Autoflight is a feedback system

Why can a correction make the oscillation worse?

A feedback controller must account for how quickly the aircraft responds.

Follow the loop rather than one command

A closed-loop system compares a desired state with measured feedback, processes the error, commands an actuator and measures the resulting state again. An open-loop command does not use that feedback to correct its result. The target, error detector, control law, actuator, aircraft response and feedback source form a complete chain. A correct target does not guarantee an appropriate command if the feedback is wrong.

The aircraft responds over time. If a controller applies excessive correction while sensing and motion are delayed, it may continue correcting after the response is already sufficient, then reverse too strongly. This can create oscillation rather than a smooth approach. Gain, damping, limits and response times must suit the aircraft and flight condition. The laboratory demonstrates underdamped and well-damped responses with synthetic dynamics.

Stabilisation and guidance have different targets

A stabilising channel may damp a rotational motion; a guidance mode may drive the aircraft towards a selected heading, path or altitude. These layers interact but answer different questions. A three-axis autopilot can control roll, pitch and yaw, while particular installations provide fewer axes or independent damping functions. The crew must know the actual channels and mode limitations rather than infer capability from the word autopilot.

If the measured state becomes erroneous, the controller can confidently command an undesirable response even while its internal loop appears satisfied. Monitor raw data, attitude, thrust and trajectory. Unexpected oscillation or incorrect guidance calls for the aircraft-specific intervention procedure and appropriate manual control, not repeated larger target changes. The visual loop makes the feedback source visible so the reason for a bad command can be traced upstream.

Who is moving the controls when the director is shown?

Guidance commands and physical control must be distinguished.

A director computes a command

The flight director computes guidance and presents pitch and roll commands using bars or a V-bar format. The autopilot can use related guidance to move control surfaces through its actuators. With the autopilot disengaged and the director displayed, the pilot controls the aircraft; the visible bars do not mean the aircraft is following them. Compare commanded and measured attitude to understand the remaining error.

The director and autopilot often share mode selections and data sources. Their agreement does not prove the guidance is correct. An incorrectly entered route, selected altitude or unreliable source can affect both. Check the actual flight-mode annunciation and the underlying data. If guidance is incorrect, follow the approved method for removing it and controlling the aircraft, rather than flying a command merely because it is displayed.

Engagement changes workload and control paths

Autopilot engagement normally requires appropriate trim and conditions so that taking control does not produce an unexpected step. Some aircraft move flight-deck controls with the servos; others do not. Control-wheel steering and touch-control steering are different architectures with specific mode and reconnection behaviour; neither label should be treated as a universal manual-flight mode. Know which system is commanding the surfaces at each point.

Disengagement uses specified controls and produces aural and visual cues to attract attention. The crew should be ready for the actual trim and flight condition rather than assuming the aircraft is perfectly balanced. With either pilot or autopilot controlling, the other pilot monitors trajectory and annunciations according to the operator’s task sharing. The model holds actual bank and director command separately so that a correctly displayed request cannot be mistaken for a completed manoeuvre.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.