B737-300 CREW HAD ALTDEV IN ZLA CLASS A AIRSPACE AFTER THE AUTOPLT REVERTED TO CTL WHEEL STEERING MODE.

Narrative

I WAS THE FO/PF FOR THIS LEG. WE WERE AT FL280 AND SPD RESTR BY ZLA TO 280 KIAS. THE CTLR CLRED US TO DSND AT PLT'S DISCRETION TO FL240 AND RESUME NORMAL SPD. THE 'B' FLT AUTOPLT WAS ENGAGED IN VNAV AND LNAV MODES. WHEN THE CLRNC WAS RECEIVED AND ACKNOWLEDGED BY THE CAPT; SET FL240 IN THE MODE CONTROLLER ALT WINDOW; WHICH I ACKNOWLEDGED PER SOP. I THEN DESELECTED VNAV AND PUT 300 KIAS IN THE MCP SPD WINDOW. THE AUTOTHROTTLES MOVED FORWARD IN RESPONSE TO THE NEW COMMANDED SPD. IT WAS NOT MY INTENT TO START DOWN TO FL240; YET SINCE WE WERE STILL ABOUT 80 MI FROM TOP OF DSCNT; I THEN MOVED MY ATTN AWAY FROM THE MCP TO GET MY CHARTS FOR THE ARR INTO LAX AND THE CAPT PERFORMED SIMILAR TASKS. AFTER ABOUT 30 SECONDS THE CAPT ASKED ME IF I KNEW WE WERE IN CTL WHEEL STEERING MODE. I STATED NO; IT WAS NOT MY INTENT. AT THIS POINT; I BECAME AWARE THE 'B' AUTOPLT WAS IN CTL WHEEL STEERING MODE ON THE EADI AND THE ACFT WAS NO LONGER AT FL280 BUT; BECAUSE OF THE THROTTLE ADVANCE; HAD CLBED. I IMMEDIATELY DISENGAGED THE AUTOPLT AND AUTOTHROTTLES AND STARTED A DSCNT TO FL240. THE CAPT CALLED LEAVING FL280 FOR FL240 TO ZLA. AT FL240 I RE-ENGAGED THE AUTOPLT IN THE LNAV AND VNAV MODES. WHEN LEVELED AT FL240; ZLA CTLR THANKED US FOR A GOOD JOB AND GAVE US A FREQ CHANGE TO THE NEXT SECTOR. THE NEXT DAY; WE FLEW THE SAME ZZZ-LAX RTE IN A DIFFERENT ACFT. AGAIN MY LEG; AND AGAIN RECEIVED THE SAME CLRNC. I RECREATED THE SAME COMMANDS OF THE AUTOPLT AND MCP. WHEN A NEW ALT IS SELECTED IN THE MCP MODE AND VNAV IS DESELECTED; THE AUTOPLT IMMEDIATELY REVERTS TO CTL WHEEL STEERING MODE. BECAUSE A NEW ALT IS SELECTED IN THE ALT WINDOW; AN ALTDEV ALARM IS NOT SOUNDED UNTIL THE ACFT APCHS THE ALARM PARAMETERS FOR THE NEW ALT. THERE WILL BE NO ALARM FOR THE OLD ALT. I HAVE FLOWN THE B737-300/500 FOR OVER 4 YRS AND WAS NOT PREPARED FOR THIS RESPONSE IN THE AUTOFLT SYS. I EXPECTED THE AUTOPLT TO REMAIN IN ALT HOLD UNTIL A DSCNT MODE WAS SELECTED EITHER VNAV; LEVEL CHANGE; OR VERT SPD. BUT BECAUSE I HAD ONLY SELECTED A SPD MODE; THE AUTOFLT SYS REVERTED TO CTL WHEEL STEERING. THE LACK OF DEV ALARM IS ANOTHER 'GOTCHA.' I AM NOW VERY AWARE OF THESE AUTOFLT LIMITATIONS; AND HOPE MY EXPERIENCE CAN PREVENT OTHERS FROM FALLING INTO THE SAME TRAP.