IAD97FA060 · Cessna 337D

The pilot of the unpressurized airplane was cleared to climb to 25,000 feet by air traffic control (ATC) and he was observed going through the assigned altitude and leveling at 27,700 feet. Then the aircraft was observed at 26,000 feet, and subsequent contact was lost. The pilot did not respond when ATC queried him about exceeding his assigned altitude. The aircraft sustained an inflight breakup during an uncontrolled descent, and came to rest in a tree. According to the surviving passenger, the airplane was refueled and a portable oxygen bottle was filled prior to takeoff. She stated that they were going to take aerial photographs at four separate locations during this flight. She said that they shot three of the locations and landed at Williamsburg, Pennsylvania where the pilot filed a flight plan and setup the portable oxygen system for their use during the next flight. They departed and climbed to 10,000 feet, and the pilot told her to put her oxygen mask on, and he did the same. The last altitude she remembered the pilot calling out was 20,000 feet, and he asked her how she was doing and if she felt okay and she said yes. When asked, 'who turned on the oxygen?' she stated that she did, started to but wasn't sure how, so, the pilot reached back and turned it on. She said she knew it was on because she could feel the cool air and that there was a little valve in both lines and they went from red to green indicating the oxygen was flowing. She said that she remembered him saying that they had just crossed 20,000 feet and she began feeling dizzy, she said that her eyes would not focus, and that she felt like she was cross eyed. She said she told the pilot that she was dizzy but she thought that he was talking to the tower because he did not respond. She recalled that she felt better when she closed her eyes, when she did, that was the last thing she clearly remembered until after the crash. Analytical testing of the contents of each cylinder used to fill the pilot's oxygen bottle found that they contained compressed breathing air at about 21 percent oxygen instead of aviation oxygen. Postmortem examination revealed the pilot's death was a result of hypoxic hypoxia due to insufficient oxygen reaching the blood.

Verbatim NTSB analysis and probable cause from the NTSB dataset