Narrative:

ZLA cleared us pilot's discretion from FL350 to FL310 and to expect salton at FL240. We understood and read back clearance to cross salton at FL240. Copilot set FL240 in MCP altitude window. Aircraft began descent out of FL310 for FL240 at descent point as aircraft was leaving 30800 ft MSL I asked first officer to confirm if the clearance was to cross or expect to cross salton at FL240. First officer transmitted to ZLA leaving FL308 for FL240. Center replied we're to maintain FL310 at FL306. I disconnected autoplt and returned aircraft to FL310. During this period TCASII gave TA. Traffic was at approximately our 4-5 O'clock position. Center asked what our altitude was and we reported level at FL310. In short an expect clearance was interpreted as a clearance to FL240. Talked to center via land line after arrival san and they were quoted as saying that they were treating incident as a non event. Looking back fatigue was a definite factor in this incident as we were nearing end of 8 block hours flying time and my failure to confirm the clearance prior to FMC beginning vertical navigation to FL240 and copilot transmitting leaving FL308 for FL240 instead of what I asked him, that being to confirm if clearance was to FL240 or if it was to expect FL240.

Related reports:

Google
 

Original NASA ASRS Text

Title: B757 CREW HAD ALTDEV.

Narrative: ZLA CLRED US PLT'S DISCRETION FROM FL350 TO FL310 AND TO EXPECT SALTON AT FL240. WE UNDERSTOOD AND READ BACK CLRNC TO CROSS SALTON AT FL240. COPLT SET FL240 IN MCP ALT WINDOW. ACFT BEGAN DSCNT OUT OF FL310 FOR FL240 AT DSCNT POINT AS ACFT WAS LEAVING 30800 FT MSL I ASKED FO TO CONFIRM IF THE CLRNC WAS TO CROSS OR EXPECT TO CROSS SALTON AT FL240. FO XMITTED TO ZLA LEAVING FL308 FOR FL240. CTR REPLIED WE'RE TO MAINTAIN FL310 AT FL306. I DISCONNECTED AUTOPLT AND RETURNED ACFT TO FL310. DURING THIS PERIOD TCASII GAVE TA. TFC WAS AT APPROX OUR 4-5 O'CLOCK POS. CTR ASKED WHAT OUR ALT WAS AND WE RPTED LEVEL AT FL310. IN SHORT AN EXPECT CLRNC WAS INTERPRETED AS A CLRNC TO FL240. TALKED TO CTR VIA LAND LINE AFTER ARR SAN AND THEY WERE QUOTED AS SAYING THAT THEY WERE TREATING INCIDENT AS A NON EVENT. LOOKING BACK FATIGUE WAS A DEFINITE FACTOR IN THIS INCIDENT AS WE WERE NEARING END OF 8 BLOCK HRS FLYING TIME AND MY FAILURE TO CONFIRM THE CLRNC PRIOR TO FMC BEGINNING VERT NAV TO FL240 AND COPLT XMITTING LEAVING FL308 FOR FL240 INSTEAD OF WHAT I ASKED HIM, THAT BEING TO CONFIRM IF CLRNC WAS TO FL240 OR IF IT WAS TO EXPECT FL240.

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.