SMA ACFT ON NIGHT TRAINING FLT; DISTR BY ERRONEOUS FUEL GAUGE; INSTRUCTOR PLT ALLOWED TRAINEE TO FLY INTO CLASS D AIRSPACE WITHOUT CLRNC.

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ON A VFR NIGHT XCOUNTRY FROM RYY TO MCN; I ACCIDENTALLY FLEW INTO WARNER ROBINS AFB AIRSPACE (CLASS D) WITHOUT ESTABLISHING 2-WAY COMS. WE HAD CONTACTED MCN APCH; APPROX 30 MI OUT AND HAD STARTED OUR DSCNT FROM 5500 FT MSL TO TFC PATTERN ALT WHEN I NOTICED THAT THE FUEL GAUGE OF OUR L TANK WAS ONLY INDICATING 1/4 OF A TANK AND THE L ONE WAS INDICATING EMPTY. AS I HAD PERSONALLY PREFLTED THE AIRPLANE (MY STUDENT WAS LATE) AND HAD IT TOPPED OFF TO 24.5 GALLONS; I KNEW THERE WAS NO WAY THE 1 HR FLT COULD HAVE CAUSED SUCH A FUEL CONSUMPTION AND I BECAME WORRIED ABOUT A POSSIBLE UNSECURED GAS CAP OR A STUCK DRAIN THAT MAY HAVE CAUSED US TO DUMP FUEL. AS WE WERE APCHING MCN WE ASKED APCH IF THERE WAS A POSSIBILITY TO GET FUEL AFTER XA00. THEY UNSUCCESSFULLY TRIED TO CONTACT THE LCL FBO AND TOLD US TO SWITCH TO UNICOM. THE FBO TOLD US THERE WAS NO WAY TO GET FUEL AND WE CAME BACK ON THE APCH FREQ TO HEAR THE CTLR TELL US THAT 'RADAR SVCS WERE TERMINATED AND TO SQUAWK VFR.' AT THIS MOMENT; MY PREOCCUPATION TO FIND FUEL/DETERMINE THE ORIGIN OF THE PROB; CAUSED ME TO STOP MONITORING MY STUDENT FOR APPROX 2 MINS. IN THE MEANTIME; WE LEFT THE ARPT UNDER OUR L WING AND FLEW INTO WARNER ROBINS AFB AIRSPACE. I REALIZED THE PROB WHEN I SAW THE AFB BEACON LIGHTS AND THE VOR/DME INDICATED WE HAD PASSED MIDDLE GEORGIA. INITIATED A TURN BACK TO THE N WHEN THE FSS CALLED ON MIDDLE GEORGIA FREQ INDICATING HE HAD RECEIVED A CALL FROM THE RADAR SVCS INDICATING WE WERE FLYING TOWARDS THE AFB. WE LANDED SAFELY AT MIDDLE GEORGIA AND FOUND OUT THE GAUGE WAS THE CULPRIT AND HAD 17 GALLONS LEFT. WE FLEW BACK WITHOUT ANY FURTHER PROB. THE PROB WAS CAUSED BY A MINOR DYSFUNCTION PERCEIVED AS A POSSIBLE THREATENING SIT; BY THE INTERRUPTION OF RADAR SVCS; BY THE RELUCTANCE TO DECLARE AN EMER AND ASK FOR HELP; ALSO FATIGUE (THE FLT STARTED 14 HRS AFTER THE BEGINNING OF THE WORKING DAY) PLAYED A ROLE AND I FELT MY DECISION MAKING ABILITIES/RESPONSE TIME WERE SLOWED DOWN; CAUSING ME TO STOP MONITORING MY STUDENT AND HAVE THE NAVIGATIONAL TASK/LOCATION OF THE ARPT LOSE PRIORITY FOR A FEW MINS AT A CRITICAL MOMENT AS MIDDLE; GA; AND ROBINS AFB AIRSPACE ARE SO CLOSE TO ONE ANOTHER.