Final Report: Turbine blade fatigue felled Bell 407

 - November 25, 2009, 5:59 AM

Bell 407, DeLand, Fla., Jan. 7, 2008– A total loss of engine power due to the fatigue failure of a third-stage turbine wheel blade downed the single-engine helicopter, the NTSB determined. The 407–operating as an EMS transport– was nearing its patient pickup destination when the pilot reported hearing a loud bang, which was followed by a yaw to the left. The pilot also noted squealing metallic sounds along with the sounding of the Fadec warning horn, the low-rotor-rpm warning horn and the illumination of several warning lights, before he initiated an autorotation to an unfinished roadway.

A witness on the ground reported seeing a “big orange glow” and sparks emitting from the right side of the helicopter before it began its descent. Upon landing, the pilot and paramedic escaped the helicopter without injury. The helicopter was substantially damaged. The engine was removed and returned to the manufacturer for examination.

Inspection of the turbine section revealed that a third-stage turbine wheel blade had cracked, exhibiting damage consistent with fatigue. Review of data from the Fadec showed that at an undetermined time, the 407’s power turbine speed had reached 104.56 percent, and that the unit had been operated above 102 percent for a cumulative 138.43 seconds. Finally, the engine torque had previously reached 108.4 percent, and had been above the torque limit for 0.1 seconds.