AN LGT TRANSPORT; USING 1 ENG WITH INOP GENERATOR ON TAXI OUT; STARTS THE R ENG WITH APU PNEUMATICS; APU DROPS OFF AND ACFT ROLLS FORWARD. ACFT ON BATTERY PWR WHEN APU RESTARTED AND BRAKES APPLIED. EMER DECLARED DURING EVENT.

Narrative

DURING TAXI OUT TO RWY 26L AT ATL; THE SECOND ENG START SEQUENCE WAS BEGUN AS THE ACFT WAS STOPPED ON TXWY E SHORT OF TXWY E13. WHEN THE STARTER ENGAGED; THE APU DROPPED OFFLINE. THE FIRST ENG WAS ALREADY STARTED BUT HAD THE GENERATOR MCO INOP. THE ACFT DROPPED PWR TO STANDBY MODE. AS WE ATTEMPTED TO RESTART THE APU THE BRAKES CEASED TO HOLD THE ACFT AND IT SLOWLY BEGAN TO TURN S OFF THE TXWY TOWARD THE MAINT HANGAR DOWN THE HILL. WITH THE PWR FAILURE; IT WAS DIFFICULT TO IDENT WHAT SYS WERE OPERATING AS THE BRAKE GAUGE WAS INOP. AS THE ACFT APPEARED IT WOULD CONTINUE OFF THE TXWY; AN EMER WAS DECLARED WITH REQUESTS FOR CHOCKS AND A TOW TRACTOR. AT THIS POINT THE APU CAME BACK ONLINE (WE HAD COMMENCED ANOTHER APU START SEQUENCE BUT THERE WERE NO INDICATIONS AVAILABLE ON STANDBY PWR) RESTORING FULL PWR AND BRAKES. THE ACFT WAS STOPPED IN ITS POS UNTIL THE GND EQUIP CONFIRMED WE WERE STILL ON THE TXWY AND HAD NOT STRUCK ANY TAXI LIGHTS. THE SECOND ENG WAS STARTED AND ALL SYS SUBSEQUENTLY CHKED GOOD. THE EMER WAS CANCELED AND AFTER CONFIRMING WITH MAINT WE RECEIVED PERMISSION TO CONTINUE THE FLT TO SLC UNEVENTFULLY. IF THE R ENG HAD BEEN STARTED AT THE GATE; THE ADDITIONAL GENERATOR WOULD HAVE AVOIDED THE ENTIRE SIT BECAUSE THE APU WOULD NOT HAVE RECEIVED AN OVERLOAD AND THERE WOULD HAVE BEEN FURTHER BACKUPS FOR THE BRAKES. NORMAL BRAKES RAN OFF THE R HYD SYS.