Investigators have established two possible icing scenarios that could have restricted fuel flow on the British Airways Boeing 777-200ER which lost power and crashed at London Heathrow earlier this year.
One scenario envisions an accretion of ice over a period of time, probably in an area downstream of the main fuel tanks' forward boost pump connection to the fuel manifold, and upstream of the engines' high-pressure pump inlet.
The other puts forward the possibility that ice, having accreted throughout the fuel-feed system, was released during a demand for increased fuel-flow.
Both scenarios match the known data obtained from the aircraft, says the Air Accidents Investigation Branch. The AAIB today released its latest findings in an interim report into the 17 January crash.
The inquiry is focusing on the fuel path from the main tank boost pumps and fuel manifold, to the engines' low-pressure pumps, fuel/oil heat exchanger, and high-pressure pumps.
For the first possibility to have developed, says the AAIB, the ice would have had to block about 95% of the cross-sectional area of the fuel system to induce cavitation of the high-pressure pump, and generate the response seen by the 777's Rolls-Royce Trent 800 engines.
Tests have shown that ice accretion on the face of the engines' fuel/oil heat exchanger is "unlikely" to have caused the restriction.
Partial restriction upstream of the low-pressure pump is consistent with the 777's flight data. But the AAIB says: "Testing has not yet been able to duplicate such a restriction with ice. Nevertheless this possibility is still being evaluated."
In the late stages of final approach, the 777's autothrottle commanded a series of four thrust adjustments. The second of these peaked at a relatively high 5,500kg/h (12,000lb/h) and the fourth marked the onset of the loss of power that led to the accident.
The AAIB's second scenario suggests ice built up throughout the fuel-feed system but suddenly shifted when the autothrottle requested increased fuel-flow.
"In this case, the ice might then travel and be 'caught' in the pipeworkthereby causing a restriction in the fuel flow," it states.
For ice to accrete within the fuel system requires long periods at low fuel-flow and temperatures below the critical icing temperature - the temperature at which ice crystals begin sticking to their surroundings.
Although the fuel system supplied fuel at a high rate as the aircraft departed Beijing, the flow rate afterwards did not rise beyond about 4,000kg/h (8,900lb/h) until the final approach. During the flight the fuel's temperature fell to a low point of minus 34°C where it remained for about 80min.
"Although the exact mechanism in which the ice has caused the restriction is still unknown, in detail, it has been proven that ice could cause a restriction in the fuel feed system," says the AAIB. "The risk of recurrence needs to be addressed in the short term while the investigation continues."
Flight's safety and operations editor David Learmount's blog Learmount - 777 crash at Heathrow: what chance of another such accident?