What you're referring to is the DLC
- direct lift control - , one of the features of the Tristar.
This is - very simplistically described - how it works :
From all six spoilers on each wing, spoilers 1 to 4 are extended seven degrees out of the stowed position, when one is in the final approach configuration : flaps at setting 30 or more.
On that final approach, any yoke action - either pilot or A/P induced - will have a primary effect on the 1 to 4 spoilers : a demand for a shallower path would partially retract them and a demand for steeper path will extend them further (to a maximum of 14° ). It's only when the demand exceeds the parameters of the DLC
spoilers that the tailplane will intervene.
That system was monstruously complex and it shines on the Lockheed engineers to have pulled the whole system up :
- Mechanically, they had to add DLC
to the spoiler roll control, the speedbrake system.... the spoiler mixer was something to behold.
- Electronically, itr was even less simple : consider that as a matter of fact, DLC
was controlled by EFCS signals, as a stabilizer out-of-trim signal.
And finally consider the whole tree of conditionals : air/ground; no GoAround.....
The myth is that Douglas, Boeing and Airbus tried to emulate the system, but could not (some patents could have helped !)
Two very beneficial effects arose from the DLC
1/- Approach stability was phenomenal ( we are talking mid-to-late seventies )
2/- In case of a go-Around, the DLC
spoilers would retract with a bang, giving the pilot an immediate 5 tons of lift... and added to the fact that DLC
caused a slightly higher thrust setting, hence a quicker engine acceleration, no wonder the L-1011 has had the lowest decision height ever
at a few airports I've visited : We tested 12 ft at CDG
without a hitch and settled for 15.
Why isn't it used any more ?
Mainly because the newer digital A/Ps were accurate and smooth enough to make a final approach as smooth as the Tristar 35 year's ago... Of course, we'd hit the runway during a go-around at decision height, but it's smooth and quite acceptable... plus, the systems are a lot less complex.
But I still miss the elegance of the Tristar approach.