PRACTICAL


A


sk a bunch of pilots if they could handle an electric trim runaway in flight, and the likelihood is that some will say “Yeah, sure, how hard can it be...?” The answer to that is ‘potentially very hard indeed’. Just ask anyone who’s simply flown by error out of trim, manual or electric, and they’ll tell you precisely that, depending on things like the size of the elevator, the aircraft’s speed and the control mechanisms, the pull or push on the yoke or stick can be worse than you might think.


Some accidents have occurred where forces of around 20kg have been needed to keep the aircraft straight and level – some might think ‘Is that all?’... But when did you last try lifting that for a sustained period of time while trying to fly accurately? Don’t believe it? If an electric trim runs away, it happens quickly. Depending on the type, just three seconds can put you significantly out of trim, and within five seconds an aircraft can be almost unmanageable.


Just think about that for a moment: if left badly out of trim mid-flight, how would a pilot on their own cope with the heavy yoke/stick forces, while trying to manage the myriad other tasks such as engine management, navigation and radio? This abridged report by Robert I. Snow* of a flight in a light aircraft type answers some of those questions and provides a sobering insight into the issues involved. “At lift-off I was holding an annoying amount of forward pressure to maintain the proper climb angle,” he says. “It didn’t seem excessive, but I was immediately aware that there was no way we could do this all the way to our destination. “Returning to the airfield, even at reduced speed, the amount of forward pressure needed to maintain control, inconsequential at first, had become exhausting by midfield on the downwind leg. I needed both hands to control the pitch. “I thank my lucky stars that there were two of us aboard. The boss handled the throttles, landing gear and radio. By the time we turned base, he was helping me on the yoke, taking off some of the pressure whenever he didn’t need his hands for something else. Relief did not come until we started the flare. Finally, I was able to ease off forward pressure to raise the nose. Although I knew


‘As more and more weight suddenly and progressively comes onto the yoke, some pilots will be confused'


philosophically that the trim is a powerful part of the control system, I had never really experienced its potential before. Trimming away pressure had become such a reflex that I virtually never had to use more than minimal force on the yoke. “If you are the kind of person who has to see things on your own, try it in flight some time. While straight and level (at sufficient height), roll in enough trim in either direction to force you to exert some effort to maintain pitch attitude, and then hold it for a while. You will quickly learn the lesson.”


That scenario related to a nose-up trim malfunction. In the nose-down case, the forces could make it difficult to slow the aircraft down, and/or configure it for landing.


The problem with electric trim malfunctions is that, as more and more weight suddenly and progressively comes onto the yoke, some pilots will initially be confused while trying to understand what’s happening. As the situation quickly worsens, coping with the problem becomes harder by the second — it’s an issue that needs immediate action. To see how a pilot would cope we gave an unsuspecting trainee commercial pilot, who was flying straight and level in a simulator, a runaway trim in the nose-up sense. It was fascinating to watch. At first, as the yoke came backwards the pilot reacted (as most would) by instinctively pushing forwards. As the yoke continued to push back with increasing weight, the pilot’s thumb went instinctively onto the forward trim


SUMMER 2019 CLUED UP 15


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