DC-10
The Lockheed L-1011 and the McDonnell Douglas DC-10 are wide-body transports in a weight class between that of the 707 and the very heavy 747. Both aircraft are powered by three high-bypass-ratio turbofan engines located in a new configuration arrangement; one engine was mounted under each wing, and the third engine was mounted at the rear of the aircraft. The L-1011 and the DC-10 were initially designed to an airline requirement for a high-capacity transport with transcontinental range, but growth versions of each are presently available with intercontinental capability.
Initial flights of' both aircraft occurred in 1970. An early version of the DC-10 entered airline operation in 1971, and the L-1011 began service in 1972. Both aircraft were in wide Use throughout the world. The three-engine configuration employed on both aircraft, in which two of the engines are located near the aircraft center of gravity, offers ail advantage in aircraft balance over an arrangement in which all three engines are mounted at the rear of the fuselage (Boeing 727, for example). Placement of two of the engines under the wing also allows the horizontal tall to be mounted in the highly desirable low position, as contrasted with the T-tall arrangement. The large lateral distance between the wing-mounted engines, however, causes larger yawing moments following loss of' power of one of these engines as compared with a similar power loss in the rear-mounted engine arrangement.
The method of mounting the rear engine was seen to be quite different on the L- 1011 and the DC-10. The L-1011 utilizes a mounting arrangement similar to that of the Boeing 727. The center engine was mounted in the aft end of the fuselage and was connected through an S-shaped duct to the large inlet mounted on top of the fuselage. In contrast, the center engine of the DC-10, including inlet and exhaust nozzle, was integrated with the fin above the fuselage. The engine efficiency resulting from this straight inlet-engine-nozzle configuration, as compared with the S-shaped duct arrangement, was thought to more than offset the structural complexity (and probable weight increase) of integrating the engine with the fin. The high performance of both aircraft, however, suggests that either method of engine installation can be made to operate successfully.
The Lockheed L-1011-200 was powered with three Rolls-Royce RB.211-524 engines of 48 000 pounds thrust each. The McDonnell Douglas DC-10-30 was powered by three General Electric CF6-50CI engines of 52 500 pounds thrust each but was also available with a version of the Pratt & Whitney JT9D engines.
The main landing gear of the L-1011 has two struts to which are attached four-wheel bogies. Early versions of the DC-10 employed a similar arrangement. The heavier DC-10-30, however, employs a third strut, equipped with a two-wheel bogie mounted on the fuselage centerline between the other two main landing-gear struts. This arrangement helps to distribute the weight of the aircraft on the runway and thus keeps the runway-bearing stress within acceptable limits.
The aerodynamic design of both of the three-engine jet transports was conventional. The wings of both aircraft have about 35° of sweepback with aspect ratios in the range of 7.0 to 7.5 and feature transonic airfoils of advanced design. The wings have double-slotted trailing edge flaps and leading-edge slats. Lateral control was provided by a combination of ailerons and spoilers. The spoilers are also used to control lift and drag when deployed symmetrically. Longitudinal control of the L-1011 was provided by a variable incidence stabilizer to which the elevator was mechanically linked. The DC-10 employs separately actuated elevators and stabilizers. Neither aircraft employs longitudinal trim tabs. The maximum lift-drag ratio of the two aircraft was estimated to lie in the range between 17.0 and 17.5.
American Airlines Flight 191 was a McDonnell Douglas DC-10-10 that crashed on May 25, 1979, after its left engine and pylon assembly separated from the wing during takeoff from Chicago's O'Hare International Airport. The NTSB determined the probable cause was an asymmetrical stall and loss of control resulting from damage caused by improper maintenance procedures.
The separation of the engine was traced back to maintenance conducted approximately eight weeks before the crash, where the engine and pylon assembly were removed as a single unit using a forklift, a procedure that deviated from the manufacturer's recommendations. McDonnell Douglas specified removing the engine from the pylon before removing the pylon from the wing. To save time (around 200 work hours), American Airlines (and other carriers like Continental and United) developed an in-house procedure to remove the engine and pylon as a single unit.
A large forklift was used to support the combined engine/pylon assembly. During the maintenance on the accident aircraft, the pylon became jammed, and the movement caused an undetected crack in the aft bulkhead flange, a critical structural component. This initial damage developed into a fatigue crack that worsened with each subsequent flight until the structure failed during the takeoff roll of Flight 191.
When the engine and pylon broke free, they severed critical hydraulic lines along the leading edge of the left wing. The loss of hydraulic pressure caused the outboard leading-edge slats on the left wing to retract. The right wing's slats remained extended. This configuration meant the left wing produced significantly less lift and had a higher stall speed than the right wing. The flight crew, unaware of the slat retraction (as the warning systems were also disabled by the loss of power from the #1 engine), followed standard engine-failure procedures, which led the aircraft to an unrecoverable asymmetrical stall and subsequent crash.
The NTSB cited the vulnerability of the pylon design to maintenance damage and deficiencies in FAA surveillance as contributing factors to the accident. The crash led to significant changes in maintenance practices and DC-10 design modifications (such as mandatory stick shakers for both pilots and slat relief valves).
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