PRACTICAL
Know the circuit breaker's position
How long could you hold full elevator for?
switch to try to ease the load. However, it quickly became obvious that the switch was having no effect, because the load continued to increase — that’s when you could almost see the thought bubble saying “what the heck’s going on...” meanwhile, the seconds ticked by. Realising that the trim button wasn’t working, as the aircraft moved out of stable flight, the pilot went for the red ‘electric trim disconnect’ button on the yoke. This solved the immediate issue before they then (correctly) pressed the ‘electric trim off’ button on the panel, to be sure. With the electric trim off, manual trim restored and a stable flightpath, some problem-solving went on. This meant checking the circuit-breaker location in case it needed to be pulled.
In total, the event took the best part of a minute to resolve and there was a clear 15-20 seconds of “what’s going on?” confusion, as height and heading started to wander. In reality it was quite a quick response, but then this was a pilot going through commercial training, who had already learned about this malfunction. But how quickly would the average GA pilot react to the surprise and take the correct steps to deal with it before the situation became much worse? First of all, make sure you know the checklists and procedures for your autopilot and trim systems. Most electric trim and autopilot systems have multiple methods of disengagement and pilots need to know them all.
The first, and closest, is often the disconnect button or switch on the yoke (if fitted), as our pilot showed. There might also be an ‘electric trim off’ button on the panel, as with the simulator aircraft.
Electric trims operated by an autopilot can also sometimes be disconnected via the mode buttons on its control panel. The electric trim in some types can also be overridden by using the manual trim wheel if fitted, though that can be challenging because it means taking a hand off the yoke at a potentially difficult
‘Prepare for a trim malfunction by thinking through the actions you'd take and practising them’
time. However, in some failures simply ‘switching off’ won’t completely solve the problem. With the loads increasing, altering speed can ease some of the yoke pressures. However, remember that an airspeed which is physically more comfortable, might not be appropriate for approach and landing. In some types it can be essential to know from memory which circuit- breaker to pull to stop the trim motor
Would manual trim override?
before the loads become too high. Some owners make the relevant circuit breaker identifiable, to ensure it is easy to locate. Bear in mind, though, that some circuit-breakers can power more than one function. Check the flight manual and only pull the circuit-breaker if it says to do so. Remember, too, that valuable information about the autopilot and trim systems might not only be in the main body of a flight manual or set of procedures, but might be contained in supplements. Pre-take-off, don’t just check the electric trim for full and free movement, also 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 procedures, and if that involves pulling a circuit breaker know where it is. Prepare for a trim malfunction by thinking through the actions you’d take and practising them for each type you fly. Electric trim malfunctions and runaways are quite rare, but they can and do happen. Trim runaway was a possible scenario identified by the 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.
* Robert I Snow writes for AOPA in the U.S. SUMMER 2019 CLUED UP 17
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