FLC OF AN LTT ACR ACFT TOOK EVASIVE ACTION TO AVOID ANOTHER ACFT DSNDING INTO ITS FLT PATH WHILE AT AN ASSIGNED APCH ALT IN A TCA.

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LEVEL AT 9000 FT MSL IN VMC; I NOTICED THE TAIL OF ANOTHER ACFT BEHIND THE WINDSHIELD SPAR. I LEANED TO THE SIDE TO LOOK AND SAW AN SMT DSNDING AND CONVERGING ON US FROM THE R. I CALLED OUT THE TFC TO MY FO (PF) AND HE IMMEDIATELY DISCONNECTED THE AUTOPLT AND BEGAN A DSCNT. I CALLED BWI APCH AND TOLD HIM WE WERE OUT OF 9000 FT TO AVOID TFC. THE CTLR SAID THE TFC DSNDED TO AVOID CLOUDS. THE SMT PLT RESPONDED THAT HE DSNDED TO AVOID WAKE TURB FROM A MIL MLT THAT HAD PASSED OVER HIM EARLIER. WE WERE IN CLASS B AIRSPACE ON AN IFR FLT PLAN; AND THE SMT WAS VFR. THE CTLR WAS NOT BUSY. HE SHOULD HAVE HAD TIME TO CALL THE TFC TO BOTH OF US. EVEN THOUGH VERT SEPARATION WAS LOST BTWN THE SMT AND US; THE SEE AND AVOID CONCEPT ALWAYS APPLIES TO ACFT IN VMC. TCASII WOULD HAVE PROBABLY PREVENTED THIS CONFLICT; BUT WE DID NOT HAVE TCASII BECAUSE FAR PART 135 DOES NOT REQUIRE IT UNTIL LATE 1995. BUT TCASII USERS SHOULD BEWARE OF BECOMING COMPLACENT; TCASII CAN'T PICK UP EVERY ACFT OUT THERE AND IT CAN'T REPLACE THE EYES.