A B737-700 FO ADMITTED THAT HE AND THE CAPT FORGOT TO RUN THE BEFORE START CHKLIST; DUE TO A CHANGE IN PROC AND OTHER DISTRS.

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AT XXXX CENTRAL; STARTED THE APU. BOTH CAPT AND I WERE IN OUR SEATS. AS I WAS PUTTING THE APU ONLINE; 'A' FLT ATTENDANT ENTERED THE FLT DECK TO BRIEFLY SPEAK WITH THE CAPT. AS THEY WERE WRAPPING UP; OPS AGENT HANDED ME LOAD MANIFEST AND I ENTERED THE DATA INTO THE OPC AND FMC; AND SHOWED IT TO THE CAPT. ENTRY DOOR CLOSED AND I CALLED FOR PUSH. AS WE BEGAN THE PUSH; WE STARTED ENG #1. PRIOR TO STARTER CUTOUT; THERE WAS A LOUD 'BANG' AND THE CAPT IN COORD WITH PUSHBACK CREW SET BRAKES. PUSHBACK CREW VERIFIED THAT THE TOWBAR SHEAR PIN HAD BROKEN; AND THAT THE NOSE GEAR LOOKED FINE. WE HAD BEEN PUSHED BACK FAR ENOUGH TO TURN OUT AND TAXI WITHOUT FURTHER PUSH; SO THE TOWBAR WAS DISCONNECTED AND PUSHBACK CREW DEPARTED. AT THAT TIME; WE COMPLETED THE BEFORE START CHKLIST; STARTED #2 ENG; COMPLETED THE PRETKOF CHKLIST DOWN TO THE LINE; CALLED FOR TAXI; AND CONTINUED THE FLT. THIS EVENT WAS DIRECTLY CAUSED BY OUR FAILURE TO ACCOMPLISH THE BEFORE START CHKLIST. I HAD BEEN THINKING ABOUT THE CHKLIST; BUT WHEN THE LOAD MANIFEST WAS HANDED TO ME; I WENT INTO 'GO' MODE; CALCULATED AND ENTERED THE TKOF DATA; REVIEWED IT WITH THE CAPT; AND CALLED FOR PUSH. I CERTAINLY SHOULD NOT HAVE DEALT WITH THE TKOF DATA UNTIL CHKLIST COMPLETED. I DO BELIEVE THAT A CONTRIBUTING FACTOR IS THE FACT THAT THERE IS A LOT OF PRESSURE FOR THE FO TO GET THE DATA DONE; AND OPC STOWED; SO THAT PUSHBACK MAY COMMENCE. I THINK THAT THE NEW (RELATIVELY) PROC OF COMPLETING ALL DATA PRIOR TO PUSH; AND PRETKOF CHKLIST DOWN TO THE LINE PRIOR TO TAXI IS A GOOD THING. ABOUT THE ONLY NEGATIVE IS THAT THE FO IS NOW VERY PRESSURED TO GET DATA DONE. USUALLY; THE LOAD MANIFEST ARRIVES IN THE FLT DECK AT PUSH TIME; IF NOT A MIN OR TWO LATE. NOW THERE IS THIS BIG CRUSH TO GET DATA DONE; AND GET PUSHED; ETC. THIS IN NO WAY EXCUSES OR JUSTIFIES THE OMISSION OF THE CHKLIST. IT JUST IS A FACTOR THAT PUT ME INTO THE 'GO' MODE AND I NEVER THOUGHT ABOUT THE BEFORE START CHKLIST AGAIN UNTIL THE TOWBAR SHEAR PIN BROKE. IN THE FUTURE; I WILL NOT CALCULATE TKOF DATA UNTIL I VERIFY THAT THE BEFORE START CHKLIST HAS BEEN COMPLETED. IF IT COSTS AN EXTRA 30 SECONDS OR MIN; THAT IS A SMALL PRICE WHEN COMPARED TO BROKEN EQUIP AND SAFETY RISK.