Aviation Investigation Report A98H0003

1.6.4  Auto Flight System

The MD-11 is equipped with an auto flight system (AFS) that is an integral part of the automatic and manual control system of the aircraft. The AFS consists of two, dual-channel flight control computers (FCC) with two integrated autopilots, flight directors (FD), autothrottle, and engine trim controls. Manual override of the automatic flight controls and autothrottle is always available.

The AFS hardware consists of the two FCCs, a dual-channel flight control panel (FCP), an automatic flight system control panel, a duplex flap limit servo, a duplex elevator load feel servo, a duplex autothrottle servo, and two control wheel force transducers. The AFS provides fail-operational Category IIIB auto-land through ground roll-out, and integrated windshear detection/warning with autopilot, FD, and autothrottle guidance escape capability.

The FCP, located on the glareshield control panel, provides the interface between the flight crew, the AFS, and the flight management system (FMS). The AFS incorporates airspeed and flight path protective features that automatically override the selected airspeed or flight path commands or both to prevent over or under speed.

Each dual-channel FCC has two similar functioning lanes. Each lane has two central processing units, which continually monitor the health of the other lane. A detected fault in the operating lane will automatically disconnect that function. For example, an autopilot fault will result in the autopilot disconnecting. Should this happen, the autopilot disengage warning system would activate a flashing red "AP OFF" alert on the flight mode annunciator and a cyclic (warbler) aural warning tone. The warbler can be reset, after at least one cycle of the tone has been completed, by pushing either of the autopilot disconnect switches installed on the outboard horn of both control wheels or by re-engaging the autopilot.

Each FCC receives inputs from the following sources:

  • Inertial reference units 1, 2, 3 (IRU-1, -2, -3);
  • Air data computers 1, 2 (ADC-1, -2);
  • Radio altimeter 1, 2 (RA-1, -2);
  • Both instrument landing systems (ILS), Flight Management Computer 1, 2 (FMC-1, -2);
  • All three full-authority digital electronic control (FADEC) engine control units, flight control sensor data, selected references from the FCP; and
  • Other information, such as weight on wheels, and gear and flap position.

The FCCs send digital signals to the electronic instrument system (EIS) for display, and control signals to actuators for control of pitch, roll, yaw, and engine thrust.

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