Amateur-built COZY MARK IV N95SF accident near Cullman, Alabama

Probable cause

The pilot’s exceedance of the airplane’s critical angle of attack while maneuvering at low altitude, which resulted in an aerodynamic stall and subsequent loss of control. Contributing to the accident was the pilot/owner’s improper installation of engine mount hardware during recent maintenance, which resulted in the partial separation of the engine and likely degraded controllability of the airplane.

NTSB analysis

A witness reported that the pilot/owner of the experimental amateur-built airplane had recently replaced the aircraft’s propeller and had removed the engine to install a new propeller governor. He indicated that, following the completion of maintenance, the pilot informed him that he intended to fly over the witnesses’ property and perform a flyby. After performing a low pass over the witness’s field, the pilot initiated a climbing left turn, during which several witnesses observed an unusual “wiggle” in the rear of the airplane and heard a droning noise from the propeller with fluctuating pitch. The airplane then stalled and subsequently impacted a field in a level attitude.

The witness reported that the pilot did not perform any unsafe flight maneuvers and both witness statements and recorded data indicated the engine was producing power throughout the flight.

Postaccident examination of the engine revealed evidence of power at the time of impact and no indications of preimpact internal mechanical malfunctions. However, during recent maintenance the engine had been removed and reinstalled with improper washers. Although the engine remained within the cowling, it was partially separated from the engine mount. All four engine mount bolts and nuts were present and tight, but the required bushings and large-area washers on the engine side of the mount were missing. Both upper engine vibration isolator bolts lacked the required washers beneath the nuts, and the upper two engine mount bolt/nut assemblies appeared to have pulled through their respective rubber vibration isolators. The two lower engine mount bolts were severely bent and exhibited thin, non-aircraft-grade washers with hex-shaped extrusion marks indicative of the nuts bearing against the washers under load. Two of the nonstandard washers were recovered within the engine cowling near the lower attachment points.

This improper installation resulted in the partial separation of the engine which likely degraded the handling characteristics of the airplane; however, the investigation could not determine the degree of degradation. The airplane’s degraded controllability, combined with the pilot’s exceedance of the airplane’s critical angle of attack when maneuvering, resulted in an aerodynamic stall and loss of control at an altitude too low for recovery.

Postmortem toxicological testing detected ethanol at 0.038 g/dL in the pilot’s blood and at 0.01 g/dL in his urine; ethanol was not detected in vitreous fluid. Based on the fact that ethanol was not detected in vitreous fluid, that ethanol was detected only at a very low level in urine, and that markers of likely postmortem microbial activity were detected in blood, it is likely that at least some of the detected ethanol was from postmortem sources and it is unlikely that ethanol effects contributed to the accident.

Additionally, toxicology results indicated that the pilot had used the sedating antihistamine medication diphenhydramine and could have been experiencing associated cognitive and psychomotor impairment at the time of the accident. However, due to diphenhydramine’s potential for postmortem redistribution, the investigation was not able to determine whether the pilot may have been impaired by diphenhydramine at the time of the accident.

Text from the NTSB. Open the NTSB docket for ERA23FA382 for the full record.