Secondary flight controls help pilots fine tune an aircraft after the main controls have set its basic motion. While ailerons, elevators, and rudders create primary roll, pitch, and yaw, secondary controls make the airplane easier to manage, slow, or descend. Trim tabs, spoilers, and speed brakes are especially important during long flights, approaches, landings, and steep descents.
Understanding them helps students see how aircraft balance lift, drag, and pilot workload.
Understanding Aviation: Secondary Flight Controls
Trim works by changing the small aerodynamic forces acting near a control surface. In many light aircraft, a trim tab sits on the trailing edge of an elevator. When the tab moves one way, air pushes it in a way that moves the elevator slightly the other way.
This creates a steady pitching effect without the pilot continuously pulling or pushing the yoke. Larger aircraft may use an adjustable stabilizer instead of a small tab.
The whole horizontal stabilizer changes angle. This is powerful, so pilots use it carefully and check its position before takeoff.
Good trim does not fly the aircraft by itself. It removes the unwanted control force after the aircraft has been set to a chosen speed and attitude. For example, when a pilot adds power, the aircraft may tend to pitch up because of its design and changing airflow.
The pilot first holds the desired attitude with the elevator. Then the pilot adjusts trim until the holding force becomes small. If airspeed changes, flap settings change, or fuel moves between tanks, the trim requirement can change too.
Students should connect trim with balance. An aircraft is trimmed when the aerodynamic moments around its center of gravity are in balance for the current flight condition.
Spoilers are panels on the upper wing surface. When closed, they fit smoothly into the wing. When raised, they interrupt the smooth flow of air that normally helps produce lift.
The wing then carries less of the aircraft's weight, while its resistance through the air rises. On some aircraft, both sides deploy together after landing. This transfers more weight onto the wheels, which improves wheel braking.
On other aircraft, a spoiler may rise more on one wing during a turn. Reducing lift on that wing helps it drop, supporting the roll command. Spoilers must be used with care at low altitude because a sudden loss of lift can make an aircraft sink quickly.
Speed brakes use the same basic idea of increasing aerodynamic resistance, but their main purpose is controlling speed and descent path. They may be wing spoilers, panels on the fuselage, or other surfaces depending on the aircraft. During a descent, reducing engine power alone may not be enough to stay below a speed limit or reach a planned altitude.
Extending speed brakes lets the aircraft descend more steeply without gaining too much speed. They are useful when air traffic control gives a late descent instruction or when the aircraft is high on approach. Their use can create buffeting, noise, and a changed pitch tendency.
Pilots therefore monitor airspeed, vertical speed, attitude, and the aircraft flight manual limits. Many aircraft restrict speed brake use with flaps extended or near the ground, since the extra drag and reduced lift can affect handling.
Key Facts
- Trim tabs reduce the force a pilot must hold on the control yoke or stick.
- Spoilers rise from the wing to reduce lift and increase drag.
- Speed brakes increase drag so an aircraft can slow down or descend more steeply.
- Lift decreases when airflow over the wing is disturbed: lower effective lift means less upward force.
- Drag force increases with speed: D = 1/2 rho v^2 Cd A.
- Roll is rotation around the longitudinal axis, and spoilers can assist roll by reducing lift on one wing.
Vocabulary
- Trim tab
- A small movable surface on a larger control surface that helps hold the aircraft in a desired attitude with less pilot force.
- Spoiler
- A panel on the wing that rises into the airflow to reduce lift and increase drag.
- Speed brake
- A surface used mainly to increase drag so the aircraft can slow down or descend without gaining too much speed.
- Lift
- The aerodynamic force that acts mostly upward and supports an aircraft in flight.
- Drag
- The aerodynamic force that acts opposite the aircraft's motion through the air.
Common Mistakes to Avoid
- Confusing secondary controls with primary controls: primary controls directly command roll, pitch, and yaw, while secondary controls mostly fine tune handling, lift, and drag.
- Thinking trim makes the airplane fly by itself: trim reduces control pressure, but the pilot or autopilot must still monitor and control the aircraft.
- Assuming spoilers only slow the airplane: spoilers also reduce lift, which is why they help with descent, landing rollout, and sometimes roll control.
- Using speed brakes as if they create lift: speed brakes increase drag and often reduce performance, so they help slow or descend rather than support the aircraft.
Practice Questions
- 1 An aircraft is descending at 250 knots and the pilot deploys speed brakes. If drag increases from 18,000 N to 27,000 N, by how many newtons did the drag increase?
- 2 A spoiler reduces lift on one wing from 52,000 N to 46,000 N while the other wing remains at 52,000 N. What is the lift difference between the two wings, and which wing will tend to drop?
- 3 During a long cruise, a pilot must constantly pull back slightly on the control yoke to keep the nose level. Explain which secondary control should be adjusted and why it reduces pilot workload.