U.K. Board Urges 777 Fuel-Blockage
Graham Warwick graham_warwick@aviationweek.com 
Measures to prevent ice in fuel lines causing a flow restriction and thrust rollback are recommended by the U.K. Air Accidents Investigation Branch (AAIB) in its second interim report on the crash of a Rolls-Royce Trent 800-powered Boeing 777-200 at London Heathrow on Jan. 17, 2008.
The AAIB says further, and more representative testing of the 777's fuel system has confirmed that ice can accumulate within the fuel lines and be suddenly released to cause a flow restriction in the fuel/oil heat exchanger (FOHE) on the Trent 800.
Investigators make five recommendations. The first is that Boeing and Rolls-Royce devise measures to prevent blockage of the heat exchanger by ice in the fuel system. The U.K. engine manufacturer is developing a modification to the FOHE which it expects to certificate within a year.
The AAIB recommends that regulatory authorities mandate any measures the manufacturers propose. The U.S. National Transportation Safety Board has issued an urgent safety recommendation calling on the FAA and EASA to require installation of the redesigned FOHE within six months of its certification.
The Board also recommends that the authorities investigate the more widespread use of an anti-icing fuel additive, such as FSII, already widely used in military aircraft and some corporate jets. Two final recommendations call for more research into the phenomenon of fuel system ice accumulation.
The second interim report released today details further testing conducted to investigate how ice could accumulate within the fuel system and whether the ice could be suddenly released to block the heat exchange and cause a fuel flow restriction and thrust rollback.
Most of the fuel delivery system from the right wing of the accident aircraft, G-YMMM, was reassembled in an environmental chamber which sought to replicate as closely as possible the fuel and ambient temperatures encountered in flight.
On the 777, fuel is fed to the engines through approximately 50 feet of pipes, says the AAIB. Fuel in the pipes is not heated, but on reaching the engin passes through the heat exchanger, which performs the dual function of warming the fuel and cooling the oil.
The tests showed that "with reasonable repeatability" a layer of ice accumulated inside the fuel pipes in quantities exceeding that previously found to cause blockage of the FOHE. The ice deposits were a mixture of water and fuel and were soft and "mobile" rather than a hard glazing of ice, the report says.
The AAIB's tests also demonstrated that increasing the fuel flow, as well as changes in temperature, could lead to the accumulated ice being released and carried to the face of the heat exchanger in sufficient quantity to cause a fuel-flow restriction.
Photo: Rolls-Royce
