A C150 INSTRUCTOR AND STUDENT MISCALCULATED THEIR FUEL BURN; REQUIRING A PRECAUTIONARY LNDG IN A FIELD.

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WE WENT TO TRAINING APPROX XA30. I INITIALLY LEFT FROM 2W5 AND FLEW FOR 30 MINS. WE ANTICIPATED THAT WE STILL HAD 3 HRS OF FUEL ON BOARD (WITH 30 MINS RESERVE) SINCE WE HAD 21 GALLONS USABLE AND I USED 5.5 GALLONS ON MY FIRST LEG. HOWEVER; ON THE WAY BACK TO HEF AROUND XA50; WE HEARD A NOISE AND MY INSTRUCTOR; WHO WAS FLYING AT THE TIME; SENSED THAT WE WERE RUNNING OUT OF FUEL. HE IMMEDIATELY FOUND THIS FARM LAND TO LAND; AND WE EXECUTED AN EMER LNDG. WE GOT FUEL THERE AND TOOK OFF. I THINK HE MADE A PERFECT CHOICE SINCE WE WERE ABOUT TO GO OVER A HILL. SUPPLEMENTAL INFO FROM ACN 536802: AT APPROX XA15; WE BEGAN AN INST TRAINING FLT IN VMC FROM HEF TO MRB AND BACK; PLANNING TO DO MULTIPLE PRACTICE APCHS AT MRB. PRIOR TO THE FIRST APCH AT MRB; WE WERE GIVEN INSTRUCTIONS TO HOLD AT THE MRB VOR. PRIOR TO THE FLT; THE STUDENT TOLD ME THAT THE TANKS WERE FULL PRIOR TO HER DEP FROM MARYLAND ARPT (2W5) AND THAT HER FLT TOOK 22 MINS. I VERIFIED FULL FUEL INDICATIONS ON THE GAUGES AND NEARLY FULL FUEL LEVELS IN THE TANKS. I CALCULATED THAT AT A FUEL BURN OF 5.5 GPH AND 21.5 GALLONS OF USABLE FUEL WE HAD ABOUT 3 HRS 54 MINS ENDURANCE. THIS WOULD ALLOW US TO STAY UP UNTIL XD15 WITH A 30 MIN RESERVE. THE 25 MINS OF HOLDING OVER MRB WOULD HAVE REDUCED THAT ENDURANCE TIME. AT ABOUT XC50 WHILE ABOUT 20 NM FROM HEF; THE ENG MOMENTARILY SPUTTERED AND I DECIDED TO MAKE A PRECAUTIONARY LNDG IN AN OPEN FIELD; SUSPECTING AN IMPENDING FUEL STARVATION. ON LNDG; WE CHKED THE FUEL LEVEL AND FOUND SMALL AMOUNTS OF FUEL REMAINING IN EACH TANK. WE PUT 10 GALLONS IN THE PLANE; GOT THE OWNER'S PERMISSION; AND DEPARTED THE FIELD FOR MANASSAS.