AIR TRACTOR AT602 near Inverness, MS — 2020-08-20

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 narrative

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.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have avoided using beta mode during flight to prevent PCL from jamming.

NTSB coding

• Sys/Comp malf/fail (non-power) · Approach-IFR final approach

Evidence available

• Photos
• 11 docket documents

Docket documents11

• PILOT/OPERATOR AIRCRAFT ACCIDENT REPORT, NTSB FORM 6120.1form
• RELEASE OF AIRCRAFT WRECKAGE, NTSB FORM 6120.15form
• STATEMENT OF PARTY REPRESENTATIVES TO NTSB INVESTIGATIONform
• AIR TRACTOR SERVICE LETTER 364other
• AIRPLANE FLYING HANDBOOK EXCERPT - BETA RANGEother
• CORRESPONDENCE - PILOTother
• INVESTIGATION SUMMARYother
• PRATT & WHITNEY CANADA PT6A-60 CONTROL RIGGINGother
• TABULAR ENGINE DATAother
• PHOTOSphotos
• COMPUTED TOMOGRAPHY SPECIALIST'S FACTUAL REPORTreport

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