What happened
During a flight intended to practice engine failure procedures at 6,000 feet mean sea level, the certified flight instructor (CFI) shut off the fuel using the right fuel selector valve. Following this action, a propeller blade separated from the right engine, and an unidentified part penetrated the aircraft windshield. The CFI reported that the right engine nearly disintegrated and detached from the aircraft within two seconds.
The resulting loss of altitude prevented the crew from maintaining level flight. Although the pilot maintained sufficient airspeed to keep the aircraft under control during the descent, a forced landing in a field was necessary. During the maneuver, the Cessna (aircraft type not specified, but context implies single engine) impacted the side of a creek and came to rest right side up in approximately three feet of water. The occupants were not injured.
The investigation
The NTSB Materials Laboratory examined the Hartzell two-bladed propeller hub and various components from the propeller assembly. The investigation focused on the right engine's hub, which was a left-hand rotating (counterclockwise viewed from the rear) propeller.
Inspection of the hub revealed a fracture around the socket for blade #1, extending through both the forward and aft halves of the hub. The fracture surface exhibited features consistent with fatigue crack progression originating generally away from the grease fitting hole. This fatigue region measured approximately 3.1 inches in length from the grease fitting hole to near the forward centerline of the blade socket.
Findings
- The propeller hub suffered a fracture around the blade socket.
- Fatigue crack progression was identified moving away from the lubrication/grease fitting hole.
- A recent Alert Service Bulletin (HC-1SB-61-297) issued by Hartzell Propeller Inc. on September 17, 2007, had previously addressed blade separation events originating in the lubrication hole of left-hand rotating propellers.