History of Flight
On April 10, 2017, about 1014 Pacific daylight time, a Beech A36TC, N755R, was involved in a flight control incident near Livermore, California. The pilot was not injured, and the airplane sustained minor damage. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
The pilot departed earlier from Livermore for a local flight including practice landings at Tracy Municipal Airport. Preflight checks were uneventful, but the autopilot preflight check was not performed. He flew northeast approximately 30 miles, then southwest toward Tracy with the autopilot engaged, using the heading bug. During landing approach, he disengaged the autopilot and performed four uneventful practice landings. He returned to Livermore, approaching from the east, and used the autopilot again, engaging it after trimming for level flight.
He contacted Livermore tower and was cleared for a straight-in approach to runway 25L. Shortly after, he disengaged the autopilot using the A/P DISC. TRIM INTERRUPT button on the control wheel but did not hear the standard aural warning chime. The airplane immediately pitched aggressively up about 50° with an immediate loss of airspeed. The pilot applied full forward pressure on the control wheel, and the airplane pitched nose-down, then back up. With full forward pressure, he could maintain level attitude, but needed both hands and a knee to keep the control wheel forward.
He declared an emergency to the tower, reporting a “stuck cable,” and was cleared to land on runway 25R. He could not reach the autopilot circuit breaker as it was on the far-right side of the panel. The autopilot disconnect button broke off during the struggle. He attempted to adjust the elevator trim wheel but it would not move. He continued the approach, regulating pitch with engine power and holding the control wheel fully forward.
During the approach, he inadvertently knocked off his glasses and headset, losing communication. He selected landing gear down and extended flaps; he heard gear extend and saw gear lights illuminate. However, the tower controller warned that the gear was not extended, but the pilot did not hear. The airplane touched down on runway 25R with the gear retracted and slid to a stop on its belly. The pilot egressed immediately.
Wreckage and Impact Information
The airplane was examined by the NTSB, FAA, and a Honeywell Aerospace specialist.
Landing Gear The landing gear selector switch was found in the down position. The landing gear and auxiliary fuel pump circuit breakers were tripped. During recovery, the gear doors were closed with no damage. Testing with the airplane on jacks showed no anomalies; the gear and fuel pump functioned correctly. The instrument panel lighting rheostat was in the “dim” position, making gear light status difficult to determine. The pilot later suggested that during attempts to disable the autopilot, he may have accidentally pulled the circuit breakers, and that the sound of flaps operation may have been mistaken for gear extension. He also noted difficulty reading indicators due to cabin lighting.
Autopilot Operation The airplane was equipped with a Bendix/King KFC 200 flight control system, a two-axis autopilot with flight director. Pitch control was via a servo driving the elevator through a bridle cable. A pitch trim servo drove the elevator trim tabs. The autopilot included manual and automatic electric pitch trim, altitude hold, vertical trim, nav modes, and control wheel steering (CWS).
Autopilot Examination on the Airframe The red cap from the A/P DISC. TRIM INTERRUPT button had broken off and was found in the footwell. With the cap missing, the switch required asserted effort to activate. The elevator trim tab gauge indicated 18U (18° nose-up), and the tabs were in the matching position. Maximum trim positions were 27° nose-up and 10° nose-down. At 150 knots, 296 lbs of force would be needed to hold the elevator neutral. The elevator flight control system was intact with no binding or failure.
A pre-flight check with a metered vacuum source showed all autopilot annunciators illuminated, but the “autotrim trim failure” light flashed once instead of four times. After reseating the A/P DISC switch, the check was normal except for a manual electric trim time of 64 seconds (nominal 34–50 sec).
The elevator trim wheel could be moved by hand, not locked as the pilot indicated. However, the left side rocker of the manual electric trim switch intermittently failed to return to neutral after selecting trim-down. In that state, the trim servo solenoid remained engaged, preventing manual wheel movement. Pulling the trim circuit breaker disengaged the solenoid.
Autopilot Component Examination Components were examined at Honeywell. Two discrepancies: intermittent sticking of the left manual electric trim switch, and seizure of the KS270A Pitch Servo drive motor, which did not operate initially. Disassembly revealed binding of the motor shaft; after freeing, the anomaly could not be repeated, and the motor operated within specification.
A simulation with the pitch servo motor held immobile showed that if the capstan wheel engaged pitch trim microswitches, after a 3-second delay, the pitch trim servo would move. No errors or enunciations occurred. Engaging the left trim switch activated the solenoid; pressing the A/P DISC switch or turning off avionics master disengaged it.
The pitch servo motor was manufactured in 1992; it had not been returned for service since. No life limit or overhaul schedule existed, and similar failures were not recorded.
Additional Information
A service bulletin for the KC 295 Flight Computer Adapter Board appeared to have been accomplished. The KFC-200 flight manual supplement provided disengagement procedures and cautions about autotrim mistrim forces. The emergency procedures included pressing the AP DISC/TRIM INTERRUPT switch, turning off avionics master, manually retrimming, and pulling the pitch trim circuit breaker.
The avionics master switch and AP engage lever were within reach, but with the control wheel fully forward, the control column obscured the trim indicator and circuit breakers. The pilot had mounted a GPS and avionics cradle on the control column, but they did not restrict access. The pilot stated he had not experienced similar problems before and followed standard autopilot procedures.