What happened
The pilot utilized a cannula attached to a portable oxygen system during the climb phase beginning at 11,000 feet mean sea level. Upon reaching flight level 170, he switched to a mask connected to the aircraft's built-in oxygen system. At flight level 250, the pilot reported experiencing stomach pains for approximately thirty minutes. He later informed his son that the oxygen had an unpleasant odor and that the mask was difficult to use and fit poorly. Consequently, the pilot attempted to use both the mask and cannula simultaneously.
The aircraft's oxygen system was inspected and indicated positive flow with a bottle pressure of 1500 psi. Despite this indication, the pilot began exhibiting signs of hypoxia and subsequently drifted in and out of consciousness. The pilot's son assumed control of the aircraft, declared an emergency, and flew until the final approach phase, at which point the pilot regained control. During short final approach, the pilot dived the aircraft, prompting his son to pull back on the yoke. The aircraft contacted the ground short of the runway.
The investigation
A mechanical inspection of the aircraft's oxygen system revealed no malfunctions. However, analysis showed that the oxygen bottle reportedly had a dew point of approximately -35 degrees Fahrenheit. Aviation regulations require aviation-grade oxygen to have a dew point of -83 degrees Fahrenheit or lower. The excessive moisture content likely caused the bad odor and poor fit issues reported by the pilot, contributing to his inability to receive adequate oxygen.
Findings
The primary factor in this accident was the pilot's hypoxia resulting from the use of oxygen with insufficiently low dew point. The moisture in the oxygen supply compromised the effectiveness of the breathing apparatus, leading to incapacitation. The involvement of the pilot's son highlights the critical need for proper oxygen system maintenance and adherence to aviation-grade gas specifications.