Bell 206B N5743W accident near Cresco, Iowa

Probable cause

The loss of engine power due to the high-cycle fatigue failure of the third-stage turbine wheel.

NTSB analysis

The commercial pilot reported that the helicopter departed with 30 gallons of fuel and about 90 gallons of chemical to spray a soybean field. About 5 minutes into the application flight and while 3 to 5 ft above the soybeans, the pilot heard a loud screech for about 2 seconds and pulled up to gain altitude. The pilot then heard the low rotor rpm horn and noticed that the main rotor rpm was at 85 percent. The pilot initiated an autorotation to a waterway between two cornfields. Upon touchdown on uneven terrain, the main rotor blades contacted and severed the tailboom, and the helicopter then came to rest upright.

Disassembly of the engine revealed that the third-stage turbine wheel was missing five airfoils (blades), that several other airfoils were damaged, and that portions of the shroud were liberated. Materials examination of the third-stage turbine wheel revealed that the five airfoils had cracked in fatigue, initiating from the trailing edge root area and progressing forward until final fracture occurred in overload. All five cracks had initiated and progressed in high-cycle fatigue. Extensive testing by the engine manufacturer could not identify a single root cause for the liberation of the airfoils. As a result of the investigation, the engine manufacturer planned to release a new wheel design and process improvements to improve the robustness and capability of the turbine wheel.

Text from the NTSB. Open the NTSB docket for CEN15LA370 for the full record.