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.