On 26 November 2008, a Boeing 777-200 powered by Rolls Royce RB211 Trent 800 series engines and being operated by Delta Air Lines on a scheduled passenger flight from Shanghai Pudong to Atlanta was in the cruise at FL390 in day Visual Meteorological Conditions (VMC) in the vicinity of Bozeman MT when there was an uncommanded thrust reduction or ‘rollback’ of the right engine. Prescribed drills were followed and a descent to FL310 was made after which normal thrust control was regained and the flight continued to the planned destination. None of the 263 occupants were injured.
An investigation was carried out by the National Transportation Safety Board (USA) (NTSB) who were already aware of progress in the UK AAIB investigation of the British Airways Boeing 777 thrust loss accident at London Heathrow earlier in 2008 and of the similar engines fitted. Flight Data Recorder (FDR) data was available but Cockpit Voice Recorder (CVR) data had been overwritten.
The flight crew reported to the investigation that they had had some doubt about the applicability of the uncommanded engine rollback Quick Reference Handbook (QRH) procedure because they were more than 3 hours from the top of the descent and had not yet performed the required ‘Cold Fuel Operations’ procedure or recently operated either engine at a high thrust setting. However, they had followed it, retarding both thrust levers and descending with the reported successful outcome.
It was established that the flight had not encountered any severe weather or abnormally low temperatures prior to the rollback event and the fuel had not reached a low enough temperature for it to freeze or solidify. All relevant systems on the aircraft were found to have been operating as designed and certified. The water content and chemical makeup of the fuel was also found to meet all international test standards.
Based on flight crew statements and FDR data, the Investigation found that about 55 minutes before the engine rollback occurred, the right engine oil temperature began to increase relative to that of the left engine oil temperature with no corresponding thrust increase or other potential explanation. It was concluded that this period of elevated oil temperature occurred because the inlet face of the fuel-oil heat exchanger (FOHE) was partially obstructed by ice, resulting in the fuel flowing through a reduced number of tubes in the FOHE, which reduced its oil cooling efficiency both prior to and after the rollback. When the right engine fuel flow had returned to normal after the ice blockage cleared, the engine oil temperature had dropped, which was considered to indicate that the FOHE oil cooling effectiveness had been restored. The oil temperature excursion was considered to provide evidence that the FOHE had been partially restricted for about 55 minutes before the engine rollback and during it.
Examination of the right engine HP fuel pump found evidence of cavitation, as previously found by the UK AAIB during the Heathrow Boeing 777 Investigation. Laboratory testing indicated that “fuel with normal amounts of entrained water could form ice crystals and that, when localised fuel temperatures are in the ‘sticky range’ (-5 to -20 °C), ice can accumulate along the internal components of the fuel system and that this ice could be released during higher fuel flows and subsequently travel downstream through the system”. This testing also led to the conclusion that “the Rolls-Royce Trent 800 FOHE inlet face could be obstructed by a concentration of ice sufficient to restrict the fuel flow to a value similar to that recorded during the incident flight and that “reducing the volume of the fuel flow through the FOHE allows the heat of the oil to melt the ice, allowing for a recovery to normal operation” as had occurred.
The investigation determined that the following sequence of events had occurred during the incident flight:
- ice formation from the normal amounts of entrained water in the fuel.
- ice accretion and subsequent release of ice from the area of accretion (possibly at more than one distinct time).
- ice travel through the fuel system to collect on, and obstruct the flow through, the FOHE inlet face.
Although the Federal Aviation Administration (FAA) had concluded that the aircraft type fuel system had met existing certification requirements, it noted that “ice accumulation in the aircraft and engine fuel feed system upstream of the FOHE had not been anticipated” by the engine manufacturers or the regulators and that “ under certain conditions, the ice could release and cause a restriction at the FOHE that would limit fuel flow to the engines”.
In March 2009, in response to the ongoing investigations into both the Heathrow accident and the Delta Incident which was the subject of their investigation, the NTSB issued the following four Safety Recommendations in respect of the design of the FOHE fitted to the Trent 800 series engine to ensure that future designs are tolerant to the threat of ice concentrations:
- That the FAA require that Rolls-Royce redesign the RB211 Trent 800 series engine fuel/oil heat exchanger (FOHE) such that ice accumulation on the face of the FOHE will not restrict fuel flow to the extent that the ability to achieve commanded thrust is reduced.
- That the FAA, once the fuel/oil heat exchanger (FOHE) is redesigned and approved by certification authorities, require that operators of Boeing 777-200 airplanes powered by Rolls-Royce RB211 Trent 800 series engines install the redesigned FOHE at the next scheduled maintenance opportunity or within 6 months after the revised FOHE design has been certificated, whichever comes first.
- That the EASA require that Rolls-Royce redesign the RB211 Trent 800 series engine fuel/oil heat exchanger (FOHE) such that ice accumulation on the face of the FOHE will not restrict fuel flow to the extent that the ability to achieve commanded thrust is reduced.
- That the EASA, once the fuel/oil heat exchanger (FOHE) is redesigned and approved by certification authorities, require that operators of Boeing 777-200 airplanes powered by Rolls-Royce RB211 Trent 800 series engines install the redesigned FOHE at the next scheduled maintenance opportunity or within 6 months after the revised FOHE design has been certificated, whichever comes first.
Subsequent action as recommended resulted in all Rolls Royce-powered Boeing 777s being fitted with a new design of FOHE during September 2010
The Probable Cause was determined "An accumulation of ice in the fuel system, which formed from the water normally present in jet fuel during commonly encountered flight conditions, which accreted and released, restricting the fuel flow at the right engine fuel-oil heat exchanger inlet face".
A Contributory Factor was also identified as:
- Certification requirements (with which the aircraft and engine fuel systems were in compliance), which did not account for the possibility of ice accumulating and subsequently releasing in the aircraft and engine fuel feed system upstream of the fuel-oil heat exchanger.
The NTSB Final Report of the Investigation was adopted on 27 June 2011.