Air Tractor AT602 N604MG accident near Inverness, Mississippi

Probable cause

The pilot’s decision to move the power control lever into beta during flight, which resulted in the power control lever and associated linkage becoming stuck in the beta position.

NTSB analysis

The turbine-powered agricultural airplane was on short final to land at the private airstrip. In attempt to slow down the airplane, the pilot actuated the power control lever (PCL) into beta, which caused the PCL, cam box, and associated linkage to get stuck in place. The airplane required more engine power to make the runway so he attempted to advance the PCL, but it would not move. The pilot made a forced landing in a field short of the runway, the airplane came to rest upright, and sustained substantial damage to both wings.

The engine data revealed that the PCL was moved to beta while the airplane was on short final. The beta signal was still on after engine shutdown, which indicates that the cam box was likely stuck in beta.

A service letter issued by the airplane manufacturer states that the use of beta mode in flight is not authorized and can result in a dangerous situation, leading to a loss of control.

In this event, when the PCL was actuated into beta in flight and the propeller servo pressure dropped to maintain control of the propeller, the servo pressure went into a negative range, and the propeller blade angle was quickly forced to the max reverse stop, which allowed the beta ring to move to the most forward position. Since the beta valve does not allow this amount of movement, the additional movement of the beta ring was transferred to the cam box and the cam box jammed in place.

Text from the NTSB. Open the NTSB docket for CEN20LA353 for the full record.