Narrative:

We departed ind with a restr of cyyz. Approaching yxu, ZOB cleared us to cyyz via yxu direct ywt direct yyz. Shortly after passing yxu, toronto center instructed us to descend to FL190 initially, then down to 12000 ft. While in the descent to 12000 ft, we were cleared to 'descend to 7000 ft via the runway 5&6R profile descent.' the first officer was serving as PF. I repeated the clearance to him and told him I would insert the profile descent routing into the FMS. We did not review the altitude and airspeed restrs on the profile descent at that time. I had mentioned the probability of receiving clearance for the profile descent and assumed he had reviewed it instead. I made the mistake of immediately going heads down to reprogram the FMS. The profile descent called for us to cross 15 NM southwest of ywt between 14000 ft and 12000 ft and then continue the descent to cross 9 NM northeast of ywt (26 DME yyz) at 7000 ft and 210 KIAS. I do not recall confirming any change to the altitude set on the flight control panel with the first officer (nor being asked to confirm any altitude change). However, when I looked up from programming the FMS, we were descending and approaching 10000 ft while still approximately 25 NM southwest of ywt (10 NM prior to the point we could descend below 12000 ft!). I immediately instructed the first officer to level off and informed toronto center we were below the profile descent. The controller appeared unconcerned and reclred us to maintain 10000 ft and descend to 7000 ft on the profile descent. The remainder of the descent, approach, and landing were uneventful. We heard nothing else from the controller regarding our apparent altitude deviation. After landing, the first officer and I discussed what happened. He understood our clearance to be descend to 7000 ft and then cleared for the profile descent. If he was correct, no altitude deviation occurred. If my understanding was correct, we descended too early. Regardless, we both learned several important lessons (again!). First, it is imperative that we both understand and agree on our clearance. Had we discussed it at the time, we could have clarified our different understandings with ATC and avoided any potential problem altogether. Second, as PNF, it is much more important to review our clearance and back up the PF than to be immediately concerned with reprogramming the automation. Better to resort to raw data than bust an altitude. Finally, if we had followed flight manual policy of confirming changes to altitude set on the flight control panel, we could have discovered our misunderstanding and prevented a possible deviation. They policy is there for a reason!

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Original NASA ASRS Text

Title: AN ACR FLT DSNDS BELOW THE STAR RELATED ALT PRIOR TO THE 15 DME SW OF YWT FOR A PROFILE DSCNT INTO CYYZ, ON.

Narrative: WE DEPARTED IND WITH A RESTR OF CYYZ. APCHING YXU, ZOB CLRED US TO CYYZ VIA YXU DIRECT YWT DIRECT YYZ. SHORTLY AFTER PASSING YXU, TORONTO CTR INSTRUCTED US TO DSND TO FL190 INITIALLY, THEN DOWN TO 12000 FT. WHILE IN THE DSCNT TO 12000 FT, WE WERE CLRED TO 'DSND TO 7000 FT VIA THE RWY 5&6R PROFILE DSCNT.' THE FO WAS SERVING AS PF. I REPEATED THE CLRNC TO HIM AND TOLD HIM I WOULD INSERT THE PROFILE DSCNT ROUTING INTO THE FMS. WE DID NOT REVIEW THE ALT AND AIRSPD RESTRS ON THE PROFILE DSCNT AT THAT TIME. I HAD MENTIONED THE PROBABILITY OF RECEIVING CLRNC FOR THE PROFILE DSCNT AND ASSUMED HE HAD REVIEWED IT INSTEAD. I MADE THE MISTAKE OF IMMEDIATELY GOING HEADS DOWN TO REPROGRAM THE FMS. THE PROFILE DSCNT CALLED FOR US TO CROSS 15 NM SW OF YWT BTWN 14000 FT AND 12000 FT AND THEN CONTINUE THE DSCNT TO CROSS 9 NM NE OF YWT (26 DME YYZ) AT 7000 FT AND 210 KIAS. I DO NOT RECALL CONFIRMING ANY CHANGE TO THE ALT SET ON THE FLT CTL PANEL WITH THE FO (NOR BEING ASKED TO CONFIRM ANY ALT CHANGE). HOWEVER, WHEN I LOOKED UP FROM PROGRAMMING THE FMS, WE WERE DSNDING AND APCHING 10000 FT WHILE STILL APPROX 25 NM SW OF YWT (10 NM PRIOR TO THE POINT WE COULD DSND BELOW 12000 FT!). I IMMEDIATELY INSTRUCTED THE FO TO LEVEL OFF AND INFORMED TORONTO CTR WE WERE BELOW THE PROFILE DSCNT. THE CTLR APPEARED UNCONCERNED AND RECLRED US TO MAINTAIN 10000 FT AND DSND TO 7000 FT ON THE PROFILE DSCNT. THE REMAINDER OF THE DSCNT, APCH, AND LNDG WERE UNEVENTFUL. WE HEARD NOTHING ELSE FROM THE CTLR REGARDING OUR APPARENT ALTDEV. AFTER LNDG, THE FO AND I DISCUSSED WHAT HAPPENED. HE UNDERSTOOD OUR CLRNC TO BE DSND TO 7000 FT AND THEN CLRED FOR THE PROFILE DSCNT. IF HE WAS CORRECT, NO ALTDEV OCCURRED. IF MY UNDERSTANDING WAS CORRECT, WE DSNDED TOO EARLY. REGARDLESS, WE BOTH LEARNED SEVERAL IMPORTANT LESSONS (AGAIN!). FIRST, IT IS IMPERATIVE THAT WE BOTH UNDERSTAND AND AGREE ON OUR CLRNC. HAD WE DISCUSSED IT AT THE TIME, WE COULD HAVE CLARIFIED OUR DIFFERENT UNDERSTANDINGS WITH ATC AND AVOIDED ANY POTENTIAL PROB ALTOGETHER. SECOND, AS PNF, IT IS MUCH MORE IMPORTANT TO REVIEW OUR CLRNC AND BACK UP THE PF THAN TO BE IMMEDIATELY CONCERNED WITH REPROGRAMMING THE AUTOMATION. BETTER TO RESORT TO RAW DATA THAN BUST AN ALT. FINALLY, IF WE HAD FOLLOWED FLT MANUAL POLICY OF CONFIRMING CHANGES TO ALT SET ON THE FLT CTL PANEL, WE COULD HAVE DISCOVERED OUR MISUNDERSTANDING AND PREVENTED A POSSIBLE DEV. THEY POLICY IS THERE FOR A REASON!

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.