On April 29, 1983, a Beech 77 (registration N67140) operated by Fancher Flyways was involved in an aviation accident near Renton, WA. Investigators recorded the probable cause as: The locking nut on the #1 cylinder exhaust valve adjusting screw was loose, allowing the adjusting screw to back out of the rocker arm due to pushrod action during takeoff, which prevented the exhaust valve from opening. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a post-overhaul engine run-in takeoff, the aircraft experienced gradual power loss and rough running followed by total power loss. The pilot attempted to return but the aircraft impacted the runway nose-first. Investigation found a loose locking nut on the #1 cylinder exhaust valve adjusting screw.
Accident Overview
An aircraft was taking off for a one-hour engine run-in following a top overhaul. Shortly after liftoff, the engine began to experience a gradual loss of power and ran roughly. The pilot attempted to turn back to the runway, but a total loss of power occurred. The aircraft had turned 45 degrees when the nose dropped, the wings rotated 180 degrees to the left, and the aircraft impacted the runway nose-first.
Crew and Maintenance Information
The mechanic who performed the removal and reinstallation of the cylinders for the top overhaul was occupying the left seat. The maintenance supervisor, who was the pilot-in-command, was in the right seat.
Findings
Examination revealed that the locking nut on the #1 cylinder exhaust valve adjusting screw was loose. The adjusting screw had backed out of the rocker arm due to pushrod action during takeoff, which negated rocker arm action and prevented the exhaust valve from opening. The locking nuts for all other valve adjusting screws were found to be properly torqued.
Probable cause
The locking nut on the #1 cylinder exhaust valve adjusting screw was loose, allowing the adjusting screw to back out of the rocker arm due to pushrod action during takeoff, which prevented the exhaust valve from opening.