I copied this out of CAPTDOUG's thread
DAL rollback update Trent895
DAL rollback update/Trent895
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SUBJECT: 777-200/Trent 895 N862DA Thrust Rollback During Cruise - 26
November 2008
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UPDATE 1 UPDATE 1 UPDATE 1 UPDATE 1
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Reference a) provides Boeing's previous fleet communication of a 777-200
with Trent 895 engines that experienced an uncommanded thrust rollback on
one engine while in level flight cruise at FL 390. The thrust rolled back
to a level above idle on that engine, approximately 40 minutes after a VNAV
step climb. The flight crew performed the Engine Response non-normal
checklist, which restored full capability to the engine for the remainder of
the flight. The other engine operated normally throughout the flight.
Based on the FDR data and the characteristics of the roll-back, it is
suspected that accumulation of water ice in the fuel path of the Fuel Oil
Heat Exchanger is the cause of the subject incident. Although the
circumstances are slightly different, the subject incident is believed to
have been caused by similar factors as those experienced in the 777-200ER
G-YMMM accident at London, Heathrow airport on 17 January 2008 as described
by Reference b).
Investigation Status
It should be emphasized that the investigations into both events are not yet
complete, with the G-YMMM accident under an open investigation by the UK
AAIB and the subject roll-back incident under an open investigation by the
US NTSB. Boeing, Rolls-Royce, the operators and other organizations are
supporting both investigations.
Recommended Operator Action
The circumstances of the subject thrust roll-back incident have led Boeing
to review the cold fuel operations procedures released by Reference c).
Based on this review, it is believed to be prudent to revise the interim
mitigating procedures to account for what was learned in the subject
roll-back incident. Changes to the interim procedures include:
Reduce the window at top of descent from 3 to 2 hours;
Assure the crossfeed valves are closed;
Assure minimum idle thrust for 30 seconds during initial descent, and;
Clarify the Condition statement in Engine Response Non-Normal Checklist
(NNC).
Reference d) provides a list of publications from Boeing that give the full
definition of the revised interim mitigating procedures. The FAA and EASA
are expected to release regulatory action to mandate use of the revised
procedures. These procedures are interim measures that will remain in place
until a permanent solution can be defined, tested, certified, and deployed
to the fleet.
Other Airframe/Engine Combinations
These interim mitigation procedures only apply to 777s powered by
Rolls-Royce Trent 800 engines and do not apply to other airframe/engine
combinations. 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.
Studies of the applicability of the thrust roll back circumstances to other
engine types and airframes are in work and will continue. Boeing will
notify operators of the results of those studies as appropriate.
Tom Dodt
Chief Engineer - Air Safety Investigation
The Boeing Company
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