What happened
The pilot was operating Cessna 172 at an altitude of 4,000 feet mean sea level when the engine abruptly quit without warning. With no option to return to the departure airport or reach a suitable alternate field, the pilot initiated a forced landing on rolling farm terrain. During the impact sequence, both main landing gear assemblies separated from the airframe. The aircraft then slid across the ground, passing through a barbed wire fence before coming to rest and catching fire.
The investigation
A metallurgical examination was conducted on the failed engine components to determine the root cause of the sudden power loss. The inspection focused on the Bendix fuel injection system, specifically part number 2424273-10. Analysis revealed that the idle set screw, which controls the diaphragm setting within the unit, had suffered a fatigue failure.
The stem of this screw failed due to reversed bending stresses. These stresses were caused by side-to-side forces acting on the component. This mode of failure was considered unusual because the primary force applied to the adjustment screw during normal operation would typically be in a twisting motion rather than a lateral side-to-side force. The presence of these lateral forces suggests an external influence or installation anomaly not immediately obvious from standard operational parameters.
Findings
The investigation determined that the fuel exhaustion was not the cause, but rather a mechanical failure within the fuel delivery system. The specific contributing factor was the fatigue fracture of the idle set screw in the Bendix fuel injection unit. The reversed bending stresses leading to this failure were inconsistent with the expected twisting forces, indicating an anomalous loading condition on the adjustment mechanism.
Safety message
This incident highlights the critical importance of regular inspections of fuel injection system components, particularly those subjected to complex stress patterns. Maintenance personnel should be aware that fatigue failures can occur in unexpected orientations if lateral forces are introduced into systems designed primarily for axial or torsional loads.