Narrative:

Toronto approach instructed flight X to turn right to FL250 heading and intercept, descend on the GS. Previous altitude assigned was 4000 ft. During the intercept, aircraft was allowed to descend to 3600 ft MSL and the controller caught it and told flight X to maintain altitude until on the GS. Autoplt had been disconnected by PF. Different phraseology by canadian controllers, ie, descend on GS vice maintain 4000 ft until established may have contributed to this error. Supplemental information from acn 449079: at 4000 ft on a 210 degree heading to intercept localizer, ATC cleared us to descend on GS. Captain dialed in 2400 ft and began slow descent. I was doing flaps and checklist. I saw descent simultaneously with ATC call telling us to level at present altitude until GS intercept. We were at 3500 ft. There was no conflict and call to ATC said they had no problem. Confusion was due to difference between canada and united states clearance which would have been maintain 4000 ft until GS intercept.

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Original NASA ASRS Text

Title: B757 CREW DSNDED BELOW THE GS IN CANADIAN AIRSPACE.

Narrative: TORONTO APCH INSTRUCTED FLT X TO TURN R TO FL250 HDG AND INTERCEPT, DSND ON THE GS. PREVIOUS ALT ASSIGNED WAS 4000 FT. DURING THE INTERCEPT, ACFT WAS ALLOWED TO DSND TO 3600 FT MSL AND THE CTLR CAUGHT IT AND TOLD FLT X TO MAINTAIN ALT UNTIL ON THE GS. AUTOPLT HAD BEEN DISCONNECTED BY PF. DIFFERENT PHRASEOLOGY BY CANADIAN CTLRS, IE, DSND ON GS VICE MAINTAIN 4000 FT UNTIL ESTABLISHED MAY HAVE CONTRIBUTED TO THIS ERROR. SUPPLEMENTAL INFO FROM ACN 449079: AT 4000 FT ON A 210 DEG HDG TO INTERCEPT LOC, ATC CLRED US TO DSND ON GS. CAPT DIALED IN 2400 FT AND BEGAN SLOW DSCNT. I WAS DOING FLAPS AND CHKLIST. I SAW DSCNT SIMULTANEOUSLY WITH ATC CALL TELLING US TO LEVEL AT PRESENT ALT UNTIL GS INTERCEPT. WE WERE AT 3500 FT. THERE WAS NO CONFLICT AND CALL TO ATC SAID THEY HAD NO PROB. CONFUSION WAS DUE TO DIFFERENCE BTWN CANADA AND UNITED STATES CLRNC WHICH WOULD HAVE BEEN MAINTAIN 4000 FT UNTIL GS INTERCEPT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.