The UK Air Accidents Investigation Branch (AAIB) has today issued an update on its investigation of the crash of the British Airways Boeing 777 at Heathrow last month. It appears to rule out several of the most-discussed possible causes, and does not strongly point to any particular cause. It mentions a couple of further lines of enquiry.
Overall, the mystery persists. I've summarised the key points below. You can read the whole thing at the AAIB site.
1. There was plenty of fuel remaining on board (10,500kg).
2. At no point in the flight did the fuel become cold enough to freeze, although conditions during the flight were notably cold. The Met Office says "not exceptional".
3. There is no sign of the fuel having been contaminated with water or anything else.
4. The autopilot, authothrottle, and engine electrical controls all worked as they should.
5. There is no evidence of engine mechanical defect, birdstrikes, or ice ingestion.
6. There are signs of "abnormal cavitation on the press-side bearings and the outlet ports" of both high-pressure fuel pumps. "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."
7. Some small bits of debris (tape, a small scraper, piece of paper, etc) were found in the fuel tanks. "The relevance of this debris is still being considered."
8. It played no role in causing the accident, but a quirk of the way BA implemented the Boeing checklist in the event of an evacuation, led to valves designed to shut off fuel to the engines being left open and allowing fuel to spill around the wreckage. Boeing has accepted a safety recommendation made by the AAIb last week.