Accident Overview
On July 20, 2023, at approximately 1255 mountain daylight time, a Beech C35 airplane, registration N620D, was substantially damaged during an accident near Rock Springs, Wyoming. The pilot and passenger suffered minor injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal cross-country trip.
Pilot Report
The pilot stated that during the flight, he activated an instrument flight plan and climbed to 15,000 feet mean sea level to get above clouds. During the climb, the pilot and passenger heard a pop from the engine. The passenger reported seeing something fly up from the engine that stuck to the windshield, describing it as five small black specks of thick viscosity. The pilot checked engine instruments, which indicated normal ranges. About five minutes later, the airplane could not maintain altitude and was cleared down to 14,000 feet msl. Engine rpm dropped slightly, but all indications remained in the green. The pilot decided to continue to the destination airport. After the airport was in sight, he canceled the instrument flight plan and entered a straight-in approach for runway 3. About a mile from the airport, he realized he was too high and performed a slipping maneuver for about three seconds to decrease altitude. He stopped before reaching his desired altitude due to an uncomfortable descent rate. Engine rpm was then 1,250, and he declared an emergency, announcing a landing on runway 27. His altitude dropped considerably, and he decided to land on a taxiway but then saw an airplane holding short and opted to turn left for an off-field landing. The airplane impacted powerlines (about 300 feet northeast of the main wreckage) and subsequently struck the ground.
Engine Monitoring Data
Data from an onboard engine monitoring instrument was downloaded and showed the entire flight. It indicated a drop in rpm from 2,500 while at about 15,000 feet msl, with rpm remaining between about 1,600 and 2,300 for the rest of the flight. During the last 15 minutes, rpm dropped then spiked about three minutes before the data ended, accompanied by an immediate rise in manifold pressure.
Postaccident Examination
Postaccident inspection 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. One nut was secured with a cotter pin; the other nut had mechanical damage, with threads visible below it and no identifiable cotter pin. The damaged nut was adjacent to the propeller pitch control bearing assembly, and the actuator bearing assembly was positioned unevenly on the hub assembly. Brass-colored metallic material was removed from the damaged No. 1 pitch control rod castellated nut. The end of the pitch control bolt was visible halfway through the nut. The nut was removed and its castellated side was damaged; the cotter pin was not found, and the bolt's cotter pin hole was clear. The No. 2 pitch control rod castellated nut was secured with a cotter pin. The actuator bearing assembly was removed from the hub; the attachment hole and surrounding area for the No. 1 pitch control rod were covered in dark oily residue, while the No. 2 hole showed clean surfaces. The No. 1 blade was removed from the hub, and grease was noted throughout. Both yoke lock nuts remained secured to the pitch control bolts and were safety-wired. The No. 1 pitch change block was undamaged. Both pitch change blocks had stamped numbers “1” and “2” but were found on opposite blade pitch assemblies. The No. 1 blade ball bearings and race halves were cleaned and showed light spalling signatures. The No. 2 blade was removed from the hub; its pitch change block (blade bushing) remained attached to the hub end, and the pitch change block, bolt, and yoke had impact damage. Grease was noted throughout. The No. 2 blade ball bearings and race halves also showed light spalling signatures. The propeller electric motor assembly was attached to an electrical source and operated normally.
Maintenance Records
Review of the airplane’s maintenance records indicated that the propeller overhaul was completed on September 15, 2021, about 252 flight hours before the accident. The last inspections on the airplane, engine, and propeller were completed on December 1, 2022, about 95 flight hours before the accident. No additional records documenting the overhaul were available, and the personnel who completed the overhaul could not be reached for interview.