“The autopilot won’t hold altitude.” The other captain had written the light-twin plane up five weeks before, but the chief-of-maintenance had (as usual) blown off the report. And it was crunch season for the mechanics, because in addition to the charter planes, and teaching plane, they had to keep two spray planes going and going and going. We had a procedure for working around a cranky autopilot, but it’s not fun, and there are times when having one that will do all that it is supposed to is very, very good.
Finally, the chief-of-maintenance [I’ll call him Bob] had to fly that plane. He landed stomped in and said, “Autopilot’s not working. Don’t use it with passengers on board. Don’t use it in the clouds.”
“OK, sir, when can you fix it?” I ventured to ask.
“Someone has to trouble shoot it. I don’t have time.” Which was not entirely BS, but I knew better than to point that out. Bob was not an easy person to work with at the best of times, and the company needed him a lot more than it needed me. Pilots are easy to replace. Mechanics with FAA Inspection Authority are scarce and valuable.
However, for once the Fickle Finger of Fate pointed at someone else, and I managed to have a long empty leg, in that plane, over a sparsely inhabited area a week or so later. And the weather was very good, with excellent visibility and no clouds. This is important, because 1) some things you don’t want to do over the Twin Cities metro area or 2) do them in the clouds.
So, there I was, over farmland, at 5000′ above the ground. I got out notepad and pen, triple-checked my safety harness, and set the autopilot. I’d already tied down everything in the plane. All seemed to be in order, I made note of the conditions and engine settings, and hit “Engage.”
Otto Pilot held everything steady for thirty seconds, then started a very gentle climb, on the proper heading. Apparently altitude hold was Otto’s problem. I made a note. After climbing 50 feet or so, the nose came down and the plane started to descend. After 75 feet, it began to climb once more. This time it went to 100 or so feet above the set altitude, and pitched down 150 feet. Keep in mind, I had not changed the engine settings, so as we went up, the plane slowed, then accelerated as we went downhill. At some point, this starts to get sporting.
When the plane performing altitude deviations of over a thousand feet, and I was having to work the throttles to keep from over-stressing the plane on the downhill runs, I turned Otto Pilot off. I had a good data set, and I didn’t care to see just how bad the phugoid could get if left completely unattended. I returned to base, wrote up my notes, and handed them in.
Bob glanced at them, tossed them in the garbage, and sent me to go wash one of the other charter planes (without giving me time to change out of a skirt-suit). That terminated any remaining personal respect I had for Bob.
It was not until the Boss got to “enjoy” the autopilot’s antics that it was pulled from the plane and replaced. If memory serves, there was a pin-hole in the bellows that triggered the altitude correction servos, and that led to the auto-pilot mis-reading altitude hold or something. Bob wasn’t good about explaining why something had gone “kSPRIOng!” unless, of course, it was pilot error. Then we got to hear about it in gory, painful, constant detail for weeks at a time.
I learned a lot from Bob.