An emergency airworthiness directive (AD) has been issued by the US Federal Aviation Administration over concerns that General Electric GE90 engines powering Boeing 777-200LR/300ERs could suffer dual thrust reductions on take-off, which could leave the aircraft without sufficient power to become airborne.

The FAA says in its emergency AD that was issued on 30 September that it has come as a result of two occurrences of engine thrust rollback during takeoff on 777-300ERs, which are powered by GE90-115B engines.

In both cases only one engine was affected but there are concerns that “a dual-engine thrust rollback could occur just after V1 (takeoff decision speed after which takeoff is to proceed even after an engine failure), which would result in the airplane not having adequate thrust to safely complete the takeoff”, says the FAA.

“A dual-engine thrust rollback, if not corrected, could result in the airplane failing to lift off before reaching the end of the runway or failing to clear obstacles below the takeoff flight path,” it adds.

Investigations found that the two cases in which there were single-engine thrust reductions during takeoff were the result of a software algorithm in the Full Authority Digital Engine Control (FADEC).

To comply with the emergency AD, operators of GE90-115Bs powering 777-300ERs as well as GE90-110Bs powering 777-200LRs must revise their flight manuals to prohibit takeoffs at less than full-rated thrust.

“Airplane takeoffs are often performed with engine thrust levels at less than the maximum engine thrust approved for the aircraft. This is done to reduce wear on the engines, increase fuel efficiency, and maximize passenger comfort,” says the FAA.

“Full-rated thrust takeoffs with the thrust levers at the full forward position are not exposed to the potential thrust rollback caused by the software anomaly” in the FADEC system.

It adds: “This is considered to be interim action. The engine manufacturer has advised that it currently is developing a modification that will eliminate the unsafe condition addressed by this AD. Once this modification is developed, approved, and available, we may consider additional rulemaking.”

The FAA says that in the two reported cases of thrust rollback during takeoff, the N1 thrust level on the affected engine “progressively dropped resulting in a thrust loss of 65-77% due to an erroneous N1 command” computed by the FADEC system. It adds that in each case the engine recovered to the proper thrust level as the aircraft climbed beyond 400ft (122m) above ground level.

In one case the cockpit crew returned the aircraft to the departure airport but in the other the crew elected to continue to the destination airport.