Somebody in the PMDG forum was just complaining that their MD-11 was unstable in the pitch on the approach. The gist of the replies was that's what MD-11s are like and well done to PMDG for modelling it.
One of them pointed to this page
http://www.airlinesafety.com/faq/faq9.htm which make the MD-11 sound pretty hairy under those conditions.
The guts of it is that to reduce drag the tailplane assembly was made 40% smaller than on similar airliners and the CoG was moved right back. This has resulted in an warm fuzzy sounding condition known as "relaxed stability" (i.e. it's unstable as buggery) and very light control forces, which makes for a bit of a lottery when doing an approach in suboptimal conditions, with overcorrections almost a given.
This from a high-hour MD-11 pilot ...
QUOTE
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 aeroplane 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...[/quote]
Add to that a quite astonishing number of electrical fires, highly suspect fireproofing materials, and the small tailplane assembly suddenly becoming even smaller if you're silly enough to stall at altitude. and you start to see why sales stopped at 200.
And you start to get an inkling of why this MD-11 did what it did, in spite of the best efforts of the passengers in the front seats.
Gary
Posted: Mon Mar 23, 2009 2:32 pm
That is quite a sight.
