Thursday, September 04, 2008

AAIB Interim Report on the Jan. 2008 British Airways Accident at Heathrow

AAIBThe United Kingdom's Air Accidents Investigation Branch (AAIB) has issued an Interim Report on the progress of the investigation of the accident involving a British Airways Boeing 777-236ER (registration number G-YMMM) at London's Heathrow International Airport (LHR) earlier this year. Readers will recall that on January 17, 2008, British Airways Flight BA 038 landed short of runway runway 27L at LHR after both of the aircraft's engines failed to respond to throttle inputs while the aircraft was on final approach. The aircraft was arriving at Heathrow on a scheduled flight from Beijing. There were no fatalities or serious injuries among the 136 passengers and 16 crew members on board.

The AAIB's newly released report describes in detail the research done by the investigation team using data obtained from the accident aircraft and its systems, as well as from similar aircraft in the British Airways fleet. The report also includes several safety recommendations directed toward civilian aviation agencies in the United States and Europe.

The new report indicates that both of the Rolls‑Royce RB211 Trent 895‑17 turbofan engines on the accident aircraft are believed to have lost power in the final minute of flight because the fuel flow to each engine was restricted, probably due to an accumulation of water ice crystals within the engine fuel feed system. Excerpt from the Interim Report's Summary:
The investigation has shown that the fuel flow to both engines was restricted; most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment; although, G-YMMM was operated within the certified operational envelope at all times.[AAIB: G-YMMM Interim Report, p. 19]
The AAIB report is careful to note that all aviation fuel contains water that cannot be completely removed, and that "if the fuel temperature drops below the freezing point of the water, it will form ice. The majority of flights have bulk fuel temperatures below the freezing point of water and so there will always be a certain amount of ice in the fuel." Addressing this problem, the AAIB report says:
To prevent the ice causing a restriction requires either: the fuel system must be designed in such a way that the ice in the fuel does not pose a risk of causing an interruption of the fuel supply to the engine or; prevention of the water from becoming ice in the first instance. Changes to the fuel system design could make the system more tolerant, but would take time to implement and would certainly not be available within the near term. Therefore, to reduce the risk of recurrence interim measures need to be adopted until such design changes to the fuel system are available.

One option would be to prevent the water from becoming ice, such as through the use of FSII. Alternatively, operational changes to reduce the risk of ice formation causing a restricted fuel flow at critical stages of flight could be introduced. Such changes could be implemented quickly, but must not compromise the safe operation of the aircraft.[AAIB: G-YMMM Interim Report, p. 20]
"FSII" refers to "Fuel System Icing Inhibitor," a fuel additive used in military jets in the U.S. and the U.K., and in some small civilian jet aircraft. FSII is not commonly used in large public transport category aircraft at the present time, however the AAIB report notes that it is approved for use in the Boeing 777.

The AAIB report states that since it has been proven that ice could cause a restriction in the fuel feed system, the risk of recurrence needs to be addressed in the short term, even though the investigation into this accident is still underway. Therefore the AAIB's Interim Report includes the following safety recommendations:
  • Safety Recommendation 2008-047: It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency, in conjunction with Boeing and Rolls-Royce, introduce interim measures for the Boeing 777, powered by Trent 800 engines, to reduce the risk of ice formed from water in aviation turbine fuel causing a restriction in the fuel feed system.
  • Safety Recommendation 2008-048: It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency should take immediate action to consider the implications of the findings of this investigation on other certificated airframe / engine combinations.
  • Safety Recommendation 2008-049: It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency review the current certification requirements to ensure that aircraft and engine fuel systems are tolerant to the potential build up and sudden release of ice in the fuel feed system.
Here is the link to the full text of the AAIB report: G-YMMM Interim Report - AAIB, Sep. 2008, (21-page 'pdf' file)

RELATED: Click here to view all posts about British Airways Flt 038 on Aircrew Buzz.