Trimming

Why aircraft trim

The reasons to trim are worth stating as the syllabus wants them:

relief — a sustained force on stick, wheel or pedals fatigues the pilot and degrades precision. Trimmed to zero force, the aircraft can be flown with fingertips, and its own stability does the routine work.

necessity after every change — the longitudinal lesson showed that every new speed, thrust setting, configuration or CG position re-poses the balance question. Each one leaves a new steady force that must be trimmed away, which is why trimming is continuous housekeeping, not a one-off setting.

hands-off flight — a trimmed aircraft holds its condition alone, the premise on which stability (and every autopilot) operates.

Trim tab versus the balance tabs

The trim tab looks identical to the tabs of the control-forces lesson (a small hinged surface at the trailing edge), and the syllabus asks precisely for the difference. The balance-tab family (balance, anti-balance, servo, spring) operates whenever the pilot's flight controls operate: the balance and anti-balance tabs are geared to the surface's own movement, the servo and spring tabs are driven by the control run itself, but all four work with every input, modifying its force. The trim tab is connected to a separate cockpit control (a trim wheel or electric switch) and to the pilot's stick not at all: it is set to a position and stays there, however the control surface then moves. One family works with each deflection; the trim tab holds one chosen deflection at zero pilot force. (The two can be combined: a geared tab that is also trim-adjustable.)

The trim tab at work

The working principle is the tab mechanism pointed at a steady job. Suppose holding level flight needs a little up-elevator, and with it a steady pull. The pilot winds the trim wheel: the tab deflects down (opposite to the required surface deflection), and the air load on the tab, acting at the longest arm behind the elevator's hinge, holds the elevator up by itself. The pull falls to zero; the pilot lets go; the surface stays where the flight condition needs it. Cockpit indication: the trim wheel or switch drives an indicator showing the direction and amount of trim set: a scale from nose-down to nose-up around a marked neutral, so the pilot can see the setting at a glance and, before take-off, set the marked position the loadsheet demands.

Stabiliser trim

Transport aircraft trim differently: not by holding the elevator deflected, but by rotating the entire tailplane. A jackscrew (an irreversible screw drive at the stabiliser's leading edge, the surface pivoting about its rear attachment) changes the incidence of the whole stabiliser, setting the tail's balancing force directly while the elevator stays faired, streamlined in line with it. Flight-deck indication: a stabiliser trim indicator reading in units, with a marked green band for permissible take-off settings; plus, classically, trim wheels that turn visibly whenever the stabiliser moves, commanded by the pilot's trim switches, the autopilot, or (in a FBW aircraft) the auto-trim.

Against the trim tab, the trade reads almost all one way, which is why every airliner chose it:

Advantages — trimming with the stabiliser leaves the elevator's full authority available in both directions (a tab-trimmed elevator is already part-deflected, and has given up some of its range); the faired elevator and absent tab deflection mean less drag; and the whole-surface incidence change is powerful enough to trim the large CG and speed ranges of a transport, which a tab could not.
Disadvantages — mechanical complexity and weight (jackscrew, motors, brakes), slower response, and failure cases that matter far more, because the surface being driven is the most powerful pitch device on the aircraft: the jam and the runaway of the closing section.

CG, the take-off setting, and what errors do

Before take-off the stabiliser is set to a position computed from the CG on the loadsheet (the load-and-balance calculation), and the relationship follows straight from the longitudinal lesson: a forward CG makes the aircraft nose-heavy, needing a larger balancing download: the stabiliser is set further toward nose-up trim; an aft (rearward) CG needs less, and the setting moves toward nose-down. The green band is that relationship drawn on the indicator: every legal CG maps to a setting inside it.

Set it wrong (or load the aircraft differently than the loadsheet claimed) and rotation shows it. Trimmed too far nose-down (or the CG further forward than assumed): the elevator must fight the mistrim as well as the stability, and rotation needs an unusually heavy pull, comes late, and consumes runway. Trimmed too far nose-up (or the CG further aft than assumed): the nose comes up early and abruptly, on unusually light forces: over-rotation and tailstrike risk. Both are why the take-off configuration warning checks the stabiliser against the green band, and why rotation feel is briefed: stick force at rotation is the loadsheet being checked against reality.

The jammed and the runaway stabiliser

Two failures, opposite characters:

runaway — the stabiliser moving without being commanded: a failed motor or switch driving the most powerful pitch surface steadily toward a stop. Out-of-trim forces grow rapidly; the drill is immediate: oppose with elevator, cut the trim system out (cutout switches, then retrim with the manual wheel). Left alone, a runaway can overpower the elevator entirely, which is why the cutouts are memory items.
jam — the opposite: the stabiliser stuck where it is, with the elevator still working. The aircraft is controllable, but it can no longer be retrimmed, so every speed and configuration change from now on must be held on the elevator, against a growing steady force.

The consequences of a jam sort by flight phase:

During take-off — jammed at the take-off setting, the aircraft flies, but the setting was computed for rotation speed, so as speed builds toward the climb and clean-up, growing elevator force is needed, and the crew plans an early return at moderate speeds.
During landing — the approach is flown at a speed chosen to keep the out-of-trim force manageable (often faster and with a reduced flap setting) because the elevator must hold the whole mistrim through the flare with enough authority left to flare at all.
During a go-around — the demanding case, where full thrust from underslung engines pitches the nose up, the speed and configuration change rapidly, and every new condition's trim demand lands on the elevator at once, close to the ground; a jammed-stabiliser approach is briefed precisely so that the go-around, if flown, is flown gently: reduced thrust if performance allows, shallow acceleration, no aggressive pitch demands.