A couple of weekends ago I flew my RV-4 back from Sunriver to Hillsboro with my 17 year old son (175 lbs) in the back. Naturally the elevator trim setting was quite different than where it is for 90% of my flights that are solo. Last weekend I went out to do some flying and check out the homebrew AOA probe for the Dynon D10 I had just installed - solo of course.
Mistake #1: Inadequate preflight - such a high percentage of my flights are solo and I didn't use a checklist that I failed to check/set the elevator trim. On climbout the nose is noticeably heavy from the trim being set for the 175lb in the back on the previous flight. No biggie of course, just hit the trim button on the stick..... No response. Hmmm.... must have blown a fuse, that hasn't happened before. OK, lets deal with this in a minute, its not an emergency just an inconvenience.
Level off at 3K AGL, at slow cruise speeds it takes a surprising amount of force pulling back to maintain level flight. Check the fuses, none blown. I forget that the trim is connected to a separate circuit breaker I had placed in a position where I could shut it off in case of runaway trim. Not in sight, but just under the main panel on the RV-4 is a cross member where I had it mounted easily accessible in flight. Mistake #2: Inadequate systems familiarity (even though I built the thing!).
Land normally and taxi back. Tach time 0.3Hrs
Ok, enough confessions & mistakes. Trim servo failed - ground debugging shows that if I reset the circuit breaker I can run nose down trim, but if I try to run nose up trim it blows immediately. Checking the current - nose down trim is a few hundred mA, nose up trim is >10AMPS!
Take out the trim servo, disassemble on the bench looks like this: (Sorry for the poor quality Iphone pics
A little more work yields this, be careful - lots of little parts to fall out or damage:
Checking with an ohm meter, one of the two diodes shown is dead shorted. A trip to Radio Shack - buy two 1N4001 for $0.99, replace both just because and it looks like this:
Re-assemble everything and bench test. Works great. Install, go flying and test (after a thorough preflight!). Voila, problem solved, total expense $0.99 and about 1 hour labor.
Lessons learned:
1) Preflight
2) Systems familiarity
I found that I had become too familiar and used to flying solo and had skipped a checklist that would have identified the trim failure on the ground. In this case it was no big deal. Secondly, after 8 years of flying my RV-4, even though I built it - its easy to forget systems and is worthwhile to re-familiarize oneself with where things are. In this case, resetting the 1A circuit breaker wouldn't have helped anyway - and arguably it would be better to get on the ground and debug rather than try to do so in the air and risk (however small) an overload condition and burning up wiring.
Steve
N144SH
RV-4
MAC Trim Servo failure - and repair (and Lessons learned the easy way)
Linear Mode
