National Transportation Safety Board Report 213

runway with the pylon structure. There was no evidence that the pylon and engine assembly struck any critical aerodynamic surfaces of the aircraft or any of the flight control surfaces.

Since the loss of thrust provided by the number one engine and the asymmetric drag caused by the leading edge damage would not normally cause loss of control of the aircraft, the safety board sought to determine the effects the structural separation had on the aircraft's flight control systems, hydraulic systems, electrical systems, flight instrumentation and warning systems, and the effect, if any, that their disablement had on the pilot's ability to control the aircraft.

The severing of the hydraulic lines in the leading edge of the left wing could have resulted in the eventual loss of number three hydraulic system because of fluid depletion. However, even at the most rapid rate of leakage possible, the system would have operated throughout the flight. The extended No. 3 spoiler panel on the right wing, which was operated by the number three hydraulic system, confirmed that this hydraulic system was operating. Since two of the three hydraulic systems were operative, the Safety Board concludes that, except for the number two and number four spoiler panels on both wings which were powered by the number one hydraulic systems, all flight controls were operating. Therefore, except for the significant effect that the severing of the number three hydraulic system's lines had on the left leading edge slat system, the fluid

leak did not play a role in the accident.

During takeoff, as with any normal takeoff, the leading edge slats were extended to provide increased aerodynamic lift on the wings [see fig. 16.3]. @en the slats are extended and the control valve is pulled, hydraulic fluid is trapped in the actuating cylinder and operating lines. The incompressibility of this fluid reacts against any external air loads and holds the slats extended. This is the only lock provided by the design. Thus, when the lines were severed and the trapped hydraulic fluid was lost, air loads forced the left outboard slats to retract. While other failures were not critical, the uncommanded movement of these leading edge slats had a profound effect on the aerodynamic performance and controllability of the aircraft. With the left outboard slats retracted and all others extended, the lift of the left wing was reduced and the airspeed at which that wing would stall was increased. The simulator tests showed that even with the loss of the number two and number four spoilers, sufficient lateral control was available from the ailerons and other spoilers to offset the asymmetric lift caused by left slat retraction at air-