Boeing links two Trent 895 incidents on Boeing 777-200ERs
Posted on Wednesday, February 04 @ 06:03:55 GMT by admin
Boeing says "similar factors" were likely at play in two Trent 895
thrust rollback incidents on Boeing 777-200ER aircraft last year.
By John Croft - Flight GlobalThe information was revealed in an update on the incidents the airframer recently sent to operators.Though
Boeing in the update admits the cir*****stances are "slightly different",
the airframer says water-ice ac*****ulation in the fuel path of the
engine fuel-oil heat exchanger systems in the powerplants appears to
have played role in both situations. The most recent incident
involved a Delta Airlines 777-200ER (N862DA) enroute from Shanghai to
Atlanta on 26 November 2008 with 232 passengers and 15 crew. Pilots
experienced an uncommanded rollback of the right Rolls-Royce engine
while in cruise at 39,000ft approximately 40 minutes after a programmed
step-climb. The crew was able to recover the engine after performing
flight manual procedures related to the problem and descending to
31,000ft.More high profile was the 17 January 2008 crash of a
British Airways Trent 895-powered 777-200ER (GYMMM) that crashed short
of the runway at London Heathrow after both engines experienced an
uncommanded rollback. Though the aircraft was destroyed in the
accident, none of the 136 passengers and 16 crew were killed. The UK
Air Accidents Investigation Bureau continues to investigate the
accident with input from the US National Transportation Safety Board
(NTSB), Rolls-Royce and others.The common failure mechanism in
the two incidents would appear to be water ice that ac*****ulated at the
front of a series of tubes that carry the fuel to the engine. Part of
the fuel-oil heat exchanger, the tubes are located next to fluid lines
carrying heated engine oil. From a design standpoint, the mutually
beneficial system allows the engine oil to be cooled by the cold fuel
while the fuel itself is heated for better combustion performance. Under
certain conditions however, Boeing engineers working in the laboratory
have found that the heat generated by the Rolls-Royce designed fuel-oil
heat exchanger is not adequate to prevent moisture in the fuel from
freezing, a condition that tests have shown will produce ice the blocks
the entrance of the fuel to the exchanger, starving the engines. In
the most recent note to operators, Boeing says that 777s powered by GE
and Pratt & Whitney engines are not prone to the problem. "Based on
our knowledge of the system configurations, scenario studies and
laboratory test results, we do not believe that immediate action is
necessary or warranted for 777s powered by other engine types or
non-777 airframes regardless of engine type," the letter states. Fuel-oil
heat exchangers for GE- and Pratt & Whitney-built engines for the
777 family include a feedback loop that cycles some of the heated fuel
back to the front of the device, pre-heating the incoming flow.Boeing
in September proposed interim steps aimed at preventing the problem,
actions that were codified by the FAA in an airworthiness directive.
Included are "periodic" climbs to higher altitudes using maximum thrust
when the main tank fuel temperature is below minus 10°C advancing the
throttles to maximum thrust for 10 seconds or until the airspeed
reaches M0.86 before descents if fuel temperature is below minus 10°C
and the aircraft has been in cruise for three hours, and running fuel
pumps for a maximum of one minute during refuelling operations if the
fuel in the main tanks is not expected to rise above 0°C before the
next flight.Boeing in the new guidance to operators recommends
reducing from three hours to two hours the window at the top of the
descent, assuring that cross-feed valves are closed and reducing thrust
to idle on both engines for 30 seconds during initial descent, an
action officials say will reduce fuel flow to the point that engine oil
heat can melt ice that may have ac*****ulated. Longer term, Boeing anticipates that aviation regulators may call for design changes to the Rolls-Royce heat exchanger. Rolls-Royce declined to comment on the incidents while the investigations are ongoing.
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