What happened
On June 6, 2023, a Cessna 207 operating as an aerial application flight was involved in an accident near Valley International Airport (HRL) in Harlingen, Texas. The pilot was performing a mission to distribute sterile fruit flies and had completed three previous flights that morning.
After refueling and reloading the aircraft, the pilot departed and reached an altitude of approximately 500 feet above ground level. After setting the engine controls to 2,500 rpm and 25 inHg manifold pressure, the engine speed immediately decreased to idle. Unable to return to the airport, the pilot performed a forced landing in a field. The pilot was not injured, but the aircraft sustained substantial damage to the left-wing spar.
The investigation
Post-accident examinations determined that the throttle control lever had fractured, causing it to become disconnected from the associated hardware. The engine and fuel control valve had been overhauled and reinstalled on April 14, 2023, and had accumulated 86.3 hours since that overhaul.
A metallurgical examination by the NTSB Materials Laboratory revealed that the throttle control lever attachment point showed plastic deformation and significant wear. The inner surface of the attachment point bore exhibited a shallow impression of the splined collar, which was also worn and uneven. These conditions meant the attachment point no longer provided sufficient interference to secure the rod end attachment hardware.
Investigators also noted that the throttle linkage hardware configuration on N91157 did not match the diagram provided in the airplane service manual, as a washer was missing from the assembly.
Findings
- The throttle linkage hardware assembly was installed differently than specified in the service manual.
- A missing washer in the linkage assembly likely allowed the hardware to loosen over time.
- This looseness caused excessive wear to the splined collar and the lever arm attachment point, ultimately leading to the fracture of the throttle control lever.