Narrative:

Passing linng at 250 KTS, 10000 ft, we were cleared to 'fly the youth 1 arrival.' believing that allowed descent also to cross vixan 'at or above 3100 ft' as depicted on the chart, we set 3100 ft in the MCP and began a VNAV descent. When approach control noticed our descent (through about 9600 ft), they cleared us to maintain 9000 ft, which we did. That clearance clued us to the fact that we should have stayed at 10000 ft and that the arrival was on RNAV/STAR and not a profile descent as we were accustomed to doing into yyz. Also, earlier in the day, going into newark, we were cleared to 'descend via the dylin 1' which allowed descent to restrs as depicted on the chart. This set a train of thought, we both agree, which was a factor in our actions on the youth 1 arrival. After landing, tower requested I contact toronto approach control. I acknowledged that beginning descent was my mistake and that altitude changes must be given by ATC. We also determined that the note at vixan intersection (on this new procedure) reads 'at or above 3100 ft' and the canadian note reads '3000 ft'). A note on the commercial chart saying something like 'descend only as cleared by ATC' would be helpful. There was no traffic conflict with this altitude deviation. I also cite fatigue as a contributing factor as we were on duty 13 hours 15 mins at the time of the incident.

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Original NASA ASRS Text

Title: A B734 CREW, DSNDING INTO CYYZ, MISINTERPED THE ARR, DSNDING PREMATURELY WITHOUT AN ATC CLRNC.

Narrative: PASSING LINNG AT 250 KTS, 10000 FT, WE WERE CLRED TO 'FLY THE YOUTH 1 ARR.' BELIEVING THAT ALLOWED DSCNT ALSO TO CROSS VIXAN 'AT OR ABOVE 3100 FT' AS DEPICTED ON THE CHART, WE SET 3100 FT IN THE MCP AND BEGAN A VNAV DSCNT. WHEN APCH CTL NOTICED OUR DSCNT (THROUGH ABOUT 9600 FT), THEY CLRED US TO MAINTAIN 9000 FT, WHICH WE DID. THAT CLRNC CLUED US TO THE FACT THAT WE SHOULD HAVE STAYED AT 10000 FT AND THAT THE ARR WAS ON RNAV/STAR AND NOT A PROFILE DSCNT AS WE WERE ACCUSTOMED TO DOING INTO YYZ. ALSO, EARLIER IN THE DAY, GOING INTO NEWARK, WE WERE CLRED TO 'DSND VIA THE DYLIN 1' WHICH ALLOWED DSCNT TO RESTRS AS DEPICTED ON THE CHART. THIS SET A TRAIN OF THOUGHT, WE BOTH AGREE, WHICH WAS A FACTOR IN OUR ACTIONS ON THE YOUTH 1 ARR. AFTER LNDG, TWR REQUESTED I CONTACT TORONTO APCH CTL. I ACKNOWLEDGED THAT BEGINNING DSCNT WAS MY MISTAKE AND THAT ALT CHANGES MUST BE GIVEN BY ATC. WE ALSO DETERMINED THAT THE NOTE AT VIXAN INTXN (ON THIS NEW PROC) READS 'AT OR ABOVE 3100 FT' AND THE CANADIAN NOTE READS '3000 FT'). A NOTE ON THE COMMERCIAL CHART SAYING SOMETHING LIKE 'DSND ONLY AS CLRED BY ATC' WOULD BE HELPFUL. THERE WAS NO TFC CONFLICT WITH THIS ALTDEV. I ALSO CITE FATIGUE AS A CONTRIBUTING FACTOR AS WE WERE ON DUTY 13 HRS 15 MINS AT THE TIME OF THE INCIDENT.

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.