AN ATR WITH A DEFERRED FUEL QUANTITY SYSTEM ANNUNCIATED A FUEL LOW LEVEL LIGHT AFTER TAKEOFF. THE CREW FORGOT THIS WAS A NORMAL INDICATION WITH THE KNOWN FAULT AND IN ERROR THEY RETURNED TO LAND.

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UPON PREFLIGHT; IT WAS NOTICED FUEL INDICATOR READ 1200 POUNDS LOW DESPITE THAT THE NORMAL FUEL LOAD WAS PUMPED INTO THE AIRCRAFT. A MAGNETIC INDICATOR TEST WAS PERFORMED BY THE MECHANICS AND TRANSLATED IN THE MANUAL TO COMPUTE EXACT FUEL LOAD. INDICATOR SYSTEM WAS FOUND DEFECTIVE AND DEFERRED. AFTER TAKEOFF UPON CLIMBING OUT OF APPROXIMATELY 5000 FT THE FUEL INDICATOR WENT TO ALL ZEROS. THE FUEL LOW LEVEL LIGHT CAME ON PROMPTING A MASTER CAUTION AND SIMULTANEOUSLY ACTIVATING THE SYSTEM'S BACKUP ELECTRIC FUEL PUMP CAUSING A RUN LIGHT. GIVEN ALL KNOWLEDGE AT HAND; INCLUDING A 2 1/2 HOUR CRUISE TO ZZZ1 WITH A BACKUP SYSTEM OPERATING INTO KNOWN THUNDERSTORMS; ADDED TO LOW VISIBILITY AND WINTRY SNOW AND FREEZING RAIN AT 100 MILE RADIUS SURROUNDING AIRPORTS ADDED TO THE SYSTEM FAILURE; THE CAPTAIN DECIDED TO RETURN TO ZZZ FOR A PRECAUTIONARY LANDING. THIS EVENT OCCURRED AFTER DEPARTING FROM ZZZ WITH A MAINTENANCE DEFERRAL OF THIS SYSTEM. THE SYSTEM RESTORED ITSELF TO NORMAL AND BEGAN INDICATING AN ACCURATE 3600 POUNDS OF FUEL IN THE RIGHT TANK. PAPERWORK WAS REFILED; THE MECHANIC SIGNED OFF THE DEFERRAL AFTER OPS CHECKS WERE SATISFACTORY; AND THE FLIGHT TOOK OFF AND FLEW TO ZZZ1 WITHOUT FURTHER COMPLICATIONS AND WITH A 2 HOUR DELAY. THE EVENT COULD HAVE BEEN AVOIDED HAD MANY FACTORS NOT CAUSED HIGH TENSION SUCH AS THE FREIGHT BEING LATE AND DEALING WITH DOING THE MAGNETIC INDICATOR TEST OF THE AIRCRAFT AND MANUALLY COMPUTING THE FUEL LOAD; AND RECALCULATING THE CENTER OF GRAVITY. THE FLIGHT WAS ALREADY PUSHING A DELAY AND ADEQUATE TIME WAS NOT TAKEN TO STOP AND BREATHE THEN DISCUSS ALL POSSIBILITIES OR AT LEAST THE MOST COMMON ONES RELATING TO THIS SYSTEM. EXAMPLE; IF THE INDICATOR DROPS BELOW 352 POUNDS OF FUEL; THE LOW LEVEL LIGHT ON THE FUEL INDICATOR PANEL AUTOMATICALLY ILLUMINATES WITH A MASTER CAUTION AND SIMULTANEOUSLY ACTIVATES THE RUN LIGHT ON THE BACKUP ELECTRIC FUEL PUMP. THIS IS A SCENARIO THAT BOTH CREW MEMBERS KNEW WAS A POSSIBILITY; HOWEVER; DID NOT HAVE -- TAKE TIME TO DISCUSS IN THE HURRY OF AN ALREADY LATE FLIGHT DURING PEAK SEASON. THE PRECEDING EVENTS COMBINED WITH THE CURRENT AND UPCOMING WEATHER AHEAD CAUSED THE CAPTAIN'S DECISION TO RETURN TO ZZZ. I WOULD SUGGEST POSITIVE REINFORCEMENT TO THE CAPTAIN FOR TAKING THE SIDE OF SAFETY IN AN UNCERTAIN SITUATION AND TRY AND USE IT AS A LEARNING TOOL THAT SUCH A SITUATION COULD HAVE BEEN SAFELY FLOWN TO THE DESTINATION. THIS COULD HAVE BEEN ACCOMPLISHED WITHOUT THE SNOWBALL EFFECT AND CONTRIBUTING FACTORS ON THE GROUND PRECEDING OUR TAKEOFF. THE BEST THING TO LEARN FROM THIS EVENT IS THAT SINCE BOTH CREW MEMBERS KNEW THE SYSTEMS WELL ENOUGH TO EXPECT WHAT HAPPENED; WE WEREN'T MENTALLY PREPARED FOR IT TO HAPPEN ON INITIAL CLIMBOUT. I THINK THE MEL SHOULD HAVE ALLUDED TO THE FACT TO IGNORE WARNINGS FROM THIS SYSTEM INCLUDING THE SCENARIO PREVIOUSLY STATED. FURTHERMORE; WE (THE CREW) SHOULD HAVE TAKEN A FEW MINUTES LONGER DELAY ON THE GROUND TO DISCUSS OUR SITUATION AND ITS POSSIBLE COMPLICATIONS PERTAINING TO THE WEATHER OPPOSED TO TAKING OFF WITH A SMALLER DELAY AND BEING UNPREPARED FOR A SITUATION WE HADN'T THOUGHT OUT. AT THE END OF THE DAY; SAFETY REMAINED THE #1 PRIORITY AND EVERYONE LEARNED FROM THE MISTAKE WHICH IS WHY WE ARE ALL HERE -- TO TRANSPORT CARGO FROM 'POINT A' TO 'POINT B' SAFELY; LEGALLY; AND ON TIME IN THAT EXACT ORDER.