F100 EXPERIENCED A TCASII RA IN CZYZ'S AIRSPACE AT FL350.

Narrative

JUST W OF BORDN WHILE ENRTE TO ORD FROM YUL ON J553 AT FL350; YYZ CTR GAVE US A TURN TO 200 DEGS FOR SPACING TO ORD. WHILE ON THAT HEADING WE RECEIVED A TA AND LOOKED AT THE NAV DISPLAY IN TIME TO SEE A TARGET AT 10:30 TO 11 O'CLOCK POS; AND ABOUT 5 MI AT FL350. THE TA ALMOST IMMEDIATELY BECAME AN RA TO DSND. AT THAT SAME TIME I HEARD THE OTHER ACFT (AN ACR HVY) RPT THAT THEY WERE RESPONDING TO AN RA TO CLB AND WERE LEAVING FL350. THE CAPT STARTED OUR DSCNT IMMEDIATELY AND WE RECEIVED A CLRNC TO DSND TO FL330. WE COMPLIED AND LEVELED AT FL330. THERE WAS APPROX 1200 FT OF SEPARATION VERTLY AS THE HVY PASSED DIRECTLY OVER US IN OUR DSCNT. THERE IS NO DOUBT IN MY MIND THAT HAD IT NOT BEEN FOR TCASII; WE WOULD HAVE COLLIDED WITH THE OTHER ACFT. TO COMPOUND MATTERS; THERE WAS AN OBSCURE LAYER AT OUR ALT THAT MADE IT IMPOSSIBLE TO SEE THE OTHER ACFT UNTIL IT WAS ALREADY ON TOP OF US. I HAVE HAD RA'S IN THE PAST; ALL OF WHICH WOULD HAVE BEEN CLOSE CALLS. IN MY VIEW; THIS WOULD NOT HAVE BEEN A CLOSE CALL BUT RATHER A MIDAIR COLLISION THAT NO ONE WOULD HAVE SURVIVED. IT IS VERY LIKELY THAT THE CTLR SIMPLY FORGOT US ON THE HEADING AND WAS DISTRACTED WITH OTHER TFC. WE DID NOT RECEIVE ANY ATC DIRECTIVE TO ANOTHER ON COURSE HEADING PRIOR TO THE RA.