A B757 IS DISCONNECTED FROM THE TOW BAR WITHOUT THE BRAKES SET DUE TO A POSSIBLE COM FAILURE. ACFT MOVED FORWARD UNTIL CAPT APPLIED BRAKES.

Narrative

PRIOR TO PUSHBACK; WE WERE NOTIFIED VIA ACARS THAT THE HEADSET FOR THE PUSH CREW WAS MALFUNCTIONING AND HAND SIGNALS WOULD BE USED. THE PUSHBACK CREW THEN CONTACTED US VIA FLT INTERPHONE LOUD AND CLR. NO MENTION WAS MADE BY THE PUSH CREW ABOUT THE HEADSET OR USING HAND SIGNALS. PUSHBACK WAS COMMENCED NORMALLY FOR A W TAXI USING FLT INTERPHONE. WE WERE GIVEN CLRNC TO START BOTH ENGS DURING PUSHBACK. WHEN STOPPED; FACING W; AND STARTING THE R ENG; 'TOW BAR DISCONNECTED' MESSAGE WAS HEARD VIA FLT INTERPHONE. I ACKNOWLEDGED AND FELT US STARTING TO MOVE FORWARD. LOOKING UP; I SAW THE TUG PULLING AWAY AND THOUGHT WE WERE STILL CONNECTED AND THAT HE WAS PULLING US FORWARD. ONCE I REALIZED THAT WE WERE DISCONNECTED; I APPLIED THE BRAKES. FORTUNATELY; NO DAMAGE OR INJURY RESULTED AND WE ACCEPTED SALUTE AND RELEASE. AT NO TIME DID I RECEIVE OR ACKNOWLEDGE 'SET BRAKES' COMMAND EITHER BY INTERPHONE OR HAND SIGNAL. THE BRAKES WERE NOT SET. HUMAN FACTORS: 1) THE HEADSET MAY HAVE MALFUNCTIONED AGAIN DURING PUSHBACK; CAUSING COM FAILURE. WE SHOULD HAVE USED HAND SIGNALS FOR THE ENTIRE PUSHBACK/ENG START/RELEASE SEQUENCE. 2) WHILE MONITORING THE R ENG START; I HEARD 'TOW BAR DISCONNECTED' ON FLT INTERPHONE; AND SHOULD HAVE IMMEDIATELY REALIZED THE 'SET BRAKES' COMMAND HAD NOT BEEN GIVEN. HEARING AN EXPECTED ROUTINE STATEMENT; MADE IT EASY TO MISS THE FACT THAT A PRIOR STATEMENT WAS NOT SPOKEN. 3) THE INITIAL VISUAL ILLUSION THAT WE WERE BEING TOWED FORWARD; CAUSED CONFUSION AND A DELAY IN BRAKE APPLICATION.