Never Again:

Bucking bronco autopilot

February 1, 2011

Never Again

Illustration by Sarah Hanson

Each of the three passengers had weighed themselves and their camping gear and dutifully telephoned to me the results so that I could calculate the weight and balance for our flight from East Kansas City to Sun ’n Fun in Lakeland, Florida.

It had been an unusually cold April in the Midwest and each of us looked forward to the warmth of Florida. We all were pilots, each eager to see, hear, and maybe experience some of the new airplanes and products sure to be at the show.

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As an experienced pilot with more than 3,500 hours over several decades, I had flown the trip in this airplane and myriad other aircraft roughly 10 times before. The route, fuel-stop airports, the arrival procedures, the bustle of competing for a safe space in the queue over Lake Parker, and the usual command for “landing on the orange dot” were quite familiar to me. I had no apprehension about the flight, but I had not flown as much as normal in the preceding 12 months because of various improvements we had made to the aircraft: new leather interior, new yoke, new six-point engine monitor.

The 1965 Beech Bonanza S35 (the “fast one,” we brag) looked absolutely pristine when I opened the hangar door and started loading the morning of our departure. Resplendent in its white and red with black trim colors and fairly glistening in the early morning sun, it was as well equipped and outfitted as any airplane I had ever flown. Low times on both airframe and engine, and every avionics advantage—including an S-Tec System Fifty Five X autopilot with yaw damper—made her a safe and exceptionally competent cross-country aircraft. Packing would have to be carefully done to keep the center of gravity forward, particularly for landing.

There would be four flight segments down and back. We would stop in Clanton, Alabama, to refuel before heading for the Lake Parker arrival. I carefully checked the figures for both takeoff and landing regimes, and the load was fine. Weather would be no consideration this day; it was clear and a million “all the way to Cuba” with light winds aloft. At 9,500 feet there would be little turbulence. It was a morning filled with the marvel of flight, camaraderie, and talk of flights past over the four-place intercom. Soon it came time for the descent into Clanton, a little more than half way to the show. No turbulence or gusty conditions were expected.

The VNAV feature of the GPS signaled the moment to begin descent. Using the vertical speed feature on the autopilot was a snap. At 1,000 feet agl I clicked off the autopilot on the yoke-mounted switch and turned to enter a left base leg to Runway 8. Continuing the descent toward final, the airplane felt heavy.

I held the yoke hard up, much harder than normal, to halt a descent. Heck, I must’ve misfigured the weight and balance! The controls hardened as we sank through 500 feet agl and time to turn final. I could barely hold level altitude and cranked in “up” trim to help. I didn’t turn to final; I couldn’t make myself turn under these circumstances.

To the others I shouted: “Something’s wrong; we are going around! Something’s not right!” We had already done the GUMP check, so I added power and swung the gear, pulling for all I was worth on the yoke. Slowly, slowly the airplane began to ascend in our path opposite the base leg. As we approached 1,000 feet I began a turn to the west. My co-pilot said, “You’re going to make a long straight-in, aren’t you?” Nodding in the affirmative, I began a labored climbing turn north.

Then there was a violent surge upward. The airplane literally jumped up. I pushed the yoke down hard impulsively to level flight. Then, just as abruptly, a pitch down followed, just as hard as the movement up had been. Things like pens and sandwich bags flew up in the air. My right hand was on the yoke, and I instinctively thumbed at the autopilot disconnect switch with my left. This made the airplane even more unstable in a wild pitch up, so I again thumbed at the switch. Forcing the airplane to climb, I hollered to my co-pilot to “Find the breaker for the autopilot and yaw damper and pull them, pronto, hurry!” Soon the co-pilot located and pulled the autopilot breaker, and the airplane immediately responded. I could hold level flight with just routine pressure against the trim, which had been full aft.

As things smoothed, normalcy returned to the cockpit, and we began to take stock. One of the headsets had fallen to the floor. One rear passenger had, with seatbelts tight, bumped the headliner. Our camping gear appeared secure, but several small articles were displaced. I returned the trim to neutral and made a series of shallow turns and up/down movements. All seemed well, so I turned for that long straight-in final approach. The landing was smooth and easy, and taxi to the pumps was normal.

One of our crew was an A&P mechanic and went over the airplane carefully as I refueled. No damage or abnormalities could be found anywhere. I suspected the problem was in the autopilot. One thing was for sure—those breakers would remain pulled. The final leg was smooth and uneventful.

Looking back on my response, I could have brought the airplane under full control much more quickly by simply turning off the master switch, located on the side panel just inches from my left hand. I know I thumbed the autopilot disconnect switch, but the problem persisted. After we reached cruise, we reconnected the circuit breakers and engaged the autopilot, then the yaw damper. They reacted normally with no hint of problem. The autopilot vendor had a booth at the show, and I had some pointed questions for them. My discussions in their booth were pleasant and informative, and brought out a possibility I hadn’t considered—perhaps I had inadvertently pressed the control wheel steering button on the yoke when I meant to disengage the autopilot. I had never used the CWS feature, but the STC requires it to be there.

The return flight home went off without a hitch. I had read many times that pilots must fully understand and master their autopilots, but I didn’t fully appreciate the accuracy of those warnings. I had been fixated on possible CG problems and how the airplane was loaded. My unfamiliarity with the new yoke and button placements exacerbated the problem. I should have turned off the electrical master switch immediately, and I should have briefed the right-seat pilot on the location of the various breakers that could affect flight. There is also a light on the face of the autopilot that indicates when it’s engaged, and I will never again engage or disconnect the autopilot without verifying that it is properly illuminated.

Jerry Witherspoon, AOPA 1502495, of Blue Springs, Missouri, is a private pilot with multiengine and instrument ratings and a glider rating. He is a co-owner of a Bonanza 35 and has more than 3,850 hours.