N620D

On July 20, 2023, about 1255 mountain daylight time, a Beech C35 airplane, N620D, was substantially damaged when it was involved in an accident near Rock Springs, Wyoming. The pilot and passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal cross-country flight.

The pilot reported that during the flight he activated an instrument flight plan and climbed to 15,000 ft mean sea level (msl) to get above the clouds. During the climb the pilot and passenger heard a pop sound from the engine. The passenger thought he saw something fly up from the engine that subsequently stuck on the windshield. The passenger described it as five small black specs of a thick viscosity. The pilot checked all of the engine instruments, which were in the green. About 5 minutes later, the airplane could not maintain altitude and was cleared down to 14,000 ft msl. The engine rpm had a slight drop, and all the engine indications were still in the green. At this point the pilot decided to continue to his destination airport. After the airport was in sight, he cancelled the instrument flight plan and entered a straight-in approach for runway 3. About a mile from the airport, he noticed that he was too high for the approach and performed a slipping maneuver for about 3 seconds to decrease altitude. He stopped the maneuver before getting to his desired altitude because of the uncomfortable descent rate. The engine rpm was now 1,250 rpm, and he declared an emergency and announced that he was now landing on runway 27. His altitude had dropped considerably, and he decided to land on a taxiway but then saw an airplane holding short of the runway and opted to turn left for an off-field landing. The airplane impacted powerlines and subsequently impacted the ground. The powerlines were about 300 ft northeast of the main wreckage.

The data from an onboard engine monitoring instrument was downloaded and revealed the entire accident flight. The data showed a drop of rpm from 2,500, while the airplane was at about 15,000 ft, and for the remaining flight the rpm remained between about 1,600 and 2,300. During the last 15 minutes of the flight, the rpm dropped then spiked about 3 minutes before the end of the data, along with an immediate rise in manifold pressure.

A postaccident 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. One of the nuts was secured to the bolt with a cotter pin and the other nut was attached to the bolt and had mechanical damage. Threads were visible below the damaged nut and a cotter pin could not be identified. The damaged nut was positioned adjacent to the propeller pitch control bearing assembly. The actuator bearing assembly was positioned unevenly on the hub assembly. (See figure 1.)

Figure 1. Actuator bearing assembly and the No. 1 pitch control bolt and nut.

The brass-colored metallic material was removed from the damaged No. 1 pitch control rod castellated nut. (See figure 1.) The end of the pitch control bolt was visible halfway through the nut. The nut was removed, and the castellated side of the nut 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 and 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.

The No. 1 blade was removed from the hub and grease was noted throughout the interior. 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 their surfaces showed light spalling signatures. The No. 2 blade was removed from the hub. The 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 were cleaned and their surfaces showed light spalling signatures.

The propeller electric motor assembly was attached to an electrical source and operated normally.

Figure 2. Operation and Service Instructions, 215 Propeller excerpt.

Review of the airplane’s maintenance records revealed that the propeller overhaul had been completed on September 15, 2021, about 252 flight hours before the accident. The last inspections on the airplane, engine, and propeller had been completed on December 1, 2022, about 95 flight hours before the accident. No additional records documenting the overhaul were available for review, and the personnel who completed the overhaul could not be reached for interview.