L-1011's Special Features
Lockheed L-1011 TriStar Intro
The Lockheed L-1011 TriStar belongs to the first generation of medium to long-range widebodies built in the 70s which also includes the 747 and the DC-10. Despite fewer sales, the L-1011 was the most advanced of all three aircraft. Lockheed used their military know-how to produce a flying fortress with plenty of redundancy. Even today, the L-1011 is a very unique aircraft. Let's look at some of its special features.
Active Control System
The L-1011-500 is known as being a very stable aircraft even in rough turbulence. The Active Control System contributes for this increased stability. While McDonnell Douglas was studying and testing winglets in its DC-10 aircraft - which were eventually introduced on the MD-11 - Lockheed believed that a more efficient way of reducing induced drag for better fuel economy was by increasing TriStar's wingspan giving a higher aspect ratio to the wing. However, increasing the wingspan usually requires wing reinforcement and thus higher aircraft weight. The solution found was to design an active system which works for wing load alleviation while deflecting the outboard ailerons.
Accelerometers installed in the fuselage and wingtips of the L-1011, detect vertical accelerations induced by turbulence or maneuvers. The system uses the information from accelerometers to symmetrically deflect the outboard ailerons in order to redistribute lift forces across the wingspan and to reduce wing bending.
The amount of deflection of the ailerons is function of airspeed changes. The Active Control System actuation is 100% automatic. It benefits fuel consumption and gives the L-1011-500 the smoothest ride of all commercial aircraft.
The L-1011's air conditioning system totally renews air in the cabin every 3 minutes, resulting in a much more enjoyable atmosphere. Remember that the majority of other commercial aircraft (even the modern ones) use air recirculation sacrificing passengers' comfort for fuel efficiency purposes.
The majority of commercial aircraft flying only have two anti-collision lights (the red rotating/ flashing lights), one on the top of the fuselage and other one under the belly. For redundancy purposes, the L-1011 has two on the top and two under the belly. If one failed, the aircraft could still be dispatched without delaying the flight. As a curiosity, standard length versions of the TriStar had real rotating beacons while the Dash 500 was equipped with flashing ones.
Autopilot / Autoland
The L-1011-500 was also the first wide-body commercial aircraft to have a digital autopilot (Collins FCS-240) instead of an analog system. It's still considered one of the most accurate auto-pilots ever built. Its high level of accuracy in conjuction with the Direct Lift Control system led Lockheed to easily attain CAT IIIb autoland capability on the TriStar.
Direct Lift Control
The DLC is a system particular to the L-1011 which was designed to control inboard spoilers' deflection during final approach to provide vertical speed control without significant changes in pitch attitude. In the case of the L-1011-500, when flaps are more than 30º, the 4 inner spoilers will come up to a new neutral position of 9º. If you are above your glide path, then the spoilers will vary from 9º to 20º, in order to kill lift. If you are below your glide path they will vary between 9º and stowed position. The DLC results in a smooth and constant pitch attitude approach and works automatically both in manual or autopilot operation. For those who have flown gliders, this concept is similar to the principle of using spoilers in the final approach, controlling the rate of descent without changing the glider's speed.
The L-1011 doors slide up into the fuselage whether in electrical, emergency or manual mode. This means that, in case of emergency, no outside obstacles can prevent the doors from being opened. Also, in electrical operation, there is no way of inadvertently deploy a slide-raft since the system logic wouldn't allow the door to be opened until the selector is in the "Detach" (disarmed) position. In manual mode the doors are opened with a hand crank, mainly used by maintenance.
Because of the slide-up movement, the forward doors (where the diameter of the fuselage is less) are not longitudinally alligned to each other. Also, the aft doors are 10 cm smaller with respect to height.
As an option, Lockheed offered built-in airstairs that serviced door R3 on standard lenght TriStars. Very few airlines took this option, LTU being one of them.
When an L-1011 pilot pushes or pulls the control column, he deflects the whole horizontal stabilizer. The function of the geared elevators is only to increase the chamber of the surface. The main goal of the flying tail concept is to give maximum pitch control, needed in jet upset and mistrimmed take-off configuration situations.
The hydraulic systems power the control surfaces of an aircraft such as ailerons, elevators, rudder, slats/flaps, etc. Without them it's impossible to fly a large commercial aircraft. The L-1011 has four independant hydraulic system lines for more redundacy. The DC-10 has only three. Only one hydraulic system is sufficient to safely fly the L-1011. Associated with the four hydraulic lines we have a total of eleven pumps (4 Engine Driven Pumps, 2 Power Transfer Units, 2 Air Turbine Motors, 2 AC Motors and 1 Ram Air Turbine) which allow for multiple system reconfigurations without losing hydraulic fluid pressure. The L-1011 was the first wide-body aircraft to incorporate a Ram Air Turbine. The RAT is a small propeller stored in the TriStar's belly which extends in the very unlikely event of all three engine failure.
RAT - Ram Air Turbine
When it came out, the L-1011 was known as the Whisperliner. Its Rolls-Royce RB211 engines were innovative with their triple shaft concept. Having one more shaft than usual, the overall engine rotation speed could be reduced resulting in less noise. The L-1011 was the quietest wide-body of its time and still complies with Stage III regulations without any engine modification required. The TriStar's RB211 engines have a by-pass ratio of 5:1, driving some 70 percent of the thrust around the outside of the core. Also, its wing-mounted engines are placed farther from the fuselage.
Pre-preduction RB211-22B engines were equipped simultaneously with both cascade vanes and clamshells reversers. The clamshells when deployed deflected the hot stream from the engine core. The clamshells were deactivated later since they proved unreliable. As a result, a shorter engine afterbody was designed to achieve less drag, a design delivered as a standard for all RB211-524B engines.
Performance Management System
This is another system particular to the L-1011 which is incorporated in the Flight Management System. Lockheed noticed that optimum cruise Mach number was unstable to keep resulting in lots of throttle movements and corrections. In other aircraft, many airlines would opt for a higher Mach number (more stable) which also led to higher fuel burn. The PMS was designed to keep the airspeed precisely in optimum conditions by changing, with small variations, the aircraft's attitude. If the airspeed is above the selected figure, a nose-up command is sent to the horizontal stabilizer making the aircraft rise a maximum of 50 ft and lose speed; other way, if the airspeed falls below the selected figure, a nose-down command is sent and the L-1011 sinks a maximum of 50 ft to gain speed without throttle correction. The benefits from this system were better fuel efficiency, extended engine life and less auditory fatigue for the passengers due to thrust variations.
Unlike its rival - the DC-10 - which has the engine number two mounted in the fin, in the L-1011 the engine #2 is embedded in the rear fuselage and is fed by an S-duct. This design allowed for a bigger and more effective rudder (remember that airplane's height was limited by the height of the hangars) and easier access for maintenance.
Last modified: 2020-03-27