May 2004Departments

Flying Smart : What It Looks Like

Pitch trim mechanism

Most airplanes have primary and secondary control surfaces. The three primary control surfaces are ailerons for bank; rudder for yaw; and elevator (or, in the case of most Piper singles, stabilator) for pitch. Secondary control surfaces include flaps, for lift and drag; and trim tabs.

Trim fine-tunes the position of primary control surfaces in flight and relieves control forces, allowing the airplane to maintain the desired airspeed and attitude with minimal control inputs from the pilot.

Proper trim setting is vital to aircraft control and stability. Without pitch trim, airplanes would be very difficult to fly. Depending on speed, the pilot of a trim-less airplane would have to exert a lot of forward or aft pressure on the elevator or stabilator to maintain level flight.

The pilot adjusts the pitch trim tab, or tabs, on the elevator or stabilator by moving a wheel or crank, or by using an electric trim switch.

On the Piper Arrow pictured here, the pitch trim control in the cockpit is connected to a steel-strand cable that runs back to and winds around a small drum in the tailcone. Adjusting the pitch trim adjusts the cable forward or aft, which causes the drum to turn clockwise or counterclockwise; this in turn moves a jackcrew up or down. A rod assembly connects the jackscrew to the pitch trim tab. When the jackscrew twists up, it pulls the rod assembly forward, which brings the trailing edge of the trim tab down. This deflects the trailing edge of the stabilator or elevator up, which causes the nose to pitch up. That's nose-up pitch trim. Nose-down is just the opposite.

Cessna uses a mechanical trim actuator instead of a drum and jackscrew arrangement, and some other manufacturers use their own systems.

The entire trim mechanism is inspected periodically to ensure cables are tensioned and lubricated properly, bushings are within tolerances, and the hinges that fasten the trim tabs to the primary control surfaces are not worn and loose.