Trim runaway: preparation starts before the warning

The UK Civil Aviation Authority's current safety notice on autopilots and trim runaways highlights correct system function, pre-flight checks, common warning signs, immediate actions, training and reporting. The subject is relevant to pilots and to organisations that operate, manage or maintain affected aircraft.

Why this deserves rehearsal

A trim problem can create rapidly increasing control forces and a high workload. The first priority remains flying the aircraft. The exact actions, including use of controls, disconnects or circuit protection, are aircraft-specific and must come from the approved flight manual, checklist and training.

Build a useful briefing

• Know how the installed autopilot and trim system are armed, disengaged and indicated.
• Complete the published pre-flight checks rather than assuming the system is serviceable.
• Brief the warning signs and immediate actions for the exact aircraft.
• Make sure switches and relevant circuit protection can be identified without a prolonged search.
• Report faults and do not normalise intermittent or unexplained behaviour.

Generic internet advice cannot safely replace the aircraft's approved procedure. Use the CAA notice as a prompt to review that procedure with a qualified instructor or engineer.

What this means in practice

A runaway or uncommanded trim condition can create increasing control force, distraction and startle. The correct response is type-specific because switch design, autopilot integration, circuit protection and trim mechanisms vary. Generic memory items copied from another aircraft can be actively dangerous.

Preparation has three parts: prove the system behaves correctly during the published checks, recognise unexpected movement or control force promptly, and rehearse the approved immediate actions until the relevant controls can be identified without a prolonged search. Intermittent faults deserve the same discipline as obvious failures.

A verification workflow

• Study the exact AFM or POH description of electric trim, manual trim, autopilot coupling, disconnects and warnings.
• Complete every published pre-flight system check and stop when the indication or movement differs from the expected result.
• Locate controls and relevant circuit protection physically in the aircraft; cockpit photographs may not match a modified panel.
• Practise recognition and approved actions with a qualified instructor at a safe time and in accordance with limitations.
• Record and report unexplained trim or autopilot behaviour and prevent normalisation of an intermittent defect.

Scenario to rehearse

Shortly after levelling, the pilot notices increasing control force and unexpected trim movement. Troubleshooting the navigation display first would allow the condition to develop. The rehearsed sequence begins with flying the aircraft, recognising the system involved and applying the exact approved actions. Once stable, the pilot communicates, chooses a suitable landing and ensures the defect is investigated.

Limits of this briefing

This briefing intentionally does not publish universal switch or circuit-breaker actions. Those instructions must come from the approved documentation for the installed system. Pulling an unidentified circuit breaker, fighting an autopilot without recognising its mode or using advice for another model can compound the emergency.

Debrief questions

• How is uncommanded trim likely to feel and appear in this aircraft?
• Which approved checks should detect a fault before takeoff?
• Can every disconnect and manual control be identified immediately?
• What maintenance action follows intermittent or unexplained behaviour?

A source-reading exercise