BMA Autopilot installation problem

12 Nov 2005 BMA Autopilot installation problem

This is not exactly a flight testing issue, but I thought this would be a good place to put this information. Here is a post from the VansAirforce.net forums, written by Glen M. Thompson:

All BMA autopilot owners CAUTION!

A VERY scary thing happened yesterday when I went out to fly 331JH. It was a beautiful morning and Jim is geting REALLY excited since there are less than 10 hours left in testing for him to take his first ride in his new 10!

I sat down and turned on the master and immediatly there was a loud squeal from the left wing tip(servo installed there) and the aft fuselage. At the same time the stick abruptly jammed to the left and aft. (Efis 1 powered by standby battery and autopilot controller powered by the main bus through a button CB) I immediatly turned off the master and found the elevator free but the ailerons jammed almost full travel to the left!

What happened was, the servos went nuts for whatever reason ( Brent spoke to larry and he cannot see how this happened, Bob and possibly Greg are coming here next week to cure our gremlins hopefully) and went full travel left and up. Now, here is where EVERYBODY should take note. Yes…. the directions and common sence says to ABSOLUTELY make sure the servos CANOT go over center, and we thought we prevented that with the aileron stops, But………We had the servo set up with the arm pointing down. Apperently, when the servo went nuts, it pushed so hard on the pushrod going to the bellcrank, (3.5 feet or so long), it bent the pushrod sufficiently for the servo arm to travel over center and the arm to then jam itself against the upper wing skin!. Ok, so I disconnected the controller power plug and went to fly. For grins, I tried to fly with the ailerons deflected the same amount to the left as they were when the controls jammed. Nuh uhhh, I could not maintain level flight with even HALF the aileron travel as when they were jammed and full rudder. Goes without saying what would have happened if this occured inflight.

I recommend that unless your servos can travel 360 degrees without jamming your controls or you have hard stops on your servos, that you consider pulling the breaker until further inspection.

Questions or comments, please call or write….With BMA coming next week we will keep everybody posted so we can all learn and be safe from this.

Glen 561 – 670 – 6095

Be careful with your autopilot installations – try to imagine failures that “can’t happen”.