Accident Overview
The aircraft, equipped with an air sampling system, was returning to the airport after collecting air samples at various altitudes over oil fields. While being vectored for an instrument approach, the airplane overshot the runway's extended centerline. The pilot then reported a fire on board. The airplane lost altitude rapidly, and radar contact was subsequently lost.
Wreckage Examination
Examination of the accident site revealed that the airplane struck the ground at high velocity and at a low impact angle. One piece of the shattered Plexiglas windshield exhibited soot streaking on its exterior surface, but the soot did not extend onto the fracture surface. This pattern indicated that the smoke source was upstream of the windshield and that the smoke exposure occurred before the windshield broke apart during impact.
Both nose baggage compartment doors were found approximately 2 miles south of the main wreckage. Their proximity suggested they separated at nearly the same time, most likely before the pilot's distress call. Although the doors showed no soot deposits, thermal damage, or deformation consistent with a high-energy explosion, the separation could have resulted from an overpressure caused by the ignition of a fuel-air mixture within the nose portion of the airplane.
Potential Overpressure Scenario
The ignition of fuel-air mixtures can create overpressure events when occurring in confinement. An overpressure in the nose baggage compartment may have stretched the airframe sufficiently to allow the doors to push open without deforming the latches. If the mixture was lean, it would likely leave no soot residue, which aligns with the absence of soot on the doors.
Post-Accident Examination Findings
Post-accident examination found no evidence that the air sampling system, which was strapped to the seat tracks behind the copilot's seat, caused the fire. The combustion heater, mounted in the right front section of the nose baggage compartment, bore no evidence of fuel leakage; however, a fuel fitting was found loose.
Probable Cause
Based on the investigation, the probable cause is the loose fuel fitting on the combustion heater that leaked a lean fuel-air mixture into the nose baggage compartment. The mixture was most likely ignited by the combustion heater, blowing off the nose baggage compartment doors and initiating an in-flight fire.