Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 75; Flight Crew Total 16000; Flight Crew Type 1000
Confusion; Distraction; Situational Awareness; Time Pressure
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Maintenance; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure; Aircraft
I departed ZZZ1 bound for ZZZ as Aircraft X. After reaching cruise altitude of 8;000 feet I observed the right wing tank fuel gauge showed an increase of several hundred pounds of fuel. I then observed that the filler cap for the right tank stuck BELOW the right wing attached to its lanyard with fuel being vented from the fill opening. The wing tanks feed the main tanks and there was plenty of fuel to reach the destination ZZZ. I diverted with concern that spreading jet fuel over the most populous city in an environmentally conscious state might not be well received. ZZZ2 provided a close alternate over sparsely populated mountains without need to fly over the city proper.As I slowed and descended into ZZZ2 and put approach flaps down; the cap that initially dangled below the wing moved up to the top of the wing still attached to its lanyard and fuel stopped venting from the open filler hole in the tank. This was not a situation where a faulty fuel cap vented fuel 'in place'. I had 30 gallons of fuel added to the right-wing tank in ZZZ2 by an experienced fueler who opined that the cap was secure and apparently worked properly. I was not aware that I should have had a mechanic inspect the cap - and continued the flight to ZZZ in violation of company and FAA policy.
A pilot reported the fuel filler cap came off in flight. The reporter landed and refueled but failed to have a mechanic inspect the cap.