Narrative:

During climb through 19000 ft for 28000 ft, an amended clearance to 21000 ft was issued and acknowledged. Navigation was solely by omega, but one or both omega units appeared to be in error, and the PNF was very absorbed trying to xchk position and determine which omega was more accurate. The PF's attention was also diverted by the problem. After hearing a faint chime from the altitude alerter, the PNF observed the altitude at 21500 ft and called it to the attention of the PF, who began an immediate descent toward 21000 ft. The ATC controller, a couple of seconds later, said 'the amended clearance was to 21000 ft but climb to 28000 ft and expedite through 23000 ft.' an aircraft was observed on TCASII 8 mi north and 500 ft above our aircraft. The incident was caused by a distraction to the flight crew at a critical time. Contributing to the problem was the altitude alerter, which issued a single chime for a warning when the aircraft was 500 ft beyond the selected altitude. This type alerter is installed in a very small number of our aircraft. The more common type alerter sounds a much more noticeable horn 300 ft past the altitude and would have allowed a much more timely correction. An altitude select mode on the autoplt would have avoided the problem completely.

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Original NASA ASRS Text

Title: AN ACR LGT CLBED THROUGH ITS ASSIGNED ALT. ALTDEV. NON ADHERENCE TO ARTCC AMENDED CLRNC ALT ASSIGNMENT. ERROR ADMITTED.

Narrative: DURING CLB THROUGH 19000 FT FOR 28000 FT, AN AMENDED CLRNC TO 21000 FT WAS ISSUED AND ACKNOWLEDGED. NAV WAS SOLELY BY OMEGA, BUT ONE OR BOTH OMEGA UNITS APPEARED TO BE IN ERROR, AND THE PNF WAS VERY ABSORBED TRYING TO XCHK POS AND DETERMINE WHICH OMEGA WAS MORE ACCURATE. THE PF'S ATTN WAS ALSO DIVERTED BY THE PROB. AFTER HEARING A FAINT CHIME FROM THE ALT ALERTER, THE PNF OBSERVED THE ALT AT 21500 FT AND CALLED IT TO THE ATTN OF THE PF, WHO BEGAN AN IMMEDIATE DSCNT TOWARD 21000 FT. THE ATC CTLR, A COUPLE OF SECONDS LATER, SAID 'THE AMENDED CLRNC WAS TO 21000 FT BUT CLB TO 28000 FT AND EXPEDITE THROUGH 23000 FT.' AN ACFT WAS OBSERVED ON TCASII 8 MI N AND 500 FT ABOVE OUR ACFT. THE INCIDENT WAS CAUSED BY A DISTR TO THE FLC AT A CRITICAL TIME. CONTRIBUTING TO THE PROB WAS THE ALT ALERTER, WHICH ISSUED A SINGLE CHIME FOR A WARNING WHEN THE ACFT WAS 500 FT BEYOND THE SELECTED ALT. THIS TYPE ALERTER IS INSTALLED IN A VERY SMALL NUMBER OF OUR ACFT. THE MORE COMMON TYPE ALERTER SOUNDS A MUCH MORE NOTICEABLE HORN 300 FT PAST THE ALT AND WOULD HAVE ALLOWED A MUCH MORE TIMELY CORRECTION. AN ALT SELECT MODE ON THE AUTOPLT WOULD HAVE AVOIDED THE PROB COMPLETELY.

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.