What happened
During an enroute descent, a commercial pilot with 6,127 hours of flight time discovered that the throttle was locked and would not advance. Despite attempts to wiggle the control, the engine RPM could not be increased beyond 1,000 RPM. As the pilot prepared for a forced landing, a sudden movement caused the throttle to pop loose, allowing it to move forward and aft; however, this change had no effect on the engine's performance.
The aircraft subsequently landed on rough and uneven terrain, resulting in the airplane nosed-over during the ground roll. The occupants were not injured.
The investigation
A review of the airframe logbooks indicated that the Cessna had undergone previous repairs following prior mishaps. Notably, the airframe had been completely rebuilt in July 2003, which included the replacement of the firewall during a scheduled overhaul. Additionally, a newly overhauled engine had been installed approximately 3.5 hours before the accident.
The investigation focused on the mechanical failure of the throttle linkage and revealed that a throttle cable support angle was missing from the firewall. There was no evidence to confirm whether this support angle had been reinstalled when the engine firewall was last replaced.