On May 26, 2021, about 1430 Pacific daylight time, a Boeing A75N1 Stearman, N75016, was substantially damaged when it was involved in an accident near Frazier Lake Airpark (1C9), Hollister, California. The pilot and pilot-rated passenger sustained minor injuries.  The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he purchased the accident airplane 6 days before the accident and hired a local maintenance shop to perform an annual inspection for the airplane. A few days later, the airplane was returned to service, and the pilot received a “thorough” walkaround and preflight covering the details of the airplane. The next day (the day of the accident), the pilot and pilot-rated passenger completed a preflight inspection and an engine run-up with no anomalies noted. They performed 10 full-stop takeoffs and landings and then flew to Hollister Municipal Airport (CVH), Hollister, California to refuel the airplane. The pilot performed another takeoff and landing after returning to 1C9. He reported that, during the initial climbout, the engine rpm began decreasing, and the airplane stopped gaining altitude. The pilot estimated that the airplane was at an altitude of about 600 ft above ground level at the time. The passenger reported that the airplane was neither gaining nor losing altitude and that he verified that the propeller lever, throttle lever, mixture control lever, and fuel selector lever were all in their correct positions. The pilot further reported that, although the “RPMs were just dropping,” the airplane “never lost complete power.” Shortly after, the pilot initiated a right turn to maneuver away from telephone lines and completed a forced landing in a soft dirt field about 0.3 nm away from 1C9. The airplane came to rest upright, and the lower part of the left and right wings sustained substantial damage. Postaccident examination of the wreckage revealed no preimpact anomalies or mechanical failures with the airframe. A subsequent engine run-up was performed, and no anomalies were noted. A follow-up examination of the engine revealed no preimpact mechanical or mechanical failures. The propeller governor installed on the engine was removed from its drive pad for further testing. A review of the maintenance logbooks revealed that the governor had not been overhauled. An external examination of the governor revealed no body damage, and the drive gears turned freely. Moreover, no metal or contaminants were observed in the oil expelled when the gears were manually rotated. The flywheel assembly, pilot valve, and thrust bearing were also unremarkable. Bench testing revealed a high rpm setting of 2,400 rpm and a relief pressure setting of 125 psi. According to the manufacturer, the high rpm setting should be 2,300 rpm, and the relief pressure setting should be 180 to 200 psi. The governor was tested in a feather setting, and the 1,200 rpm that was observed was within the manufacturer’s specifications. The governor’s internal leakage and pumping capacity of 8 quarts per hour and 10 quarts per minute, respectively, were within limits. A lag was observed as the rpm rose when the simulated throttle setting was increased; conversely, a lag was not observed when the throttle setting was decreased. A teardown examination of the relief valve assembly, which consisted of a bushing, plunger, spring, and plug with a locking washer within the governor body, revealed that the relief valve spring was shorter in length than an exemplar spring, as shown in the figure below. No broken pieces were seen or recovered in the governor. Subsequently, the spring was replaced with a standard-length spring, and the governor was again bench tested. The governor produced 200 psi in relief pressure, and a lag was not observed as the rpm rose when the throttle setting was increased. A review of the maintenance logbooks did not reveal when the propellor governor was last overhauled.

Figure. Removed propeller governor (left) with close-up view of relief valve spring (upper right) and a standard-length relief valve spring (lower right). (Source: Wings West Exchange & Overhaul, Inc)