Primary Flight Controls

While comfortably seated on a jet airliner flying many miles above the ground, at speeds close to the speed of sound, one may wonder just how the pilot is able to keep the aircraft so smoothly under control. This article will explain how flight controls allow this 20-ton aircraft to be maneuvered so gently and precisely.

Unlike an automobile or boat, an aircraft can rotate about three axes. To turn, an aircraft must bank; while raising one wing and lowering the other, the aircraft rotates or "rolls" about its longitudinal axis. An aircraft rotates about its vertical or "yaw" axis to change direction or "heading." When climbing and descending, the nose must be pitched up or down about the lateral axis.

To rotate about the longitudinal axis a combination of ailerons and spoilers are used. The low speed ailerons are located on the aft or "trailing edge" of the wing tip. These operate in opposing directions, that is, as the right aileron extends upward, the left aileron extends downward. Additional ailerons are located about halfway to the wing tip. These high speed ailerons are used to make slower, more gentle changes in bank while at cruising speeds.

Centrally located on the wings are the spoilers. These deploy upward to make small changes in bank by eliminating or "spoiling" lift. Spoilers can be used simultaneously on both wings to significantly reduce lift. By using spoilers, the pilot can make a descent without having to lower the nose. Keeping the pitch attitude level with the horizon is more comfortable for passengers and makes cabin service easier. Upon touchdown the spoilers deploy automatically, creating a firm downward pressure on the landing gear, which aids in braking effectiveness.

On the tail or "empennage" of the aircraft are two more flight controls. The horizontal stabilizer and elevator provide the pitch control to establish climbs and descents. The rudder is located on the trailing edge of the vertical stabilizer. Unlike a ship, in which the rudder is used to steer, the rudder on an aircraft is used to coordinate turns. While an airplane is turned by banking the wings, the rudder is used to match the rate of turn with the bank. That's why the beverage in your glass on the tray table does not slosh when the airplane turns. The slip skid indicator on the instrument panel shows the pilot how much rudder is necessary for perfectly smooth turns.

The pilot actuates the elevator and ailerons by use of a control yoke. Rotating the yoke to the left and right moves the ailerons and spoilers. Moving the yoke forward and aft deflects the elevator for pitch control. Next to the pilot's feet are pedals to deflect the rudder. The rudder pedals also turn the nosewheel used for steering while on ground taxi. At the top of the rudder pedals are toe brakes used to stop the aircraft's ground roll.

It may sound as though the flight deck is a confusing place of levers, yokes and pedals, but the controls have been so exquisitely constructed that their use becomes very natural with practice. As you can see, the aircraft has been designed with the highest priority placed on passenger comfort. Next month's column will examine how flaps and slats allow the wing to operate efficiently within a typical flight's wide range of air speeds.

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