Accident Sequence
On May 24, 2023, at approximately 0900 mountain standard time, a Cessna 172S, registration N60372, sustained substantial damage during a forced landing in a field near Casa Grande, Arizona. The aircraft was operated as a personal flight under Title 14 Code of Federal Regulations Part 91. The pilot and a pilot-rated passenger, serving as a safety pilot, were not injured.
The flight was being conducted under simulated instrument flight rules using a view-limiting device, with the safety pilot, who was also a certified flight instructor, monitoring. After completing an approach into Casa Grande Airport (CGZ), the crew entered a holding pattern at 5,500 feet mean sea level (msl). The area was uncontrolled airspace commonly used for instrument training, and pilots reported their positions on the Common Traffic Advisory Frequency (CTAF). Following another aircraft's announcement, the pilot descended to 5,000 ft msl. After leveling off, the safety pilot observed a decrease in airspeed and noted the throttle was fully open, but the engine speed was only 1,900 rpm. The pilot advanced the mixture to full rich, engaged the fuel boost pump, and cycled the throttle, but engine power did not change. The safety pilot announced the engine anomaly over CTAF and stated they were proceeding direct to CGZ.
The aircraft reached the airport at 3,000 ft msl and began a circling descent. The engine speed remained at approximately 1,900 rpm during the turn to final approach. The pilot attempted to reduce throttle without success. Flaps were deployed to reduce speed, and the safety pilot, now flying, assessed landing options. Unable to decelerate adequately, he chose to land in a field at the end of runway 5. He reduced the mixture to idle/cutoff, and the engine stopped. The airplane touched down normally, but during the landing roll it struck a fence, and the left wing subsequently impacted a tree, resulting in substantial damage to the left wing and fuselage.
Postaccident Examination
Examination of the engine revealed that the throttle linkage had separated from the throttle arm at the fuel servo. The throttle system uses a cable connecting the throttle lever to the throttle arm of the fuel servo, which is mounted on a plenum below the oil sump. The linkage threads into a rod end attached to the throttle arm. When the linkage was reconnected, the engine operated smoothly and continuously at run-up power with no mechanical anomalies observed.
Maintenance History
The aircraft maintenance manual specifies a 50-hour inspection interval for the throttle assembly, which includes checking for freedom of movement, proper travel, security of attachment, and evidence of wear. The last recorded major engine overhaul was completed eight months before the accident, which was likely when the throttle linkage was last installed. The airplane's most recent annual inspection predated that engine installation. A review of maintenance records indicated that the throttle linkage assembly was not inspected within the prescribed interval. The mechanic who performed subsequent maintenance after the engine overhaul stated he was unaware of the inspection interval for the throttle linkage.