Control Loops and Definitions - ATPL 022 — Instrumentation

Before any autopilot can be understood, one idea has to be owned: the control loop. This lesson defines what automatic flight control is for, splits its work into control and guidance, builds the closed loop element by element, and ends with the loop's characteristic failure mode, the self-induced oscillation, and what a crew does about it.

Why flight goes automatic

An automatic flight control system (AFCS) is the collective name for the machinery this module covers, and the syllabus gives it two purposes:

enhancement of the flight controls — a machine can measure a deviation and answer it faster, finer and more consistently than a human hand, so the aircraft flies more precisely and more stably with the system engaged than without.
reduction of pilot workload — flying an aircraft by hand for hours is fatiguing, and every ounce of attention spent holding a heading is attention not spent monitoring, planning and deciding.

The automatics take the holding so the crew can do the thinking.

Control and guidance

The AFCS does two distinguishable jobs:

Aircraft control — stabilising the aircraft around its centre of gravity (CG): the rotations of pitch, roll and yaw all happen about the CG, and the control task is to damp disturbances and hold the commanded attitude against them.
Aircraft guidance — guiding the aircraft's flight path: once the aircraft is steady, something must decide where that steadiness should point, holding a heading, an altitude, a radio beam or a route.

Control steadies the aircraft; guidance steers the steadied aircraft, and the next lesson builds the autopilot in exactly those two layers.

Open loop, closed loop

The syllabus names two automatic control principles:

closed loop — feedback from the action or state is compared to the desired action or state: the system watches the result of its own output and keeps correcting until measured and desired agree. A room thermostat is a closed loop: it heats, measures, and stops when the measurement matches the setting.
open loop — there is no feedback: the system delivers its output and never checks the result. A heater with a plain on-off switch is an open loop: it heats the same whether the room is freezing or sweltering.

Flight control that matters is closed-loop; the aircraft's state is far too changeable for a system that never looks.

The closed loop, element by element

The syllabus lists five elements, and every automatic system in this module is this same loop wearing different hardware:

Input signal — the desired state or action: the heading the crew has dialled, the altitude to hold, the attitude to keep.
Error detector — compares the feedback with the input signal. The difference between them is the error, and the error is the only thing the rest of the loop ever works on: no difference, no action.
Signal processor — turns the raw error into a measured output signal according to set criteria or laws (the control laws: the rules deciding how strongly and how quickly to respond to a given error).
Control element — the muscle, such as an actuator: a motor or jack that physically moves a control surface by the amount the processor ordered.
Feedback signal — the measured actual state, returned to the error detector for comparison with the input signal, closing the loop and starting the next round.

When the loop fights itself

A closed loop is not automatically a stable one. If the system overcompensates for deviations, answering each error with a correction bigger than the error deserved, the correction itself creates a new deviation on the far side of the desired state, which draws a still larger correction back the other way. The result is a self-induced oscillation: the aircraft swings across the desired state with growing amplitude, each pass worse than the last, driven not by the atmosphere but by its own control system.

Recognising it is the crew's job, and the signature is exactly what the figure shows: a rhythmic oscillation the pilot did not command, whose swings grow instead of settling, felt in an axis or seen on the instruments and in the repeated, reversing movement of the controls. The effects escalate with the amplitude: aircraft controllability degrades as the swings widen and the controls spend their travel fighting the loop's own motion, and aircraft safety follows, through growing structural loads, altitude and attitude excursions, and ultimately the threat of loss of control.

The mitigation the syllabus names is timely manual intervention: the oscillation is driven by the automatic loop, so the pilot breaks the loop, disconnecting the automatics and taking over by hand, early, while the amplitude is still small. Positive control is then kept with deliberate technique: smooth, small inputs toward the desired state rather than sharp reversals chasing each swing, since a pilot who answers every excursion with a large opposite correction has simply replaced the machine's overcompensation with their own. Settle the aircraft first; diagnose the misbehaving system second.

That is the vocabulary of the whole module: control and guidance, loops closed by feedback, and the discipline of watching for the loop that overcompensates. The next lesson wires these loops into the machine that flies the aircraft: the autopilot.

What the exam asks

The five loop elements and the oscillation logic carry the marks here. The overview, following the learning objectives:

Describe the AFCS purposes: enhancement of flight controls, reduction of pilot workload

Define the two AFCS functions: control (stabilise around the CG) and guidance (flight path)

Describe closed-loop and open-loop control

List the five elements of a closed loop and their functions

Describe how overcompensation produces self-induced oscillation

Explain detection, effects and mitigation of self-induced oscillations: controllability, safety, timely manual intervention, positive-control technique