Accident Details
On August 20, 2020, at about 0900 central daylight time, an Air Tractor AT602 airplane, registration 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.
Pilot Report
The pilot stated that during short final, the airplane required more engine power to reach the runway. Attempting to advance the power control lever (PCL), he found it would not move. He subsequently made a forced landing in a field, sustaining substantial damage to both wings. An initial examination revealed the PCL was stuck near the idle position and could not be moved.
Regarding the accident flight, the pilot reported positioning the PCL friction lock for smooth movement, describing the friction as a “medium range.” He noted that during previous flights he had used beta in flight to quickly slow the airplane. He could not recall if he had placed the PCL into beta range before it became stuck but acknowledged it was possible.
Beta Range and Control Jam
The MVP-50T engine monitor records the actual commanded position of the PCL. Recorded data showed the PCL was moved into beta during the accident flight, and the beta signal remained on after engine shutdown.
According to the Airplane Flying Handbook, the beta range comprises power lever positions from flight idle to maximum reverse. Movement of the PCL into beta reduces propeller blade angle below flight idle. Under certain flight conditions, blade angles below flight idle can cause blade aerodynamic forces that decrease propeller servo pressure, leading to loss of control by the propeller governor. The propeller blade angle may then be aerodynamically forced to the max reverse stop without pilot action.
When the propeller blade angle reaches the reverse stop, the beta ring moves to its most-forward position. If the propeller is not controlled by the governor/beta valve, moving the PCL out of beta positions the beta valve forward without corresponding aft movement of the beta ring. With the beta ring and valve in these positions, further movement of the reversing cable/reversing cam is prevented, potentially jamming the cam box—the intermediate linkage between cockpit levers, propeller governor, and fuel control unit. If the cam follower pin is in the reversing range of the cam, forward PCL movement is blocked.
Aircraft Service Bulletin
Air Tractor Service Letter 364 states that the use of beta mode in flight is not authorized on Air Tractor aircraft and can result in a dangerous situation, leading to loss of control. Beta mode and reverse thrust are authorized only for ground operations, such as taxi to reduce brake wear, or during landing rollout after all three wheels are on the ground.