What happened
During a training flight involving two simulated engine-out maneuvers, a Cessna (type not specified) experienced a sudden mechanical failure. As the flight instructor reached for the mixture control to begin a third simulation, a loud bang occurred, and one propeller blade along with part of the right engine's propeller hub separated in flight. The aircraft yawed violently to the right and began losing altitude.
The instructor attempted to return to his home airfield but found it impossible to maintain sufficient altitude. Consequently, he performed a forced landing in the only available clear area, which was a cemetery. During the landing, the aircraft collided with a wood chip pile and various monuments before coming to rest adjacent to a mausoleum. There were no fatalities reported in the accident.
The investigation
Investigators examined the right propeller hub and determined that it had fractured and separated. While one blade and a portion of the hub were not recovered, the remaining components revealed that the fracture originated from fatigue cracking. This cracking began at the intersection of a grease-fitting hole and a chamfer, specifically near the threads closest to the interior surface of the hub.
Technical analysis showed that the fatigue crack started just outside thread remnants that had been plastically deformed during a chamfering process. The grease fitting was located in a region where the hub wall was at its thinnest and where there was a slight change in the axial profile of the propeller. This specific geometry created high stress levels at the edges of the hole.
Findings
- The fracture was caused by fatigue cracking originating from a grease-fitting hole.
- High stresses were concentrated at the edge of the hole due to the thinness of the hub wall and the change in profile.
- Although the manufacturer had retrofitted chamfers to both the interior and exterior surfaces of the hub to reduce stress, this modification decreased the amount of material in the wall, potentially increasing overall stress.
- This event marked the first instance of fatigue cracking emanating from a grease-fitting hole in a two-bladed propeller.