(Reuters) – General Electric Co (GE.N) and Boeing Co (BA.N) have alerted airlines about a potential problem with engines on Boeing’s long-range 777 jumbo jet that caused the engines to shut in mid-flight twice this year.

The problem affects about 118 so-called transfer gear boxes made between September and March. The part, made by Italian company Avio SpA, are on about 26 in service 777-300ER jets and another 44 aircraft in production, GE said.

There are more than 1,150 of the GE90-115B engines in service and the gearbox has been a reliable part for more than 15 years, GE said. The cause of the problem appears to be with an anomaly in the material that caused gears to separate, although the exact cause remains unknown, GE said.

The companies told airlines to inspect or replace the transfer gear boxes produced during the six-month period, ensuring that at least one engine on the plane has had an inspection or a replacement made before September.

GE is sending replacement parts to airlines.

The gearbox transfers power from the engine to run fuel pumps and other vital engine functions, GE said. Failure causes the engine to shut down.

The incidents in which engines shut down during flight occurred in February and on May 9, GE said. One of the incidents occurred on an Air China plane. The other could not immediately be identified.

In both cases, only one engine shut down, and the twin-engine 777 is able to continue flight with the remaining engine.

“You don’t have a fire, you don’t have an explosion,” when the gearbox fails, said Rick Kennedy, a GE spokesman.

While the cause of the failures remains unknown, the FAA is expected to make the inspections mandatory through an Airworthiness Directive. The FAA did not respond to requests for comment.