Trim Control

Proper trim technique is an important and often overlooked basic flying skill. An improperly trimmed airplane requires constant flight control pressures from the pilot, produces tension and fatigue, distracts the pilot from outside visual scanning, and contributes to abrupt and erratic airplane attitude control inputs.

 

Trim control surfaces are required to offset any constant flight control pressure inputs provided by the pilot. For example, elevator trim is a typical trim in light GA airplanes and is used to null the pressure exerted by the pilot on the pitch flight control, which is being held to produce the tail down force required for a specific angle of attack (AOA). [Figure 3-9] This relieves the pilot from holding a constant pressure on the flight controls to maintain a particular pitch attitude and provides an opportunity for the pilot to divert attention to other tasks, such as evaluating the airplane’s attitude in relation to the natural horizon, scanning for aircraft traffic, and maintaining situational awareness.

Figure 3-9. Elevator trim is used in airplanes to null the pressure exerted by the pilot on the pitch flight control.

Figure 3-9. Elevator trim is used in airplanes to null the pressure exerted by the pilot on the pitch flight control.

Because of their relatively low power, speed, and cost constraints, not all light airplanes have a complete set (elevator, rudder, and aileron) trim controls that are adjustable from inside the cockpit. Nearly all light airplanes are equipped with at least a cockpit adjustable elevator trim. As airplanes increase in power, weight, and complexity, cockpit adjustable trim systems for the rudder and aileron may be available.

In airplanes where multiple trim axes are available, the rudder should be trimmed first. Rudder, elevator and then aileron should be trimmed next in sequence; however, if the airspeed is varying, continuous attempts to trim the rudder and aileron produces unnecessary pilot workload and distraction. Attempts to trim the rudder at varying airspeeds are impractical in many propeller airplanes because of the built-in compensation for the effect of a propeller’s left turning tendencies. The correct procedure is when the pilot has established a constant airspeed and pitch attitude, the pilot should then hold the wings level with aileron flight control pressure while rudder control pressure is trimmed out. Finally, aileron trim should then be adjusted to relieve any aileron flight control pressure.

A properly trimmed airplane is an indication of good piloting skills. Any control forces that the pilot feels should be a result of deliberate flight control pressure inputs during a planned change in airplane attitude, not a result of forces being applied by the airplane. A common trim control error is the tendency for the pilot to overcontrol the airplane with trim adjustments. Attempting to fly the airplane with the trim is a common fault in basic flying technique even among experienced pilots. The airplane attitude must be established first and held with the appropriate flight control pressures, and then the flight control pressures trimmed out so that the airplane maintains the desired attitude without the pilot exerting flight control pressure.