CONVAIR 580 CARGO FLT ABORTS TKOF AFTER UNSECURED CARGO SLIDES AFT AND CAUSES TAIL TO STRIKE RWY DURING EARLY STAGES OF ACCELERATION.

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TLH-ENW PART 121 SUPPLEMENTAL OPS. CARGO CREW ARRIVED AT TLH TO PICK UP FREIGHT AND DELIVER TO ENW. AFTER LOADING FREIGHT AND STRAPPING IT DOWN IN TLH; WE ARRIVED IN ENW AND UNSTRAPPED THE FREIGHT TO OFFLOAD. DUE TO THE APCHING TSTMS WE DECIDED TO LEAVE THE CARGO DOOR CLOSED UNTIL THE FORKLIFT ARRIVED. AFTER 1.5 HRS OF WAITING FOR THE FORKLIFT WE WERE ASKED TO GO TO MKE; WE DID THE REQUIRED PAPERWORK AND CALLED MKE APCH TO CHK THEIR LCL WX. WE REMOVED THE CHOCKS FROM THE MAINS AND CLOSED THE ENTRANCE DOOR. WE GOT INTO THE SEATS TO GO TO MKE. WE FORGOT WE HAD UNSTRAPPED THE FREIGHT AND BEGAN OUR TAXI. DURING TKOF ROLL; ABOUT 300 FT DOWN THE RWY; THE NOSE CAME OFF THE GND AND THE TAIL HIT THE RWY ABOUT 600 FT DOWN. REALIZING THAT WE COULD DO NO MORE; WE SHUT DOWN THE ENGS. I BELIEVE THAT THE CAUSE OF THIS INCIDENT WAS THE FACT THAT WE NORMALLY OFFLOAD FREIGHT ONCE WE ARRIVE; THE FACT THAT WE WERE READY TO OFFLOAD AND DIDN'T; MADE ME FORGET THAT WE HAD UNSTRAPPED THE FREIGHT. I BELIEVE THAT IF THERE WAS A CHK ON THE CHKLIST FOR A SECURE LOAD; THIS WOULD HAVE BEEN AVOIDED.