Interesting regulatory challenge posed by the GEnx engine, as detailed in this FAA policy determination
Seems that the engine's innovative system for catching post shut-down fuel drainage---while more environmentally friendly than previous designs--sometimes results a bit of fuel vapor being discharged via the engine inlet or exit. A careful reading of Part 34.11
reveals that this is a no-no.
The wrinkle: Part 34.11 was written in 1973, in response to what FAA dubbed "short-sighted environmental practices of the time," which included dumping up to a gallon of fuel post-shutdown. The GEnx system is a different animal, so FAA did the sensible thing.
After working with the OEMs (the situation wasn't discovered until GEnx flight-testing on the 787 and 747-8) and EPA to verify that the GEnx fuel vapor discharge's environmental impact is indeed minimal, FAA said it will allow the engines to be certified as good to go with their airframes (the 747-8 and 787) while the agency takes another look at the rule in light of current technologies.