What happened
During the preflight inspection of the aircraft, the first officer opened the main oxygen supply valve located in the cockpit. Upon opening the valve, a loud hissing sound was heard. Almost immediately following this event, flames engulfed the cockpit area.
The copilot managed to escape the aircraft and was not injured. The fire caused substantial damage to the cabin interior, melted the pressure reducer-regulator assembly of the oxygen system, and burned a hole in the right forward side wall of the aircraft. The fire was eventually extinguished by ramp personnel.
The investigation
A laboratory analysis of the components involved in the incident was conducted. This analysis revealed that a deposit containing oil was present on the interior of the oxygen cylinder.