POTENTIAL CONFLICT OCCURS BTWN 2 ACFT WHEN ONE DSNDS THROUGH THE ALT OF WBOUND TFC AT FL280; 50 NM NW OF LVZ VOR; PA.

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THE CAPT WAS THE PF AND I WAS THE PNF. WE WERE LEVEL AT FL410 AND WERE PROCEEDING DIRECT TO THE LVZ VOR. WE WERE GIVEN A XING RESTR AT LVZ AT FL180. AFTER THE PF INITIATED THE DSCNT; I PICKED UP THE DEST ARPT ATIS ON COM #2. AFTER SWITCHING BACK TO COM #1; THE PF ADVISED ME THAT WE WERE NOW TURNING TO A HEADING FOR THE DSCNT AND THAT HE WAS INCREASING HIS RATE OF DSCNT. A FEW SECONDS LATER; ATC RECLRED US DIRECT TO THE LVZ VOR. SHORTLY AFTER THIS WE RECEIVED AN ALERT ON TCASII (TA MODE); BUT I WAS HAVING DIFFICULTY INTERPRETING THE TCASII DISPLAY. I SCANNED OUTSIDE BUT DID NOT SEE ANY ACFT. THE PF ASKED ME TO SWITCH TO TA/RA MODE. SHORTLY THEREAFTER THE PF BEGAN TO SHALLOW HIS DSCNT AND EVENTFULLY LEVELED OFF AT APPROX FL270. ATC ASKED IF WE WERE LEVEL AT FL290. THE ALT ALERTER WAS STILL SET AT FL180. WE LATER DETERMINED THAT WHILE I WAS OBTAINING THE ATIS; THE PF RECEIVED AND ACKNOWLEDGED AN AMENDED CLRNC CONSISTING OF A HEADING AND A DSCNT TO FL290. HE DID NOT IMMEDIATELY SET THE ALT ALERTER WHEN HE RECEIVED THE AMENDED CLRNC AND FORGOT ABOUT IT. I BELIEVE THE FAILURE OF THE PF TO IMMEDIATELY RECORD THE AMENDED ALT CLRNC ON THE ALT ALERTER CAUSED THIS EVENT. NORMALLY THE PNF SETS THE ALT ALERTER UNLESS HE IS BUSY USING THE OTHER COM RADIO FOR NON ATC RELATED COM. IN THIS CASE; THE PF ASSUMES ALL DUTIES INCLUDING SETTING THE ALERTER. I BELIEVE THE CONTRIBUTING FACTORS INCLUDED THE INABILITY OF THE PF TO KEEP THE AMENDED ALT CLRNC IN SHORT TERM MEMORY; FATIGUE ASSOCIATED WITH AN XA30 AM DEP; LACK OF FAMILIARITY WITH SOME OF THE TCASII DISPLAYS/MESSAGES; AND THE POSSIBILITY MY TCASII/IVSI DISPLAY MAY HAVE STILL BEEN DIMMED FROM EARLIER IN THE FLT. THE IMMEDIATE CORRECTIVE ACTIONS DURING THE EVENT INCLUDED REDUCING AND EVENTUALLY STOPPING THE DSCNT. THE ACTIONS TAKEN AFTER THE FLT INVOLVED DISCUSSING THE EVENT WITH MGMNT; INCLUDING WHAT OCCURRED; WHY IT OCCURRED; AND HOW TO PREVENT IT. ALSO; I WILL BE SPENDING SOME TIME WITH THE TCASII PLT'S GUIDE. SUPPLEMENTAL INFO FROM ACN 576535: CONTRIBUTING FACTORS: MIS-PRIORITIZING THE R HANDED TASKS; AUTOPLT PITCH; AUTOPLT HEADING AND ALT ALERT. USING THE TCASII FOR TFC AWARENESS ONLY AND RELYING ON ATC FOR SEPARATION. TCASII IN RA MODE WOULD HAVE GIVEN MORE TIME TO PREVENT THE CONFLICT. SHOULD HAVE REACTED MUCH MORE RAPIDLY TO THE NUMBERS THAT WE SAW ON THE TCASII. TRIED TO USE VISUAL SEPARATION TECHNIQUES IN POSITIVE CTL AIRSPACE.