Beech C-99 N55RP accident near Litchfield, Maine

Probable cause

A horizontal stabilizer pitch trim anomaly that was not identified in the available evidence.

NTSB analysis

The newly hired pilot and an experienced flight instructor were conducting the third flight of the pilot’s initial training. After conducting an instrument approach in visual meteorological conditions, they conducted a missed approach and the airplane proceeded to a holding pattern at 3,000 ft, consistent with a normal training profile. As the airplane was completing the first turn in the holding pattern, it pitched over and accelerated. The airplane impacted wooded terrain in a nose-down attitude in excess of 20°, a slight right bank, and at an airspeed exceeding 250 knots. No distress calls were heard from the crew, who were not communicating with air traffic control at the time and were not required to.

The highly fragmented wreckage was indicative of a high-energy impact. The engines and propellers were separated during the impact sequence and showed evidence of operation at high power. All structural components of the airframe were found within the confines of the debris field.

The airplane was equipped with a dual electric horizontal stabilizer trim system. The horizontal stabilizer was driven concurrently by two arms (cylinder rods) on the horizontal stabilizer actuator, mounted in the aft fuselage. An examination of the wreckage revealed that the right-side cylinder rod end became disconnected before the accident; the connecting bolt, nut, and washers were missing. This hardware also secured the control rod for the motion-indicating gearbox (MIG) that provided the pilots with aural and visual indications in the cockpit of stabilizer movement and takeoff trim position; disconnection of the MIG would have resulted in missing or incorrect reporting of the stabilizer and trim positions to the flight crew.

Although either actuator cylinder rod was capable of independently operating the stabilizer trim, the airplane manufacturer considered operation with only one cylinder rod a “limited emergency operation” mode. The horizontal stabilizer trim was likely near the full-nose-down position at impact. The investigation was not able to determine the effects of the pitch trim operating with only one cylinder rod attached for an undetermined but possibly extended duration. Functional testing of the components was not possible due to impact damage.

A pilot receiving instruction and flight instructor in another airplane performing maneuvers near the holding pattern reported observing the accident airplane make an abrupt turn to their right, which they described as a near-miss. The investigation reviewed flight track data from both flights to evaluate the possibility that the abrupt maneuver performed in response to the near-miss was related to the accident; however, of the accident airplane’s flight track showed the airplane maintaining the holding pattern for another 8 miles after the near-miss and before the descent and collision with terrain.

Text from the NTSB. Open the NTSB docket for ERA23FA346 for the full record.