Aviation Investigation Report A98H0003

1.6.15  Hydraulic System

Hydraulic power for the MD-11 is derived from three parallel, continuously pressurized systems. Each system is powered by two engine-driven hydraulic pumps. Different combinations of two of the three systems provide parallel power to each of the primary flight control actuators. Two back-up electrically driven hydraulic pumps are also available. If necessary, one of these pumps can be driven by electrical power from the ADG.

In the event of an in-flight engine shutdown, if the aircraft is in a take-off or land configuration (the flaps, slats, or landing gear are extended), hydraulic power is transferred automatically from an operating system to a non-operating system by reversible-motor pumps. In the cruise configuration, hydraulic power is not transferred.

During the investigation, various components of the hydraulic system were examined to determine whether any anomalies in the hydraulic system could have had an adverse effect on aircraft controllability. The shut-off valves associated with the reversible-motor pumps were found to have been closed at the time of impact when it would be expected that, given the configuration of the aircraft, at least one set of valves would have been open, allowing one of the reversible-motor pumps to operate. (STI1-28) Although the reason for the valves being in the closed position could not be determined, it could be attributed to several scenarios associated with fire-related electrical anomalies. (STI1-29)

Engine 2 was shut down by the pilots approximately one minute prior to the time of impact (see Section 1.12.9). The shutdown of Engine 2 and the loss of automatic hydraulic power transfer through a reversible-motor pump would have resulted in an eventual loss of, or reduction in, Hydraulic System 2 operating pressure. However, the functions of the primary flight controls operated by Hydraulic System 2 would have been picked up through a parallel operating system. Therefore, the anomaly would have had little or no adverse effect on aircraft controllability. (STI1-30)

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