Narrative:

Close to the end of a long day and the third day of a 4 day trip, we were descending into philadelphia, PA. The first officer was flying. We were instructed to cross johnstown VOR at FL190. The first officer started down in plenty of time to make the crossing. I was preoccupied with some personal family issues. When I heard the controller tell another aircraft that the altimeter was 30.74 inches, I, for some unknown reason, started to reset the captain's altimeter to 30.74 inches. The first officer noticed what I was doing and disengaged the autoplt. He then leveled off at FL190 using his altimeter for reference. The controller asked our altitude and I responded 'FL180 -- no, FL190!' the controller cleared us to FL180. I reset my altimeter to 29.92 inches and we descended to FL180. I have never done that before in 30 yrs of airline flying. Thankfully the first officer was sharp enough to keep us from busting our clearance, although it may have appeared so to the controller as the altitude reporting comes off of the captain's altimeter. The only corrective action I can recommend is to increase vigilance on long duty periods.

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Original NASA ASRS Text

Title: CAPT OF A B737 INADVERTENTLY RESET HIS ALTIMETER TO THE LCL SETTING WHEN ABOVE FL180 CAUSING THE ARTCC CTLR TO BELIEVE THAT HIS ACFT HAD DSNDED BELOW ASSIGNED ALT. HOWEVER, THE FO, FLYING, NOTICED CAPT'S MISTAKE AND FLEW THE ACFT AT THE CORRECT ALT BY HIS ALTIMETER. THE CAPT IMMEDIATELY CORRECTED HIS ALTIMETER SETTING REFLECTING THE CORRECT ALT TO ATC.

Narrative: CLOSE TO THE END OF A LONG DAY AND THE THIRD DAY OF A 4 DAY TRIP, WE WERE DSNDING INTO PHILADELPHIA, PA. THE FO WAS FLYING. WE WERE INSTRUCTED TO CROSS JOHNSTOWN VOR AT FL190. THE FO STARTED DOWN IN PLENTY OF TIME TO MAKE THE XING. I WAS PREOCCUPIED WITH SOME PERSONAL FAMILY ISSUES. WHEN I HEARD THE CTLR TELL ANOTHER ACFT THAT THE ALTIMETER WAS 30.74 INCHES, I, FOR SOME UNKNOWN REASON, STARTED TO RESET THE CAPT'S ALTIMETER TO 30.74 INCHES. THE FO NOTICED WHAT I WAS DOING AND DISENGAGED THE AUTOPLT. HE THEN LEVELED OFF AT FL190 USING HIS ALTIMETER FOR REF. THE CTLR ASKED OUR ALT AND I RESPONDED 'FL180 -- NO, FL190!' THE CTLR CLRED US TO FL180. I RESET MY ALTIMETER TO 29.92 INCHES AND WE DSNDED TO FL180. I HAVE NEVER DONE THAT BEFORE IN 30 YRS OF AIRLINE FLYING. THANKFULLY THE FO WAS SHARP ENOUGH TO KEEP US FROM BUSTING OUR CLRNC, ALTHOUGH IT MAY HAVE APPEARED SO TO THE CTLR AS THE ALT RPTING COMES OFF OF THE CAPT'S ALTIMETER. THE ONLY CORRECTIVE ACTION I CAN RECOMMEND IS TO INCREASE VIGILANCE ON LONG DUTY PERIODS.

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.