A swap with working 310 autopilot led me down a new path towards fixing my STEC 55X porpoising problem.
STEC Update from Twin Cessna Flyer September 2012 Issue
My STEC 55X saga continues with some good news. When I last wrote about this, the working theory was that a worn copilots control column was causing a “catch” as small worn spots on the column passed over the rollers holding the column in place. I found a salvaged column in good shape and had it painted and ready to install when I got a call from Robin Howard of Howard Aviation in LaVerne, CA. Robin had a customer with a 303 who had the identical porpoising problem, only more severe. He had a spare 55X that he had removed from a 310J. As a test, he swapped the 310 autopilot with the one in the 303 and it worked perfectly. He suggested I try it and, of course, I jumped at the chance since it might mean we wouldn’t have to take apart my panel to install the new control column. The autopilot arrived just prior to my flight to Oshkosh. I took off and flew the first hour of the flight with the old autopilot just to confirm that it was porpoising under our flight conditions and weight and balance. It was. I let the copilot fly manually while I swapped boxes and for the next couple of hours the 310 autopilot flew my 303 perfectly. It did so on the return flight as well. Chad Howard, the Customer Service rep for STEC was at Oshkosh and I explained the situation to him. He was glad to have the new information and said we could adjust the settings on my 55X to match those of the 310 autopilot.
There is some FAA paperwork involved and we are working on that now. I’ll have to ship my unit back to STEC for a few days, or perhaps fly to Mineral Wells
for a flight test, but after that I’m pretty sure my problem will be solved. So why didn’t my autopilot, certified specifically for the 303, not work properly in my airplane and apparently some other 303’s as well? Chad explained that when they certify an autopilot they set the adjustments to match their particular test aircraft. Sometimes the test aircraft is not an accurate representative of the fleet as a whole. It may be slightly mis-rigged or have an undetected anomaly in the control system that makes it fly differently from the rest of the fleet. Thus, you get an autopilot that produces problems in a significant portion of that higher. The view may not be as good, but the increased peace of mind, for me, is priceless.