Narrative:

Descending from phx-dtw on polar 1 arrival, we were cleared to cross giber intersection at FL230. The first officer was flying and had set the altitude in the FCU altitude window and set the restr in the FMGC. At about FL250, I went off frequency to say good bye to the passenger. When I came back on frequency, I noticed that the aural and visual altitude alerter were going off. The altitude indicated FL225. The first officer was engaged in conversation with ZOB. He asked cle 'what do you show now?' 'FL225' came the reply. The first officer then inched the aircraft back up to FL230. There was no traffic in the area as far as I could see on this clear night and we did not receive any TCASII messages. I asked the first officer what had happened. He explained that while descending on autoplt in VNAV, he reset the barometric reference window to the local dtw altimeter setting of 30.42 in preparation for our descent and approach checklist. The aircraft captured altitude based on 30.42 instead of 29.92 before he reset the altimeters to 29.92. Our company policy as spelled out through our flight crew operating manual says not to reset the altimeters till passing through FL180. It has become a matter of pilot technique to reset the altimeters early on, then reselect 29.92 on the barometric reference knob. Then when accomplishing the descent and approach checklist, all one has to do is push the barometric reference knob in and he'll have the correct altimeter setting. I've asked our local alpa central air safety committee to get the word out to emphasize not resetting the altimeters until actually passing through FL180.

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Original NASA ASRS Text

Title: WRONG ALTIMETER SETTING SELECTED BY FO, PF, LEADS FLT INTO AN ALTDEV ALT OVERSHOT ON DSCNT IN AN A320. ALT XING RESTR NOT MET. CAPT FAILED TO XCHK ALTIMETERS.

Narrative: DSNDING FROM PHX-DTW ON POLAR 1 ARR, WE WERE CLRED TO CROSS GIBER INTXN AT FL230. THE FO WAS FLYING AND HAD SET THE ALT IN THE FCU ALT WINDOW AND SET THE RESTR IN THE FMGC. AT ABOUT FL250, I WENT OFF FREQ TO SAY GOOD BYE TO THE PAX. WHEN I CAME BACK ON FREQ, I NOTICED THAT THE AURAL AND VISUAL ALT ALERTER WERE GOING OFF. THE ALT INDICATED FL225. THE FO WAS ENGAGED IN CONVERSATION WITH ZOB. HE ASKED CLE 'WHAT DO YOU SHOW NOW?' 'FL225' CAME THE REPLY. THE FO THEN INCHED THE ACFT BACK UP TO FL230. THERE WAS NO TFC IN THE AREA AS FAR AS I COULD SEE ON THIS CLR NIGHT AND WE DID NOT RECEIVE ANY TCASII MESSAGES. I ASKED THE FO WHAT HAD HAPPENED. HE EXPLAINED THAT WHILE DSNDING ON AUTOPLT IN VNAV, HE RESET THE BAROMETRIC REF WINDOW TO THE LCL DTW ALTIMETER SETTING OF 30.42 IN PREPARATION FOR OUR DSCNT AND APCH CHKLIST. THE ACFT CAPTURED ALT BASED ON 30.42 INSTEAD OF 29.92 BEFORE HE RESET THE ALTIMETERS TO 29.92. OUR COMPANY POLICY AS SPELLED OUT THROUGH OUR FLC OPERATING MANUAL SAYS NOT TO RESET THE ALTIMETERS TILL PASSING THROUGH FL180. IT HAS BECOME A MATTER OF PLT TECHNIQUE TO RESET THE ALTIMETERS EARLY ON, THEN RESELECT 29.92 ON THE BAROMETRIC REF KNOB. THEN WHEN ACCOMPLISHING THE DSCNT AND APCH CHKLIST, ALL ONE HAS TO DO IS PUSH THE BAROMETRIC REF KNOB IN AND HE'LL HAVE THE CORRECT ALTIMETER SETTING. I'VE ASKED OUR LCL ALPA CENTRAL AIR SAFETY COMMITTEE TO GET THE WORD OUT TO EMPHASIZE NOT RESETTING THE ALTIMETERS UNTIL ACTUALLY PASSING THROUGH FL180.

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.