To understand the above mentioned SFC and OEW figures better, here is some History of the 747-8 program (see also my posting of April 5 2012 as quoted by Stitch.) :
- Early in the 747-8 program an usual issue was noticed : Operating Empty Weight (OEW) was above design target. The exact amount is stil a well kept secret, but somewhere between 4000 and 6000 lbs in the early built airframes will be a good guess. This figure was also in line with the later published plans of Boeing to remove at least 5000lbs of the OEW gradually in later produced airframes.
- On top of that a more serious problem emerged : the TSFC of the GEnx-2B67 engine was 2.7% above specification.
- Consequently the aircraft was below the payload range performance, promised by Boeing to the airlines.
- To restore the payload range of the aircraft, the maximum operating weights were increased, after evaluation of the actual flight loads during the test flights showed that it was possible to increase these maximum structural weights, without structural reinforcement. The Max Zero Fuel Weight (MZFW) was increased to keep the maximum design payload the same. The Maximum T/O Weight (MTOW) was increased from the design 970.000 lbs to 987.000lbs. The increase of 17.000 lbs was used to restore the max. range at max payload by allowing an increase in fuel qty ( 11.000-13000 lbs) at MTOW. (The 747-8F is not fuel limited at MTOW with max payload) .
- After evaluation of the actual fuel consumption during the first months of airline operation, Boeing (the messenger : Randy Tinseth) has stated in Randy's Journal that the actual performance is 1% better than the zero reference. This was very good news, aerodynamically the aircraft must have been better than expected (lower drag).
The result was from the start (EIS) an aircraft with an exactly or even better payload-range than designed.
However the promised fuel consumption target was not met, due heavier aircraft weights (0.5-1.0%) and under performing engines(-2.7%).
The exact performance of this IGW-aircraft was now calculated and measured during the final stages of the flight test program and used as a zero reference for the FMC data base and performance calculations used for flight planning.
Seen the improvements of the GEnx-2B PIP engine (with aircraft/engine certification scheduled before the end of 2013) with a TSFC improvement of 1.8% and the OEW overweight reduced to a few hundred pounds, this will produce an aircraft exactly (or even slightly better) than the design fuel consumption target, but with a much better than expected payload-range performance due the higher than original designed maximum operating weights of the aircraft.
747-8 Aircraft status at the end of 2013.
- OEW on spec or a few hundred pounds above target : SFC = 0,1 % above target
- GEnx-2B/P1 engine .....................................................: TSFC = 0,9 % (2,7 % - 1,8%) above target
- Aerodynamic airframe performance ...........................: SFC = - 1,0 % below target
- The aircraft/engine combination is exactly on target, but the engines are still 0,9 % above TSFC target.
- Payload-range is better than original spec, due increased operating weights.
[Edited 2013-05-22 14:03:07]
Operating a twin over the ocean, you're always one engine failure from a total emergency.