What happened
On June 9, 2007, a private Rockwell Commander 114, registration EC-HST, was performing a local flight under visual flight rules (VFR) departing from Bilbao Airport. The flight was carrying a pilot and one passenger under favorable meteorological conditions.
During the return approach, approximately 5 nautical miles from the airport and at an altitude of 2,300 feet, the pilot noticed a sudden change in engine sound, which became higher-pitched and accelerated. Simultaneously, engine oil sprayed onto the windshield, severely obscuring visibility. The pilot was unable to regulate the engine speed and declared an emergency to the control tower. The aircraft subsequently landed on taxiway C4, crossing runway 12/30 and coming to a stop approximately 150 to 200 meters from the threshold of runway 28. There were no injuries to the crew or passengers, though the aircraft sustained significant damage.
The investigation
The investigation focused on the mechanical failure that led to the propeller separation. Post-flight inspection revealed that the propeller had detached from the engine and was not recovered. Examination of the engine compartment showed that all six bolts securing the propeller to the engine flange had been sheared, with only the threaded studs remaining. One recovered bolt head still contained a fragment of safety wire.
Metallographic analysis of the broken bolts determined that the failure resulted from a simultaneous fatigue process caused by low-level bilateral bending loads. The study concluded that the fatigue was driven by an insufficient preload in the bolts. This lack of adequate preload could have resulted from either an insufficient torque applied during installation or the loosening of the bolts during service due to improper installation of the safety wiring.
Findings
- The propeller separation was caused by the failure of the six bolts connecting the propeller to the engine flange.
- The fatigue process in the bolts occurred during the approximately six hours of flight time accumulated since the propeller and governor had been overhauled and reinstalled.
- The investigation identified two possible causes for the insufficient preload: either the bolts were tightened with insufficient torque during the post-overhaul assembly, or the bolts loosened during flight due to inadequate safety wiring.
- The investigation ruled out the propeller control components and the expired airworthiness certificate as contributing factors.