Post-accident investigation by the
National Transportation Safety Board (NTSB) showed that the leading cause of the accident was Denver's inability to switch fuel tanks during flight. The quantity of fuel had been depleted during the plane's flight to Monterey and in several brief practice takeoffs and landings Denver performed at the airport immediately before the final flight. His newly purchased amateur-built Rutan aircraft had an unusual fuel tank selector valve handle configuration. The handle had originally been intended by the plane's designer to be between the pilot's legs. The builder instead put it behind the pilot's left shoulder. The fuel gauge was also placed behind the pilot's seat and was not visible to the person at the controls.
[51][52] An NTSB interview with the aircraft mechanic servicing Denver's plane revealed that he and Denver had discussed the inaccessibility of the cockpit fuel selector valve handle and its resistance to being turned.
[51][52]
Before the flight, Denver and the mechanic had attempted to extend the reach of the handle using a pair of
Vise-Grip pliers, but this did not solve the problem, and the pilot still could not reach the handle while strapped into his seat. NTSB officials' post-accident investigation showed that because of the fuel selector valves' positioning, switching fuel tanks required the pilot to turn his body 90 degrees to reach the valve. This created a natural tendency to extend one's right foot against the right rudder pedal to support oneself while turning in the seat, which caused the aircraft to
yaw (nose right) and pitch up.
[51][52]
The mechanic said that he told Denver that the fuel sight gauges were visible only to the rear cockpit occupant. Denver had asked how much fuel was shown. He told Denver that there was "less than half in the right tank and less than a quarter in the left tank". He then provided Denver with an inspection mirror so he could look over his shoulder at the fuel gauges. The mirror was later recovered in the wreckage. Denver said that he would use the autopilot in flight to hold the airplane level while he turned the fuel selector valve. He turned down an offer to refuel, saying that he would be flying for about an hour.
[51][52]
