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Old 09-14-2002, 02:02 AM   #8 (permalink)
Sgt Caribou
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This copied from a website, http://www.airlinesafety.com/faq/faq9.htm, and basically reiterates what was said above. This site does make for some interesting reading.

[The MD-11 was designed with a smaller horizontal stabilizer than other airliners. That, plus the shifting of its center of gravity further aft, all to reduce drag and thus fuel burn, causes it to be unusually light on the controls. That design, known as “relaxed stability,” is common to fighter planes but is not normally found in the pitch axis of a civilian airliner. It makes it more likely that the pilot will overcontrol and exacerbate the situation, during a recovery attempt after a high altitude upset or during a bounced/hard landing.

I have never flown an MD-11. However, I do have a description of its handling characteristics from a pilot that has had considerable experience in that cockpit:

“The MD-11 is not fly-by-wire. It is, however, fly by CONSTANT pilot input. The geniuses at MD decided to make the empennage 40% smaller than the DC-10 to save on both parasitic drag and induced drag by keeping the c.g.[center of gravity] near the aft limit during high-speed cruise.

This airplane doesn't really have a "slot" when you are on final; it doesn't seem to really stay at a trimmed AOA [angle of attack] /deck angle at a specific power setting/airspeed. As such, the pilot is constantly making little corrections, like flying a dynamically unstable fly-by-wire fighter with the computer out. This is unlike any transport aircraft I've flown. Part of the problem is a system called the Longitudinal Stability Augmentation System (LSAS) which is a computer that constantly trims the stab to make up for the shortcomings of the tail size. The landing is also unique. As soon as the plane touches down I have to push on the yoke to counteract a severe pitchup from the spoilers coming to 2/3 extension. Less than a second later, the autobrakes kick in, so you have to pull back on the yoke to gently lower the nose to the runway.

Somebody once said they should let Lockheed design all the airplanes, Boeing build them...and McDonnell-Douglas market them! And let the French guys stick to making Citroens and Peugeots...”

In my view, that unstable pitch mode constitutes defective design, which is directly responsible for all the deaths and injuries that have occurred during high altitude upsets and the resulting violent pitch oscillations.

I also think it reasonable conjecture that the Anchorage, Newark and Hong Kong landing accidents might not have happened at all if the MD-11 was designed with a stable pitch mode as are Boeing airliners. The kinds of conditions encountered in those accidents (heavy weights, short runway, wake turbulence, gusting crosswinds), are personally known to airline pilots of high experience. That is when the skills of the pilot are put to the ultimate test, and the design quality of the plane is revealed. It is precisely the time when the pilot, and all on board, need everything going for them.]

The pilot must be skilled enough to instantly recognize what control inputs are needed, and the plane must be designed so as to respond instantly to those commands without excessive oscillations. The brain of the pilot and the brain of the design engineer must be simpatico. If they are not, then in my opinion, you get what happened in some of the accidents listed above.
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