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.