A B727 FLC DSNDING IN BOS; MA; LEAVES 6000 FT TO BEGIN THE APCH; THEN THEY WONDER IF THEY WERE REALLY CLRED FOR THE APCH.

Narrative

WHILE ON APCH TO BOS RWY 4R ON LOC AND GS THE ALT ALERTER IN THE COCKPIT SOUNDED. WE WERE AT 5900 FT DSNDING AND THE ALT ALERTER WAS SET AT 6000 FT. WE WERE UNSURE IF WE HAD BEEN CLRED FOR THE APCH TO FOLLOW GS DOWN SO WE LEVELED OFF ABOUT 5500 FT AND STARTED A SLOW CLB BACK TO 6000 FT (NOW WELL ABOVE THE GS). THE APCH CTLR THEN CONFIRMED WE WERE CLRED ILS RWY 4R APCH SO WE REINTERCEPTED GS AND LANDED UNEVENTFULLY. SUPPLEMENTAL INFO FROM ACN 395320: OUR DSCNT WAS INITIATED USING THE GS AS A REF. PARTICULAR ATTN WAS PAID TO ACFT SPACING AS THERE WERE NUMEROUS ACFT ON THE EXTENDED FINAL TO RWY 4R. OUR ALTDEV 'MAY' HAVE OCCURRED AS WE CONTINUED ALONG THE GS BELOW 6000 FT MSL. WE RECEIVED CLRNC FOR THE ILS MOMENTS AFTER WE DSNDED BELOW 6000 FT MSL. WE LANDED WITHOUT INCIDENT OR COMMENT FROM THE CTLR. THIS EVENT FOCUSES THE REQUIREMENT FOR VIGILANCE EVEN ON A CLR DAY WITH VIRTUALLY NO DISTRACTIONS IN EVIDENCE. OUR DESIRE TO 'FOLLOW' THE GS AND MAINTAIN SPACING (TCASII) WAS JUST ENOUGH TO LEAVE THE CREW WONDERING ABOUT THE POSSIBLE DEV.