ACR X ON VISUAL APCH HAD CONFLICT WITH VFR B747 IN CLASS D AIRSPACE. SEE AND AVOID CONCEPT.

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THE DSCNT FROM ALT TO THE VISUAL APCH FOR RWY 14L AT GRANT COUNTY (MWH) WAS ROUTINE. I HAVE FLOWN THE PROFILE WE WERE GIVEN MANY TIMES IN THE PAST. WE ENTERED VMC AT ABOUT 7000 FT MSL; AND AT ABOUT 10 NM FROM THE ARPT. OUR SWITCH FROM APCH CTL TO TWR WAS CLOSE TO THE AIRFIELD; WHICH WAS NORMAL. AT NO TIME; ON EITHER APCH CTL OR TWR FREQ; WERE WE GIVEN ANY TA CONCERNING B747 TFC. AS WE COMMENCED THE R HAND ROLLOUT FROM THE TURN TO FINAL; AT LESS THE 1500 FT AGL; WE SAW A B747 APPEAR OUT FROM UNDERNEATH OUR ACFT. THE B747 WAS FASTER THAN WE WERE; MOVING AHEAD OF OUR ACFT. THE B747 WAS APPROX 300 FT BELOW US; AND LESS THAN 2 PLANE LENGTHS AHEAD. WE STOPPED THE ROLLOUT AND COMMENCED A RAPID TURN TO THE R; AWAY FROM THE NORMAL TFC PATTERN OF THE B747 ACFT. AFTER WE WERE IN THE TURN; THE TWR MADE A COMMENT INQUIRING WHICH TYPE OF RETURN TO THE PATTERN WE WOULD LIKE. THIS WAS THE FIRST INDICATION WE HAD THAT THE TWR WAS AWARE OF THE SIT. WE FLEW A R-HAND 360 DEG PATTERN; AND LANDED ON RWY 14L. THE ACFT WE WERE IN HAD A TCASII UNIT INSTALLED. THE UNIT HAD PERMANENT PLASTIC TIES AROUND THE CIRCUIT BREAKERS. THE UNIT ALSO HAD AN 'INFO' STICKER IN THE LOGBOOK AND ON THE EQUIP. THE UNIT WAS NOT DEFERRED AS PER MEL PROCS. THE TWR LATER STATED THAT THEY HAD US BOTH IN SIGHT; AND THAT THE B747 WAS VFR. I AM UNABLE TO COMMENT ON THE RELEVANCE OF THOSE STATEMENTS. TCASII IS INSTALLED IN THIS AND OTHER ACFT; BUT IS NOT AVAILABLE FOR USE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE COMPANY WAS INSTALLING TCASII IN ACFT BUT WOULD NOT LET FLCS USE TCASII UNTIL THEY HAD SEEN A VIDEO ON THE USE OF TCASII AND ITS OP. LOCKING OFF TCASII WAS THE COMPANY'S WAY OF CTLING USE OF TCASII. RPTR WAS INFORMED TO CALL FACILITY MGR REF ATC INCIDENTS.