|Quoting billreid (Thread starter):|
The MD-11 was an aircraft that did not live up to specifications. When was this discovered and when did sales drop because of this.
During design, the engine manufacturers supply to the airframe manufacturer "engine decks" which are computer programs that simulate the performance of their engine. The decks supply fuel flow, thrust, etc. for a given power setting. The decks are for uninstalled performance, so they must be corrected for installation effects. Also the decks are typically optimistic. Most manufacturers audit the decks and reduce the optimism, based upon experience.
On the MD11, McD had a very limited development budget, so they didn't audit the engine decks. When they got to flight test, they discovered that the fuel flows were higher than had been predicted for given flight conditions. A big aerodynamic improvement effort was embarked on and the airframe drag was reduced. At the same time, the engine manufacturers tweaked their engines and got slightly better performance out of them. Despite this, the aircraft didn't live up to the original design guarantees.