Last evening, while enjoying glass-smooth air at about 4000' just after sunset, my RV9a unexpectedly pitched down hard. I had just reached over to pull a little cabin heat when it happened.
My first thought was I just hit a massive downdraft - something was pushing my nose down hard. I could pull back to counter it, but had to pull on the stick hard and it wasn't letting up. I pulled throttle back and it eased up a little. My second thought was "how could cabin heat cause this" when I didn't have an autopilot that might be affected by the changing pressure in the cabin.
Then, I tried pitch trim up (electric trim, with a switch on my panel just above the throttle) and it just as quickly corrected itself.... problem solved!
But, it certainly got my heart racing, and also thinking about how the pilots in the recent 737 Max8s that crashed must have felt as something 'external' was pushing their nose to the ground.
I certainly didn't think my right hand finger was pushing the trim button at the same time my left hand was pulling the cabin heat knob, but my right hand was resting on the throttle where it normally does, and I suppose it could have happened as I slightly leaned forward to pull that knob. I don't suspect a faulty Ray Allen switch, but I suppose that could have happened too.
I also took out my original ray-allen trim indicator on my panel when I installed my GRT efis, moving the trim indicator to the EFIS. However, the trim indicator isn't shown when I'm in PFD mode with a split screen with moving map, so I didn't know the trim had moved.
I checked the forums this morning and found several other threads describing similar incidents where people had bumped the trim buttons on their stick, or had malfunctioning switches. They described very similar symptoms as I saw...
The best suggestion I saw was to install a "Safety-Trim
" which limits any press of the trim button to 3 seconds before you have to release and press it again. I'd like to hear from anyone who has installed one of these because it seems that the limit should be 1 or 2 seconds instead of three. I think it could happen again similarly with a 3 second press of the button unless I slow the trim motor down a lot. (which might actually be a good think to make it more sensitive to fine adjustments)
I also thought that perhaps installing a big red "TRIM" light on my panel would alert me whenever the trim motor is activated. That might get annoying though during regular use though and I'm not sure how to wire it.
My third thought is to see if I can program the EFIS to display an error when the trim reaches a certain point away from center. That would have helped me identify that the trim was out of normal range. Has anyone done this on a GRT EFIS?
At the VERY LEAST, I STRONGLY ENCOURAGE EVERYONE with electric trim to practice dealing with unexpected trim by establishing cruise speed at a decent altitude,press the trim button up or down for 4 or more seconds to see what it feels like! See how much back or forward pressure you need on the stick to counter it! I wish I would have done this when people talked about these issues before so I would have been prepared for it when it happened.
In any case, I'm sharing this to hopefully avoid others from experiencing the same thing, and to discuss the best possible options to keep it from happening again... or at least quickly identify the issue when it does.
Please, don't discuss manual vs. electric trim here... I have electric, and I'm not changing it now, although I might put manual in my next build.
Discussions in previous threads:
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