FLT INSTRUCTOR OF SMA TRAINING ACFT INADVERTENTLY RAN OUT OF FUEL AND LANDED OFF ARPT.

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IN THE RETURN PORTION OF THE FLT FROM THE CENTERVILLE AREA N OF HOUSTON; STRONG HEADWINDS WERE ENCOUNTERED. MY JUDGEMENT OF HOW MUCH EVIDENCE I HAD AVAILABLE WAS BASED ON AN AVERAGE ESTIMATE FROM THE HANDBOOK; USING PREVIOUS EXPERIENCES IN THE AIRPLANE; I ESTIMATED THE AIRPLANE WOULD HAVE 4 HRS OF FUEL ON BOARD. I RELIED ON THESE CALCULATIONS RATHER THAN THE FUEL GAUGES; BECAUSE OF THE NOTORIOUSLY UNRELIABLE NATURE OF THOSE INSTS IN THESE SMALL AIRPLANES. CONSEQUENTLY; I FLEW BY RADISONVILLE AND HUNTSVILLE ARPTS. MY OTHER CONCERN WAS THAT IF I DID DIVERT FOR FUEL; THERE WOULD BE NONE AVAILABLE THERE; THUS LEAVING ME WITH NO POSSIBLE MEANS OF GETTING BACK TO HOOKS; BUT I DID NOT THINK TO USE THE UNICOM TO CHK; HAD I FOUND OUT THERE WAS FUELING AVAILABLE; I WOULD HAVE DIVERTED. THIS IS NOT AN 'AFTER-THE-FACT' THOUGHT. ONCE I HAD GOTTEN RADAR CONTACT FROM HOUSTON APCH; I ASKED THEM FOR MY GND SPD AND DISTANCE TO THE ARPT. THEIR REPLY WAS 60 KTS AND 21 MI. AT THIS POINT; I ESTIMATED STILL ON 40-50 MINS OF FUEL L. SOON AFTER THIS; HOWEVER; NERVOUS ABOUT MY EXACT FUEL STATE; I ELECTED TO CLB ANOTHER 500 FT FROM 2500 FT TO SEE IF I COULD PICK UP ON A RPTED TAILWIND AT THAT ALT. UNFORTUNATELY; THERE WAS NO NOTICEABLE CHANGE IN GND SPD. I WAITED LATE TO BEGIN MY DSCNT; BUT; WHEN I DID; IT WAS A SLOW DSCNT (300-400 FPM); STAYING ABOVE THE VASI GLIDEPATH; IN THE HOPE THAT THE EXTRA SPD WOULD MAKE UP FOR SOME OF THE SLOWER PROGRESS EARLIER. I HAD BECOME ACUTELY AWARE OF OUR TIME ALOFT AS NIGHT FELL; AND THE FACT THAT THE FUEL GAUGES WERE GETTING LOW. HOWEVER; THE FIRST LINK IN THE CHAIN HAD BEEN WHEN I DID NOT CHK THE EXACT TIME OFF THE GND; EVEN THOUGH I MEANT TO; AND SO MY TIME CALCULATIONS WERE AN ESTIMATE. I ALSO BACKED MYSELF INTO A CORNER WHERE I HAD TO MAKE HOOKS ARPT OR CRASH; BECAUSE I TOOK THE MOST DIRECT RTE POSSIBLE. AS IT TURNED OUT; THE AIRPLANE DID NOT HAVE 4 HRS OF ENDURANCE BUT JUST OVER 3 AND 1/3; HOWEVER; I WAS NOT CONVINCED OF THAT AT THE TIME. I ALSO BELIEVE THAT I FELL INTO A 'GET HOMEITIS' TRAP.