CEN24LA239

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 ina 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.

On June 17, 2024, about 0710 central daylight time, a Thrush Aircraft S2R-G10 airplane, N529KC, sustained substantial damage when it was involved in an accident near Hebron, North Dakota. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. According to the pilot, the purpose of the low-level flight was to apply a liquid chemical to a rural wheat field, in an area with rolling hills. He departed from the operator’s airstrip, Chase Airstrip (6NA5), Hebron, North Dakota. 6NA5 consists of a single turf runway that is 3,800 ft long. After circling the wheat field several times, the pilot completed the A-B pass (a process to determine and mark point A and point B, which denote the start and end of the spray pass). Shortly after, the pilot set the airplane up to perform a C pass (a process to determine and mark point C, which denotes the boundary opposite of points A and B). The pilot reported that 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 6NA5. GPS data recovered from onboard avionics revealed that during the flight to 6NA5, the airplane flew to the west of the Glen Ullin Regional Airport (D57), Glen Ullin, North Dakota. The flightpath, which tracked to the northwest, was about 2.25 miles to the west of D57 at its closest point. D57 has a single asphalt runway that is 3,799 ft long and is publicly owned. As the airplane flew closer to 6NA5, the pilot maneuvered the airplane for a left downwind leg for runway 13 and the engine lost all power. The pilot maneuvered the airplane for a forced landing. During the forced landing sequence, the pilot performed an emergency hopper dump (to reduce the weight of the airplane by dumping the liquid chemical out of the bottom of the hopper while in flight), although he was concerned that the nose of the airplane would pitch up hard due to the rapid change of the airplane’s weight and balance configuration. After the successful completion of the emergency hopper dump, the pilot maneuvered the airplane for 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 pilot was able to egress from the airplane without further incident. The airplane sustained substantial damage to the rudder. Postaccident examination of the airframe revealed no preimpact anomalies that would have precluded normal operation. Both intact wing fuel tanks contained Jet-A fuel at the 66-gallon fuel level marker. Fuel freely flowed from the airframe to the engine. Airframe-to-engine control continuity was established. The airplane was not equipped with a cockpit-basedengine monitoring system (that tracks critical turbine engine parameters), nor was it required to be by the FAA. The engine was placed onto a propeller test stand at the engine manufacturer’s test facility. An engine run to a partial flight power setting confirmed the reported engine instability that the pilot observed during the accident flight. A CT examination of the Woodward 8210-263 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 the engine manufacturer’s unapproved field adjustment. Maintenance records revealed that a mechanic performed this adjustment during an engine inspection in May 2024. 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 a follow-on test found the PG within operational limits. A subsequent teardown of the PG did not find any internal anomalies or mechanical faults. Examination of the oil passages and the in-line fittings of the engine’s accessory gearbox found a small red piece of FOD within the barrel of the check valve (manufactured by The Lee Company). The FOD appeared to be positioned between the valve ball and seat. It appeared to be the same color and consistency as 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. The FOD could not be seen in the CT image because the debris was made of a low-density material. The FOD and a sample of the torque stripe paint material from the engine line fittings were sent to the engine manufacturer’s materials laboratory for analysis; Fourier transform infrared analysis was performed on the FOD and the torque stripe paint sample material, which confirmed the chemical similarity between the FOD and the torque stripe paint material. The Honeywell Aerospace Technologies TPE331 engine series has a NTS system, whose purpose is to provide propeller drag reduction by driving the propeller blade angle toward feather at an optimal rate in case of an engine failure or shut down. The secondary purpose of the NTS system is to assist with the air start procedure. The NTS system check valve installed on the accident engine contained a 0.030-inch diameter orifice located between the oil screen of the PG mounting flange and the check valve ball/seat. The size of the FOD recovered from within the check valve of the engine was about 0.071 inches long, 0.035 inches wide, and 0.025inches thick. The investigation was unable to determine how or when the FOD entered the PG oil system. The FAA-approved Honeywell Aerospace Technologies Maintenance Manual 72-00-27 contains the following guidance about FOD awareness for maintenance personnel: Exercise extreme care to prevent dirt, dust, and foreign particles from entering the engine. If any foreign particle is dropped into the engine during maintenance, stop work until the foreign particle is located and removed, even if this may cause considerable disassembly. For added protection to the engine, cover large openings with plastic sheeting. Cap open tube ends. If the engine is to be left for even a short period in a partially disassembled state, cover or cap all openings. Honeywell Aerospace Technologies published Service Bulletins TPE331-72-2164 and TPE331-72-2168 on April 13, 2010. Both service bulletins discuss the installation of a new check valve. This new check valve contains an integral orifice, downstream of the check valve’s ball/seat, and an integral filter screen designed to protect the check valve orifice from blockage resulting from oil-borne contamination. The engine did not have the modifications specified in these two service bulletins, nor was compliance required by the FAA. The Lee Company’s publication, “An Engineer’s Guide to Selecting a Check Valve,” discusses potential failure modes for check valves: The most common failure mode for a check valve is damage due to ingestion of foreign material, or simply contamination. Unfortunately, fluids can contain contaminants of various sizes and materials, or those contaminants can be introduced by the system itself. This contamination can damage the valve’s internal components or may become lodged inside the valve. This may cause the valve to leak or even become stuck in either the open or closed position. In a worst-case scenario, contamination can become trapped in a place that prohibits the valve from closing, such as between the sealing surfaces, allowing fluid to unexpectedly flow in the checked direction. Adequate protection against contamination should be incorporated upstream of the check valve. A review of the FAA-approved Thrush Aircraft S2R-G10 Airplane Flight Manual did not find any guidance for pilots on how to perform an emergency hopper dump (commonly known as an “e-dump” in the aerial application industry), nor was the AFM required to contain such guidance by the FAA.