Investigation Summary
The aircraft was nearly brand new, having been manufactured only five weeks prior to the accident. While climbing from 6,000 to 8,000 meters, the No. 4 propeller pitch control mechanism malfunctioned due to a manufacturing defect. This failure caused the propeller blades to move into a reverse thrust position, creating massive drag on the right wing. The crew, faced with a sudden and violent yaw and roll, were unable to immediately identify which of the two right-wing engines was failing and feathered both propellers on that side. The aircraft entered a steep descent, during which the aerodynamic loads exceeded the structural limits of the airframe, tearing off both ailerons. Although the pilots briefly regained some level of control at low altitude, the lack of lateral control surfaces made the final impact inevitable.
Final Conclusions
The investigation concluded that the primary cause was a defect in the oil sealing rubber gasket on the AV-68I propeller's oil transporter. This defect was traced back to incorrect assembly at the factory. The faulty gasket led to a failure of the pitch control mechanism and the speed regulator, which allowed the propeller to transition into reverse thrust while in flight. This condition was impossible for the crew to detect or rectify through standard cockpit procedures and could only have been identified during a detailed physical inspection of the propeller assembly.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
AV-68I Propeller
The specific propeller model that suffered a pitch control mechanism failure due to a manufacturing defect.
Propeller Pitch Control
The system responsible for adjusting blade angle; its failure led to uncommanded reverse thrust.
Ailerons
Flight control surfaces that detached from the aircraft due to excessive aerodynamic stress during the uncontrolled descent.
Interesting Facts
- 01The aircraft had only 154 flight hours and 68 pressurization cycles at the time of the crash.
- 02The No. 4 engine initiated reverse thrust during the climb, a catastrophic failure mode for a turboprop at altitude.
- 03Both ailerons were torn off the wings due to extreme aerodynamic loads during the emergency descent.
- 04The investigation used laboratory simulations to confirm the failure was caused by a defect in an oil sealing rubber gasket.
- 05The aircraft was manufactured on February 26, 1963, and had been in service for just over a month.
Safety Actions & Advisories
Propeller Assembly Quality Control
The investigation highlighted that the defect was caused by incorrect assembly at the factory, leading to stricter oversight of the AV-68I propeller manufacturing process.