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BOEING 727 FLIGHT CONTROL SYSTEM

 

Roll Control...

Roll control on the 727 is provided primarily by inboard and outboard ailerons that are augmented by flight spoilers. Both inboard and outboard ailerons are fitted with balance tabs to reduce the required control pressures and assist primary flight control movement by relieving aerodynamic forces.

When the wing flaps are up, the outboard ailerons are locked in a faired position with the wing. As flaps are extended the slow-speed ailerons become operative, providing increased roll control at lower airspeeds.

The maximum amount of outboard aileron deflection is determined by the position of outboard trailing edge flaps. When the flaps are retracted, the outboard ailerons are faired. At Flaps 5, approximately 80% of full outboard aileron travel is available. At Flaps 40, full travel is available.

Ailerons go into "manual revision" with the loss of hydraulic pressure providing a direct cable link between the pilot's yoke and the inboard aileron tabs (also outboard aileron tabs if flaps are extended).

Pitch Control...

Pitch is controlled by the elevators and by the horizontal stabilizer. The control wheels cause the elevators to move using System A and B hydraulic pressure. The horizontal stabilizer may be operated by a main electric trim motor, an autopilot trim motor, or by manual control cable. When pitch trim is applied, the leading edge of the stabilizer is rotated to a new position. The leading edge of the stabilizer will always move opposite the desired movement of the nose. Caution should be used during the stabilizer's trim check so that the captain's and the first officer's trim switches are not applied in opposite directions.

Yaw Control...

The Boeing 727 has two rudders...upper and lower. Power for the upper rudder is supplied by hydraulic system B while power for the lower rudder is supplied by hydraulic system A.

The rudders have no provision for manual revision. However, if System A pressure is lost, the lower rudder can be powered by the Standby Hydraulic System.

The rudder trim wheel on the control stand repositions a centering mechanism in the rudder power control units. A change in rudder trim position will have the same effect as pilot movement on the rudder pedals. The rudder will be hydraulically held in the new position and the rudder pedals will be deflected.

The rudders are equipped with anti-balance tabs. These tabs are displaced in the same direction as the rudder movement, providing the same effect as a larger rudder.

The Boeing 727 has Yaw Dampers that operate on the upper and lower rudders. They counteract the dutch roll tendency. When they automatically counteract yaw in flight, the only indication on the flight deck is movement of the rudder indices. The rudder pedals do not move. Maximum altitude and speed restrictions apply should any yaw damper become inoperative.

Additionally, the rudders are equipped with Rudder Load Limiters that regulate the amount of hydraulic pressure the rudders receive. With flaps up, both rudders receive 800 PSI of hydraulic pressure and, therefore, will deflect equally. With flaps deflected, the lower rudder receives about 2,400 PSI of pressure which will give the aircraft more rudder authority in low-speed flight and during crosswind takeoff and landing conditions.

High-Lift Devices...

Extension and retraction of all flaps and slats is normally controlled through a single flap handle located on the right side of the control stand. This flap handle has detents for trailing edge flap positions a 0, 2, 5, 15, 25, 30, and 40. Some leading edge slats and flaps are extended at Flaps 2, while the remainder are deployed at any setting greater than Flaps 2.

Flight controls affected by extension of trailing edge flaps are:

Spoilers...

In addition to augmenting roll control, the spoilers are used as speed brakes that greatly increase drag. During flight, they will slow the airplane or allow an increased rate of descent. Position of the spoilers is controlled by the speed brake lever located on the left side of the control stand. Speed brakes and flaps can't be used at the same time during flight.

On the ground, spoilers destroy lift and increase wheel braking action. The flight spoilers will deploy anytime the speed brake handle is moved aft. Ground spoilers are armed only when the aircraft is on the ground. The air-ground sensor is located on the left main landing gear.

Spoilers are inoperative without hydraulic pressure.

Surfaces Operated by the Autopilot...

The following surfaces are operated by the autopilot:

Note that ground spoilers, and both upper and lower rudders are not controlled by the autopilot. At least one yaw damper must be engaged before the autopilot can be engaged.