During flight testing for CAA certification it was found that in a jet upset type scenario the ACS (as it was called on the tri-motor) would effectively be 'adding' to the elevator up and would cause the a/c to increase the pitch up and thus exacerbate the situation. This was a result of the very short fuselage for the -500 variant that put the extended wingtips almost at the same longitudinal position as the stabiliser.
And also to prevent jet upset all L1011 series used a stabilator or an "all flying stabiliser" as it was called.
Besides ACS it also had the Maneuvering Direct Lift Control wich was an extension of DLC/Autospoilers function which improved pitch control characteristics at high speed (above Mach 0.65) in function of positive accelerations.
The ACS and MDLC make the TriStar500 the smoothest plane to ride ever built and with great turbulence response. No modern aircraft can beat it.
Just to let you know, the neutral angle of active ailerons in flight cruise was 2º down!
As for DLC it worked during approaches (after flaps more than 30º) to provide vertical speed control without significant changes in pitch attitude.
When flaps are more than 30º, DLC makes the 4 inner spoilers raise to a new neutral position of 7º. If you are are above your glidepath than the spoilers vary from 7º to 14º to "kill" lift. If you are under you glide path they vary between 7º and 3º. And voila, you also have the smoothest and precise approach.
You can see DLC working on this video of a wingview approach:
http://www.tristar500.net/filmes/landing.wmv
More things could be said... Also it's the only commercial aircraft that do not re-circulate the cabin air. Instead, the total cabin air is renewed every 4 minutes!!! And you got a healthier flight!
A trully remarkable aircraft with some tech still ahead of OUR time.
It will be sorely missed!
Best regards