A new modification for the most advanced version of the GEnx-1B engine alters one of the fuel-saving features added to the Boeing 787 propulsion system.

Despite the required design change, manufacturer GE Aviation says the closely scrutinised fuel burn performance for the Boeing 787 fleet will not be affected.

“No performance impact [is] expected,” GE tells Flightglobal.

GE engineers are shaving about one-tenth of an inch from a layer of abradable seal material surrounding the ring of fan blades.

GE issued a new service bulletin recommending the design change on 11 March. The US Federal Aviation Administration (FAA) followed up a week later with an urgent proposal to make the change mandatory on all GEnx-powered 787s.

The change was prompted by an in-flight shutdown caused by ice build-up on the fan blades. When the ice abruptly sheds from the blades, the sudden change causes an imbalance in the fan. As a result, the blades rub too hard against the abradable seal, causing so much damage inside the engine that it could not be restarted in flight.

An investigation determined that the problem was unique to the most advanced version of the GEnx-1B engine – the performance improvement package (PIP)-2, which allowed GE to meet Boeing’s fuel burn targets on the 787.

One of the fuel-saving PIP-2 features included tightening the space between the blades and the abradable seal material, reducing air flow leakages that make the engine less efficient.

But that change also appears to have made the engine more susceptible to damaging rubbing between the blade and the abradable seal.

“Reduced fan clearance, if not corrected, could result in engine damage and a possible in-flight, non-restartable power loss of one or both engines,” the FAA says in a proposed airworthiness directive.

GE says all affected engines will be re-worked before a self-imposed deadline on 25 March. Although the FAA notice suggests the modification requires 40h of labour, GE says the rework takes only 16h to complete.

The FAA also proposes to require operators to adopt a new procedure for flight crews. When flying in icing conditions above 12,500ft while the fan is rotating below 85% of maximum speed, the pilots should increase the fan speed to 85% momentarily every 5min. A high engine vibration level is another reason to initiate the new procedure, the FAA says.

Source: Cirium Dashboard