Amateur-built AIRCAM N848HP accident near Deland, Florida

Probable cause

The uncommanded movement of the left propeller into reverse during approach for undetermined reasons, resulting in an in-flight loss of control of the airplane at low altitude during approach.

NTSB analysis

The pilot proceeded toward the destination airport in the experimental, amateur-built, twin-engine airplane that was equipped with a customized propeller pitch control system consisting of a standard electronically controlled propeller pitch device, plus various selectors and other limit switches, enabling the propellers to provide reverse thrust capability. After reducing power while on a base leg of the airport traffic pattern with both engines operating within about 30 rpm of each other, the pilot believed the propeller for the right engine went uncommanded into reverse. He said that he had not moved the propeller control guarded switches (which would have been one of several steps required to command the propeller to reverse) and that he did not see any propeller mode light illuminate on the instrument panel. Within 1 to 3 seconds, the airplane was uncontrollable and entered a spin, which, according to a witness, was to the left, and the airplane quickly lost altitude and impacted the ground.

Based on recorded data for various airplane parameters, about the time and location where the pilot likely reduced power, the left engine’s speed increased within 1 second from 4,630 rpm to about 5,330 rpm, consistent with propeller blade angle decreasing toward fine or low pitch, while the right engine’s speed remained about 4,660 rpm to 4,680 rpm. During the next 21 seconds, the left engine’s speed decreased and remained below 3,360 rpm, while the right engine’s speed increased to a maximum of 5,370 rpm, consistent with the maximum value recorded at takeoff.

During that time, the airplane rolled left briefly before rolling to and remaining in a right roll, the maximum extent of which was about 8°. During the next 13 seconds (to the end of the recorded data), the airplane’s indicated airspeed decreased to the lowest value of 47 kts, the left engine rpm values increased slightly while the right engine rpm decreased then stabilized, with both engines ending within 90 rpm of each other. The airplane’s bank angle values and directions changed, continuing in a left bank that increased during the last 4 seconds of the recorded data.

Postaccident examination of the wreckage revealed that the left propeller actuator was retracted, consistent with the propeller being in reverse, which matched the position of the left propeller blades, and the right propeller actuator was extended, consistent with the propeller being in cruise, which matched the position of the right propeller blades. Both propeller control guarded switches were found closed, consistent with the pilot’s statement.

Thus, based on the engine data parameters and the wreckage evidence, when the pilot reduced the throttles in preparation for landing, the variable-pitch left propeller moved uncommanded to, and likely past, the fine or low pitch position into reverse range. The reason for the uncommanded pitch change was not determined.

Although the pilot mistakenly perceived that the issue was with the right propeller and responded by applying left rudder (which would have exacerbated a spin to the left), given the airplane’s low altitude, the pilot had little time to assess and address the condition and likely could not have successfully regained control of the airplane before the airplane impacted the ground.

Text from the NTSB. Open the NTSB docket for ERA24LA208 for the full record.