AN AX10 BALLOON SINGLE PLT MADE AN INADEQUATE PREFLT AND MISSED A CRITICAL ITEM ON HIS FUEL SYS WHICH CAUSED HIM TO MAKE A PRECAUTIONARY LNDG.

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I; ALONG WITH SEVERAL OTHER PLTS; CONTACTED FLT SVC PRIOR TO THE FLT AND DETERMINED THAT IT WAS SAFE TO FLY. I GAVE PAX A SAFETY BRIEFING AND PROCEEDED WITH A VISUAL INSPECTION OF THE BALLOON. DID NOT NOTICE ANY PROBS WITH THE BALLOON OR ITS EQUIP. I PROCEEDED WITH THE INFLATION AND LOADED THE PAX INTO THE BASKET. I THEN TOOK OFF TO 1800 FT AGL. ABOUT 10 MINS INTO THE FLT; I NOTICED A STRONG SMELL OF PROPANE. I PROCEEDED TO FIND THE SOURCE OF THE SMELL AND NOTICED ICING ON THE #2 VALVE OF ONE OF THE TANKS. I IMMEDIATELY ISOLATED THE FUEL BY SHUTTING OFF THE VALVE AND PROCEEDED TO FIND A SAFE PLACE TO LAND AS SOON AS PRACTICAL. I TOUCHED DOWN ON AN EMPTY DIRT LOT AWAY FROM ANY HOMES OR BUILDINGS. I ESCORTED MY PAX OUT OF THE BASKET AND PROCEEDED TO FIND THE SOURCE OF THE FUEL LEAK. I LATER DETERMINED THAT THE AIR RELIEF VALVE ON THE TANK WAS OPEN; SLIGHTLY CAUSING THE ICING OF THE TANK VALVE; AND THAT THERE WAS NOTHING ELSE WRONG. AFTER EXAMINING THE SIT; I BELIEVE THAT I PROBABLY COULD HAVE CHKED THE BLEEDER VALVES ON THE TANKS A LITTLE BETTER BEFORE TAKING OFF.