Accident Overview
The aircraft was in cruise flight at an altitude of 7,500 feet with the engine producing satisfactory power. The pilot reported that the engine slowly and smoothly lost power over a duration of 10 to 15 seconds. The engine then completely lost power while over an unlighted rural area. During the subsequent forced landing, the airplane struck trees and collided with terrain.
Maintenance History
Examination of aircraft records revealed that the engine and the fuel control unit had been overhauled by their respective manufacturers. The engine was installed on the aircraft two weeks prior to the accident and had accumulated 14.6 hours of operation since the overhaul.
Mechanical Examination
Inspection of the engine itself found no mechanical deficiencies. However, examination of the fuel control servo revealed that a lock nut was no longer installed. Initial flow tests on the servo produced a flow rate equivalent to the idle cut-off position. When a test nut was installed, the unit flowed "perfectly."
Inspector's Statement
When questioned about the performance of the fuel servo and the engine with the outer nut missing, the inspector stated, "Basically, what it does is it shuts off the fuel when that nut backs off. It's just like pulling it to idle cut off."
Conclusion
The investigation identified that the missing lock nut in the fuel control servo resulted in a fuel flow condition that terminated engine power. No other mechanical issues were found with the engine or airframe.