Pilot Debrief

Air Tractor AT 400 near Grand Forks, ND — 2018-08-21

Final reportWPR18LA234
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Date
2018-08-21
Location
Grand Forks, ND, USA
Airport
MN81
Aircraft
Air Tractor AT 400
Registration
N7300M
Category
Airplane
Highest injury
Serious
Fatalities
0
Phase of flight
Maneuvering

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 narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have aborted the final turn if aware of engine issues.

NTSB coding

Evidence available

  • Photos
  • 8 docket documents
View NTSB final reportView NTSB docket

Docket documents8

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