B727 CREW HAD ALT BUST ON DSCNT.

Narrative

WE WERE ON A CRUISE DSCNT INTO OKC. OUR ORIGINAL CLRNC WAS PLT'S DISCRETION TO FL240; THEN AMENDED DURING THE DSCNT TO FL210. THE ALTS WERE SET PROPERLY IN THE ALT ALERTER DURING THE DSCNT; ON AUTOPLT. WE WERE STARTING TO BRIEF THE APCH INTO OKC. WE WERE SLIGHTLY BEHIND BECAUSE THE FE WAS LATE GETTING THE LNDG CARD READY; BUT DID NOT FEEL RUSHED. AS WE PASSED THROUGH FL208; I REALIZED WE DIDN'T HEAR THE ALERTER; CLICKED OFF THE AUTOPLT; AND CLBED BACK TO FL210. BOTH THE CAPT AND I OBSERVED OUR ALT TO BE AS LOW AS FL206. AS LUCK WOULD HAVE IT; THERE WAS TFC AT FL200. WE GOT A TCASII TA; BUT NO RA. THE DSCNT CONTINUED AFTER PASSING TFC. WE WERE BOTH DISTURBED THAT WE MISSED THE 1000 FT CALL AND ALERTER. IT TURNS OUT THE ALERTER DID NOT WORK. THE REST OF THE WAY INTO OKC WAS A CONTINUOUS DSCNT TO A VISUAL APCH. LEAVING OKC; WE DISCOVERED THE ALERTER TO BE INOP; WHICH WE WROTE UP. BOTTOM LINE: IT IS VERY EASY TO TAKE THE 'GADGETS' FOR GRANTED; EVEN ON A VERY BASIC AIRPLANE; AND WHEN THEY FAIL; IT TAKES YOU BY SURPRISE. IT IS A STRONG REMINDER TO DO YOUR JOB FIRST AND NOT GET DEPENDENT ON THE BACK-UP TOOLS OF AUTOMATION. MUCH EASIER SAID THAN DONE; THOUGH. SUPPLEMENTAL INFO FROM ACN 425465: DURING DSCNT TO FL210 ON ACR FLT ABCD; OUR ALT ALERT SYS FAILED -- NO LIGHT; NO TONE. WE WERE BRIEFING THE APCH. ACFT DSNDED TO FL206 BEFORE WE TOOK CORRECTIVE ACTION. ACTION WAS TAKEN BEFORE CTR ALERTED US. THERE WAS OTHER ACFT AT FL190. FAILURE OF ALT ALERT SYS CONTRIBUTED BUT BRIEFING COULD HAVE BEEN ACCOMPLISHED EARLIER IN DSCNT OR IN A DIFFERENT MANNER.