BA 777 Crash Probe Focuses on Fuel

British air accident investigators probing the Jan. 17 crash landing of British Airways Flight 88 at London Heathrow continue to focus on fuel supply issues as the culprit in the spectacular total aircraft loss, the first-ever crash of a Boeing 777-236ER (G-YMMM). A week after the crash landing, the British...

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British air accident investigators probing the Jan. 17 crash landing of British Airways Flight 88 at London Heathrow continue to focus on fuel supply issues as the culprit in the spectacular total aircraft loss, the first-ever crash of a Boeing 777-236ER (G-YMMM).

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 started probing 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 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 has said little about the accident, but released a progress report May 12 saying they suspected that the plane's fuel flow became restricted somewhere between the engines and the fuel tanks. But the report did not indicate what they thought caused the blockage. Ice collecting in or near an engine component has emerged as a prime suspect.

According to the May 12 AAIB statement: "Extensive examination of the aircraft and detailed analysis of the recorded data have revealed no evidence of an aircraft or engine control system malfunction. There is no evidence of a wake vortex encounter, a bird strike or core engine icing. There is no evidence of any anomalous behavior of any of the aircraft or engine systems that suggests electromagnetic interference. The fuel has been tested extensively; it is of good quality, in many respects exceeding the appropriate specification, and shows no evidence of contamination or excessive water. Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages. The spar valves and the aircraft fuel boost pumps were serviceable and operated correctly during the flight. The high pressure (HP) fuel pumps from both engines have unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure. The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected."

The UK accident investigators noted that during the flight the Boeing 777 passed through a region of particularly cold air (with ambient temperatures as low as -76 degrees C) between the Urals and Eastern Scandinavia.

The focus of the investigation continues to be the fuel system of both the aircraft and the engines, in order to understand why neither engine responded to the demanded increase in power when all of the engine control functions operated normally. Under the direction of the AAIB, extensive full-scale engine testing has been conducted at Rolls- Royce, Derby, and fuel system testing is ongoing at Boeing, Seattle.

The engine test cell at Rolls-Royce was altered to enable the introduction of calibrated restrictions at various locations in the engine and aircraft fuel feed to replicate the engine fuel and control system response.

The AAIB report said "the primary challenge at Boeing is to create the environmental conditions experienced on the flight over Siberia, at altitudes up to 40,000 ft, in which to test a representation of the aircraft fuel system. These tests are collectively aimed at understanding and, if possible, replicating the fuel system performance experienced on the day and the potential for formation of restrictions."

In addition, work has commenced on developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine.

Meanwhile, a data analysis team, working with statisticians from QINETIQ, are reviewing and analyzing recorded data from a large sample of flights on similar aircraft. "No individual parameter from the flight of G-YMMM has been identified to be outside previous operating experience. The analysis is concentrating on identifying abnormal combinations of parameters." The report stated.

The Washington Post quotes Bill Voss, president of the Flight Safety Foundation, saying "this is a great mystery, and I never expected this accident to be this difficult to solve, given the state-of-art tools on the plane and the fact that the aircraft was largely intact. This has potentially broad implications that go beyond this one airplane, depending on what they find."

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 stated.

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 superseded 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.