Narrative:

The flight was cleared from FL390 to FL310 by ZLA. Before reaching FL310, a new clearance to FL280 at pilot's discretion was received. After the captain leveled the aircraft at FL310, the first officer left the #1 radio to obtain ATIS and gate information on the #2 radio. During the first officer absence from the #1 radio, the captain set 24000 ft in the altitude selector, thinking this was the descent clearance limit and started the descent from FL310. The first officer saw 24000 ft in the altitude selector and assumed the captain had received a new clearance. A frequency change was given by ATC and when the captain checked in on the new frequency, ATC advised to stop the descent and climb to FL280. Supplemental information from acn 383760: the captain decided to leave FL310 set in the altitude alert window then told first officer to go to radio #2 and he would monitor ZLA. First officer was getting ATIS and gate information. After leveling at FL310, the captain set in FL240 in alert window and began descent. First officer returned to work radio #1 and noticed 24000 ft set in the altitude alert. First officer thought that a new clearance to FL240 had been issued, however, passing FL260 center called and told us to immediately climb to FL280. The captain had misunderstood the initial clearance or forgotten the original flight level at pilot discretion. The aircraft was re-established at FL280.

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Original NASA ASRS Text

Title: 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.

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.