The US Federal Aviation Administration is requiring General Electric to modify its GEnx-series engines by end-2012 to prevent fuel vapour leaks, in order to comply with a 1973 environmental regulation.
The ruling, published 25 July on the Federal Register, would affect all Boeing 787s powered by GEnx-1B engines and 747-8s, which are exclusively powered by the -2B model.
The leaks must be eliminated from all new-build GEnx engines by 31 December 2012 and in-service engines will require retrofit by 31 December 2014 in order to comply with the ruling.
According to the FAA, about .16l (5.5oz) of fuel from the GEnx is being released "intermittently under certain conditions" and is "vapourised on contact with hot surfaces inside the engine, resulting in the visible fuel vapours that have been observed when they emit from either the inlet or exit plane of the engine".
The 1973 rule, which the FAA called "short-sighted", prohibited a then common practice of dumping up to 3.8l (1gal) of raw fuel after engine shutdown, but the prohibition specifically sited "elimination of intentional discharge" prescribing specific intent of the operator.
While the FAA determined "no safety and minimal environmental effects are evident" after consultation with the Environmental Protection Agency, it said it remains "concerned about compliance with the intent of the current regulation" to prevent the intentional discharge of fuel.
Even though the rule does allow "normal fuel seepage from shaft seals, joints, and fittings", the GEnx fuel discharge is from none of the three allowed sources, thus creating a ambiguous interpretation and application of the regulation and as a result has prompted the requirement to modify the GEnx engines.
Boeing's 747-8 freighter is set to enter service before the end of September, with the passenger model to be delivered to a completion centre for outfitting in VIP configuration by the end of the year. Boeing has not identified a launch customer or timeline for the GEnx-1B's certification on the 787.