Trim runaway has been a factor in a number of GA accidents. This is the topic of this article and the latest edition of Sunny Swift. There are 3 key messages:
- If you have electric trim in any aircraft you fly, know how to disconnect it in accordance with the flight manual procedure.
- Read the article to understand more about this important topic.
- Download, share and print the Sunny Swift on Trim Runaway in all EU languages here.
Can you handle a trim runaway?
Ask yourself, can you handle a trim runaway? If it happened in flight, would you know what to do?
Trim runaway was a possible scenario identified by the UK Air Accidents Investigation Branch (AAIB) in a report into a 2017 Piper PA-31 fatal accident. The pilot reported pitch control problems and diverted to Caernarfon Airport where the aircraft crashed on an attempted landing. The elevator trim was found in a significantly nose-down position. You can read the full AAIB report in the AAIB March 2019 bulletin.
If the trim runs away, depending on the elevator size, aircraft speed aircraft’s and control mechanisms, significant forces can be needed to push or pull on the yoke or stick to keep the aircraft straight and level. This doesn’t seem to be much but can quickly wear you out, considering that you still have to take care of navigation, avoid traffic, terrain, weather and an-authorised airspace, manage the radio and communicate, manage the engine, operate the landing gear and land the aircraft. Relief will only come when you will start the flare: you will then finally be able to ease off forward pressure to raise the nose.
It might not happen often but it happens quickly so be ready to act
Electric trim malfunctions and runaways are rare events. And they happen and develop quickly. Depending on aircraft type, you can get out of trim in just 3 seconds and the aircraft can become unmanageable in only 5 seconds. Failure to recognise and handle trim runaways promptly and adequately can result in a loss of control.
Handling Trim Runaway
Experience how it feels with an instructor
Simulating a nose up trim malfunction: while straight and level (at sufficient height), introduce enough trim in either direction to bring you to exert some effort to maintain pitch attitude and hold it for a while. You will quickly experience how much force is needed to compensate for the trim effect! This exercise will also help you recognise a trim-related event. Practice it with an instructor.
Simulating a nose-down trip malfunction: it is not recommended to simulate such a situation in-flight, as the forces could make it difficult to slow the aircraft down and configure it for landing.
Before take-off: Check trim equipment, know the emergency procedures and mentally prepare of a trim event!
Check the electric trim for full and free movement, check that the trim wheel is moving in the correct sense, and that the disconnection mechanism works (on both yokes, if fitted).
Know the emergency procedure and prepare for a trim malfunction by minding and practicing the actions you would have to do for each type you fly (there might be differences).
In case of electric runaway, as more and more weight suddenly and progressively comes onto the yoke, it could be difficult to understand what is happening. As the situation quickly worsens, coping with the problem becomes harder by the second: you need to react immediately!
Know the checklists and procedures for your autopilot and trim systems
Most electric trim and autopilot systems have multiple methods of disengagement: consult regularly the flight manual to learn and refresh your knowledge of the procedures.
Use the forward trim switch and if this doesn’t work, push the electric trim disconnect button or switch on the yoke immediately
Your aircraft can also be equipped with an electric trim off button on the panel.
Electric trims operated by an autopilot can also sometimes be disconnected via the mode buttons on its control panel.
Refer to the flight manual of the aircraft you fly. Procedures can vary across types and information about the trim systems and autopilot might not only be in the main body of a flight manual but may be contained in supplements.
If the flight manual procedure says to disconnect the circuit breaker then focus everything you have on doing this.
Pull the circuit-breaker if the flight manual allows to do so!
Pulling the circuit breaker will stop the trim motor. Know which circuit-breaker to pull: it is a good idea to the mark the relevant circuit breaker in the panel to locate it quickly in case of need and avoid pulling a wrong breaker in flight.
CAUTION: Depending on the aircraft type, some circuit-breakers can power more than one systems! Check out the flight manual and only pull the circuit-breaker if it allows to do so!
Share tasks with others and work as a team
If you fly with a passenger and if he or she has the necessary competencies, sharing tasks will greatly help. Your passenger can also help you cope with the startle and surprise effect, control stress and stay calm. While you fly the aircraft, he or she can check the navigation, scan outside for terrain, traffic and weather, handle the throttle and the radio communications, and prepare the landing gear before landing. He or she could also help you on the yoke, taking off some of the load, when not busy with other tasks. Communicate among yourself and work as a team!
Beware of reduced speed!
With the loads and drag increasing, speed will decrease, which will ease some of the force you have to exert on the yoke. Beware that lower speed which is physically more comfortable, may not be appropriate for approach and landing.
Don’t hesitate to declare an emergency!
If things go wrong for any reason, for instance if you get tired and hardly can keep control, if you get lost because losing track of navigation, if you risk to enter unauthorised airspace, if you run out of fuel because of increased drag or lose altitude and risk to enter bad weather or IMC, do not hesitate to declare an emergency! Controllers will guide and assist you until you safely land.
Electric trim malfunctions and runaways are rare and develop quickly but can be managed if you react quickly and appropriately. Have a safe flight and enjoy flying!
References UK AAIB Air accident monthly bulletin March 2019
UK CAA CAP 174 In Focus Special – Handling a trim runaway, March 2019
AOPA Trim Malfunction – Fighting out-of-trim aircraft will quickly wear you out, by Robert I. Snow, 2019
AOPA Pitch Trim Runaway - After Take-Off, it’s a recipe for disaster, 2017
FAA Airplane Flying Handbook FAA-H-8083-3B) Chapter 16
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