Air Tractor AT 400 N7300M accident near Grand Forks, North Dakota

Probable cause

The loss of engine power while maneuvering due to the failure of the power turbine blade assembly for reasons that could not be determined based on available evidence.

NTSB analysis

The pilot was maneuvering the airplane during an aerial application over a field. A witness noticed that the airplane’s final turn was different than previous turns. Just before the airplane impacted the ground, the witness noticed a puff of smoke or liquid coming from the airplane. As the witness was a distance away, she was not certain. During the impact, the airplane sustained substantial damage.

Examination of the engine revealed circumferential rubbing marks due to contact between the compressor turbine disk assembly and power turbine (PT) disk assembly. Of the 41 PT blades, 18 were recovered, and those blades exhibited fracture surfaces consistent with overstress fracture under elevated temperatures. Impact marks were identified inside the exhaust duct. The fracture of these blades likely occurred at ground impact when the engine was operational.

It is likely that one or more of the unrecovered PT blades failed for an undetermined reason during the flight and impacted adjacent PT blades and components, including the exhaust duct. The internal engine damage from the released blades caused the PT disc assembly to fail, resulting in a loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR18LA234 for the full record.