The investigation into why a British Airways Boeing 777 on Jan. 17 crashed (with no fatalities) short of the runway at London's Heathrow airport has determined "that there are two possible scenarios" that match the data collected from the flight -- both involve ice in the fuel system. The Air Accidents Investigation Board (AAIB) has found that of all flights flown with similar equipment (about 140,000), less than 0.2 percent had been subjected to fuel temperatures at or below the minus 34 degrees centigrade recorded for the accident aircraft. The accident aircraft is also noted for operating in those temperatures at very low fuel flows, but within certified operational limits. As a result, Boeing 777 pilots will be required to cycle the thrust of their engines (maximum thrust for 10 seconds prior to descent) to clear the system of ice before landing and vary their altitudes en route when fuel in the main tank is below 10 degrees Celsius for more than three hours. There are rules for low-temperature ground operations, too. All instructions apply to 220 777s worldwide. This short-term fix aims to address concerns while the exact root of the problem is further investigated and (for now) acts in place of retooling the Rolls-Royce Trent 800 fuel feed systems on the jets. For the accident itself, the AAIB has detailed its two most likely causal scenarios.
One AAIB theory involves "ice accreted over a period of time" downstream of the forward boost pump connection into the fuel manifold and upstream of the high-pressure pump inlet. "This ice would have had to have accrued to an extent to block approximately 95 percent of the cross sectional area," inducing pump cavitation and resulting in loss of power. Investigators have not been able to reproduce that scenario, but the possibility is still being evaluated.
The second theory is that ice had accreted throughout the fuel system and was released when the pilots commanded a second acceleration on final approach that was not met by the engines. Central to both theories is a requirement for the fuel system's extended exposure to both low fuel flows and temperatures "below the Critical Icing Temperature." The report states in summary that the investigation has shown that both engines suffered from restricted fuel flow "most probably due to ice within the fuel feed system." Again, it is noted that the aircraft was operated within its certified operational envelope at all times, but also for long periods "with low fuel flows, in an unusually cold environment." Find the AAIB's full Interim Report here (PDF).