Beech C35 N620D accident near Rock Springs, Wyoming

Probable cause

Maintenance personnel’s failure to properly secure the propeller pitch control rod castellated nut hardware, which resulted in an in-flight failure of the pitch control bearing and subsequent loss of engine RPM and thrust.

NTSB analysis

The pilot was conducting a personal cross-country flight in the single-engine airplane when the accident occurred. During the flight the pilot and passenger heard a pop sound from the engine. The passenger thought he saw something fly up from the engine and they subsequently saw some residue stuck on the windshield. The passenger described it as five small black specs of a thick viscosity. A few minutes later the airplane could not maintain altitude and the engine rpm had a slight drop even though all of the engine indications were still in the green. The pilot decided to continue to his destination airport. After maneuvering at the destination airport, the maximum engine rpm decreased to 1,250 rpm. The pilot declared an emergency and switched runways; however, due to airport ground traffic, he decided to land off-airport. The airplane contacted powerlines and the ground during the landing.

Examination of the wreckage revealed that the propeller pitch control bearing forward cage and ball bearings had separated and were not found with the wreckage. The bearing inner and outer races were covered in a dark residue. The castellated nuts on the actuator bearing assembly remained attached to the pitch change bolts. However, the nut on the No. 1 pitch control rod had backed out of place and sustained mechanical damage. A cotter pin was not found in the nut. The nut on the No. 2 pitch control rod remained in place and a cotter pin was installed in the nut. The damaged nut was positioned adjacent to the propeller pitch control bearing assembly. The actuator bearing assembly was positioned unevenly on the hub assembly.

Based on this evidence, the damaged castellated nut was likely not properly installed with a cotter pin. The missing cotter pin allowed the nut to back off to the end of the pitch change bolt, resulting in contact and catastrophic damage to the propeller pitch control bearing. It is likely that the unevenly positioned pitch control rods resulted in a reduction of RPM and thrust.

The actuator bearing assembly was removed from the hub; the attachment hole and surrounding area for the No. 1 pitch control rod was covered in a dark oily residue. The No. 2 pitch control rod attachment hole showed clean surfaces. Therefore, the No. 1 pitch control rod was likely loose for some time, resulting in the dark oil residue in the attachment area.

Text from the NTSB. Open the NTSB docket for WPR23LA283 for the full record.