Accident Sequence
A pilot was conducting a personal cross-country flight. En route, he contacted air traffic control to report a fuel pump issue and requested a diversion to the nearest airport, stating the request was precautionary. No emergency was declared. As the airplane approached the diversion airport, witnesses observed it flying low and rolling to the left before impacting terrain. A postcrash fire ensued.
Examination Findings
Examination of the airframe revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation. A postaccident examination and review of recorded data indicated that the left engine was secured and in the feather position, while the right engine was operating at a high RPM setting. The left engine-driven fuel pump was found fractured. Further examination of the fuel pump revealed fatigue failure of the pressure relief valve. The fatigue failure initiated in upward bending on one side of the valve disk and progressed around both sides of the valve stem. As cracks grew, the stem separated from the disk on one side and began to tilt, causing the lower end of the stem to rub against the valve guide, creating wear marks. The increasing stem tilt likely caused the valve to stick in the closed position, preventing it from opening and allowing fuel pressure to rise above the set point. Because the pump was engine-driven, the pilot could not shut it off or bypass it, resulting in variable fuel pressure as the valve became stuck and unstuck.
Examination of the spring seat and diaphragm plate in the fuel pump assembly revealed wear marks indicating prior separation and reinstallation in a different orientation, suggesting someone had attempted to troubleshoot a fuel pump problem before the accident flight.
Flight Parameters and Loss of Control
The last radar data point showed the airplane at a groundspeed of about 98 knots, having passed north of the airport and traveling southwest. The minimum control speed for single-engine operation was 88 knots. If the pilot initiated a left turn back toward the airport, the right engine torque and wind (14 knots gusting to 24) would have necessitated a higher speed. Because appropriate control inputs and airspeed were not maintained, the airplane rolled in the direction of the feathered engine (due to the left fuel pump problem), resulting in a loss of control.
Toxicological Report
The pilot's toxicology report was positive for cetirizine, sumatriptan, gabapentin, topiramate, and duloxetine. All these drugs act on the central nervous system and can be impairing alone or in combination. The investigation could not determine the reason for the pilot's use of these drugs, but they are commonly used to treat chronic pain syndromes or seizures. It is likely that the pilot was experiencing impairment from multiple impairing medications, which contributed to his inability to respond to the in-flight urgent situation and safely land the airplane.
Probable Cause
The probable cause, as stated in the investigation, is the pilot's loss of airplane control due to his failure to maintain appropriate control inputs and airspeed after shutting down an engine because of a progressive failure of the pressure relief valve in the fuel pump, which resulted in variable fuel pressure in the engine. Contributing to the loss of control was the pilot's use of multiple impairing medications.