The Murphy Builders Site

Had an incident that I thought might be of interest to some. While doing
some airspeed calibrations this past week on my SR-2500 - 60% cruise power
at 3,500 feet, 122 k indicated - the nose suddenly went up into a pretty
steep climb. I was able to overcome this with a very strong forward push.
I found that the elevator trim had run to the full nose-up position
(uncommanded). While I have emergency trim cut-off switches to remove all
power from a trim system in just such a case, I was too slow witted to
realize what had happened and to disable the elevator trim system before
it had run to the full up position - and was not movable by any of the
three trim switches. In my configuration, I have trim switches on both
sticks as well as the rocker switch in the panel.

With a lot of arm force I found that I had sufficient elevator authority
to overpower the trim, though I wouldn't want to hold it for any great
length of time. Reducing power and speed greatly reduced the push needed
to overcome the trim, and in fact - in the lading configuration (A/S about
70 k and flaps) the trim was about right for a hands off approach. A
go-around might have been interesting, though.

The problem was found to be in the Ray Allen grip switch (a five button
grip - up, down, left ,right & PPT). Evidently the up button either
shorted or was stuck. I didn't like this grip anyway, as it was difficult
to reach all buttons with my thumb or fingers. I have a single hat-switch
type grip on the right side with PPT and like it much better and will
replace the defective one with this type.

It was a bit of an alarming situation, but glad to report that the
airplane was fully controllable even with a fully mis-trimmed condition.
I'll try testing with a full down to see if that is also controlable.

Charlie Starr SR-2500 #065