Narrative:

During initial descent into roc, ZOB cleared air carrier X to descend from FL220 to 16000 ft. The altitude was set in the altitude alerter, however, the altitude alerting system was inoperative and placed on MEL. Prior to flight, the first officer and I talked rather extensively about the procedures for altitude awareness with an inoperative alerter. We discussed the serious ramifications of this MEL and both of us made every effort to avoid an altitude deviation. Nevertheless, I inadvertently descended through 16000 ft to 15000 ft before realizing my error. Contributing to the deviation was the following. Upon commencing the descent out of FL220, I began to brief the approach at roc, believing I had plenty of time to conclude the briefing and then turn my attention to ensuring I leveled off at 16000 ft. However, a discussion about a NOTAM and field conditions ensued after the briefing, and it was during this time my attention was diverted from the altitude. Most aircraft types now are equipped with autoplt altitude capture. Not so in the DC9, making the loss of the altitude alerter very serious. Consideration should be given to removing this MEL from the book due to altitude deviation and CFIT implications. If a deviation can happen to 2 pilots who were trying hard not to let it happen, it can happen to anybody.

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Original NASA ASRS Text

Title: CAPT OF A DC9-30 OVERSHOT DSCNT ALT DUE TO AN INOP ALT ALERTER.

Narrative: DURING INITIAL DSCNT INTO ROC, ZOB CLRED ACR X TO DSND FROM FL220 TO 16000 FT. THE ALT WAS SET IN THE ALT ALERTER, HOWEVER, THE ALT ALERTING SYS WAS INOP AND PLACED ON MEL. PRIOR TO FLT, THE FO AND I TALKED RATHER EXTENSIVELY ABOUT THE PROCS FOR ALT AWARENESS WITH AN INOP ALERTER. WE DISCUSSED THE SERIOUS RAMIFICATIONS OF THIS MEL AND BOTH OF US MADE EVERY EFFORT TO AVOID AN ALTDEV. NEVERTHELESS, I INADVERTENTLY DSNDED THROUGH 16000 FT TO 15000 FT BEFORE REALIZING MY ERROR. CONTRIBUTING TO THE DEV WAS THE FOLLOWING. UPON COMMENCING THE DSCNT OUT OF FL220, I BEGAN TO BRIEF THE APCH AT ROC, BELIEVING I HAD PLENTY OF TIME TO CONCLUDE THE BRIEFING AND THEN TURN MY ATTN TO ENSURING I LEVELED OFF AT 16000 FT. HOWEVER, A DISCUSSION ABOUT A NOTAM AND FIELD CONDITIONS ENSUED AFTER THE BRIEFING, AND IT WAS DURING THIS TIME MY ATTN WAS DIVERTED FROM THE ALT. MOST ACFT TYPES NOW ARE EQUIPPED WITH AUTOPLT ALT CAPTURE. NOT SO IN THE DC9, MAKING THE LOSS OF THE ALT ALERTER VERY SERIOUS. CONSIDERATION SHOULD BE GIVEN TO REMOVING THIS MEL FROM THE BOOK DUE TO ALTDEV AND CFIT IMPLICATIONS. IF A DEV CAN HAPPEN TO 2 PLTS WHO WERE TRYING HARD NOT TO LET IT HAPPEN, IT CAN HAPPEN TO ANYBODY.

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.