History of Flight
On November 26, 2008, about 1930 coordinated universal time, a Boeing B777-232ER, registration N862DA, operated by Delta Air Lines as flight 18, experienced an uncommanded thrust reduction (rollback) of the right Rolls-Royce RB211 Trent 895 engine during cruise at flight level (FL) 390 (approximately 39,000 feet). The flight was a regularly scheduled passenger service from Shanghai Pudong International Airport, China, to Hartsfield-Jackson Atlanta International Airport, Georgia, operating under 14 CFR Part 121. Visual meteorological conditions prevailed with an instrument flight rules flight plan.
The flight crew, having recently been relieved by the incident crew, executed the climb to FL390 using VNAV and CLB power settings. About 30 minutes after reaching FL390, the first officer (pilot flying) noted fluctuations in the right engine pressure ratio (EPR), N1, and exhaust gas temperature indications. The crew reported an "ENG RESP" message on the engine indicating and crew alerting system (EICAS), though no such message exists on the B777. Aware of the January 2008 accident involving a B777 at London Heathrow (G-YMMM), they suspected engine icing.
The Delta B777 Operations Manual Quick Reference Handbook contained a revised U ENG RESPONSE checklist (unannunciated) issued on October 31, 2008, as a result of recommendations from the Air Accidents Investigation Branch (AAIB) investigation of G-YMMM. The checklist was to be used when an engine did not reach commanded thrust or rolled back after performing the Cold Fuel Operations procedure or after high thrust settings. The first item was to descend with thrust levers closed. The crew initially questioned the checklist's applicability because they had not performed the Cold Fuel Operations procedure and were more than 3 hours from top of descent, but they retarded both thrust levers and descended to FL310. The right engine recovered and responded normally for the remainder of the flight. The flight continued to Atlanta and landed without further incident.
Aircraft and Crew Information
The captain, age 52, held an Airline Transport Pilot certificate with type ratings in B777, B767, B757, and DC-9, and had 13,409 total flight hours, including 87 hours in the B777. The first officer, age 54, held an ATP certificate with type ratings in B777, B767, B757, and TBM 700, with 10,412 total hours, of which 1,896 were in the B777.
N862DA was delivered new to Delta in December 1999 and had accumulated about 41,222 flight hours and 5,670 cycles. The right engine (serial number 51150) had 33,773 flight hours. The fuel-oil heat exchangers (FOHE) were manufactured by Sumitomo Precision Products; the right engine FOHE (serial 504) was installed in November 2007. The fuel system includes three tanks, with center tank override/jettison pumps feeding each engine; crossfeed valves separate left and right systems. The Trent 800 engine has a main engine-driven fuel pump with low-pressure and high-pressure stages, with the LP stage feeding through the FOHE to cool engine oil and warm fuel.
Meteorological Conditions
At the time of the rollback, the aircraft was in visual meteorological conditions above an undercast layer. The flight encountered light to moderate turbulence while passing a low pressure area south of the Aleutian Islands. Recorded static air temperature at FL390 was about -57°C.
Flight Recorder Data
Review of the digital flight data recorder (DFDR) and quick access recorder (QAR) showed that about 45 minutes after reaching FL390, the right engine EPR dropped from approximately 1.25 to 1.1, and fuel flow reduced to about 5,000 pounds per hour. All engine parameters were consistent with the reduced fuel flow. The engine control system attempted to increase fuel flow by driving the fuel metering valve fully open, but no increase occurred. The thrust asymmetry compensation system automatically adjusted the rudder. The rollback persisted for about 23 minutes until the crew executed the descent. The ENG THRUST caution message (EPR shortfall) was not displayed. About 55 minutes before the rollback, right engine oil temperature began to rise by about 31°C, then decreased during the rollback but remained elevated; after recovery it returned to normal. Fuel temperature at the time of the rollback was -22°C. The aircraft had performed four step climbs at fuel flows exceeding 11,000 pounds per hour, the latter two at fuel temperatures below 0°C.
Tests and Research
Postincident examination of the fuel system found no preexisting component failures. However, cavitation marks were observed on the HP stage of the right engine fuel pump, consistent with a fuel flow restriction upstream. Fuel samples showed no anomalies in water content or chemical composition. Research by Rolls-Royce and Boeing, conducted in support of the AAIB investigation, confirmed that ice crystals can form from water in jet fuel at sub-zero temperatures and accrete on the FOHE inlet face during high fuel flows, restricting fuel flow and causing rollback. Testing showed that reducing fuel flow by retarding thrust levers clears ice from the FOHE within seconds.
Analysis indicated that the engine control system cycled the fuel metering valve every 25 to 30 seconds due to surge and stall recovery logic, preventing the ENG THRUST message from being set because the engine continued to run above idle and experienced periodic fuel flow increases.
Operational Changes
In response to the G-YMMM accident, Boeing issued a flight crew operations manual bulletin in September 2008, including a Cold Fuel Operations supplement procedure and an engine response non-normal checklist. Following this incident, Boeing revised the procedure to require the cold fuel operations procedure within 2 hours of top of descent (instead of 3 hours) and to retard throttles to minimum idle for 30 seconds during initial descent. The Federal Aviation Administration issued Airworthiness Directive (AD) 2009-05-11, superseding AD 2008-19-04.
Subsequent safety recommendations led to a redesigned FOHE with flush-face fuel inlet tubes, which was certified in 2009 and mandated by European Aviation Safety Agency AD 2009-0142. Delta completed installation of the redesigned FOHE on its Trent 895 engines by December 2009.
No other reportable events occurred, and no injuries or damage were sustained.