6.2 Aircraft Control Surfaces and Flight Axes

6.2 Aircraft Control Surfaces and Flight Axes

To navigate the three-dimensional sky, an aircraft must be able to rotate around three distinct imaginary axes that intersect at its Center of Gravity (CG). Managing this rotation requires specialized moving parts on the airframe known as control surfaces. These surfaces are categorized into primary flight controls, which dictate the fundamental trajectory of the aircraft, and secondary flight controls, which alter the aerodynamic characteristics of the wing or assist the pilot in maintaining control pressures. Mastering the axes of flight and the corresponding control surfaces is an absolute necessity for aspiring aviators taking the ASTB-E.

The Three Axes of Flight

An aircraft in flight is completely free to move in three dimensions. We define this movement in relation to three perpendicular axes that intersect at the aircraft's Center of Gravity. Whenever an aircraft maneuvers, it rotates around one or more of these axes.

1. The Longitudinal Axis (Roll)

The longitudinal axis is an imaginary line running lengthwise through the fuselage from the nose to the tail of the aircraft. Movement around this axis is called roll. When an aircraft rolls, one wing goes up while the other wing goes down, which banks the aircraft and initiates a turn.

2. The Lateral Axis (Pitch)

The lateral axis is an imaginary line running crosswise from wingtip to wingtip. Movement around this axis is called pitch. When an aircraft pitches, its nose moves up or down relative to the horizon. Pitching nose-up typically causes the aircraft to climb and lose airspeed, while pitching nose-down causes it to descend and gain airspeed.

3. The Vertical Axis (Yaw)

The vertical axis is an imaginary line running vertically through the center of the aircraft, from top to bottom. Movement around this axis is called yaw. When an aircraft yaws, the nose moves left or right. Yaw is primarily used to counteract adverse aerodynamic effects (like adverse yaw during a roll) and to keep the aircraft's nose aligned with the direction of the relative wind, resulting in coordinated flight.

Primary Flight Controls

The primary flight controls are the movable surfaces attached to the trailing edges of the wings and the empennage (tail section) that allow the pilot to rotate the aircraft around its three axes.

Ailerons (Control Roll)

The ailerons are located on the trailing edge of the outer portion of each wing. They are connected to the control stick or yoke and operate in opposition to one another. When the pilot deflects the stick to the left, the left aileron moves up and the right aileron moves down. The upward-deflected aileron on the left wing decreases the wing's camber, reducing lift and causing that wing to drop. Simultaneously, the downward-deflected aileron on the right wing increases its camber, increasing lift and causing that wing to rise. This combined action rolls the aircraft to the left around its longitudinal axis.

Elevator (Controls Pitch)

The elevator is a hinged surface attached to the trailing edge of the horizontal stabilizer (part of the tail assembly). It is controlled by the forward and aft movement of the control stick. When the pilot pulls the stick back, the elevator deflects upward. The air flowing over the tail pushes down on the raised elevator, forcing the tail down and pitching the nose of the aircraft up around its lateral axis. Conversely, pushing the stick forward deflects the elevator downward, pushing the tail up and pitching the nose down. (Note: Some aircraft use a "stabilator," where the entire horizontal tail surface moves as one unit to control pitch, rather than a separate hinged elevator.)

Rudder (Controls Yaw)

The rudder is a hinged surface attached to the trailing edge of the vertical stabilizer. It is controlled by the rudder pedals at the pilot's feet. Pressing the right rudder pedal deflects the rudder to the right. The airflow strikes the deflected rudder, pushing the tail to the left and yawing the nose to the right around the vertical axis. The rudder is not primarily used to turn the aircraft; rather, it is used in conjunction with the ailerons to overcome "adverse yaw"—the tendency of the nose to point away from the direction of a turn due to the increased drag on the upward-moving wing—and to keep the aircraft's turns coordinated.

Secondary Flight Controls

Secondary flight controls do not steer the aircraft directly but are used to modify performance or reduce the physical workload on the pilot.

Flaps

Flaps are located on the trailing edge of the wings, usually inboard toward the fuselage. When extended, flaps increase both the camber (curvature) and the effective wing area, significantly increasing the lift produced by the wing at a given airspeed. This allows the aircraft to fly at slower speeds without stalling, which is crucial for safe takeoffs and landings. However, extending the flaps also significantly increases parasite and induced drag. This increased drag allows the pilot to maintain a steeper angle of descent during the landing approach without inadvertently increasing airspeed. There are several types of flaps, including plain flaps, split flaps, slotted flaps, and Fowler flaps, with Fowler flaps being the most efficient at increasing wing area.

Leading Edge Devices (Slats and Slots)

Slats and slots are installed on the leading edge of the wing to improve high-angle-of-attack performance. A slot is a fixed gap that directs high-energy air from beneath the wing over the upper surface, energizing the boundary layer and delaying airflow separation. Slats are movable surfaces that deploy (either automatically via aerodynamic forces or manually by the pilot) to create a slot effect, allowing the aircraft to reach a much higher critical angle of attack before stalling.

Spoilers

Spoilers are plates located on the upper surface of the wing. When deployed, they "spoil" or disrupt the smooth airflow over the wing, destroying a significant portion of lift and simultaneously creating substantial drag. Spoilers can be deployed asymmetrically (on one wing only) to assist the ailerons with roll control, or symmetrically (on both wings) to act as speed brakes in flight or to firmly transfer the aircraft's weight to the wheels immediately upon landing, improving braking effectiveness.

Trim Tabs

Trim tabs are small, adjustable hinged surfaces attached to the trailing edges of the primary flight controls (most commonly the elevator). As an aircraft changes airspeed, altitude, or power settings, the aerodynamic forces on the control surfaces change, requiring the pilot to apply constant pressure to the control stick to maintain the desired attitude. A trim tab acts like a miniature control surface for the primary control surface. By deflecting the trim tab in the opposite direction of the desired primary control deflection, aerodynamic forces hold the primary control surface in place, relieving the pilot from holding constant pressure and significantly reducing fatigue.

Use the following table to review key flight control surfaces and axes of flight: