N604MG

On August 20, 2020, about 0900 central daylight time, an Air Tractor AT602 airplane, N604MG, was substantially damaged when it was involved in an accident near Inverness, Mississippi. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot stated that when he was on short final, the airplane required more engine power to reach the runway. He attempted to advance the power control lever (PCL), but it would not move. He subsequently made a forced landing in a field and the airplane sustained substantial damage to both wings. An initial examination of the PCL revealed that it was stuck near the idle position and could not be moved.

The pilot reported that during the accident flight, he positioned the PCL friction lock such that the lever would move smoothly and characterized its friction as a “medium range.” He said that during some previous flights, he had used beta in flight to quickly slow the airplane. He could not recall if he had put the PCL into beta range before it became stuck but said it was possible.

The actual commanded position of the PCL provides an indication to the airplane’s MVP-50T engine monitor. The recorded data revealed that the PCL was moved into beta during the accident flight. Also, the beta signal was still on after engine shutdown.

According to the Airplane Flying Handbook (FAA-H-8083-3B), the beta range of operation consists of power lever positions from flight idle to maximum reverse. Beginning at power lever positions just aft of flight idle, propeller blade pitch angles become progressively flatter with aft movement of the power lever until they go beyond maximum flat pitch and into negative pitch, resulting in reverse thrust.

To move the PCL into beta, a pilot moves the thumb latch on the top of the PCL forward, which raises the idle stop and allows the PCL to move aft of the anti-reversing plate. A fence on the power quadrant keeps the idle stop in line with the anti-reversing lock plate to prevent the PCL from inadvertently moving into beta.

When the PCL is actuated aft of the idle stop position and into the beta range it reduces the propeller blade angle below the flight idle position. Under some flight conditions blade angles below flight idle result in blade aerodynamic forces that cause the propeller servo pressure to decrease and the propeller to become uncontrolled by the propeller governor. In this condition the propeller blade angle would be aerodynamically forced – without pilot action – to the max reverse stop.

When the propeller blade angle moves to the reverse stop, the beta ring moves to the most-forward position. If the propeller is not controlled by the governor/beta valve at this point, movement of the PCL out of beta range moves the beta valve to its max forward stop without corresponding aft movement of the propeller beta ring. With the beta ring and beta valve in those positions, no further movement of the reversing cable/reversing cam would be possible, and the cam box could get jammed. The cam box is the intermediate linkage between the cockpit levers, the propeller governor, and the fuel control unit. If the cam follower pin is in the reversing range of the cam when this occurs, it will prevent forward movement of the PCL.

Air Tractor Service Letter 364 states in part:

The use of beta mode in flight is not authorized on Air Tractor aircraft and can result in a dangerous situation, leading to aircraft loss of control…beta mode and reverse thrust settings are only authorized for ground operations. These settings may be used during taxi operations to reduce brake system wear and to reduce taxi speeds. Beta mode and reverse thrust may be used during the landing rollout, but only after all three wheels are on the ground.