Accident Overview
The airplane collided with ground obstacles during a forced landing after the pilots could not maintain altitude. The loss of altitude occurred when one propeller blade and part of the propeller hub from the right engine separated in flight.
Sequence of Events
The flight instructor (CFI) and a student had practiced two simulated engine-out maneuvers. As the CFI reached for the mixture control to initiate a third simulation, a loud bang was heard. The airplane yawed violently to the right and began losing altitude. The CFI turned toward his home field but struggled to maintain altitude. Realizing he could not reach the airport, he elected to make an off-field landing. The only clear spot he saw was a cemetery. During the landing, the airplane collided with monuments and a wood chip pile before coming to rest next to a mausoleum.
Propeller Hub Examination
Investigators determined that the right propeller hub had fractured and separated. One blade and the corresponding part of the hub were not recovered. The hub fractured as a result of fatigue cracking emanating from one of the grease-fitting holes. The fatigue cracking originated in the area of the grease-fitting threads closest to the interior surface of the hub, at the intersection between the grease-fitting hole and the chamfer. No material or manufacturing defect was identified as the source of the fatigue cracking. The fatigue cracking originated just outside the thread remnants that were plastically deformed during a chamfering process.
Fatigue Analysis
The grease fitting was located in a region where the wall of the hub was approximately at its thinnest and where there was a slight change in profile along the axis of the propeller. These geometrical factors would have led to high stresses at the edges of the hole. Chamfers were retrofitted to the grease-fitting holes by the manufacturer at both the interior and exterior surfaces of the hub in response to fatigue fractures that occurred in three-bladed propellers. This accident was the first occurrence of fatigue emanating from a grease-fitting hole for a two-bladed propeller. The chamfering was apparently introduced to smooth the stress distribution around the grease-fitting holes. However, the chamfer at the interior surface, while removing the stress concentration associated with the change in wall profile, also decreased the amount of material in the wall, leading to higher overall stresses. The stress concentration associated with the grease-fitting hole and the chamfer, coupled with the location in a position where the hub wall was very thin, were likely sufficient to initiate fatigue cracking. In more recently manufactured hubs, the grease-fitting holes have been relocated to positions where the hub wall is much thicker.