HDG AND ALT MISINTERP.

Narrative

ACFT X WAS NBOUND AND INBOUND TO IND. ACFT Y DEPARTED CVG CLBING OVER IIU VOR. ACFT Y INITIALLY CLRED TO CLB AND MAINTAIN FL250 AND ACKNOWLEDGED CORRECTLY. SUBSEQUENT CLRNC TO ACFT Y WAS 'FLY HDG 270' DIRECT PXV WHEN ABLE REST UNCHANGED. ACFT Y READ BACK CLBING 270 DIRECT PXV. TCASII RA'S ISSUED BY BOTH ACFT AND FAA STANDARD SEPARATION WAS LOST. PARAMOUNT TO THIS SIT WAS ACFT Y'S IMPROPER READBACK OF THE CLRNC. AT ALTS ABOVE 17000 FT ALL ATC ALT CLRNCS WILL BE PRECEDED BY THE WORDS 'FLT LEVEL.' IF ACFT Y HAD READ BACK THE CLRNC INCLUDING THE ALT INDICATORS 'FLT LEVEL' THE CTLR WOULD HAVE CAUGHT THE ERROR AND SEPARATION COULD HAVE BEEN MAINTAINED. CONTRIBUTING TO THE SIT WAS ASSIGNING A HDG THAT COULD BE MISTAKEN FOR AN ALT. ALTHOUGH A HDG OF 270 DEGS WAS A CORRECT HDG FOR PXV; A HDG OF 275 DEGS OR 265 DEGS COULD HAVE AVOIDED CONFUSION. ATC CLRNCS NEED TO BE READ BACK COMPLETELY. CTLRS ARE TRAINED TO HEAR DEVS FOR ISSUED CLRNCS -- ESPECIALLY WHEN CLRNCS AFFECT SEPARATION. IN THIS SIT THE CTLR ONLY HEARD 270 DIRECT PXV AND SEPARATION WAS LOST. ACFT Y WAS REQUESTING FL310 AND ACFT X NEEDED TO BE STARTED DOWN. CTLR WAS USING VERT SEPARATION UNTIL ACFT CLRED.