B733 FLC DO NOT PERCEIVE GRADUAL CLB AND CONFLICT WITH ANOTHER ACR WHILE UNDER ZTL CTL.

Narrative

CRUISING AT FL270; APPROX 90 NM SW OF LGC (AUTOPLT AND AUTOTHROTTLE ON); WE WERE GIVEN PLT'S DISCRETION DSCNT TO FL250 (WHICH WAS SET IN ALT WINDOW) AND ASKED TO KEEP OUR AIRSPD FAST. I DESELECTED VNAV (WHICH PUT US IN SPD MODE WHERE ACFT VARIES PITCH TO MAINTAIN AIRSPD) AND ROTATED SELECTED SPD FROM 310 KTS TO 325 KTS. APPROX 2 MINS LATER; ZTL CALLED OUT TFC AT 10 NM (FL280). AT APPROX 6-8 NM; RECEIVED YELLOW TCASII ALERT. ALL 3 HEADS (JUMP SEAT RIDER) WENT OUTSIDE. ACQUIRED TFC AT 4-5 NM. AS I WATCHED TFC; NOTICED VERT DISTANCE BTWN US WAS DECREASING. AT THAT POINT; I LOOKED INSIDE AND NOTICED AUTOPLT HAD CLBED US TO FL275. I DISCONNECTED AUTOPLT AND RETURNED ACFT TO FL270 AS TFC WAS APPROX 2 - 2 1/2 NM FROM US AND DISPLACED LATERALLY 2 - 2 1/2 MI AND 800-1000 FT VERTLY. ANALYSIS: BY HAVING ALT WINDOW SET AT FL250; WE RECEIVED NO ALTDEV WARNING AS WE CLBED. LITTLE KNOWN FACT (AND ONE I HAD FORGOTTEN) -- PLACING AUTOTHROTTLE IN SPD MODE CAUSES AUTOPLT TO VARY PITCH TO MAINTAIN AIRSPD. BY SELECTING A GREATER SPD; THE ACFT AT SOME POINT STARTED A VERY SLOW; INSIDIOUS (SIC) PITCH UP THAT WAS NOT CAUGHT TILL TFC WAS ACQUIRED VISUALLY. ALL EYES LOOKING OUTSIDE TO ACQUIRE CONFLICTING TFC. CAUSES -- 3. THIS PROB; LIKE ALL ACCIDENTS OR NEAR ACCIDENTS; OCCURRED BECAUSE OF AN UNLUCKY CHAIN OF EVENTS: ALT WINDOW SET TO LOWER ALT; A BAD COMPUTER PROGRAM THAT ALLOWS/CAUSES THE AUTOPLT TO LEAVE AN ALT WHEN IN 'ALT HOLD MODE' WITHOUT ANY WARNING; AND THIRDLY ALL EYES OUTSIDE FOR CLOSE-IN TFC. FIX: THE FIRST (ALT WINDOW SET TO LOWER ALT) AND THIRD CAUSES (ALL EYES OUTSIDE) ARE NORMAL PLT FUNCTIONS AND CAN'T BE CHANGED. THE ONE CAUSE THAT SHOULD CHANGE IS THE COMPUTER PROGRAM THAT CAUSES AN ACFT TO LEAVE AN ALT WHEN 'ALT HOLD' WITHOUT SELECTING A VERT MODE (LEVEL CHANGE OR VERT SPD OR VNAV THROUGH COMPUTER INPUT BY PLT). IMPORTANT NOTE: IF IN VNAV AND ALT WINDOW NOT SET TO ANOTHER ALT; DESELECTING VNAV TO GET SPD MODE CAUSES MODE ANNUNCIATOR TO CHANGE TO 'ALT HOLD' BUT BY HAVING A DIFFERENT ALT SELECTED; AUTOPLT CHANGES TO A PITCH MODE. WHY 2 AUTOPLTS SHOULD ALWAYS STAY IN ALT HOLD UNLESS COMMANDED BY A VERT MODE AS STATED ABOVE. SUPPLEMENTAL INFO FROM ACN 509162: HAD JUST FINISHED MY 3 DAY TRIP AND HAD TO RUN AND CATCH MY COMMUTE TO ATL FROM IAH. I WAS LOOKING FORWARD TO A NAP SINCE I HAD BEEN UP SINCE EARLY MORNING. I WAS INFORMED THAT THE FLT WAS FULL AND I WOULD HAVE TO RIDE JUMP SEAT. THE TRIP WAS UNEVENTFUL UNTIL THE FIRST TA. I WAS IN THE CTR JUMP SEAT AT THE TIME DURING CRUISE DRIFTING IN AND OUT OF CONSCIOUSNESS UNTIL I HEARD A 'TFC; TFC' TA AND LOOKED UP AT THE CAPT'S ADI AND NOTICED TFC AT 12 O'CLOCK POS AND CLOSING. I COULD NOT TELL IF THE TFC WAS ABOVE OR BELOW US AS THE SYMBOL +/- WAS ON THE LUBBER LINE AND COULD NOT BE DISTINGUISHED. STILL TRYING TO GAIN MY FULL ATTN TO THE EVENT; I NOTICED THE TARGET DSNDING AND THE TA CONVERTED TO AN RA. THE CAPT RESPONDED TO THE RA AND DSNDED. ONLY AT THAT POINT WE ALL REALIZED WE WERE AT FL272 AND CLBING. AT THAT TIME THE OTHER ACFT STATED THAT HE WAS RESPONDING TO AN RA; AT WHICH POINT THE CTLR ASKED WHAT OUR ALT WAS. AT THAT TIME WE HAD RETURNED TO FL270 AND THAT WAS WHAT WAS RPTED TO ATC. AT THAT POINT I NOTICED THAT FL250 WAS SELECTED IN THE ALT SELECTOR ON THE MCP. THE CTR CTLR ASKED THE OTHER ACFT 'HOW CLOSE DID HE COME TO US?' AND HE RESPONDED WITH '500 FT.' IT APPEARS THAT ATC HAD ISSUED US A CLRNC TO DSND AT PLT'S DISCRETION TO FL250. THE CAPT HAD DIALED IN FL250 IN THE MCP AND DID NOT HAVE A PITCH MODE ENGAGED. TO ME; IT APPEARS THAT INEFFICIENT USE OF THE FMC/FMS CONTRIBUTED TO THIS EVENT. AS FOR MY PART IN THIS; I DID NOT WANT TO BE THERE (IN COCKPIT) BECAUSE I WAS TIRED AND ANXIOUS TO GET HOME.