Boeing and General Electric are looking to formalise a third performance improvement package (PIP) for the GEnx engine family to return the 747-8's fuel burn to its original target.
The move comes as GE advances on twin packages already in development on the Boeing 787 while further reducing nacelle drag.
"We're a little short," says Boeing airplane programmes vice-president Pat Shanahan of the 747-8's performance. "We're working with our friends at GE. They always appreciate my phone calls to say we need a little more."
The 747-8 PIP has no firm date for implementation, though preliminary development planning targets 2013, confirms GE Aviation chief David Joyce.
Joyce says the company is currently flight- and ground-testing a package of internal improvements to the GEnx-1B engine for the 787, portions of which will carry over to the 747-8's 289kN (65,000lb) GEnx-2B engine.
The GEnx-2B engine features a smaller fan and pneumatically driven start system, compared to the GEnx-1B's electric bleed-less architecture, though each contains a common core and 80% common line replaceable units.
While the exact fuel consumption shortfall or excess aircraft weight have not been publicly disclosed, Lufthansa executive vice-president group fleet management Nico Buchholz says the 747-8I will be able to meet the carrier's requirements.
"All the mission profiles we want, we can meet," says Buchholz. "And actually we have very onerous points where we look for mission profiles, sometimes routes we don't fly, but we pinpoint certain areas to find if there are any weaknesses, and then we are extremely pleased when we don't find them. And I'm still pleased."
Buchholz adds that the 747-8I's PIP will require no operational change to the aircraft "except we may save a little bit of fuel".
For the twin 787 PIPs already in the works, "we will put that in the engines as soon as we finish the validation of it and we're committed", says Joyce.
GE says PIP1, which includes a revised low pressure turbine (LPT), will be test flown on aircraft ZA005 mid-year. The revision increases the blade, vane and nozzle count after weight-saving reductions in these areas reduced performance. The GEnx-2B engine is already flying with the revised LPT, as programme delays allowed its incorporation.
PIP2, which features aerodynamic improvements to the high pressure compressor (HPC), has been in ground-testing since December and GE expects to flight-test the changes in the second half of 2011. Certification of these changes is likely in the first quarter of 2012, followed by entry into service in late 2012, says GE.
"We've committed to Boeing that we'll get on the fuel burn where we need to for the engine," says Joyce.
According to a 2008 Airbus assessment of the 787, the GEnx-1B is believed to have missed specific fuel consumption (SFC) targets by 2-3%.
In addition to PIP1 and PIP2, Joyce says GE will further evaluate "not only the specific fuel consumption of the [GEnx-1B] engines, but any drag polars around the engine that may have to be cleaned up as well".
Joyce says Boeing and GE will gather flight-test data to refine the installation of the 787's GE engine and focus on "where the interaction occurs between the engine and the wing, and also the outside nacelle area and how the flow lines of the nacelle interact with the engine".
"So we've got some work to do cleaning up the installation, performance, and we've got programmes already in play for the aerodynamic, thermodynamic performance inside the engine.
"And that'll just be something we do during the flight-test programme and that's a collaborative effort with Boeing, we both own that responsibility," adds Joyce.