Aviation Investigation Report A98H0003

1.6.20  Powerplants

  1. - General
  2. - Full-Authority Digital Electronic Controls  General (STI1-38)

The occurrence MD-11 aircraft was equipped with three Pratt & Whitney model 4462 engines. The engines are referred to by number: Engine 1 mounted under the left wing, Engine 2 mounted in the vertical stabilizer (tail), and Engine 3 under the right wing.

The aircraft is also equipped with an APU mounted in the tail section. The APU on the occurrence aircraft was not used by the pilots before the stoppage of the FDR, and it was not operating at the time of impact.  Full-Authority Digital Electronic Controls (STI1-39)

The engine thrust for each engine is controlled by a dual-channel (channels A and B) FADEC that interfaces with the aircraft and engine control systems. Each channel is independently capable of controlling engine operation. Electrical power to each FADEC is supplied primarily by an engine-driven permanent magnet alternator. Each FADEC can also be powered, if necessary, by the aircraft electrical system, through a supplemental control unit (SCU); this was an optional feature installed on this aircraft. This method of powering the FADEC using the SCU is referred to as back-up power. The FADEC also receives inputs from the engine throttle resolvers located below the central pedestal and linked to the throttle levers. There are two throttle resolvers per throttle lever. One resolver provides throttle resolver angle (TRA) input to Channel A and the second to Channel B. Electrical excitation for the resolvers is provided by the FADEC.

Each FADEC channel (A and B) also receives information from three digital data buses. Two of the buses supply data from the ADCs and the other supplies data from the FCCs. FCC-1 provides data to Channel A and FCC-2 provides data to Channel B. The ADCs provide pressure altitude, fan inlet total pressure (Pt2)[42] and total air temperature (Tt2)[43] to channels A and B. The FCCs provide EPR trim, engine bleeds, Weight-On-Wheel (nose gear compressed), and "flaps/slats retracted" information.

Each FADEC channel includes non-volatile memory (NVM) that records fault information used for maintenance scheduling and troubleshooting. There are 192 continuously available NVM fault cells. Each fault is registered only one time per flight leg, but is rewritten to memory when the engine is shut down with the engine FUEL switch. The contents of the fault memory will typically span many flights. The information in these memory cells is retained until all 192 NVM cells have been filled with information, at which time the information begins to be overwritten from the beginning.

Certain faults will cause the engine to revert from the normal EPR mode to the soft reversionary N1 mode of operation. This reversion will also cause the autothrottles to disconnect. Autothrottle cannot be re-engaged if any engine is in the N1 mode. Loss of TRA input will cause the engine to go to a fixed thrust that cannot be altered through the throttle control levers.

[42]    Pt2 refers to total pressure at compressor inlet (Station 2).

[43]    Tt2 refers to total temperature of the air at the compressor inlet (Station 2). Total temperature of a moving gas is the static temperature plus the temperature rise resulting from ram effect (kinetic energy caused by air in motion).

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