What happened
On July 29, 1994, a Bell 206 L-1 helicopter, registration F-GLGD, was performing an air medical evacuation mission for the SAMU 62 service. The aircraft departed from the Arras Regional Hospital Center, heading toward a site in the commune of Humières for a medical transport mission.
During the final approach to the landing site, the helicopter was flying at an altitude of approximately 15 to 20 meters above a road. At this critical height, the pilot experienced a sudden impact and a loud noise, followed by a significant drop in rotor RPM. Due to the extremely low altitude, the pilot was unable to execute a proper autorotation to arrest the descent. The aircraft struck the ground, resulting in two serious injuries to the pilot and the passenger.
Witnesses at the scene reported hearing a heavy thud and observing both black and white smoke emanating from the engine during the descent.
The investigation
The investigation focused on the engine's performance and structural integrity. The aircraft's engine, an Allison 250 C28B, had completed 4,293.5 hours of operation and 8,407 cycles.
Metallurgical analysis of the engine revealed that the N1 turbine wheel had fractured into two pieces. This failure caused the disk to detach from the generator and be ejected radially, which subsequently destroyed adjacent components. The investigation also examined the N2 turbine stage, which exhibited similar damage patterns.
Findings
- The primary cause of the accident was an uncontained engine failure occurring at a critical altitude, which left the pilot with insufficient time or height to perform a safe autorotation.
- The loss of power was triggered by the disintegration of the N1 turbine wheel.
- The fracture was initiated by fatigue cracks located on the blade foot ring, specifically on the trailing edge side.
- Extensive fatigue cracks were observed throughout the N1 turbine inter-blade spaces, with several blades showing cracks of a few millimeters in length.
- Similar fatigue cracks were also identified on the N2 turbine stage.
- The cracks were determined to be the result of thermal cyclic fatigue, though the specific origin of these cracks could not be determined.