Whereas before investigators probing the Jan. 17 crash landing of British Airways Flight 88 at London Heathrow had concentrated on an electrical or computer fault as to the landing accident, those same air safety investigators have now switched their focus to fuel supply.
A week after the crash landing, the British Air Accidents Investigation Branch (AAIB) said the Boeing 777’s two Rolls-Royce Trent 895-17 engines did not shut down, as previously thought. Investigators said the thrust on the right engine fell first. Then eight seconds later, the thrust on the left engine also fell. Both however continued to run but with inadequate power.
It is not believed that the jetliner simply ran out of fuel, so the AAIB is now examining the complete fuel flow path to see if there was an interruption that would cause both
Trent engines to provide too little thrust. But fuel management problems don’t normally affect both engines at the very same time.
The probe into the non-fatal accident shows the danger in second-guessing air safety investigators, especially in the early days of a crash investigation. It also depicts how closer examination of evidence, including data from the aircraft’s flight data recorder and cockpit recorder, leads to ‘twists and turns’ in an aircraft crash investigation.
The AAIB had previously said that both turbofans failed to respond to demands for more power moments before the first-ever crash of a Boeing 777-236ER (G-YMMM).
The British Airways twinjet enroute from Beijing crash landed just shy of Runway 27L. There was one serious injury among the 136 passengers and 16 crewmembers onboard.
The 777 entered service in 1995 and about 687 remain in use. The accident aircraft was built in 2001.
The AAIB’s initial statement on the BA38 accident said: “Initially the approach progressed normally, with the Autopilot and Autothrottle engaged. The aircraft then descended rapidly and struck the ground, some 1,000 ft short of the paved runway surface, just inside the airfield boundary fence. The aircraft stopped on the very beginning of the paved surface of Runway 27L.
“During the short ground roll the right main landing gear separated from the wing and the left main landing gear was pushed up through the wing root. A significant amount of fuel leaked from the aircraft but there was no fire.
“Initial indications from the interviews and Flight Data Recorder analyses show the flight and approach to have progressed normally until the aircraft was established on late finals. At approximately 600 ft and two miles from touchdown, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond. The aircraft speed reduced and the aircraft descended onto the grass short of the paved runway surface.
“The investigation is now focused on more detailed analysis of the Flight Recorder information, collecting further recorded information from various system modules and examining the range of aircraft systems that could influence engine operation,” the initial report on the Boeing 777 accident stated.
Since issuance of the AAIB’s First Preliminary Report on Jan. 18, work has continued on all fronts to identify why neither engine responded to required throttle lever inputs during the final approach.
According to a Jan. 24 AAIB probe update, ” whilst the aircraft was stabilized on an ILS approach with the Autopilot engaged, the Autothrust system commanded an increase in thrust from both engines. The engines both initially responded but after about three seconds the thrust of the right engine reduced.
“Some eight 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.
“Recorded data indicates that an adequate fuel quantity was on board the aircraft and that the Autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.
“All possible scenarios that could explain the thrust reduction and continued lack of response of the engines to throttle lever inputs are being examined, in close cooperation with Boeing, Rolls Royce and British Airways.
“This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles,” the second AAIB report states.
The U.S. National Transportation Safety Board’s (NTSB) database records six previous engine failures involving the Boeing 777, including one in August 2005, when a Malaysia Airlines 777 suffered a loss of thrust 30 minutes after takeoff from Perth, Australia. The Safety Board’s investigation of that incident might shed some light on the Jan. 17 Boeing 777 loss at Heathrow and is thus worth mentioning.
According to the NTSB, on Aug. 2, 2005, a Boeing 777-200, registered in Malaysia as 9M-MRG and operated by Malaysian Airline System, experienced a pitch up about one-half hour after takeoff from Perth, Australia. This event occurred as the flight was climbing through 36,000 feet and while the aircraft was on autopilot. During the pitch up the aircraft climbed to 41,000 feet and the indicated airspeed dropped from 270 knots to 158 knots. The stick shaker and the stall warning indicator activated during the event.
The flight landed uneventfully back at Perth.
On August 29, 2005, the Federal Aviation Administration (FAA) issued emergency airworthiness directive (AD 2005-18-51) that superceded an earlier airworthiness directive. It states that operators should install ADIRU-03 software within 72 hours in their Boeing 777 aircraft. It also states that faulty ADIRU data could cause anomalies in 777 primary flight controls, autopilot, pilot displays, autobrakes and autothrottles.