Narrative:

Our flight was cleared to descend from 24000 ft so as to cross CCC (calverton VOR) at 12000 ft and 250 KTS. The controller gave the CCC altimeter setting as 29.21. We noted the low altimeter and requested verification, then set the standby altimeter to this setting. We then became preoccupied with programming the vertical descent mode of the FMC so as to make the CCC restriction. The descent had to be manually controled to make the crossing and in doing so, we passed 18000 ft without resetting our altimeters to 29.21. Thus the aircraft leveled off at 12000 ft (or 600 ft low 11400 MSL). The ZBW controller asked us to check our altitude and we then reset the altimeters to 29.21 and climbed back to 12000 ft. Contributing to this incident was that the crew had spent the last 6 days flying in europe, where altimeters are reset at low altitudes (ie, transition levels 3000-5000 ft) so a readjustment to united states rules had to be made. Also, the preoccupation of programming the VNAV computer was a large factor. Recommendations: change united states rules to allow altimeters to be set in advance to local settings if descending to a level below FL180, from a level above FL180, and stress that 1 pilot flies, the other pilot talks on the radio, programs the computer, and watches for traffic. Supplemental information from acn 204745: discovered rapidly falling altimeter pressure had put us at an actual altimeter reading of 11500 ft as discovered from latest pvd ATIS. Immediately went back to 12000 indicated. No altitude comment from either ZBW or pvd approach. Cannot remember getting altimeter update from ZBW.

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Original NASA ASRS Text

Title: ALTDEV ALT OVERSHOT IN DSCNT. ALT XING RESTRICTION NOT ADHERED TO.

Narrative: OUR FLT WAS CLRED TO DSND FROM 24000 FT SO AS TO CROSS CCC (CALVERTON VOR) AT 12000 FT AND 250 KTS. THE CTLR GAVE THE CCC ALTIMETER SETTING AS 29.21. WE NOTED THE LOW ALTIMETER AND REQUESTED VERIFICATION, THEN SET THE STANDBY ALTIMETER TO THIS SETTING. WE THEN BECAME PREOCCUPIED WITH PROGRAMMING THE VERT DSCNT MODE OF THE FMC SO AS TO MAKE THE CCC RESTRICTION. THE DSCNT HAD TO BE MANUALLY CTLED TO MAKE THE XING AND IN DOING SO, WE PASSED 18000 FT WITHOUT RESETTING OUR ALTIMETERS TO 29.21. THUS THE ACFT LEVELED OFF AT 12000 FT (OR 600 FT LOW 11400 MSL). THE ZBW CTLR ASKED US TO CHK OUR ALT AND WE THEN RESET THE ALTIMETERS TO 29.21 AND CLBED BACK TO 12000 FT. CONTRIBUTING TO THIS INCIDENT WAS THAT THE CREW HAD SPENT THE LAST 6 DAYS FLYING IN EUROPE, WHERE ALTIMETERS ARE RESET AT LOW ALTS (IE, TRANSITION LEVELS 3000-5000 FT) SO A READJUSTMENT TO UNITED STATES RULES HAD TO BE MADE. ALSO, THE PREOCCUPATION OF PROGRAMMING THE VNAV COMPUTER WAS A LARGE FACTOR. RECOMMENDATIONS: CHANGE UNITED STATES RULES TO ALLOW ALTIMETERS TO BE SET IN ADVANCE TO LCL SETTINGS IF DSNDING TO A LEVEL BELOW FL180, FROM A LEVEL ABOVE FL180, AND STRESS THAT 1 PLT FLIES, THE OTHER PLT TALKS ON THE RADIO, PROGRAMS THE COMPUTER, AND WATCHES FOR TFC. SUPPLEMENTAL INFO FROM ACN 204745: DISCOVERED RAPIDLY FALLING ALTIMETER PRESSURE HAD PUT US AT AN ACTUAL ALTIMETER READING OF 11500 FT AS DISCOVERED FROM LATEST PVD ATIS. IMMEDIATELY WENT BACK TO 12000 INDICATED. NO ALT COMMENT FROM EITHER ZBW OR PVD APCH. CANNOT REMEMBER GETTING ALTIMETER UPDATE FROM ZBW.

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.