In an automation malfunction, it’s likely that a plane’s electric trim will transform from convenience to extreme hazard.
Before you imagine flying a plane that’s out of trim, it’s important to understand what trim is, and this is widely misunderstood. Many, perhaps most, pilots think of trim as being an airfoil adjustment, and it is. On most light planes the trim adjusts the elevator trim tab or stabilator. The reason the trim tab is located so far aft is simple physics. The farther back it is, the more leverage it has over the pitch. And at its greatest throw, the pitch trim has a powerful effect on the plane’s attitude, so much so that it may be difficult or impossible to control the airplane at full trim deflection.
It’s also important to understand the relationship between trim and airspeed. We think of trim as being an easing or balancing of flight controls to the conditions of flight, and it is that, in a sense. But a more accurate way to think of trim is as a speed setting. For every configuration and power setting, there’s an airspeed at which the trim belongs, called, sensibly enough, the trim airspeed.
Most autopilots will automatically adjust this control surface to keep the controls trimmed up while the autopilot maneuvers the plane. Watch the trim wheel next time you go flying, and you can watch it at work. Increase the power a little while on autopilot in altitude hold mode, and watch the system trim the plane nose down. Reduce power, and it will trim nose up to maintain the selected trim speed.
This is all well and good, so long as things are working correctly. But when the airplane’s trim system suddenly and without warning starts trimming nose up or nose down or if the autopilot suddenly drops a wingtip toward the ground, things aren’t normal, and you need to respond.
When it comes to small planes, the problems we can have with automation are chiefly from two places, the autopilot system and the trim system, and they’re often closely related. They’re also widely misunderstood. While this won’t be an exhaustive discussion of either, I will offer some general tips on how to stay alive should you ever run up against one of these problems.
Trim issues are sometimes mechanical in nature, and they are almost always associated with electric trim. If you’re flying a plane that goes into a runaway trim situation, you are in an immediate, life-threatening situation. What you need to do is immediately disable the autopilot and/or the trim. And as soon as you do, be prepared to fly an airplane that’s badly out of trim. How bad will depend on how fast your system moves the trim and how fast you respond. If your airplane has manual trim in addition to the electric trim—most do—well, trim it up and go land somewhere now. If it doesn’t have manual trim available, then get it back on the ground as safely as you can, but be prepared to fly an airplane that might be badly out of trim and possibly really hard to control, including to land.
There are three axes of trim, though many planes have just one, pitch trim. Many light GA planes, such as the Cessna 182, have rudder trim, as well, and some airplanes have aileron trim, too. Many high-performance planes have yaw dampers, which attempt to keep the ball centered while they’re engaged and you’re maneuvering. They’re a form of always-on, automatic rudder trim.
A runaway trim or servo motor event can lead to trouble in any axis, though pitch trim and aileron (roll) trim departures are especially dangerous. Generally a roll departure from controlled flight happens as part of a mechanical failure of the autopilot system. An autopilot typically uses servos to control roll. To recover from a roll hardover, as it’s called, the pilot needs to overpower the autopilot to maintain control while immediately disconnecting the autopilot. This has to be demonstrated within that same three-second threshold at less than 60 degrees of bank. So, in three seconds you might find yourself at just less than 60 degrees of bank—with the nose almost certainly dropping precipitously, up to 30 degrees nose up or down, by regulation. And that’s within limits.