Incident Overview
On July 29, 1994, at approximately 14:30 UTC, a Bell 206 Long Ranger-1 helicopter with registration F-GLGD was involved in an accident near Humières, in the Pas-de-Calais department of France. The aircraft was operated by Héli-Ouest for Air Médical Service and was conducting a medical evacuation mission for SAMU 62. The flight originated from the Regional Hospital Center in Arras, with the destination being a road site within the commune of Humières.
Flight Details and Crew
The helicopter carried two individuals: the pilot and a passenger who was a permanent staff member of the hospital center. The pilot, aged 41, held a PPH license obtained in 1976 and had been employed by AIRMES since January 1992. At the time of the accident, he possessed 4,800 total flight hours, including 630 hours on type. His recent experience included 75 hours in the preceding three months and 28 hours in the month prior to the incident.
Accident Sequence
During the final approach to the landing site, the helicopter was at an altitude of 15 to 20 meters above the road when the pilot experienced a sudden impact followed by noise and a significant drop in rotor RPM. Due to the low altitude, the pilot was unable to execute a safe autorotation. The aircraft impacted the ground, resulting in serious injuries to both occupants. Witnesses reported hearing a dull noise accompanied by black and white smoke emanating from the engine area.
Technical Investigation
The aircraft had 4,853 total flight hours, with 1,200 hours since its last major overhaul. The Allison 250 C28B engine had accumulated 4,293.5 hours and 8,407 cycles. Metallurgical examination revealed that the N1 turbine wheel had fractured into two parts. The disk detached from the generator rotor and was ejected radially, destroying adjacent components. The failure initiated with fatigue cracks located on the blade root ring, specifically on the trailing edge side. Similar fatigue cracks were also observed on the second-stage turbine disk. These cracks propagated radially inward due to high-cycle thermal fatigue.
Conclusion
The accident resulted from a contained engine failure occurring at a critical altitude that precluded a safe autorotation. The loss of power was caused by the rupture of the N1 turbine wheel, initiated by fatigue cracks on the blade root ring. The specific origin of these initial cracks could not be determined.