N3630DSchweizer Aircraft G-164B · 1984

On May 5, 2019, at 1006 Pacific daylight time, a Schweizer Aircraft Corporation G-164B airplane, N3630D, was substantially damaged when it was involved in an accident near Sutter, California. 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, after completing 17 takeoffs that day, he departed the gravel runway to the south with about 45 gallons of Jet-A fuel and about 2,500 lbs of fertilizer onboard. He climbed to a cruise altitude of about 500 ft above ground level and set the power to 36 lbs of torque and the propeller to 2,000 rpm. Fifteen seconds after he established cruise flight and about 1 minute into the flight, the engine speed reduced to idle power without any control inputs from the pilot, and the airplane immediately began to descend. After an unsuccessful attempt to add power by advancing the throttle, the pilot jettisoned the load of fertilizer and initiated a forced landing to a nearby road. The airplane landed short of the road and impacted a group of prune trees before it came to rest. According to the pilot, the engine continued to run at idle power (about 52% Ng) after the airplane came to rest.

A review of photographs furnished by the operator revealed substantial damage to the wings and fuselage.

A detailed examination of the engine supervised by the National Transportation Safety Board (NTSB) investigator-in-charge revealed that the fuel control unit (FCU) interconnect rod was disconnected at the FCU actuating lever and the connection hardware was not recovered. The examination further showed that the reversing cam had fractured. The engine did not display any preimpact mechanical anomalies.

The airplane was powered by a Pratt & Whitney PT6A-34AG turboprop engine operated by three levers: a power lever, a fuel lever (condition lever), and a propeller lever. The power lever is used to control compressor speed and propeller pitch in beta and reverse. Power lever movements are transmitted from the cockpit to the FCU, which controls fuel flow to the engine. From the power idle to full reverse positions, the power lever can increase compressor speed and change the propeller blade angle into reverse pitch. The fuel lever delivers fuel to the combustion chamber in coordination with the lever position. The propeller lever controls propeller speed and allows the pilot to feather the propeller on the ground or during flight if necessary.

During engine operation, a reversing cam and FCU actuating lever coordinate inputs from the three cockpit controls with the FCU and propeller governor. The FCU interconnect rod joins the FCU to the propeller governor through the actuating lever and reversing cam (see figure 1). According to the engine manufacturer, if the interconnect is disconnected during flight, the FCU arm, which moves the spring-loaded compressor speed scheduling cam within the FCU, will rotate counterclockwise under spring tension and bring the operating condition of the FCU back to idle power automatically (see figure 2). The FCU arm will remain at idle as long as the interconnect rod is separated. A bolt, nut, spacer, and cotter pin are normally used to secure the FCU interconnect rod to the FCU actuating lever.

Figure 1: Fuel control unit connections

Figure 2: Fuel Control Unit Schematic

Review of the airplane's maintenance records revealed that the engine was removed from another airplane and installed in the accident airplane on May 14, 2014. Records showed that the FCU was repaired on May 22, 2017, and again on March 20, 2019. The director of maintenance reported that his mechanic would have disconnected the interconnect rod at the FCU arm and may have removed the interconnect linkage at the actuating arm during the last service for easier access to the FCU.

A NTSB material laboratory examination revealed that the reversing cam fracture faces exhibited a texture consistent with overstress and displayed no evidence of fatigue cracking. The FCU interconnect rod showed no evidence of corrosion or mechanical damage.