DISTR WITH DECISIONS AND NAV DUTIES AND THE RELIANCE ON A SYS THAT WAS NON EXISTENT ONLY; IN THIS PARTICULAR ACFT; LED TO AN ALTDEV.

Narrative

WE WERE CLRED TO FL290 AND WERE FLYING AN ACFT WITH AN AUTOPLT THAT DID NOT HAVE AN ALT FEATURE; AFTER 2 LEGS ON AN ACFT THAT DID HAVE AN ALT CAPTURE. THE CAPT SET FL290 IN THE ALT ALERT WINDOW AND THE ALERT SIGNALED PASSING FL285. THEN THE CAPT; FO; AND SO; WERE DISTRACTED BY A RADIO CALL AND A NAV DECISION. WE NOTICED AT FL294 THAT WE HAD FAILED TO LEVEL OFF; PEAKED AT FL296; THEN DSNDED BACK TO FL290. THERE WERE NO ACFT IN THE AREA. THE ZLC CTLR NOTICED OUR DEV AFTER WE HAD STARTED THE DSCNT AND CONFIRMED OUR ASSIGNED ALT OF FL290. I THINK THAT MIXING LEGS ON ACFT WITH DIFFERENT AUTOPLTS DEFINITELY LEADS TO ALTDEVS. SUPPLEMENTAL INFO FROM ACN 300892: BLOCK 4 AUTOPLT; HAD LIGHT AND AURAL TONE 1000 FT; 500 FT PRIOR. MADE REF TO 1000 FT TO GO; THEN DISTRACTED BY NAV DUTY; TRYING TO VERIFY OMEGA NAV TO ABQ. SUPPLEMENTAL INFO FROM ACN 301198: CALLED OUT 1000 FT TO GO. I NOTICED THE PLANE SLOWLY CLBING THROUGH FL295 AND IMMEDIATELY RETURNED TO FL290. THERE WAS NO IMMEDIATE TFC CONFLICT AND THE CTLR ASKED OUR ALT AS WE WERE RETURNING TO FL290. IN THE FUTURE; ALL ATTN WILL BE FOCUSED ON LEVELOFF RATHER THAN LESS ESSENTIAL TASKS.