Cessna 310 N86H accident near Hayden, Idaho

Probable cause

The pilot’s failure to maintain adequate airspeed and his exceedance of the airplane’s critical angle of attack, which resulted in a loss of control and subsequent stall during a precautionary landing. Contributing to the accident was the improper maintenance performed by unknown personnel, which resulted in a partial loss of throttle control of the left engine.

NTSB analysis

According to the pilot, who did not hold a multi-engine rating, as the airplane climbed through 4,500 ft msl, he noted that the manifold pressure gauge showed asymmetric manifold pressure readings between the two engines; the left engine appeared unresponsive and could not be reduced below 21 inches of mercury with movement of the throttle lever. The pilot briefly attempted to troubleshoot the unresponsive engine before he decided to return to the departure airport for landing. During the final approach, the airspeed decayed to 80 mph. The pilot stated that he “pushed the props forward instantly;” however, the airplane immediately rolled left and the stall horn sounded as it descended into terrain.

A video recording from a surveillance camera showed the airplane bank left and right before it pitched up, rolled to the left, and descended out of view of the camera. Witnesses near the accident site reported that the airplane appeared slow while on approach to landing. One witness, who responded to the accident site, reported that the pilot told him that the “throttle stuck.”

Postaccident examination of the engines did not reveal any preimpact mechanical failures or malfunctions that would have precluded normal operation. Additionally, both the left and right propellers exhibited chordwise, rotational scoring, which was consistent with both propellers rotating at impact.

Examination of the airframe revealed that the hardware used to attach the left engine throttle cable to its mounting bracket did not conform to the manufacturer's design. Instead of the throttle conduit, which houses the throttle cable, secured using a singular rubber-cushioned support clamp, the accident airplane had a piece of aluminum tubing wrapped around and affixed to the outside of the throttle cable conduit with 2 worm-drive band clamps. The throttle cable and aluminum portion were attached to the bracket with a rubber cushioned support clamp. When the throttle cable was moved by hand, the cable, aluminum, and cable conduit slid through the support clamp. This installation likely allowed the throttle cable assembly to move though the support clamp freely, with either band clamp moving against the support clamp or throttle conduit securement nut, restricting the movement of the travel of the throttle cable itself, or inducing enough slack within the throttle cable to not rotate the throttle control arm on the fuel control when the cockpit control throttle control lever was moved.

Because the airplane’s maintenance records were not made available for review, the investigation could not determine when the left-engine throttle cable assembly was repaired and modified from the original design.

Text from the NTSB. Open the NTSB docket for WPR24LA034 for the full record.