A BELL 206 EXPERIENCED A FUEL PUMP LOW PRESSURE LIGHT FOLLOWED BY THE FUEL QTY INDICATOR SUDDENLY DROPPING TO ZERO. COMPLETE FUEL EXHAUSTION OCCURRED DURING AN OFF ARPT PRECAUTIONARY LNDG.

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ON AUG 2006; I OBSERVED THE FOLLOWING CONDITIONS WITH 4 PAX ON BOARD. FLT PLANNING/DISTANCE/TIME/GPH/TOTAL FUEL REQUIRED; INCLUDING RESERVES; WERE CALCULATED PRIOR TO START. AT APPROX 700 FT AGL AND JUST W OF ZZZ1; I OBSERVED THE FUEL PUMP CAUTION LIGHT ILLUMINATE ON THE EMER PANEL. WITHOUT DELAY; I ADVISED THE PAX OF THE CAUTION AND THAT WE WOULD BE LNDG AT ZZZ1. WHILE TURNING INBOUND; I PRESSED THE CAUTION PANEL RESET TO WHICH THE FUEL PUMP LIGHT REMAINED LIT. AT THIS TIME; I WITNESSED THE FUEL GAUGE NEEDLE DROP FROM 13-14 GALLONS INDICATED; STRAIGHT DOWN TO ZERO. I POINTED AT THE GAUGE AS THIS OCCURRED AND WAS ALSO WITNESSED BY THE FRONT SEAT PAX. I IMMEDIATELY DECLARED AN EMER AND WAS CLRED TO LAND ON RWY. I OPTED TO MAKE AN OFF FIELD LNDG DUE TO THE FUEL GAUGE HAVING DROPPED SO RADICALLY. AT APPROX 10 FT ABOVE GND; THE ENG SHUT OFF AND I PERFORMED AN AUTO ROTATION. THE HELI SKIDS SPREAD AND THE PLT SIDE WINDSCREEN CRACKED. NO INJURIES AT ALL WERE SUSTAINED. THE HELI HAS AN STC APPROVED FUEL TANK FILLER EXTENDER WHICH DOES NOT ALLOW FOR VISUAL VERIFICATION OF FUEL ON BOARD. THERE WAS NO FUEL GAUGE REDUNDANCY; FUEL FLOW INDICATOR OR LOW FUEL LIGHT. WHAT CAN BE DONE: IF FUEL TANK FILLER EXTENDERS ARE TO BE USED; THEN FUEL GAUGE REDUNDANCY; FUEL FLOW INDICATOR; LOW FUEL WARNING LIGHT SHOULD BE REQUIRED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THIS EVENT IS BEING INVESTIGATED BY THE FAA AND THAT HE IS BEING THREATENED WITH LICENSE ENFORCEMENT ACTION. THE QUESTION THAT HE WAS ASKED IS; 'HOW DID YOU VERIFY THE AMOUNT OF FUEL ONBOARD PRIOR TO DEP?' AND HE CAN ONLY ANSWER; 'BY READING THE GAUGE' BECAUSE THE FUEL EXTENDER THAT ALLOWS THE TANK TO BE COMPLETELY FILLED ALSO REMOVES THE PLT'S ABILITY TO DIRECTLY OBSERVE THE FUEL QTY LEVEL IN THE TANK. A SIGHT GAUGE IS INCORPORATED IN THE EXTERIOR NECK THAT WILL INDICATE WHEN THE TANK IS COMPLETELY FULL BUT CANNOT INDICATE ANY OTHER LEVEL. THIS PARTICULAR HELI HAS NEVER HAD ITS TANK COMPLETELY FULL WHILE AT THIS FBO BECAUSE THE TYPICAL WX CONDITIONS AT THE ARPT WOULD NOT ALLOW IT TKOF WITH THAT 75-80 GALLON FUEL WT. THE QTY GAUGE ITSELF IS A MECHANICAL DEVICE AND ANY TYPE OF DEBRIS OR RUST CAN MAKE THE ASSEMBLY STICK AND THEREFORE READ INACCURATELY.