ACR MLG TOW BAR DAMAGED BECAUSE ACFT HYDRAULIC SYSTEM WAS PRESSURIZED DURING PUSHBACK.

Narrative

THE APU OF THIS MLG WAS ONLY ABLE TO SUPPLY PNEUMATICS SO THAT GND PWR WAS PLUGGED IN AND USED AT THE GATE. THE MECHANIC AND I DISCUSSED WHETHER TO START 1 ENG OR BOTH ENGS AT THE GATE AND HE PREFERRED STARTING BOTH. HE MENTIONED THAT AFTER ENG GENERATORS WERE ON LINE THAT HE WOULD REMOVE GND PWR. SINCE WE WERE BOTH CONCERNED WITH ON TIME DEPS; THE SOONER WE COULD GET GND PWR REMOVED THE SOONER WE COULD GET RAMP CLRNC FOR PUSHBACK. WE STARTED BOTH ENGS AFTER COMPLETING THE BEFORE START AND BEFORE PUSHBACK/TAXI CHKLISTS. AS PREVIOUSLY DISCUSSED WITH THE MECH; I GOT BOTH GENERATORS ON THE LINE AND CHKED THEM FOR OUTPUT AND THEN; EVIDENTLY; TURNED ON BOTH 'A' HYD PUMPS. NORMALLY THE 'A' PUMPS ARE TURNED ON IMMEDIATELY AFTER THE GENERATORS ARE SELECTED AND CHKED. I TOLD THE MECH THAT HE WAS CLEAR TO DISCONNECT THE GND PWR. WE GOT PUSHBACK CLRNC FROM THE RAMP AND WE PUSHED BACK ABOUT 20' AT WHICH TIME A DISTINCT NOISE WAS HEARD UNDER US. THE MECH THEN PULLED US BACK INTO THE GATE. THE TOW BAR SHEAR BOLTS WERE REMOVED AND REPLACED SINCE THEY HAD SHEARED BECAUSE THE 'A' SYS WAS PRESSURIZED. NO OTHER DAMAGE OCCURRED TO THE ACFT AND NO INJURY RESULTED. THE INCIDENT OCCURRED BY USING NORMAL 'FLOW PATTERN' PROCS DURING AN ABNORMAL CONDITION. THERE IS NO WRITTEN PROC IN OUR MANUAL FOR THE APU ELEC INOP START. THE F/O WAS IN HIS FIRST MONTH IN THAT SEAT. I SUGGEST THAT DURING ANY ABNORMAL CONDITION; WHERE SWITCHES ARE POSITIONED DIFFERENTLY FROM NORMAL; THAT APPLICABLE CHKLISTS BE REPEATED AS NECESSARY. HAD I CALLED FOR THE BEFORE PUSHBACK CHKLIST AFTER THE ENGS WERE STARTED AND THE GENERATORS SWITCHED (AND 'A' PUMPS TURNED ON); I WOULD HAVE CAUGHT THE PROB.