FLC FAILED TO MAINTAIN ASSIGNED HDG AFTER A GAR. WHILE RE-ESTABLISHING HDG FLC RECEIVED A TCASII TA. APCH CTLR ASSIGNED A NEW ALT.

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OUR ACFT WITH CAPT FLYING WAS CLRED FOR THE ILS RWY 10L APCH AT PIT. WHILE TRANSITIONING TO THE LNDG CONFIGN; I (FO/PNF) NOTICED THAT AT FLAPS 5 DEGS WE HAD AN AMBER LEADING EDGE FLAPS TRANSIT LIGHT AND A #5 LEADING EDGE SLAT AMBER LIGHT. THE CAPT CALLED FOR THE QRH. WHILE I TRIED TO FIND THE CORRECT CHKLIST; WE WERE GETTING CLOSE TO THE RWY AND NOT PROPERLY CONFIGURED FOR LNDG; SO THE CAPT CALLED FOR A GAR. I TOLD APCH CTL THAT WE WANTED TO GO AROUND AND TURN L. APCH CTL INSTRUCTED US TO PROCEED STRAIGHT AHEAD AND CLB TO 3000 FT. ON COMMENCING THE GAR WHEN GOING FROM FLAPS 5 DEGS TO FLAPS 1 DEG; THE LEADING EDGE SLATS INDICATION CHANGED TO A SAFE (GREEN LIGHTS ON #5 LEADING EDGE SLAT AND LEADING EDGE FLAP EXTENSION). APCH CTL GAVE US A TURN TO N (360 DEGS) FOLLOWED BY A TURN TO 280 DEGS. DURING THE TURN TO 280 DEGS; THE CAPT INSTRUCTED ME TO BRIEF THE PAX ON THE GAR. I DESELECTED APCH CTL (VHF 1) AND SELECTED PA ON MY AUDIO PANEL AND TURNED DOWN THE AUDIO VOLUME ON VHF. WHILE BRIEFING THE PAX ON THE PA. I WAS NOT PAYING ATTN TO THE CAPT'S FLYING OR THE ACFT'S POS. WHEN I FINISHED THE PA AND RESELECTED VHF 1 AND TURNED THE VOLUME BACK UP; ABOUT THAT TIME THE CAPT SAID SOMETHING ABOUT; WHAT IS THE AIRPLANE DOING? I LOOKED AT THE INST PANEL AND NOTICED THAT OUR HDG WAS 220 DEGS INSTEAD OF 280 DEGS. ABOUT THAT TIME APCH CALLED AND ASKED US WHAT OUR HDG WAS AND I TOLD THEM. THE CAPT IN THE MEANTIME HAD DESELECTED THE AUTOPLT AND HAD COMMENCED A TURN BACK TO 280 DEGS. APCH THEN GAVE US A TURN FURTHER R TO 320 DEGS; FOLLOWED SHORTLY BY A CLB TO 4000 FT. ABOUT THAT TIME WE RECEIVED A TCASII 'TFC' CALL. I LOOKED AT THE TCASII SCREEN AND SAW THE OTHER ACFT AT ABOUT 10 O'CLOCK POS AND 3 NM. I BELIEVE THAT THE CLOSEST WE CAME TO EACH OTHER WAS 2.5 NM WITH AT LEAST 500 FT VERT SEPARATION. WE NEVER RECEIVED AN RA. I BELIEVED THAT THIS INCIDENT COULD HAVE BEEN AVOIDED IF I WOULD HAVE STAYED IN THE LOOP BY NOT DESELECTING VHF 1 AND MONITORING THE FREQ AND THE ACFT'S HDG AND ALT INSTEAD OF BECOMING LAX AND PREOCCUPIED IN EXPLAINING TO THE PAX THE REASONS FOR THE GAR AND THUS SUFFERING A COMPLETE LOSS OF SITUATIONAL AWARENESS.