RR Working On Ice Issue For Trent 777s
By Graham Warwick 
Rolls-Royce expects to certify a modification to its Trent 800 turbofan in July that will reduce the risk of ice in fuel lines causing a flow restriction and thrust rollback, the suspected cause of the Jan. 17, 2008, crash of a British Airways Boeing 777-200 at London Heathrow.
The modification, already under development, is among measures recommended by the U.K. Air Accidents Investigation Branch (AAIB) is its second interim report on the crash, released on March 12.
The AAIB says further, more representative testing of the 777’s fuel system has confirmed that ice can accumulate within the fuel lines and be released suddenly to cause a flow restriction in the fuel/oil heat exchanger (FOHE) on the Trent 800.
Rolls-Royce is developing a modification to the FOHE flush plate — the interface between the fuel delivery system and heat exchanger — that will reduce the risk of blockage from the buildup and release of ice in the fuel lines.
The AAIB recommends that regulatory authorities mandate any measures Boeing and Rolls-Royce propose. The U.S. National Transportation Safety Board on March 11 issued an urgent safety recommendation calling on FAA and EASA to require installation of the redesigned FOHE within six months of its certification.
U.K. investigators also recommend that the authorities investigate expanded use of an anti-icing fuel additive, such as FSII, already widely used in military aircraft and some business jets. Two final recommendations call for more research into the phenomenon of fuel system ice accumulation.
The second interim report details further testing conducted to investigate how ice could accumulate within the 777’s 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 ft. of pipes, says the AAIB. Fuel in the pipes is not heated, but on reaching the engine passes through the heat exchanger, which performs the dual function of warming the fuel and cooling the oil.
The tests showed, “with reasonable repeatability,” that 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,” the AAIB says.
Based on its investigation, the AAIB believes ice accumulated in fuel feed pipes located in the area of the 777’s engine struts during the high-altitude cruise. In the later stages of the approach to Heathrow, engine acceleration — perhaps combined with turbulence, aircraft pitch changes and warming of the struts — caused the sudden release of ice into the fuel feed system for both engines. The ice traveled through the fuel pipes to block the face of the heat exchangers and cause engine thrust rollbacks.
Photo: Rolls-Royce
