FAILURE TO VERIFY 'A' HYDRAULIC PUMPS OFF PRIOR TO PUSH BACK AND ENGINE START RESULTS IN A BROKEN TOW BAR SHEAR BOLT ON A B737-700.

Narrative

DURING THE TURNAROUND; I LEFT THE ACFT TO GET US BOTH COFFEE. CAPT STARTED TAKING CARE OF THE TURNAROUND DUTIES; ATIS; ETC. DESPITE THE LAST LEG OF THE TRIP AND RUNNING JUST A FEW MINS LATE; NEITHER ONE OF US WAS SUFFERING FROM ANY GET HOMEITIS. NEITHER ONE OF US WAS RUSHING OR HURRIED. IT WAS A GOOD TRIP; NO CREW CONFLICTS OF ANY KIND. I RETURNED TO THE ACFT AND WE SOON RAN THE BEFORE START CHKLIST. CAPT HAD THE ELECTRIC 'A' HYD PUMP ON AND I DIDN'T NOTICE IT DURING THE CHKLIST; DESPITE PHYSICALLY LOOKING RIGHT AT THE SWITCHES. PUSHBACK BEGAN AND A LOUD 'CLUNK' WAS HEARD. CAPT REALIZED IMMEDIATELY WHAT HAD HAPPENED -- SHEAR BOLT/PIN IN TOW BAR HAD BROKEN. I'M REALLY DISAPPOINTED THAT I DIDN'T CATCH THIS ERROR WHEN WE RAN THE CHKLIST. I MAKE A CONSCIOUS EFFORT EVERY TIME I RUN A CHKLIST TO NOT FALL INTO THE TRAP OF JUST READING THE LIST. I NEVER HURRY IT AND I LOOK AT THE SWITCH OR SYS TO MAKE SURE THINGS ARE SET CORRECTLY. BUT; SOMEHOW; I GOOFED. I REMEMBER LOOKING RIGHT AT THE SWITCHES -- DON'T KNOW HOW I COULD HAVE MISSED IT. LESSON LEARNED: BEFORE I CALL FOR PUSHBACK; INSTEAD OF JUST ENSURING THE DOORS ARE SHUT AND THE GND PWR IS DISCONNECTED; I AM GOING TO MAKE A CONSCIOUS EFFORT TO DOUBLECHK THE 'A' HYD SYS PRESSURE IS READING '0' LIKE IT SHOULD BE. ONE MORE WAY FOR ME TO XCHK OUR OURSELVES IN CASE WE MISS THE INCORRECT SWITCH POS. I NEED TO VERIFY THAT 'A' SYS HYD PRESSURE IS READING '0' BEFORE CALLING THE BEFORE PUSH CHKLIST COMPLETE. EVEN THOUGH THAT'S NOT PART OF THE CHKLIST; I THINK THAT'S A GOOD LOGICAL PLACE FOR ME TO TRY AND CATCH THIS MISTAKE BEFORE THE PUSHBACK TUG GETS UNDER WAY. THIS STEP WOULD HAVE CAUGHT THE MISTAKE WE MADE.