Narrative:

In contact with quonset approach at 10000' MSL, captain is the pilot flying. Quonsett issues a clearance to 8000' MSL. In the descent from 10000' to 8000', we are verbally discussing the appropriate procedure to be flown and accomplishing appropriate descent checks. Quonsett approach then issues us a heading change to 180 degree and what was interpreted by the copilot as a descent to 6000' MSL. Copilot verbally acknowledges by repeating airline call sign, heading and descent to 6000'. At approximately 6800' MSL, quonsett approach issues a warning to climb immediately to 8000' and turn right to 200 degree. Seconds after issuing the warning, an aircraft is visually seen at our 10 O'clock high position and about 2 to 3 mi away. Landing at isp was uneventful and quonsett approach was called once we reached operations. Problems. 1) quonsett didn't discover altitude excursion in time to help conflict. (Other aircraft queried him.) 2) quonsett approach didn't notice copilot's readback of inappropriate altitude. 3) somewhat garbled and hard to hear approach control xmissions. 4) copilot, captain fatigue, i.e., 12 hour duty day with 7 hour and 30 min flying. 5) daytime to nighttime transition period. 6) diverted attention in a heavily saturated traffic area. 7. Captain's 1ST flight following captain's upgrade, and aircraft transition, i.e., concentration diverted to other areas of flight operations. Recommendations. Cockpit checks performed sooner so concentration can be focused more acutely to heavier, busier areas of flight profile.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.