Aviation Investigation Report A98H0003

1.6.13  Flight Control System

  1. - General
  2. - Longitudinal Stability Augmentation System
  3. - Flaps and Slats  General (STI1-22)

The MD-11 has a conventional flight control column and rudder pedal configuration for the captain and first officer. The primary flight control system comprises the inboard and outboard elevators, the inboard and outboard ailerons, and one upper and one lower rudder. The secondary flight control system comprises the inboard and outboard wing flaps and slats, the wing spoilers/speed brakes, and a controllable horizontal stabilizer. (STI1-23)

All primary and secondary flight control surfaces are hydraulically powered by two aircraft hydraulic systems. Flight control positions are displayed, normally by DU 4, on the SD by selecting the configuration page with the CONFIG cue switch on the SDCP. In addition to the SD, flap and slat positions are also shown on the PFD. Alerts will appear on the EAD and the SD.

Other than the slats, which are electrically controlled and hydraulically actuated, the flight control system is designed with a direct mechanical/hydraulic interface consisting of cables that run between the cockpit controls and the various hydraulic actuators that move the control surfaces. Therefore, with the exception of the slats, the movement of the control surfaces does not depend on the availability of electric power.  Longitudinal Stability Augmentation System

The MD-11 incorporates a longitudinal stability augmentation system (LSAS) that enhances longitudinal stability through commands to the elevators in a series mode. The LSAS holds the existing pitch attitude of the aircraft whenever the sum of the captain's and first officer's column forces is less than two pounds. In the software version that was installed in the occurrence aircraft, below 15 000 feet, there is no LSAS input when the column force is above two pounds. Above 15 000 feet, the LSAS provides an additional pitch rate damping input when the control column force is above two pounds. Automatic pitch trim of the horizontal stabilizer is also operative in the LSAS mode.

The LSAS is inoperative whenever the autopilot is engaged or when the aircraft is below 100 feet above ground level (agl). With the LSAS inoperative and automatic pitch trim unavailable, manual pitch trim is available.

As part of the investigation, simulator flights were conducted below 15 000 feet to gain an appreciation of the flyability of the MD-11 with the LSAS inoperative. There were no noticeable controllability changes in the pitch control or flyability of the aircraft with the LSAS inoperative.  Flaps and Slats (STI1-24)

The flaps and slats are controlled by the FLAP/SLAT lever on the right-hand side of the cockpit centre pedestal. In normal operation, as part of the climb-out check, the pilots would pre-select 15 degrees of flap on the DIAL-A-FLAP wheel located on the right-hand side of the FLAP/SLAT lever. When the flaps are selected down they extend to the pre-selected setting (in the case of SR 111, 15 degrees); the slats normally extend whenever the flaps are extended.

At the time of impact, the flaps were extended to about 15 degrees, and the slats were retracted. The slat system incorporates overspeed protection, which prevents the slats from extending whenever the aircraft's speed is above 280 knots and the flaps are extended less than 10 degrees. The slat overspeed protection can also be overridden by selecting a flap extension of 10 degrees or more. The slat-extend function can also be overridden by pushing a SLAT STOW button, which is used in the event of either a slat disagree alert or the loss of hydraulic systems 1 and 3. There is no indication that either of these events occurred on the accident flight. The failure of the slats to extend was most likely the result of fire damage that led to an interruption in the electrical power supply to the slat control valves.

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