How Fast Is Acting Fast?
The question is, in the case of an accidental runaway trim, how much harm could it do in each configuration and phase of flight were it to run to the stop, and how long can we expect it to take for a properly training pilot to respond and recover from the event before it gets to the point of no return?
With some problems in the air, you might have a little time to think them through. These are not those problems. Diagnosing uncommanded control input problems and responding to them is not something you can afford to do at your leisure. For every second the problem persists, the less likely it is that you’ll be able to recover from it.
With certificated airplanes, the calculus for how long you have to respond before it’s too late has been done for you by the factory flight test engineers when the plane was going through FAA certification. Those test pilots precisely determined how much time the electric trim could run before it would put the plane in dire circumstances. Be aware that the threshold for a successful recovery is slim—the length of time granted the pilot to recognize the runaway and disable the trim motor is a mere 3 seconds. If that sounds like a long time, then consider that by around the time you’ve finished reading this sentence, time is up, and the trim runaway might be catastrophic.
Just in case, it’s a good idea for pilots to get a feel for how their airplane behaves when it’s out of trim in various flight configurations, so long as the flying is done with plenty of altitude to spare and while staying well within the flight envelope, so that if you find yourself flying an airplane out of trim, at least you’ve been there before.
Safer Through Design
Many homebuilders, regardless of what the kit designer calls for, choose to add a switch on the yoke to disconnect both the autopilot and the electric trim (or just the electric trim if the plane is not autopilot equipped). Many add a dedicated circuit breaker, as well. As with the use of multiple ways to disconnect an autopilot, having multiple, easy-to-get-at ways to cut power to the electrical trim makes sense, too. The idea is to make responding to these emergencies easy and quick.
And that seems be the thinking behind the design of Garmin’s new GFC 500 and GFC 600 retrofit autopilots, the former of which I flight tested recently. The system design, I saw, makes recovery from trim or autopilot malfunctions simple, so long as you know what to do and what order to do it in. In the unlikely event that there’s a trim or autopilot malfunction, the display that’s linked with the system will annunciate the failure. The checklist, a memory item in the POH, calls for the pilot to grip the yoke firmly, press and hold the autopilot disconnect/trim interrupt button on the yoke, and then to pull the autopilot circuit breaker, keeping the disconnect button pressed until after the breaker is pulled. The procedure on the GFC600 is even simpler.
It was Garmin’s GFC700 autopilot, part of thousands of factory-installed G1000 avionics suites, that inspired the creation of these two new retrofit autopilots. These innovations include built-in attitude sensors, self-check fault logic and the use of brushless motors, which because of their more sophisticated design compared to previous generation servo-motors, are immune to the major mechanical causes of hardover failures.
The new STEC 3100 retrofit autopilot system from Genesys Aerosystems also monitors autopilot malfunctions and will annunciate the failure and automatically disengage the autopilot, letting you know with visual and audible annunciations that it has done just that.
See more about these systems’ envelope protection functions and learn about how difficult it is to disengaging them, in the accompanying story, How Safe Is Light Plane Envelope Protection?
As much convenience and safety as an autopilot can bring to our flying, it also also adds an element of risk that we need to understand thoroughly. The same is true for electric trim systems, which have been around for many decades. For you in your specific model of plane, the process for stopping trouble should it happen might be different from those we’ve described here, and if so, it might be more complicated and/or less intuitive.
Regardless, the underlying principles are the same, so understand them. Runaway trim or malfunctioning autopilots, or errors from their associated sensors, can kill. And the longer we wait to get things under control, the more dangerous our situation will become. So whatever the solution is for you in your airplane, know it by heart and be able to perform those lifesaving actions immediately. Such fast action might be the only thing that will save your life and the lives of everyone on board.