Pitch Trim Run-away

[design4p]

Hi everyone,

I am wondering under what circumstances a pitch trim run-away could occur.

I have a Sportscruiser and notice that a pitch trim run-away is mentioned twice in the TAD sheet 338. The first is with MOD-338-014 and the words used are 'unintended continuously running pitch servo trim servo' and the second mention is in section 3.5 Special Test Flying Issues 'to eliminate the possibility of pitch trim runaway'.

If such an occurrence should happen it would be a sudden shock for the pilot and because of the topic being mentioned twice I am thinking this fault is a real possibility.

I am now trying to consider under what circumstances such an occurrence could happen.

Any suggestions would be welcome - John

[AlanR]

Hi John,
As explained on the Sportcruiser forum there was a SB concerning this back in 2009/10'ish. The stick trim( Ray Allen ) on the early S/C were wired by CZAW and required a modification. I would check that this has modification has been carried out because if not it will certainly cause a trim runaway.
Here is the SB:-
http://www.czechsportaircraft.com/useru ... 1_13ss.pdf

[John Clarke]

Thank you for that SB. I fly a Zenair Zodiac with a Ray Allen 205 and I've had uncommanded "up" trim runaway a few times, the first being after only 7 hours on type (my second solo flight in a taildragger) and, yes, it scared that daylights out of me! I couldn't get the trim control to return to neutral while in the air though I realise now, after having had it happen a few times, that slowing the aircraft to 60 knots will allow a return to normal trim. The first time it happened I was commencing an overhead join and having to land with the trim fully up was quite a challenge for me.

I had always assumed that it was pilot error as it's only happened 3 or 4 times in 5 years and I thought that I had, inadvertently, pressed the '"up" trim button while pressing the PTT. I'm very careful these days to keep my thumb off the top of the stick unless I actually need to trim. What you have described, along with the SB, will have me checking my G205 tomorrow!