AOPA Project Pilot

Keeping Tabs on Trim

Mastering a pilot's little helper

As a mentor, you can help your student with this observation. Maybe you remember experiencing it yourself …

The first time a student does a practice go-around, he or she will get what is probably the first introduction to the real power of trim. There they may be, in a Cessna, power back and full flaps, all trimmed up for a slow-speed final, maybe holding a bit of back pressure on the yoke as they slow for the flare. Then - inevitably just before what they know is going to be the best landing of the day - the sadistic instructor commands, "Go around."

With full power and the nose-up trim left over from final, the airplane tries to fly just about straight up. What had been a little backpressure on the yoke becomes a firm shove forward as the student hurriedly retrims while keeping an eye on the airspeed and starting to worry about getting the flaps up.

The go-around is a graphic demonstration of the fact that trim settings correspond to particular airspeeds, power settings, and configurations. Another example is forgetting to set the trim before take-off. A mistrimmed airplane can mean increased pilot workload, more drag, and decreased fuel efficiency. In aircraft equipped with autopilots or electric trim, a "runaway" trim malfunction can lead to loss of control if not interrupted by disabling the system.

Trim Basics

General aviation aircraft may be equipped with pitch, roll, or yaw trim that affects the elevators, ailerons, or rudder, respectively. Most basic trainers have elevator trim only (some have fixed tabs on the ailerons or rudder that are adjusted on the ground). Usually, the elevator trim tab consists of a small control surface - think of it as a miniature elevator - that acts opposite the direction of the main control surface. This creates the aerodynamic force necessary to keep the elevator in a desired position, i.e., down for nose-down and up for nose-up pitch. In some airplanes, such as the Piper J-3 Cub, the whole horizontal stabilizer adjusts up or down for trim. A more rare arrangement, such as in Mooneys, is for the entire tail section to pivot up or down.

Remind your student of the three main factors that influence trim: center of gravity, airspeed, and configuration.

With a forward CG, for example, more nose-up trim will be required. A simple example of this is a Piper Cub. While its airspeed range is small, major changes in CG occur depending upon whether there are one or two people occupying the tandem seats. In larger aircraft, passenger and baggage loading affects trim (as balanced fuel loading affects roll trim).

Airspeed is an important trim consideration. Each trim setting has a corresponding speed that the aircraft will seek to hold. If you let go of the controls at a particular setting, the airplane will go through a few pitch oscillations before settling in on its trim airspeed and the pitch attitude required to maintain that speed. Most aircraft are designed so that the horizontal stabilizer, elevator, and trim tab will be more or less streamlined at typical cruise speed.

Finally, configuration changes - deploying flaps, extending landing gear, or opening cowl flaps, for example - have a marked effect on trim, although in some airplanes, it's more pronounced than in others. Generally, when flaps are extended high-wing airplanes will pitch up, and low-wing airplanes will pitch down, though there are exceptions to this rule. Adding power will require nose-down trim while cutting power requires nose-up trim to maintain level flight.

Trim Rules

Almost every before-takeoff checklist includes a reminder to set trim for takeoff. Forgetting to do so can lead to problems. With too much nose-down trim, you may have trouble getting the airplane to lift off the runway. Too much nose-up trim can cause the airplane to zoom into a nose-high attitude after rotation - an undesirable tendency just after takeoff, when airspeed is critical. Were you to take off with an aft CG and nose-up trim, the airplane could be uncontrollable.

The cardinal rule in using trim is to never initiate a pitch action with trim. Always use the primary flight controls and then trim off the pressure you're holding on the yoke. It's also a good idea not to use trim to counter temporary conditions such as up- and down- drafts or the backpressure you're holding on the yoke in steep turns.

Practice, Practice, Practice

Like most other things students learn in flying, using trim becomes more natural the more they do it. The important thing is to understand how and why it works - and how using it incorrectly can get them into trouble. As a mentor, you can help your student by sharing your knowledge about trim and continuing to provide encouragement through this and every phase of flight training.

Next month: Beswitched

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