SA-227 AC Metro III
Flight Crew Commercial
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
General None Reported / Taken
Aircraft
On approach to ZZZ; noticed RH fuel indication had dropped to 300lbs lower than the LH fuel indicator. No action taken in the air as I was on the ILS approach and total fuel was not low.Upon landing and parking; I noticed a substantial fuel leak and local airport operations and fire department were notified due to the size of the spill. The spill was contained and the aircraft grounded until a replacement part could be installed. Initially became aware of the fuel problem in flight on approach into ZZZ. The cause of the fuel problem was identified on the ground after shut down.  After conferring with ZZZ1 maintenance; it was determined that a quick drain plug had come loose and fell out of its place during flight.
SA-227 Captain reported a fuel imbalance during flight led to discovery of a fuel leak after landing.