AC50 PLT HAD A FORCED OFF ARPT LNDG CAUSED BY FUEL STARVATION; WHEN A FUEL TANK CAP CAME OFF INFLT.

Narrative

DURING FLT FROM ZZZ1 TO ZZZ AT 8000 FT; VFR ON TOP; AIRPLANE ENGS STARTED TO SPUTTER. EMER PROC FOR FUEL EXHAUSTION WAS PERFORMED; BUT DID NOT HELP SIT. THE AIRPLANE CONTINUED TO DSND THROUGH CLOUDS UNTIL GND CONTACT WAS OBSERVED (AT 2800 FT MSL). AT THAT TIME I OBSERVED A PASTURE FOR LNDG LOCATED BELOW ACFT. I IMMEDIATELY MADE A 270 DEG L TURN. WHEN I LEVELED WINGS; LNDG GEAR WAS EXTENDED. THE AIRPLANE LANDED SAFELY ON A 500 FT LONG COW PASTURE. I RECEIVED NO INJURIES; NOR DID THE AIRPLANE RECEIVE ANY DAMAGES. WHEN I EXITED THE AIRPLANE I DID NOTICE THE FUEL CAP WAS NOT ATTACHED. BEFORE THE FLT; I DID A THOROUGH PREFLT; INCLUDING ENG OIL LEVELS AND FUEL LEVEL. THE AIRPLANE WAS TOPPED OFF. AFTER DEP; I NOTICED THE FUEL GAUGE READING 80 GALLONS. ABOUT 20 MINS LATER IT READ 50 GALLONS. I MONITORED THE FUEL GAUGE AND NOTICED IT STABILIZED AT 50 GALLONS. I WAS 25 MINS FROM ZZZ AND DETERMINED I HAD MORE THAN ENOUGH FUEL TO REACH MY DEST (THE AIRPLANE BURNS 30 GALLONS OF FUEL PER HR). 5 MINS LATER; THE ENGS STOPPED. INFLT; I AM UNABLE TO SEE IF FUEL CAP IS ON OR OFF.