A B727 FLC RPTS A DSCNT OF 600 FT BELOW THE ASSIGNED ALT DURING A DSNDING TURN. THE FLC CITES FATIGUE AND DISTR.

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INCIDENT OCCURRED DUE TO DISTR WHILE IN A DSNDING R TURN. EVENT AS FOLLOWS: AFTER INITIATING AN ENRTE DSCNT FROM FL310 DOWN TO FL290; CTR VECTORED THE FLT OFF THE AIRWAY DUE TO TFC CONFLICT. THE CTLR ALSO STATED THE TFC WAS AT OUR HIGH 1 O'CLOCK POS AT FL310 FOR A FEW MI. SIMULTANEOUSLY; THE FLT ATTENDANTS PAGED THE COCKPIT. BOTH PLTS LEANED FORWARD TO OBTAIN A VISUAL WITH THE TFC WHILE THE FE TENDED TO THE COCKPIT PAGE. AFTER GETTING A VISUAL ON THE TFC; I; AS COPLT (PNF) LEANED BACK AGAIN INTO MY SEAT TO ASSESS OUR PROGRESS TOWARD THE NEW HDG AND FLT LEVEL. AT THAT TIME; I NOTED THAT WE WERE AT FL284. I QUICKLY NOTIFIED THE CAPT (PF) WHO IN TURN RAPIDLY RETURNED THE AIRPLANE TO THE CLRED ALT OF FL290. THE AUTOPLT WAS OPERATING BUT EITHER THE ALT SELECT FEATURE FAILED OR WE NEVER ENGAGED IT AFTER INITIATING THE DSCNT DUE TO ALL THE DISTRS. NO MENTION OF THE INCIDENT WAS MADE BY ATC. SUPPLEMENTAL INFO FROM ACN 335400: CONTRIBUTING FACTORS: ALL 3 CREW HAD STATED EARLIER HOW GLAD WE WOULD BE TO GET HOME. WE WERE TIRED. WE HAD 2 WEST COAST AFTERNOON PICKUPS THEN A SHORT LAYOVER (9 HRS) ON EAST COAST TIME AND AN EARLY PICKUP. THIS WAS THE 4TH LEG OF THE DAY.