Approximately 40 hours before the accident the airplane had gone in for maintenance. The owner stated that the crankshaft oil seal was replaced due to an oil leak.
He did not recall seeing a cotter pin ever installed on the hub nut, noting instead the use of a cadmium plated bolt and “Nyloc” nut.
The pilot reported that the accident flight’s ground run-up, takeoff, and initial climb were uneventful, with the airplane maintaining a climb rate of 500 feet per minute, at an engine speed of 2,300 rpm.
About five minutes later, he felt a low frequency vibration, and assuming it was an engine speed-related harmonic, reached forward to adjust the throttle. Just as he reached forward, the engine speed surged to 4,000 rpm, and he immediately retarded the throttle.
He could not see any propeller movement, and noted the engine did not appear to be producing thrust. In an attempt to diagnose the problem, he advanced the throttle again, and the engine surged to 4,200 rpm.
The pilot realized that the propeller had separated from the engine, and began to configure the airplane for a forced landing. The airplane was difficult to control because the center of gravity was altered by the loss of the propeller, but the pilot made a forced landing in a rocky field five miles from the airport. He received minor injuries.
The airplane sustained substantial damage to the right wing and firewall during the accident. The propeller and its associated hub mounting hardware were not recovered. The threaded portion of the crankshaft had been flattened by the propeller’s hub, indicating that the hub nut was in place but had backed off during the flight, most likely because the incorrect torque had been applied during its reinstallation.
The NTSB determined the probable cause was the separation of the propeller from the engine during initial climb due to the incorrect torque applied to the propeller hub nut during maintenance.
NTSB Identification: WPR13LA191
This April 2013 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.