What happened
While the aircraft was engaged in normal cruise flight, a structural failure occurred involving the propulsion system. A portion of one propeller blade detached from the assembly while the aircraft was airborne. Despite this significant mechanical failure, the pilot maintained control and successfully executed an emergency landing in an open field.
During the subsequent landing roll, the aircraft deviated from its intended path and collided with a haystack located near the landing area. This secondary impact caused additional damage to the airframe beyond that resulting from the initial propeller failure.
The investigation
Following the accident, investigators conducted a thorough examination of the recovered propeller blade to determine the root cause of the separation. The physical inspection revealed that the blade had separated at blade station 28.5.
To understand the mechanics of the failure, a metallurgical examination was performed on the fractured surface of the propeller blade. This specialized analysis provided critical insights into the material's condition and the nature of the stress that led to the breakage.
Findings
The metallurgical evidence indicated that the propeller blade did not fail due to an overload or manufacturing defect in the bulk material. Instead, the failure originated from fatigue cracking. The examination revealed that a pre-existing nick on the leading edge of the blade served as the stress concentrator that initiated the fatigue process.
This defect allowed cracks to propagate over time under normal operating loads until the remaining cross-section could no longer support the aerodynamic forces, resulting in the sudden separation of the blade portion during cruise flight. The presence of this nick was identified as the primary factor contributing to the structural failure.