British safety officials have issued recommendations for flight-data recorders (FDRs) to record engine fuel-metering information and for reviews of landing-gear failure requirements and Boeing 777 data buffering. Accident investigators call for the action among nine safety recommendations in the final report of the Jan. 17, 2008, British Airways Boeing 777-236ER accident at London Heathrow Airport, which was released on Tuesday.
Following a 10.5-hour flight from Beijing, the 777 (flying as British Airways Flight 38) lost power on its final approach to Heathrow, as both engines suffered an uncommanded “rollback” (or power reduction) at 720 feet an8d the airliner subsequently made a forced landing about 1,100 feet short of the runway. Ice in the fuel system had restricted fuel flow to the Rolls-Royce Trent 800 engines, according to the UK Department for Transport Aircraft Accidents Investigation Branch (AAIB).
The airplane struck the ground at “a recorded normal peak load of 2.9 g, and a descent rate of about 1,400 fpm” and slid 1,220 feet. One main landing-gear sheared off the airplane on impact, while the other penetrated the upper surface of the wing. One passenger suffered a broken leg when landing-gear items entered the cabin; 34 passengers and 12 crew, among 151 remaining occupants, suffered minor injuries.
The investigation determined that the fuel-flow restriction occurred at the fuel/oil heat exchanger (FOHE) on the right engine and “most likely” on the left-hand FOHE. Subsequent tests and data mining suggest the possibility of other restrictions was unlikely. The AAIB said ice can form from normal concentrations of dissolved and entrained water in the fuel, while the 777 was operating with low fuel flows over a long period. The FOHE was susceptible to restriction when presented with highly concentrated soft ice while fuel temperature was “below –10 degrees C [14 degrees F] and fuel flow above flight idle.” Relevant certification requirements did not accommodate this phenomenon as the risk had not been recognized previously.
At takeoff, the left main fuel tank temperature was –23 degrees C. Investigators said “a small percentage” of 777 flights take off with a fuel temperature below 0 degrees C. Fuel temperature reached a minimum of -34°C and minimum outside air temperature reached was -45°C, circumstances that were “unusual” but within the aircraft operating envelope. But the AAIB says the flight was unique among 175,000 flights in “having a low cruise fuel flow and a high fuel flow during approach while at a low fuel temperature.”
When fuel flow became restricted during final approach, both the left- and right-engine fuel-metering valves (FMVs) moved to full open and the electronic engine controls (EECs) entered loop-in control (LIC), with no effect on the fuel flow. “Data mining did not reveal any flight, other than the accident flight and [a later, November 2008 Delta Air Lines] flight, that had indicated an EEC LIC or had a genuine FMV position versus fuel flow mismatch.”
The fuel temperature at the time of the engine rollback was -22 °C in both events, said investigators. When the crew attempted manually to increase thrust, there was no engine response. The digital flight-data recorders did not record FMV positions, and data buffering meant the information did not reach the quick-access recorder.
“Ice probably began to accumulate in the fuel feed pipes while the warm center tank fuel flowed through cold fuel pipes that pass through the main fuel tank at the start of the flight. During the later stages of approach, accumulated ice was probably released due to engine accelerations and possibly a combination of turbulence, aircraft pitch changes, and an increase in strut temperature,” concluded AAIB.
Following earlier recommendations, Boeing introduced changes to mitigate the risk from fuel icing in Trent 800-engine 777s. The EASA has approved a modified FOHE mandated from Rolls-Royce.