DIRECT LIFT CONTROL
DLC - stands for Direct Lift Control - the L-1011 incorporated a pitch-smoothing flight control system within it's spoilers or speed brakes.
The flight spoilers are those retangle metal "boards" you see on top of the wings near the rear of the wings, on the top surface, infront of the flap system. They provide a way for the pilot to create inflight drag to slow airspeed by deploying these boards or panels in the air. The pilot can pull on a handle and extend the boards any amount the pilot wishes/needs. The maximum deployment angle is 45 degrees. Upon landing, the spoilers are also deployed (normally) automatically (always fully) to kill lift and thus plant more weight on the main gear wheels sooner - so that the anti-skid system can become armed to operate as soon as possible. This is especially critical when landing in the rain with water on the runway.
Bringing the story back to the DLC, on approach, when the flaps are set to either one of the 2 landing positions - 28 degrees or 42 degrees - a computer activates and causes all of the flight spoilers to raise on each wing 7 degrees. Passengers in the cabin can easily see this! Now, as the aircraft descends on the glide path, normally if the aircraft gets low, the pilot pulls back on the control wheel and this causes the aircraft to pitch up to fly back into the proper glideslope or glidepath. If the aircraft gets too high, the pilot pushes on the control wheel and this causes the aircraft to pitch down and fly down back into the glideslope - this is how it is on all other commercial jets - now - here is what DLC does for the TriStar: If the aircraft gets too low, the pilot still responds the same way - pulls back on the wheel - however, what happens with DLC is that the spoilers ÒretractÓ, thus removing some drag which causes the wings to create additional lift and the aircraft "lifts" itself back up and into the glidepath with minimum pitch changes. If too high, when the pilot pushes forward on the wheel, the spoilers raise higher on the wings, creating more drag and causes the aircraft to "drift own" or otherwise "settle" back down into the glide path. The end result of all of this is that up and down pitch changes on final approach are minimized for passenger comfort. You'll get a smoother ride on final in the L-1011 than on any other commercial jetliner because only the L-1011 has Direct Lift Control.
There is something about the L-1011 horizontal stablizer that makes it unique: In the early design stages, the airlines told Lockheed they wanted the pilots to have more power and control over the pitch system, so that in case of a jet upset, the crew could pull the aircraft out of a dive.
Lockheed designed this extra power into the tail: The L-1011 is the only commercial jet that can take off with full nose down trim. This was actually demonstrated to the airlines during the flight testing phase - with full nose down trim (which is a mis-set trim setting but has happened in the past on other aircraft and caused accidents) the pilot is still able to rotate the aircraft and climb away - it takes a lot of control wheel force to do it but it can be done - in other aircraft, the control column can be pulled full aft but with full nose down trim the nose won't come up for rotation. For those who don't know: Jet upset was an early event that happened more in the 1950's and early 1960' with the commercial jets than we see today - via rough air, the aircraft would get knocked out of it's stablized cruise condition until it was diving towards the ground out of control - the pilots would attempt to pull back on the wheel to get the nose to come up but most of the jets of that era did not have the power to overcome the excessive speed and thus they could not pull the aircraft out of the dive - when the L-1011 was being designed, the airlines wanted to make sure the L-1011 had enough pitch force to do this should it happen - to this day, a TriStar has never been lost as a result of jet upset and not being able to pull out of the resulting dive.
Now on to the technical description of the L-1011 pitch control system. The L-1011 incorporates an all-flying horizontal stablizer to control movement in the pitch axis. This "all flying tail" is unique in the commercial aircraft industry. First of all, pitch control on most airplanes are usually controlled by elevators - on the L-1011 however, the primary part of the tail that controls pitch is the front part called the horizontal stablizer, the leading edge of which which move up and down - the elevators are attached to the rear spar of this movable stablizer piece - the elevators do move, yes, but not under direct pilot control. The elevators move as a direct result of stablizer movement only via a physical mechanical link: When the stablizer moves it's nose downward, the elevators deflect upward to increase the camber of the entire tail - this causes a downward moment and thus pulls the entire rear portion of the aircraft in a downward direction - this causes the nose of the aircraft to pitch up. When the pilot pushes the control wheel forward, this causes the stablizer nose to move up, causing the elevators to deflect downward, increasing the camber effect to cause a lifting moment and thus pull the tail of the aircraft upward and this in turn causes the nose to pitch down.
The L-1011 has 4 hydraulic systems - by contrast, the DC-10 and MD-11 only has 3 hydraulic systems. All four of the 1011 hydraulic systems power the horizontal stablizer, any one of which can operate it. Not all of the flight controls are powered by all of the hydraulic systems - most are powered by 3 of the 4 and only the tail is powered by all four - this is because the pitch system is considered to be the most critical. Like all widebodied commercial jets, the flight controls are only powered by the hydraulics - there is no manual cable backup system - if you lose all the hydraulics to a given flight control, you lose that flight control.