What happened
During a night IFR flight, an air traffic controller observed the aircraft deviating significantly from its assigned course. The pilot, aged 68, was seen moving south of the course and correcting back slightly before deviating south again. These deviations reached as much as 90 degrees. Additionally, the aircraft's altitude fluctuated between approximately 14,400 feet MSL and 14,000 feet MSL.
The flight had been cleared to climb to 12,000 feet at 1936. By 2006, the controller reported that the aircraft was erratic, stating, "I don't know where he's going...he's all over the place...." Approximately 10 miles southeast of Helena, Montana, radar contact was lost in an area that typically maintains good coverage. The aircraft sustained an in-flight breakup before impacting in hilly terrain. The pilot sustained multiple blunt force injuries resulting in 1 fatality.
The investigation
On-scene investigators determined that a portable oxygen bottle being carried in the airplane was not being used by the pilot. It was noted that the pilot had a history of flying at altitudes above 12,500 feet, and as high as 17,000 feet, without the use of supplemental oxygen.
An autopsy of the pilot revealed atherosclerosis of coronary arteries, specifically featuring 90% stenosis of the left anterior descending coronary artery and 90% stenosis of the right coronary artery.