A C152 LOSES POWER DUE TO FUEL STARVATION TWELVE MILES FROM THEIR INTENDED REFUELING ARPT. LAND SAFELY IN FIELD.

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THE INCIDENT YOU ARE BEING INFORMED OF WAS AN EMER LNDG MADE BECAUSE OF MINIMAL FUEL. WE HAD INTENDED OUR FLT ON TAKING US APPROX 3 HRS 30 MINS. BOTH CESSNAS ARE FITTED WITH STANDARD TANKS WHICH HOLD 24.5 GALLONS OF USABLE FUEL. AFTER TAKING INTO ACCOUNT THE .8 GALLONS USED FOR TAXI AND RUN-UP; AS WELL AS THE 1.6 GALLONS OF FUEL BURNED IT TOOK US TO CLB TO 6500 FT; WE SHOULD HAVE HAD ACCORDING TO THE CESSNA 152'S POH 3 HRS 49 MINS OF FUEL AVAILABLE FOR CRUISING. IF YOU ADD THE 12 MINS AS SPECIFIED IN THE POH FOR OUR CLB AND 10 MINS FOR TAXI AND RUN-UP; THAT WOULD HAVE TECHNICALLY MEANT THAT WITH OUR 24.5 GALLONS OF USABLE FUEL WE SHOULD HAVE BEEN ABLE TO FLY FOR 4 HRS 11 MINS. OUR ENG WAS LEANED THROUGHOUT THE ENTIRE FLT EXCEPT FOR OUR CLBS AND DSCNTS. DURING THE LAST PORTION OF OUR FLT NEARLY 10 NM FROM OUR DEST AT EXACTLY 3 HRS 30 MINS INTO THE FLT; I NOTICED THAT THE RPM SUDDENLY DROPPED FROM 2400 FPM TO 2200 RPM. I IMMEDIATELY REACHED FOR THE THROTTLE AND GAVE IT FULL PWR AND WATCHED AS THE RPM CONTINUED TO DROP TO 1500 RPM. THE RPM THEN BEGAN SUDDENLY JUMPING SPORADICALLY LOW TO HIGH RPM AND EMPTY TANKS WAS MY FIRST THOUGHT EVEN THOUGH THE GAUGES READ 5 GALLONS ON THE L SIDE. WE CHKED FOR CARB HEAT JUST IN CASE BUT ELIMINATED THAT SHORTLY. I TRIMMED FOR 60 KTS; CHOSE A FIELD AS MY COPLT RAN THROUGH THE CHKLIST JUST IN CASE IT WAS NOT FUEL STARVATION. WE SQUAWKED 7700 AND CONTINUED OUR RADIO COM UNTIL WE WERE SAFELY ON THE GND AND THE ENG WAS SHUT DOWN. LUCKILY THERE WAS ENOUGH FUEL LEFT TO ALLOW THE ENG TO RUN AT IDLE BUT WOULD NOT KEEP RPM WITH FULL THROTTLE IN. WE SAFELY LANDED THE PLANE IN A LARGE HAY FIELD IN A CIVILIAN'S BACK YARD; AND WERE MORE THAN GRACIOUSLY HELPED BY THE LCL CIVILIANS AND POLICE. THE PLANE WAS THEN INSPECTED BY A MECH; THE MAINT BOOK WAS ENDORSED; AND APPROVAL TO TAKE OFF WAS OBTAINED FROM THE FSDO; LANDOWNER; AND LCL LAW ENFORCEMENT. FUEL WAS BROUGHT TO THE FIELD AND ALL EXCESS WT WAS REMOVED FROM THE ACFT TO ENSURE THE SAFEST DEP. THE PLANE TOOK OFF FROM THE FIELD AND SAFELY FLEW TO ZZZ; OUR INTENDED DEST. IT WAS A SITUATION THAT COULD HAVE BEEN A SERIOUS DISASTER BUT LUCKILY NO DAMAGE WAS DONE TO ANY PERSONS OR PROPERTY. SUPPLEMENTAL INFO FROM ACN 784635: UPON REACHING 3 HRS 30 MINS OF FLT TIME WE WERE APPROX 12 MI FROM OUR ARPT OF INTENDED LNDG.