Desk Driver,
My references tell me the following:-
Long range performance of the MD-11 left much to be desired. Both engine manufacturers (GE and P&W) had been "optimistic" in their performance figures when in fact neither engine would deliver the fuel consumption required. The initial PW4460 fuel burn was 6.7 to 8.4% above that specified in the contract, the GE CF6-80C2D1F engines were 4.5 to 5.3% above specification.
The engine manufacturers embarked on product-improvement programs, and MD did likewise in an effort to squeeze a few hundred more miles out of the airframe. The 3-point PIP (performance improvement program) addressed the issues of drag, fuel and weight.
The A-1 PIP saw a 0.1% reduction in drag by adding splitter-plates at the trailing edge of the wing. Later PIPs (1993) achieved a 1.5& drag reduction as well as a 100nm range imrovement, or a 2,5000lb payload improvement. Phase II saw the ailerons drooped by 3 degrees in cruise to improve spanwise loading. This also involved adding slat seals to the outer slat segments.
A further PIP (A-1 Package 2) saw the wing box structure, leading and trailing edges and undercarriage being beefed up to take the increased weight which had now risen from 618,000lbs to 625,000lbs. This increased fuel carrying capacity resulting in a 250nm range improvement or 7,000lb extra payload.
PIP A-1 Package 3 involved re-rigging inboard and outboard ailerons so they drooped for takeoff and assisted takeoff performance at high gross weights. Phase IIIA of the PIP saw some aerodynamic improvements which resulted in a further 1% drag reduction, mainly by very minor changes such as a pylon fillet, seals around the ailerons and a windshield fairing.
Phase IIIB in late 1994 resulted in a drag reduction of a further 1.5% and featured an increase in the diameter of the No.2 engine intake, one of the few non-retrofittable modifications.
In Feb 1995 the fourth phase of PIP was undertaken, wringing out a further 1.2% reduction in drag. This included re-rigging the elevator, wing and undercarriage seals and redesigning the flap hinge-fairings. All in all the total performance increase was about 8% and it restored the range to 7,000 miles with 298 pax. Additional modifications included eother extended standard fuel-tanks or modular add-on tanks in the cargo bay.
P&W and GE had not been idle, their efforts achieveing fuel burn improvements of up to 3% with an estimated saving per year of about $300,000 per aircraft.
To sum it up, a case of promising too much whilst delivering too little, then a hell of a lot of tweaking to achieve the promises, which didn't in fact occur in toto. What kept the MD-11 line going was the fact that it made a damn fine freight aircraft, second only to the 747-400F. The last MD-11s will almost certainly be freighters and they may well see 50 years, along with their (former dc-10) MD-10F predecessors. Fedex will almost certainly end up as the largest operator of the MD trijets in the world.
Hope this answers your query. Most of the info comes from "Douglas Jetliners" by Guy Norris & Mark Wagner. I bought a copy in Winnipeg last year. Highly recommended for any Douglas/MD enthusiast.