N6793K – GRUMMAN ACFT ENG COR-SCHWEIZER G-164B – 2024-06-25 NTSB Accident Report

On June 25, 2024, about 0554 Pacific daylight time, a Grumman-Schweizer G-164B, turbopropeller-engine equipped airplane (current type certificate held by Allied Ag Cat Productions Inc.), N6793K, was substantially damaged when it was involved in an accident near Nicolaus, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight.

According to the pilot, the operator had just purchased the airplane and hired him to reposition it from the seller’s private airstrip in Pleasant Grove, California, to a job location in Amana, Iowa. His previous experience flying Ag Cat airplanes involved only models equipped with a reciprocating engine.

To prepare for the repositioning flight, the pilot attended 3 hours of ground training provided by a pilot/mechanic who had about 7,000 hours experience flying Ag Cat airplanes, including both turbopropropeller- and reciprocating-engine equipped models. The pilot also conducted four “operational check flights” to familiarize himself with the accident airplane that totaled about 2.5 hours. For each successive flight, the pilot increased the amount of fuel in the hopper to increase the load, and he observed that the airplane departed the runway quickly with no performance issues. The pilot reported that he did not use a load as heavy as that of the accident flight.

On the day of the accident flight, the airplane was loaded with 60 gallons of jet fuel in the wing tank and 270 gallons of jet fuel in the hopper. According to the pilot, the airplane’s empty weight of 3,600 lbs, the fuel weight of 2,211 lbs, the pilot’s weight of 190 lbs, and the baggage weight of 10 lbs equated to a gross takeoff weight of about 6,011 lbs. According to an exemplar POH for G-164B airplanes, the design gross weight for the airplane was 4,500 lbs, and the Civil Aeronautics Manual (CAM) Part 8 operational gross weight was 6,075 lbs. According to the pilot, another reference (Jane’s Civil and Military Aircraft Upgrades) listed the airplane’s CAM 8 maximum takeoff weight as 7,020 lbs. The POH did not list performance data for the turbopropeller engine or provide performance charts for calculating required runway distances.

According to the ground instructor, he had flown the accident airplane in the past and from the airstrip where the accident flight departed, and he was confident that the airplane could take off with the load and in the environmental conditions of the accident flight. Based on weather conditions reported about the time of the accident from the nearest automated weather reporting station at Lincoln Regional Airport/Karl Harder Field (LHM) Lincoln, California, the calculated density altitude was about 978 ft msl.

The pilot reported that, for the accident flight, he used a ground start pack to start the engine and monitored the engine system instruments. After the engine started, he had the start pack removed. He turned on the generator and confirmed that it was operating normally. He then moved the power lever up and over the back gate to reduce engine speed and ensure the propeller stops were disengaged. He checked the engine instruments and confirmed that they were in the normal ranges. After checking the navigation equipment, the pilot then taxied to the north end of the 2,100 ft asphalt runway, again checking the engine instruments and confirming that they were still in the normal ranges. He then advanced the red engine speed control lever all the way forward and felt the engine speed increase. He then released the wheel brakes to depart.

According to the pilot, the airplane did not accelerate as it had on previous flights with less weight. During the takeoff roll, once the tail came up, he noted that about 90% of the runway had been used, and the airplane was failing to gain lift. The pilot applied an aft control stick to clear a gate located near the end of the runway, but the airplane struck the gate and then a pickup truck that was traveling on a road that ran perpendicular to the end of the runway. The airplane then impacted terrain, spun about 180°, and came to rest upright.

Postaccident examination of the airframe revealed damage to the fuselage, empennage, and the upper and lower wings, consistent with impact of multiple objects. Flight control continuity was confirmed from the flight controls to their respective control surfaces. Examination of the airframe, engine, and propeller governor identified no evidence of preaccident malfunction or anomaly that would have precluded normal operation. According to the manufacturer of the airplane, the TPE-331 engine was not a factory installation and therefore did not have performance charts for the TPE-331 in the Pilot’s Handbook.

Testing of the fuel control unit (FCU) using test specification procedure (TSP)-8519, revision A1, revealed no leaks or other anomaly when the FCU was pressurized. Fuel flow readings were collected at the lower power settings of the acceleration schedule but once Ps3 pressure reached about 95 psi, an air leak could be heard and felt coming from the main air section cover.

Removal of the Ps3 (static pressure at station 3.0) bellows cover revealed that the Ps3 diaphragm had a circumferential tear over about 90° adjacent to the plate. According to the FCU manufacturer, “The Ps3 rolling diaphragm is subject to wear. Extreme wear can result in a hole in the diaphragm sidewall. The hole allows air to leak to Pt2 (total pressure at station 2.0), causing the FCU to sense an increased Pt2 pressure. An incorrect Pt2 pressure can result in higher than desired power lever schedule fuel flows and a reduction in acceleration schedule fuel flows.”