Delta Air Lines Flight 18

About flight level 390 (about 39,000 feet), the airplane experienced an uncommanded thrust reduction (or “rollback”) of the right engine. The flight crew actions did not initiate or exacerbate the engine rollback. Although the flight crew expressed some questions about the applicable procedures, they reduced thrust as required and the right engine recovered and responded normally for the remainder of the flight. (Following the Delta incident, Boeing revised its engine response non normal procedure.) The flight did not encounter any severe weather or abnormally low temperatures aloft, and the fuel did not reach a low enough temperature to freeze or solidify. All relevant systems on the airplane were operating as designed and certified. The water content and chemical makeup of the fuel met all international test standards.

Flight crew statements and recorded flight data indicated that the flight was normal from the departure from Shanghai until about 55 minutes before the engine rollback occurred; at which time, 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. (A consistent small difference existed between the left and right engine oil temperatures, which was attributed to manufacturing tolerance variances.) 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 the oil cooling efficiency of the FOHE before and during the rollback. When the right engine fuel flow returned to normal after the ice blockage cleared, the engine oil temperature dropped, indicating that the FOHE oil cooling effectiveness was restored. Therefore, the oil temperature excursion is evidence that the FOHE was partially restricted for about 55 minutes before and during the engine rollback.

The investigation determined that the following sequence of events occurred during the incident flight: ice formation from the normal amounts of entrained water in the fuel, ice accretion and subsequent release from the area of accretion (possibly at more than one distinct time), and ice travel through the fuel system to collect on, and obstruct the flow through, the FOHE inlet face.

The recorded fuel temperature during the time period leading up to the engine rollback was -22° C. Extensive testing using Boeing 777 components in a cold fuel laboratory concluded that, jet 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. The testing further concluded 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. Testing also demonstrated 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.

Although the fuel system met certification requirements, the FAA determined that neither the FAA nor applicants anticipated ice accumulating in the aircraft and engine fuel feed system upstream of the FOHE or that, under certain conditions, the ice could release and cause a restriction at the FOHE that would limit fuel flow to the engines. Therefore, the FAA and European Aviation Safety Agency provided applicants revised acceptable methods of compliance to those applicable regulations to ensure that future designs are tolerant to the threat of ice concentrations.

Verbatim NTSB analysis and probable cause from the NTSB dataset