Narrative:

I was the first officer/PF for this leg. We were at FL280 and speed restr by ZLA to 280 KIAS. The controller cleared us to descend at pilot's discretion to FL240 and resume normal speed. The 'B' flight autoplt was engaged in VNAV and LNAV modes. When the clearance was received and acknowledged by the captain, set FL240 in the mode controller altitude window, which I acknowledged per SOP. I then deselected VNAV and put 300 KIAS in the MCP speed window. The autothrottles moved forward in response to the new commanded speed. It was not my intent to start down to FL240, yet since we were still about 80 mi from top of descent, I then moved my attention away from the MCP to get my charts for the arrival into lax and the captain performed similar tasks. After about 30 seconds the captain asked me if I knew we were in control wheel steering mode. I stated no, it was not my intent. At this point, I became aware the 'B' autoplt was in control wheel steering mode on the eadi and the aircraft was no longer at FL280 but, because of the throttle advance, had climbed. I immediately disengaged the autoplt and autothrottles and started a descent to FL240. The captain 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 controller thanked us for a good job and gave us a frequency change to the next sector. The next day, we flew the same ZZZ-lax route in a different aircraft. Again my leg, and again received the same clearance. I recreated the same commands of the autoplt and MCP. When a new altitude is selected in the MCP mode and VNAV is deselected, the autoplt immediately reverts to control wheel steering mode. Because a new altitude is selected in the altitude window, an altitude deviation alarm is not sounded until the aircraft approachs the alarm parameters for the new altitude. There will be no alarm for the old altitude. I have flown the B737-300/500 for over 4 yrs and was not prepared for this response in the autoflt system. I expected the autoplt to remain in altitude hold until a descent mode was selected either VNAV, level change, or vertical speed. But because I had only selected a speed mode, the autoflt system reverted to control wheel steering. The lack of deviation 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.

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Original NASA ASRS Text

Title: 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.

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.