MLG MADE UNAUTH DESCENT AND CREATED POTENTIAL CONFLICT WITH ANOTHER ACFT. CTLR INTERVENED. NEW CAPT ON MLG.

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IN CONTACT WITH QUONSET APCH AT 10000' MSL; CAPT IS THE PLT FLYING. QUONSETT ISSUES A CLRNC TO 8000' MSL. IN THE DSCNT FROM 10000' TO 8000'; WE ARE VERBALLY DISCUSSING THE APPROPRIATE PROC TO BE FLOWN AND ACCOMPLISHING APPROPRIATE DSCNT CHKS. QUONSETT APCH THEN ISSUES US A HDG CHANGE TO 180 DEG AND WHAT WAS INTERPRETED BY THE COPLT AS A DSCNT TO 6000' MSL. COPLT VERBALLY ACKNOWLEDGES BY REPEATING AIRLINE CALL SIGN; HDG AND DSCNT TO 6000'. AT APPROX 6800' MSL; QUONSETT APCH ISSUES A WARNING TO CLB IMMEDIATELY TO 8000' AND TURN RIGHT TO 200 DEG. SECONDS AFTER ISSUING THE WARNING; AN ACFT IS VISUALLY SEEN AT OUR 10 O'CLOCK HIGH POS AND ABOUT 2 TO 3 MI AWAY. LNDG AT ISP WAS UNEVENTFUL AND QUONSETT APCH WAS CALLED ONCE WE REACHED OPS. PROBS. 1) QUONSETT DIDN'T DISCOVER ALT EXCURSION IN TIME TO HELP CONFLICT. (OTHER ACFT QUERIED HIM.) 2) QUONSETT APCH DIDN'T NOTICE COPLT'S READBACK OF INAPPROPRIATE ALT. 3) SOMEWHAT GARBLED AND HARD TO HEAR APCH CTL XMISSIONS. 4) COPLT; CAPT FATIGUE; I.E.; 12 HR DUTY DAY WITH 7 HR AND 30 MIN FLYING. 5) DAYTIME TO NIGHTTIME TRANSITION PERIOD. 6) DIVERTED ATTN IN A HEAVILY SATURATED TFC AREA. 7. CAPT'S 1ST FLT FOLLOWING CAPT'S UPGRADE; AND ACFT TRANSITION; I.E.; CONCENTRATION DIVERTED TO OTHER AREAS OF FLT OPS. RECOMMENDATIONS. COCKPIT CHKS PERFORMED SOONER SO CONCENTRATION CAN BE FOCUSED MORE ACUTELY TO HEAVIER; BUSIER AREAS OF FLT PROFILE.