Autothrust

What it is for, and where it reports

The purpose of the autothrust system is to control the engines automatically: holding a commanded speed, setting a rated thrust for take-off or climb, and unloading the crew of the continuous small adjustments that speed-keeping by hand demands. Like every automatic in this module, it reports in one place: the FMAs are the only indication of the active autothrust modes, in the autothrust column of the strip the flight-director lesson built. The levers' position, like any switch, is a request; the column is the receipt.

The modes

The mode names vary by manufacturer, but they sort into a small set of jobs:

Take-off and go-around (TOGA) — full rated thrust for the two manoeuvres that need everything the engines certify.

Thrust-targeted modes — THR CLB, THR MCT, N1, THR HOLD, EPR: the autothrust sets a target thrust, climb or maximum continuous, or in THR HOLD simply freezes the thrust where it stands, expressed through the N1 or EPR gauges module 1 taught. The thrust is the promise; the speed becomes someone else's job.

Speed modes — SPEED, MCP SPD: the autothrust varies the thrust continuously to hold the commanded airspeed. The speed is the promise; the thrust floats.

Idle and landing modes — THR IDLE and RETARD (or ARM awaiting them): thrust brought to idle for the descent, and in the landing flare the RETARD mode closing the levers for touchdown.

Two hardware styles

The syllabus names two main variants, and they rhyme with the autopilot lesson's moving and static controls:

modes are selected on the FCU/MCP and the thrust levers move with the autothrust's commands — the levers are back-driven, so the pilot sees and feels every thrust change in the palm of the hand.
modes are selected with the thrust levers themselves, which remain static during autothrust operation — the levers sit in a detent position that names the mode, the engines move underneath without them, and awareness of what the thrust is doing must come from the gauges and the FMA, not the hand.

Turbulence, and a loop that chases

The autothrust is a closed loop on airspeed, and turbulence feeds it lies. Fluctuating airspeed indications in turbulence or wind shear can lead the autothrust to overcompensate in an oscillating manner: each gust-driven flicker of the needle draws a thrust change, the thrust change arrives after the gust has gone, and the engines end up surging back and forth chasing air that has already moved on, the control-loops lesson's overcompensation on the speed axis. When it happens, manual thrust may be required to settle the airspeed: the pilot sets a sensible average and lets the fluctuations average themselves out. The airspeed indications and trend vectors (the trend arrows of module 2's speed tape) suggest the right adjustment, but the reaction should not be too aggressive: a pilot who chases every flicker by hand has only replaced the machine's oscillation with their own.

The energy threat

The system's quiet competence carries its own threat: using autothrust can cost the pilot the sense of energy awareness, the running feel for how fast the aircraft is, how much thrust is holding it there, and which way both are trending. A hand that never touches the levers stops learning from them, most completely in the static-lever style where the levers never move at all. The defence is deliberate monitoring: speed, speed trend and the thrust gauges belong in the scan even when, especially when, a machine is managing them.

Who flies the speed

Underneath every mode pairing sits one principle: speed can be controlled by varying the engine thrust, or by varying the aircraft's pitch: push the nose down and the speed rises as surely as pushing the levers up. The automation splits these two levers between its two machines, and the split explains how the autopilot and the autothrust interact when a mode is selected for either system: choose a pitch mode that flies a path (ALT, V/S, G/S) and the autothrust must fly the speed, so it takes a speed mode; choose a pitch mode that flies the speed (LVL CHG, OP CLB or OP DES, SRS) and the autothrust must not fight it, so it holds a fixed thrust. Selecting one system’s mode swaps the other’s to match.

The combinations and their traps

The syllabus asks for the implications of the specific pairings, and they follow from the figure:

SPEED with ALT hold — thrust flies the speed, pitch holds the level: both promises are keepable at once, and the speed is protected.
SPEED with V/S in a climb — the trap: thrust promises the speed, pitch promises the rate, and the two promises compete for the same energy. If the selected rate needs more than full thrust can give, the autothrust saturates at its limit and the speed decays with nothing left in the autothrust to defend it (any rescue then belongs to the envelope protection, not the mode).
SPEED with V/S in a descent — the mirror: an excessive descent rate drives the autothrust back until it saturates at idle, and beyond that the speed rises with nothing left in the autothrust to restrain it.
N1/THR CLB with LVL CHG or OP CLB — fixed climb thrust, pitch flies the speed: the speed is protected by construction, and what floats instead is the climb rate, which becomes whatever the surplus thrust affords.
ARM/THR IDLE with LVL CHG or OP DES — idle thrust, pitch flies the speed: again the speed is safe and the rate floats, the descent steepening or shallowing to hold the target speed.