ANGLE OF ATTACK VANE DAMAGED WHEN ACFT ROLLED BACKWARDS INTO THE JET BRIDGE.

Narrative

THE ACFT HAD BEEN BOARDED AND WE WERE ABOUT TO PUSH 7 MINS LATE OF SCHEDULED DEP TIME. AS THE GATE AGENT CLOSED THE FORWARD CABIN DOOR; I RELEASED THE PARKING BRAKE (INTENDING TO DO SO MOMENTARILY TO TRIP THE 'OUT' FUNCTION OF THE ACARS AND THEN TO RESET IT WHILE COORD WITH GND CREW TOOK PLACE). THE ACFT MOVED BACKWARDS ABOUT 18 INCHES; ENOUGH FOR THE JETWAY; WHICH WAS NOT YET RETRACTED FROM THE ACFT; TO IMPACT WITH THE ALPHA VANE ON THE SIDE OF THE FUSELAGE. THE ALPHA VANE WAS DAMAGED AND WAS SUBSEQUENTLY REPLACED BY MAINT BEFORE WE DEPARTED. NO GND PERSONNEL WERE INJURED. RELEASING THE PARKING BRAKE WITHOUT FIRST CONFIRMING THAT ACTION WITH THE GND HEADSET OPERATOR WAS IMPROPER. I WAS MOTIVATED BY THE LATE PUSH TO GIVE THE BEST OUT TIME POSSIBLE TO THE GATE AGENT. AT THE TIME I RELEASED BRAKES; THE ACFT WAS ATTACHED TO THE PUSHBACK TUG AND TOWBAR. THE ACFT MOVED BACKWARDS ON ITS OWN; PULLING THE TRACTOR UP ONTO A CHK. THE TRACTOR RPTEDLY HAD ITS PARKING BRAKE SET. SUBSEQUENT TO THIS; WHEN WE COMMENCED THE ACTUAL PUSHBACK; THE ACFT DID NOT MOVE WHEN BRAKES WERE RELEASED; SO I DO NOT THINK THE RAMP SLOPED SUFFICIENTLY TO CAUSE THIS MOVEMENT.