commenced on developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine.

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