AN ACR WDB FLC DSNDED BELOW THEIR ASSIGNED ALT. THE CAPT SET THE WRONG ALT WHILE THE FO WAS OFF THE FREQ AND THE FO THOUGHT THAT IT WAS A NEWLY ASSIGNED ALT.

Narrative

THE FLT WAS CLRED FROM FL390 TO FL310 BY ZLA. BEFORE REACHING FL310; A NEW CLRNC TO FL280 AT PLT'S DISCRETION WAS RECEIVED. AFTER THE CAPT LEVELED THE ACFT AT FL310; THE FO LEFT THE #1 RADIO TO OBTAIN ATIS AND GATE INFO ON THE #2 RADIO. DURING THE FO ABSENCE FROM THE #1 RADIO; THE CAPT SET 24000 FT IN THE ALT SELECTOR; THINKING THIS WAS THE DSCNT CLRNC LIMIT AND STARTED THE DSCNT FROM FL310. THE FO SAW 24000 FT IN THE ALT SELECTOR AND ASSUMED THE CAPT HAD RECEIVED A NEW CLRNC. A FREQ CHANGE WAS GIVEN BY ATC AND WHEN THE CAPT CHKED IN ON THE NEW FREQ; ATC ADVISED TO STOP THE DSCNT AND CLB TO FL280. SUPPLEMENTAL INFO FROM ACN 383760: THE CAPT DECIDED TO LEAVE FL310 SET IN THE ALT ALERT WINDOW THEN TOLD FO TO GO TO RADIO #2 AND HE WOULD MONITOR ZLA. FO WAS GETTING ATIS AND GATE INFO. AFTER LEVELING AT FL310; THE CAPT SET IN FL240 IN ALERT WINDOW AND BEGAN DSCNT. FO RETURNED TO WORK RADIO #1 AND NOTICED 24000 FT SET IN THE ALT ALERT. FO THOUGHT THAT A NEW CLRNC TO FL240 HAD BEEN ISSUED; HOWEVER; PASSING FL260 CTR CALLED AND TOLD US TO IMMEDIATELY CLB TO FL280. THE CAPT HAD MISUNDERSTOOD THE INITIAL CLRNC OR FORGOTTEN THE ORIGINAL FLT LEVEL AT PLT DISCRETION. THE ACFT WAS RE-ESTABLISHED AT FL280.