N755R

HISTORY OF FLIGHTOn 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 stated that he departed earlier in the morning from Livermore with the intention of performing a short local flight that included practice landings at Tracy Municipal Airport. The preflight checks were uneventful, but he did not perform the autopilot preflight check. He flew northeast approximately 30 miles, then southwest toward Tracy with the autopilot engaged, while he maneuvered the airplane using the heading bug. During the 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 he had trimmed the airplane for level flight.

He contacted the Livermore air traffic control tower and was given a clearance for a straight-in approach to runway 25L. Shortly after contacting the tower, he disengaged the autopilot using the autopilot disconnect button (A/P DISC. TRIM INTERRUPT) on the control wheel and noticed that he did not hear the standard aural warning chime indicating that the autopilot had disengaged. The airplane then immediately pitched aggressively up about 50ºwith an immediate loss of airspeed. The pilot stated that the pitch up happened so fast that he 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 forward pressure, and the airplane pitched back up again. With full forward control wheel pressure, he was able to maintain an approximate level attitude, although the control forces were so great that he needed to use both hands and his knee to keep the control wheel forward.

He then called the tower, reported an emergency and that he had a “stuck cable” and was given an amended clearance to land on runway 25R. He reached over to the circuit breaker panel but could not pull the autopilot breaker because it was out of his reach on the far-right side of the instrument panel. He would have needed to release his grip on the control column to access it, and then possibly lose control of the airplane. He also attempted to disengage the autopilot with the autopilot disconnect button, but it had broken off, presumably during the struggle to maintain control.

The pilot reached down to adjust the elevator trim wheel, but it would not move, and he could not safely move himself into a position to look down and observe the elevator tab position indicator. He previously experienced an engine throttle cable failure and suspected that this time the elevator flight control system had either stuck or failed.

He continued the approach and was able to regulate pitch by adjusting engine power and holding the control wheel fully forward. He reported to the tower controller, “I have an autopilot stuck, and I can’t get it unstuck.”

The airplane continued to porpoise as he initiated a descent back to the airport. He stated that at some point during the approach and ensuing struggle, he inadvertently knocked off his glasses and headset, so was no longer able to hear the tower controller. He set the landing gear selector switch to down and extended the flaps. He stated that he heard the landing gear extend and confirmed that the landing gear lights had illuminated. However, the tower controller made multiple calls during the final approach, which the pilot did not hear due to the loss of his headset, warning the pilot that the landing gear was not extended. The airplane touched down on runway 25R, and it was then that the pilot realized the landing gear was not extended. The airplane came to a stop on its belly, and the pilot immediately egressed.

WRECKAGE AND IMPACT INFORMATIONThe airplane was examined by representatives from the NTSB and FAA, along with a technical specialist from Honeywell Aerospace, the autopilot manufacturer.

Landing Gear

Immediately after the incident the landing gear selector switch was found in the down position, and the landing gear and auxiliary fuel pump circuit breakers were both in the tripped (pulled) position. During recovery of the airplane the landing gear doors were all found completely closed, with no damage or gaps that would have indicated a partial extension.

The landing gear and fuel pump circuit breakers were recycled, and the landing gear and fuel pump were tested with the airplane configured on jack stands. No anomalies were noted, and both the landing gear and fuel pump functioned correctly. Of note, the instrument panel lighting rheostat was found in the “dim” position, and as a result, determining the landing gear light status in the daylight condition was difficult.

The findings were discussed with the pilot, who stated that it was possible that while he attempted to disable the autopilot, he may have accidentally pulled the landing gear and fuel pump circuit breakers, and the sound of what he thought was the landing gear extending, may have been the flaps operating. He also stated that due to the cabin lighting and urgency of the situation, he may have misinterpreted the landing gear indicators.

Autopilot Operation

The airplane was equipped with a Bendix/King KFC 200 flight control system (FCS). The FCS incorporated a two-axis (pitch and roll with altitude hold) autopilot and a flight director system.

Pitch control was accomplished through a servo which drove the elevator via a bridle cable attached directly to the elevator control cables. A pitch trim servo drove the elevator trim tabs via a bridle cable attached directly to the elevator tab control cables.

System capabilities included manual and automatic electric pitch trim, altitude hold, vertical trim, multiple nav capture and track modes, and control wheel steering (CWS). Pilot interface with the FCS was via both the panel-mounted KC 290 mode controller and several switches on the left horn of the pilot's control wheel. Self-test, mode select/engage, and vertical trim were controlled via the KC 290. The A/P DISC. TRIM INTERRUPT, CWS, and dual rocker electric pitch trim switches were on the control wheel.

The automatic trim allowed the KFC-200 system to trim off elevator control surface pressures while the autopilot is controlling the elevator through the pitch servo. If the autopilot is not engaged, the pilot can use the manual electric trim switch or the trim wheel (located in the center console) to trim off elevator control forces.

FAA certification regulations require that the autopilot must be capable of being manually overridden by the pilot, by using the control wheel.

Autopilot Examination on the Airframe

The red cap from the autopilot A/P DISC. TRIM INTERRUPT button on the left horn of the pilot’s control wheel had broken away from the switch and was found in the footwell immediately after the incident. The pilot stated that it likely came off as he attempted to gain control of the airplane. With the red cap removed, the switch plunger was free to sway within the switch case and required an asserted effort to activate.

The elevator trim tab gauge indicated "18U" (18° nose-up), and the elevator tabs were found in the matching tab down (nose-up) position. The maximum trim tab positions were 27 ° nose-up, and 10 ° nose-down respectively. According to representatives from Textron Aviation, with an elevator trim setting of 18U, and an airspeed of 150 knots, the pilot would have needed to apply 296 lbs of force to the control wheel to hold the elevator in the neutral position.

The entire elevator flight control and trim system was examined. All control cables were intact, and there were no indications of binding, failure, or foreign object interference.

With the A/P DISC. TRIM INTERRUPT switch cap re-installed, and a metered vacuum source connected to the airplanes vacuum system, a pre-flight check was performed in accordance with the KFC-200 flight manual supplement and all autopilot annunciators illuminated, but the “autotrim trim failure” light flashed once, rather than four times as required.

The A/P DISC. TRIM INTERRUPT switch was then reseated by movement, and the check was performed again, and no anomalies were noted, except section IV (7e.3) (manual electric trim run through full travel), which took 64 seconds to compete, rather than the nominal time of 34 to 50 seconds.

The elevator control trim wheel could be moved easily by hand, and it was not locked as the pilot had indicated. However, during the examination, it was found that the left side rocker of the manual electric trim switch would intermittently fail to return to the neutral position after the trim-down position was selected. In this state, the trim servo solenoid remained engaged, and the trim wheel could not be moved. Pulling the trim circuit breaker disengaged the solenoid and allowed for manual movement of the trim wheel.

Review of the elevator trim system electrical schematic revealed that with the left manual electric trim switch jammed in the fore or aft position, the trim solenoid would remain engaged, but the trim motor would not run.

Autopilot Component Examination

The autopilot drive and control components were removed and examined at the facilities of Honeywell Aerospace under the oversight of the NTSB. Examination revealed two discrepancies: the intermittent sticking of the left manual electric trim switch, and seizure of the KS270A Pitch Servo drive motor, which did not operate during its first test. Disassembly of the K270A unit revealed binding of its motor shaft, which could be overcome by rotating the tachometer gear end of the motor by hand. Once freed, the anomaly could not be re...