CEN24LA239 · Thrush Aircraft Inc S2R G10

The airplane departed from the operator’s airstrip to apply a liquid chemical to a rural wheat field, in an area with rolling hills. After the completion of the A-B pass and during the descent of the C pass, the engine abruptly accelerated on its own. The pilot then decided to climb out of the field to gain some altitude and assess the situation. The pilot noticed that the exhaust gas temperature and the engine torque were both high. Due to the abrupt engine fluctuations and the abnormal engine indications, the pilot decided to return to the airstrip. While maneuvering the airplane for a left downwind leg to the turf runway, the engine lost all power. The pilot maneuvered the airplane for a forced landing and performed an emergency hopper dump. After the successful completion of the emergency hopper dump, the pilot performed an off-field landing on a rural dirt road. During the landing, the airplane departed the road and came to rest upright, partially in a grass ditch. The airplane sustained substantial damage to the rudder. Postaccident examination of the airframe revealed no preimpact anomalies that would have precluded normal operation. A computerized tomography (CT) examination of the propeller governor (PG) found no internal abnormalities or contamination. The PG was further examined at the manufacturer; visual examination revealed an unusually large offset field adjustment on the maximum and minimum stop settings of the propeller speed lever. Further investigation revealed that the speed setting lever had been re-indexed on the speed setting shaft spline by two spline tooth widths relative to the manufacturer’s original factory setting, which was an unapproved field adjustment that a mechanic previously performed. During the acceptance test procedure of the as-received PG, the speed limit settings were not within factory or operational limits. The PG was returned to factory settings by adjusting the speed adjusting set screws and the follow-on test found the PG within operational limits. Examination of the engine oil passages and the in-line fittings of the accessory gearbox found a small red piece of foreign object debris (FOD) within the barrel of the check valve, between the valve ball and seat. The FOD was consistent with the torque stripe paint material that was applied to many of the external air, oil, and fuel line connectors on the engine. The CT inspection of the check valve showed the ball was not properly seated. Given the size of the FOD, it would not have been able to flow past the oil screen in the PG mounting flange, and so was not introduced upstream of the oil screen or the PG. FOD clogging this orifice would prevent oil from being correctly ported downstream to both the Negative Torque Sensing (NTS) system and the reset piston within the PG; the lack of oil pressure available to the PG hydraulic reset piston would result in an increase to the PG governing setpoint. According to the engine manufacturer, this may result in a perceived shift in engine rpm that could be overcome by re-indexing the splined joint between the speed setting lever and the speed setting shaft by two spline tooth widths. It is likely that the FOD blocking the check valve orifice passed through the orifice during the flight, enabling oil pressure to be re-established to the PG reset piston. The sudden return of oil pressure at the PG reset piston would result in a decrease of the PG governing setpoint of 5-8% rpm and subsequently create an abnormal engine control system interaction with the fuel control unit (FCU), resulting in thrust fluctuations. The investigation was unable to determine how or when the FOD entered the PG oil system. Although torque stripe paint material is used to enhance the safety of external air, oil, and fuel line connectors on both turbine and reciprocating engines, it can also become a FOD hazard. Honeywell Aerospace Technologies Service Bulletin TPE331-72-2168 recommends the incorporation of a new check valve design, which relocates the orifice downstream of the check valve’s ball/seat and adds an integral filter screen contained in a flange with a diameter of 0.070 inches. Given the dimensions of the FOD recovered from the accident check valve, it is likely that the flow of oil would not have been completely blocked and oil pressure would have been maintained on the PG reset piston had the new check valve been installed, potentially mitigating the mechanic’s improper repair and adjustment of the PG speed lever.

Verbatim NTSB analysis and probable cause from the NTSB dataset