LOSS OF SEPARATION BTWN AN AC69 AND A C172 AT 12000 FT WHEN RPTR MISSED THE INCORRECT ALT READBACK BY THE AC69 PLT OF 16000 FT VICE 11000 FT.

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ACFT #1 REQUESTED AN IFR CLRNC TO PHX. HE WAS OFF OF SEZ A POP-UP IFR. I GAVE HIM A CODE AND IDENTED HIM 5 MI S OF SEZ. I ASKED HIM IF HE COULD MAINTAIN HIS OWN TERRAIN AND OBSTRUCTION CLRNC 'TO 11000 FT.' HE SAID AFFIRMATIVE. I THEN CLRED HIM TO THE PHX ARPT VIA DIRECT TONTO AND THE FOSSL 3 ARR CLB AND 'MAINTAIN ONE; ONE -- ELEVEN THOUSAND; FLG ALTIMETER...' HE THEN READ BACK 'ROGER DIRECT TONTO; FOSSL 3 ARR; ACFT #1; UHH UP TO 16000 FT.' I SAID 'ROGER;' OBVIOUSLY NOT HEARING THE ALT READBACK. I THEN CALLED TFC FOR ACFT #2 THE ACFT THAT I STOPPED ACFT #1 FOR. THEN I CALLED TFC FOR ACFT #1 'TFC 12 O'CLOCK POS 4 MI 12000 FT A C172.' HE SAID 'ROGER WE'RE LOOKING.' I THEN BEGAN GIVING A RELIEF BRIEFING WHEN I NOTICED THAT ACFT #1 WAS AT 11600 FT CLBING. I ASKED HIM TO VERIFY LEVEL AT 11000 FT; HE SAID 'NEGATIVE WE READ BACK 16000 FT.' HIS MODE C NOW SHOWED 12000 FT. I SAID 'NEGATIVE YOUR CLRNC WAS 11000 FT; BUT CONTINUE YOUR CLB; MAINTAIN 16000 FT; TFC 12 O'CLOCK POS 1 MI C172 AT 12000 FT.' HE SAID 'LOOKING; SORRY ABOUT THAT.'