Aileron trim supplies the pilot with a mechanical substitute for continuous muscle pressure on the yoke. By moving a small tab that sits at or near the trailing edge of each aileron, it re-balances the rolling forces so the aeroplane no longer pulls toward the heavier wing. Pilots engage it once airborne whenever the aeroplane refuses to track straight by first applying any rudder trim the situation demands, then using aileron trim to cancel the residual roll command and hold the wings level until the imbalance, whether from fuel or from off-centre loading, is corrected.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an aileron trim on an aircraft?
An aileron trim is a system used to hold the aileron in a specific position. Aileron trim can be used whenever the aircraft will not maintain straight flight, and it is used in case of wing asymmetry. Aileron trim is used until you correct the asymmetry or it can be used to keep the airplane from responding to a situation where one side of an airplane is significantly heavier than the other side.
Trim is a secondary flight control system that holds the aileron in a specific position. The feature employs trim tabs - small movable sections resembling scaled-down ailerons placed at or near the trailing edge of each aileron. By setting these tabs, the pilot can cancel any roll tendency caused by uneven loading or fuel imbalance without maintaining continuous pressure on the control wheel.
What is the purpose of the aileron trim system?
An aileron trim system helps pilots relieve control pressures by neutralizing the rolling tendency that develops in flight. These systems are arranged as ground adjustable tabs which are bent to a desired position to counter unwanted roll. A pilot or engineer sets the ground adjustable trim for cruise conditions, so the wings stay level with minimal effort on the control wheel. The need for roll trim usually arises when unequal fuel in the left and right tanks causes the aircraft to bank toward the side that carries more fuel, or when an unbalanced load between the two wings produces the same effect.
How does aileron trim work?
Aileron trim works through a small hinged surface, the trim tab, hinged to the trailing edge of each aileron. Trim tabs are operated by moving a small control wheel in the cockpit and turning the wheel re-positions both tabs in the same direction. When a trim tab moves up, the airflow striking it deflects the aileron downward and when the tab moves down, the airflow deflects the aileron upward. This induced deflection is opposite to the tab's own movement, so the aerodynamic force on the trim tab balances control surface pressure and cancels the stick force that was holding one wing low.
In flight, the pilot sets the spring force applied by the wheel until the aircraft flies wing-level with feet off rudder pedals and no aileron pressure on the yoke. If the ball is not centered in its race the pilot first applies rudder pressure to center it, then fine-tunes aileron trim to remove any remaining rolling tendency. Because the tab is only in the correct position for one power setting, any large change in airspeed or power requires the pilot to re-trim.
On some aircraft the same job is done with an actuator inside the wing that tweaks the aileron's neutral position rather than with an external tab. The Cirrus SR22 does not have trim tabs and instead relies on this internal actuator. On other types, engineers usually set the ground adjustable trim for cruise conditions by bending a small tab on the aileron itself, after which the pilot's in-flight wheel gives fine adjustment. Whether through tab or actuator, the function is the same: equalize lift, cancel unwanted rolling moment, and let the pilot fly hands-off.
What is an aileron trim box?
An aileron trim box is the compact mechanical assembly that houses the linkages, gearing and limit switches needed to deflect the aileron trim tab. It is mounted inside the wing, close to the aileron hinge line, and its output arm connects to the trim tab through a short push-rod or cable. Inside the box, a small electric motor or manual cable drum rotates a worm gear that moves the arm through a defined arc. Micro-switches at each end of the arc shut the motor off when the tab reaches its maximum travel, preventing over-travel and binding. The whole unit is sealed against moisture and vibration, and it is attached to the wing structure with shear-bolts so it can be removed as a single module during overhaul.
What is an aileron trim indicator? Mounted on the cockpit pedestal, the aileron trim indicator is a miniature dial or LCD segment driven by the same shaft that moves the trim tab. A position sensor inside the trim box sends a synchronous signal to the indicator, so the pointer always displays the combined positional information of the trim tab itself. When the pilot rotates the trim switch, the needle moves in real time, giving an immediate reference for neutral and the direction of travel. What is electric aileron trim? Electric aileron trim replaces the manual crank with a servo-actuator inside the trim box. Pressing the rocker switch on the yoke energizes the motor. The servo is connected to a simple, extruded hinge trim tab and drives it through its full range in about three seconds. Limit switches and a slip clutch protect the mechanism, and the autopilot works overtime in out-of-trim conditions, so the system is designed to correct small fuel asymmetries without pilot effort.
What is an aileron trim box kit? The kit is a pre-assembled package that contains the trimmed, sealed box, the servo, mounting brackets, wiring harness and the waterproof servos advertised in the Flush Mount Trim Tab Kit. Approved for the Cantilever 177 and 210 series, the kit allows an operator to retrofit ground adjustable tabs to full electric operation in a single afternoon. No structural riveting is required because the hinge trim tab is already bonded into the trailing edge of the aileron and the box bolts into existing pilot holes.
What is the difference between aileron trim and rudder trim?
Aileron trim removes rolling pressure on the control wheel while rudder trim reduces forces on the rudder pedals related to yaw. Aileron trim offsets an out-of-balance wing or heavy fuel load, while rudder trim counters asymmetric thrust from the engines and keeps the fuselage aligned with the relative wind. Aileron trim acts on the lateral axis whereas rudder trim acts on the vertical axis. Aileron trim is adjusted with every change in weight or power, whereas rudder trim is adjusted with every change in thrust or airspeed.
How to use aileron trim?
To use aileron trim, follow the instructions given below.
- Rotate aileron trim counter-clockwise
- Adjust aileron trim to eliminate pressure
- Consult POH for guidance on trim usage
- Slowly trim aircraft until yoke pressure is no longer needed
- Operate trim tabs using cables and pulleys
- Use aileron pressure to keep wings level
- Trim out need for left aileron pressure
- Hold wings level with aileron input
- Set ground adjustable trim for cruise conditions
- Roll wheel in nose-up position to move tab down
A safe and accurate way is to achieve the desired bank attitude first, then trim until yoke pressure is no longer needed, always checking and adjusting in the same order: control first, trim second.
When is aircraft aileron trim used?Aileron trim is used whenever the pilot feels a sustained roll force on the stick that was not caused by a deliberate turn command or by turbulence. Early in the flight, unbalanced load between the two wings or unequal fuel makes the aircraft bank towards the side with more fuel. The pilot introduces aileron-hold with the thumb switch so that the wheel stays off-centre and the wings stay level while the crew prepares the fuel-cross-feed. Later, the need for aileron trim changes during the course of the flight as fuel is burned off and the weight and balance of the aircraft shifts, so the trim wheel is moved again, centring the control column without touching the primary flight-controls.
With the autopilot engaged, the pilot still uses the same switch for lateral trimming: Autopilot uses the same motor as the manual electric trim, so a commanded click is felt at once and the wheel moves under the servo, not under the pilot's hand. The flight-director adds a short opposite-wheel input until the new datum is accepted. A left-bank cue therefore means ‘trim left’ on the switch, which drives the tab up on the left wing and down on the right, rolling the airplane back to wings-level.
The only time the mechanic's hand must go straight to the aileron-cutout is during trim runaway to the stop: that fault is a dire emergency, because the servo can drive the wheel all the way to its limit in a few seconds, forcing the airplane into a steep spiral until the clutch is isolated.
