THE HELICOPTER (HEL) WAS BEING USED ON AN EXTERNAL LOAD OPN TO PSN A 2500 LB VENTILATOR TO THE ROOF OF A BLDG. ON THE NEXT TO LAST LOAD, THE PLT RADIOED THAT HE NOTED A MOMENTARY ILLUMINATION OF THE FUEL BYPASS ADVISORY LIGHT. HE ELECTED TO CONT THE OPN & RETURNED TO THE STAGING AREA TO PICK UP THE 2500 LB LOAD. WHEN THE LOAD WAS APRX 8' FM THE ROOF TOP, WITNESSES NOTED THE LOAD (& HEL) BGN TO SETTLE. THE PLT RLSD THE LOAD AS THE ROTOR RPM BGN DECREASING & MNVRD AWAY FM THE ROOF. THE HEL HOVERED MOMENTARILY OVR AN ELEVATED CONVEYOR, THEN THERE WAS AN ADNL LOSS OF RPM. AS THE HEL DSCNDD, THE MAIN & TAIL ROTORS HIT THE CONVEYOR & THE HEL FELL APRX 150' & CRASHED IN A NOSE DOWN ATTITUDE. THE FWD FUEL BLADDERSPLIT OPEN DRG IMPACT. THE AFT FUEL BLADDER CONTAINED A USABLE, BUT UNDETERMINED, AMOUNT OF FUEL. DIRT WAS FND IN THE FUEL BLADDERS & FUEL FILTERS, BUT NO FUEL BLOCKAGE WAS NOTED. FUEL X-FEED SELECTOR & VLV WERE FND IN THE OPEN PSN. THE FLT MANUAL LIMITS THE USE OF X-FEED FOR EMERGENCY & CRUISE FLT ONLY; BOOST PUMP MUST BE ON WHEN UNUSED TANK IS EMPTY.