Trim

Trim

Trim is the adjustment of control surfaces so the aircraft stays in the desired flight state without pilot force. Significantly reduces pilot fatigue.

Working principle

Untrimmed, the pilot must hold constant yoke force to maintain e.g. pitch. With trim, the control surface is mechanically or aerodynamically brought into a "neutral" position where the desired control effect occurs without pilot force.

Main-axis trim

1. Elevator trim (pitch trim)

Most important trim in any aircraft.

• Trim wheel in cockpit: rotatable.
• Trim switch (newer aircraft): electric.
• On pitch-up trim: elevator pushed slightly down → up-lift at stab → nose up.

When to use:

• After transitioning to cruise: power setting done → trim pitch.
• Approach configuration: flaps out → pitch changes → re-trim.
• After take-off: trim climb pitch.

2. Rudder trim

• Compensates P-factor (single-engine left yaw).
• In cruise: slight right trim.
• On approach: possibly different.

3. Aileron trim

• Rare in PPL trainers.
• Use: with uneven loading or asymmetric fuel.
• In some modern aircraft electrically adjustable.

Trim mechanisms

1. Trim tab

• Small moveable tab at the trailing edge of the elevator (or other control surface).
• On pitch-up trim: trim tab down → up-lift on tab → pushes elevator up → produces down-force at stab → nose-up.
• Benefit: small mechanism, large effect.
• Use: standard in C172, PA-28, DA-40.

2. Bungee trim

• Spring in the control system as additional load.
• Pilot adjusts bungee preload → control surface tends to specific position.
• Use: gliders, some high-performance GA.

3. Anti-servo tab (geared tab)

• Tab that goes up when control surface goes up.
• Produces more force in the control system, so control feels heavier → "reliable" feel.
• Use: some stabilator designs (e.g. PA-28 with stabilator).

4. All-moving stabilator trim

• Stabilator (entire moveable stab) has its own trim mechanism.
• Cherokee PA-28 with stabilator has anti-servo tab for trim.

5. Springs / hydraulics (airliners)

• Complex trim systems with hydraulic support in large aircraft.

Trim setting in cockpit

Display in cockpit: trim indicator (scale or electronic display).

• Take-off trim setting: usually centred or slightly nose-up.
• Cruise trim: set in flight.
• Landing: usually nose-up on approach.

Trim routine

1. Setup pitch attitude: fix desired pitch via yoke.
2. Hold pitch with yoke.
3. Adjust trim until yoke force becomes zero.
4. Release yoke (check briefly): aircraft maintains pitch.

Special cases

Trim runaway

Defect in trim system: trim runs uncontrolled in one direction → pilot cannot hold control surface.

• POH emergency procedure: pull trim circuit breaker, hold yoke with full force.
• Very rare but critical.

Mistrim at take-off

Wrong trim position at take-off: unexpected control force → pilot may react too slowly.

• Pre-takeoff check: trim to TAKE-OFF position (POH).

POH guidance

POH gives trim values for:

• Take-off.
• Stable cruise.
• Approach.
• Landing.

In large aircraft: detailed trim profiles per phase.

Trim and fly-by-wire

Airliners like A320, B787 trim automatically via the FBW system:

• Pilot sets only pitch wish via side-stick.
• Trim automatically tracks.

Pilot duty

• Trim is efficiency, not control: pilot flies actively, trim relieves.
• Never use trim as primary control.
• In emergency (e.g. EFATO): set trim aggressively → more yoke efficiency.