WPR17IA086 · Beech A36TC

After performing a series of uneventful landings in the single-engine airplane at a local airport, the pilot flew a return leg with the autopilot engaged. As he approached the airport, he disengaged the autopilot using the autopilot disconnect button on the control wheel. The airplane then immediately pitched up aggressively with an immediate loss of airspeed such that the pilot feared the airplane was about to stall. He applied full forward pressure on the control wheel, and the airplane pitched almost directly nose down and began to quickly build airspeed. He then reduced the control wheel pressure, and the airplane pitched back up again. He was able to maintain an approximate level attitude with full forward control wheel pressure, although the control forces were so great that he needed to use both hands and his knee to keep the control wheel forward.

During the event, the pilot reportedly attempted to adjust pitch with the trim wheel, but it was jammed. He also attempted to disengage the autopilot by pulling its circuit breaker; however, the breaker was both beyond his reach on the right side of the airplane and blocked by the right arm of the control column. Other methods of responding to this emergency were available to the pilot; however, due to the jammed trim wheel (there are no emergency procedures that cover this contingency) and a previous experience involving a throttle cable failure, the pilot was convinced he had a flight control failure rather than an autopilot anomaly and did not attempt any alternative actions. Ultimately, the pilot was able to regulate pitch with engine power and landed the airplane gear-up.

Examination of the airframe did not reveal any mechanical failures that would explain why the landing gear did not extend, and following the incident, the landing gear selector switch was found in the down position, and the landing gear circuit breaker was in the pulled (deactivated) position. It is likely that the circuit breaker was inadvertently pulled by the pilot as he tried to disengage the autopilot, and as a result the landing gear did not extend as he had thought. This misdiagnoses further highlights the stress the pilot was experiencing.

No discrepancies were observed in the airplane’s flight control system, and an important service bulletin related to pitch trim runaway had been applied.

Examination of the autopilot system found that the left side rocker of the manual electric trim switch would intermittently fail to return to the neutral position after use. In this state, the trim servo solenoid remained engaged, which locked the trim wheel. However, pulling the pitch trim circuit breaker would have released the solenoid and resolved the anomaly. The damaged rocker switch was therefore likely the reason the pilot was unable to manually adjust the airplane trim during the emergency.

Disassembly of the pitch servo revealed that a bearing in its drive motor had seized and was causing the servo to intermittently stop operating. The design of the servo was such that in the event that it locks, the motor assembly can activate the pitch trim servo to compensate, which could result in a mistrim configuration when the pilot disengages the autopilot. Other possibilities exist that could cause the airplane to be in a mistrim condition with the autopilot enabled, including the pilot manipulating the controls or adjusting the engine speed with the autopilot engaged; however, the pilot stated that he did not do this during the flight.

To prevent deactivation of the autopilot under a mistrim condition, the flight manual specifically calls for pilots to check the command bar on the flight command indicator prior to disengagement. There is no evidence the pilot did this and, as a result, was likely caught off guard by the mistrim condition. Additionally, the pilot did not perform the autopilot pre-flight check prior to flight, which may have alerted him to the anomalies with the trim switch and would have alerted him to another unrelated problem with the pitch trim servo, which was operating slower than required.

Even if the pilot disconnected the autopilot without checking the command bars, the required control forces, while high, would still have been controllable. However, the trim position as found after the incident would have required an extreme application of force (almost 300 lbs) by the pilot at the control wheel to maintain a neutral elevator position, an amount well beyond what the autopilot system could command. Therefore, it is likely that this trim finding was a result of the autotrim system reacting against the pilot’s control input while he attempted to maintain pitch control. It could not be determined why the autotrim system did not completely disengage during this phase of the flight.

The pilot’s previous experience with a throttle failure event likely affected his ability to accurately diagnose the cause and identify an appropriate remedy for the multiple conflicting system cues and failures in a timely manner; his performance was further degraded by the physical workload required to maintain control of the airplane.

Verbatim NTSB analysis and probable cause from the NTSB dataset