Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 26000; Flight Crew Type 3000
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft Aircraft Damaged
Procedure
Airport fueler was putting jet fuel in the PA31-350. I stopped him after he put 6 gallons in the right main tank. I had him top off with 100LL; wrongfully thinking a small amount would not be a problem. I flew the trip to ZZZ and back with no incident. When being parked I saw a red ALT and ALT INOP light come on the right engine; the one with the jet fuel in it. I do not remember any other negative engine indications. Being at the gate; I shut the engine down. I reported to the Director of Operations and the Director of Maintenance that I had flown the plane with 6 gallons of Jet A in A 56 gallon tank. The next day I found oil on the ground below the right engine.
PA31 pilot reported the ALT INOP light and oil leak occurred after flying with six gallons of Jet A fuel mistakenly mixed with the 100LL fuel in the right main tank.