A B737-300 FLC MISUSES THEIR FMS MODE CTL PANEL SELECTIONS AND; THROUGH AN ALTDEV; INSPIRE AN ATC INQUIRY INTO THEIR ALT.

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THE FO WAS PF. I WAS DSNDING ON THE CRESO 3 ARR INTO LAS. ZLA WANTED US TO DSND TO FL220 AT 320 KTS OR BETTER. I WAS USING THE AUTOPLT IN THE VERT SPD MODE. AT APPROX FL260 THE MODE CTL PANEL SPD FEATURE TOOK CTL. I DESELECTED THE MODE AND USING THE YOKE I BEGAN TO SLOW THE DSCNT RATE. ALT ACQUIRE MODE IMMEDIATELY TOOK CTL WITH A 5000 FPM DSCNT RATE. THE ALT ACQUIRE MODE WENT ABOUT 100 FT LOW AND BEGAN A SLOW ASCENT. ABOUT THAT TIME ZLA GAVE US A RESTR OF 12000 FT AT WHIGG. THE CAPT (NON-FLYING) PUT THE RESTR IN THE ALT WINDOW OF THE AUTOPLT. THE CAPT WENT OFF FREQ TO GET THE ATIS. I LOOKED IN THE CDU TO GAUGE THE DISTANCE TO BEGIN THE DSCNT TO WHIGG. I AND THE CAPT THOUGHT THE ACFT WAS IN THE ALT HOLD MODE. WHEN I NEXT LOOKED UP THE ACFT WAS ABOUT 250 FT HIGH. WHEN THE ALT WINDOW WAS CHANGED THE VERT SPD MODE TOOK CTL WITH A CLB. I USED THE VERT SPD WHEEL TO BEGIN A DSCNT. WE BALLOONED UP TO 350 FT. I THEN DISCONNECTED THE AUTOPLT AND TOOK OVER MANUALLY. AT THAT TIME ATC ASKED US OUR ALT AND WE RECEIVED A TA ON AN ACFT AT FL230 ABOUT 6 MI AWAY. DURING THE DSCNT WE INFORMED ATC OF OUR MISTAKE. DUE TO ALL THE ACTIVITY OCCURRING SIMULTANEOUSLY AND AN INCORRECT ASSUMPTION BY BOTH PLTS; OUR ACFT DID NOT LEVEL OFF AT THE CORRECT ALT. MORE ATTN NEEDED TO BE PAID TO FLYING THE ACFT DURING THAT CRITICAL PHASE OF FLT. THEN THE PF SHOULD HAVE IMMEDIATELY DISCONNECTED THE AUTOPLT AND BEGUN THE DSCNT SO AS TO NOT DELAY IT. SUPPLEMENTAL INFO FROM ACN 437387: THE CTLR ISSUED A CLRNC TO CROSS WHIGG INTXN AT 12000 FT. I SIMULTANEOUSLY SELECTED 12000 FT IN THE ALT WINDOW ON THE MODE CTL PANEL; AND I IMMEDIATELY CHKED OFF CTR FREQ TO OBTAIN THE CURRENT ATIS IN LAS. THE FO WAS DISTRACTED BY MY CHKING OFF AND ASSUMED ALT HOLD MODE HAD ENGAGED; WHILE IN FACT THE VERT SPD MODE HAD CAPTURED THE 200 FPM ASCENT FROM THE ORIGINAL AUTOPLT CORRECTION OF CLBING BACK TO FL220. DURING THE CONTINUING DSCNT THE CTLR AND I DISCUSSED THE SIT; IE; POSSIBLE CONFLICT; AND HE INSINUATED THAT NO CONFLICT EXISTED; BUT 'THE MACHINE' HAD TRIGGERED AN ALERT AT HIS END. I DO BELIEVE THAT WERE WE FLYING THE OLD TECHNOLOGY AUTOPLT; THIS INCIDENT WOULD NOT HAVE OCCURRED. WE NEED TO CONTINUE WITH THE IDENTICAL MINDSET ON BOTH OLD AND NEW TECHNOLOGY ACFT -- 'MIND THE STORE' DURING ALL ALT AND HEADING CHANGES. DO I LIKE AUTO LEVELOFFS -- NO. AM I MORE AWARE -- YOU BET.