ERA24LA223 · Beechcraft TC-12B

At the conclusion of a training flight that included two uneventful landings using ground fine/beta or reverse thrust settings, the pilots of the multi-engine, turbo-propeller equipped airplane returned to the home base airport. The pilot flying (PF) reported that he moved each propeller control full forward. He reported that the airplane crossed the runway threshold at 110 knots indicated airspeed (KIAS), or landing reference speed (Vref) plus 22 knots (kts). The airplane touched down with both power levers at flight idle and the “REV NOT READY” annunciator extinguished. After touchdown, the PF moved both power levers aft of the idle gate into beta, then into reverse. Recorded GPS data revealed that the airplane began deviating slightly to the left while decelerating, then about 8 seconds after touchdown, while at 66 kts groundspeed (about 72 KIAS), the airplane quickly veered to the left. The flight crew applied right rudder and right brake, but were unable to correct the airplane’s trajectory. The pilot monitoring (PM) stated that it felt like the airplane was accelerating as it departed the left side of the runway. The GPS data showed that the airplane continued to slow while rolling off the runway. The airplane continued through a 5-ft-deep depression in the grass off the side of the runway before it came to rest with the left main and nose landing gear collapsed, resulting in substantial damage to the fuselage and left wing.

Flight control continuity was confirmed from the cockpit to each control surface, while the rudder and aileron trims were neutral. No discrepancies were noted with either engine power lever or propeller control lever. There was no evidence of pre-impact mechanical failures or malfunctions that would have precluded normal operation of either propeller constant speed unit (CSU) or its attached beta valve; either propeller; or either fuel control unit. No propeller-related issues were reported by the accident flight crew during the two previous landings earlier that day using beta or reverse, nor were any propeller discrepancies reported by other pilots who had recently flown the airplane and employed beta or reverse.

Flight testing of an exemplar airplane equipped with the same engines and propellers revealed that, with each power lever at flight idle and each propeller control full forward, the propeller CSUs were still governing to 120 KIAS, while flight testing with the propeller controls set to 1,700 rpm revealed that the propeller CSUs were still governing to 90 KIAS; safety concerns prevented flight testing below that airspeed. By design, propeller blade movement into beta and reverse cannot occur unless its respective propeller CSU is in an underspeed condition.

Based on the examination of the airplane, there was no evidence of any mechanical anomaly with either propeller CSU or its beta valve that would have prevented the propellers from entering beta and then reverse as commanded if either CSU had been in an underspeed condition. Flight testing confirmed that if the propeller controls were full forward, any airspeed below 120 KIAS would have resulted in both propeller CSUs being in an underspeed condition; the airplane was well below that speed at touchdown.

When the PF moved each power lever aft of the flight idle gate and into beta, then reverse, after touchdown, the propellers should have produced symmetrical reverse thrust. However, the circumstances of the accident flight are consistent with production of asymmetric reverse thrust, which resulted in the flight crew’s inability to maintain directional control and a runway excursion. Based on the postaccident examination of the airplane and propeller systems, the reason for the asymmetric reverse thrust could not be determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset