AN F2TH CAPT RPTED IMPROPER PROGRAMMING OF HIS AP CAUSED NON COMPLIANCE WITH A DSCNT INSTRUCTION NEAR OTT VORTAC.

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WE WERE LEVEL AT FL330 WITH A CLRNC DIRECT TO OTT VOR. I WAS THE CAPT AND PF THIS LEG OF THE TRIP. THE ATC CTLR ISSUED US A CLRNC TO CROSS 85 DME SW OF OTT AT FL290. OUR PARTICULAR FMS IS APPROVED FOR V-NAV OPS; BUT 3 STEPS HAVE TO BE ACCOMPLISHED FOR IT TO PERFORM THAT TASK. FIRST; THE XING RESTR HAS TO BE PROGRAMMED INTO THE FMS. NEXT; THE ALT PRE-SELECTOR HAS TO BE SET TO THE NEWLY ASSIGNED ALT. AND LASTLY; THE MOST IMPORTANT STEP; V-NAV HAS TO BE SELECTED ON THE FLT GUIDANCE PANEL. SINCE THE AUTOPLT WAS ENGAGED; I WENT AHEAD AND PROGRAMMED THE XING RESTR INTO THE FMS WHILE THE FO SET THE ALT PRE-SELECTOR TO OUR NEW ASSIGNED ALT OF FL290. AFTER I WAS FINISHED INSERTING THE RESTR INTO MY FMS; I ASKED THE FO IF HE WOULD LIKE FOR ME TO SEND IT OVER TO HIS SIDE; SINCE THE 2 THE FLT MGMNT SYS OPERATE INDEPENDENTLY. AFTER XFERRING THE XING RESTR TO HIS SIDE; WE BOTH HAD THE SAME INFO DISPLAYED ON OUR MULTIFUNCTION DISPLAYS; INCLUDING THE 85 DME SW OF OTT FIX WITH AN ALT OF FL290 AND A TOP OF DSCNT POINT. I PREFER TO USE THE VERT SPD MODE FOR MOST DSCNTS IN THIS ACFT; DUE TO THE FACT THAT IN THE V-NAV DSCNT MODE (APCHS EXCLUDED); THE ACFT HAS A TENDENCY TO PORPOISE UP AND DOWN WHILE FOLLOWING THE VPATH; ESPECIALLY WITH A FAIRLY STRONG TAILWIND. SINCE WE WERE GOING TO BE STARTING DOWN FAIRLY SOON; AND WE KNEW WE WERE GOING TO BE SHOOTING AN ILS APCH DOWN TO ABOUT 400 FT AGL AND 1 SM IN LIGHT SNOW; WE STARTED BRIEFING THE APCH. ABOUT 3/4 OF THE WAY THROUGH THE APCH BRIEFING; THE CTLR ASKED US IF WE WERE GOING TO MAKE OUR XING RESTR. AT THIS TIME; WE WERE ABOUT 87 MI SW OF OTT; SO I PICKED UP THE MICROPHONE AND NOTIFIED THE CTLR THAT WE WERE VACATING FL330 IMMEDIATELY WITH AN EXPEDITIOUS DSCNT TO FL290. THE CTLR TOLD US THAT THE NEXT SECTOR HAD ALREADY VECTORED THEIR FL310 TFC OUT OF THE WAY; AND FOR US TO JUST DSND TO FL290. WE COULD SEE ON OUR TCASII THAT THE OTHER ACFT WAS WELL OUTSIDE THE 5 MI RING AND DEFINITELY WAS NOT A CONFLICT AT ANY TIME. I CAN SEE SEVERAL FACTORS AT PLAY HERE. THE MOST OBVIOUS BEING LACK OF ANYONE MONITORING THE PROGRESS OF THE FLT (NO ONE FLYING THE AIRPLANE). THIS WAS MOST LIKELY CAUSED BY A NUMBER OF FACTORS; THE BIGGEST OF WHICH WAS THE DISTR OF COMMENCING THE APCH BRIEFING AT A TIME WHEN OTHER MORE IMPORTANT DUTIES SHOULD HAVE BEEN PERFORMED; SUCH AS; COMMENCING THE DSCNT. A COUPLE OF POSSIBLE UNDERLYING PHYSIOLOGICAL FACTORS COULD HAVE BEEN THE LACK OF A RESTFUL NIGHTS SLEEP THE NIGHT BEFORE DEP (LOTS OF TOSSING AND TURNING); AND A PRE-OCCUPATION WITH UPCOMING ADMINISTRATIVE DUTIES. KNOWING THAT I WAS NOT WELL RESTED AND DIDN'T FEEL AS ALERT AS I NORMALLY DO; I SHOULD HAVE LET THE TYPE-RATED FO TAKE THE FIRST LEG. THIS IS ONE INSTANCE WHEN AUTOMATION COULD HAVE SAVED THE DAY. BELIEVE ME FROM NOW ON; I AM GOING TO PERFORM THAT ALL IMPORTANT THIRD STEP IN THE V-NAV PROCESS; WHICH IS SELECTING V-NAV ON THE FLT GUIDANCE PANEL. EVEN IF IT CHASES THE VPATH; AT LEAST IT WILL START THE ACFT DOWN SHOULD THE CREW BECOME DISTR FOR WHATEVER REASON. IT IS A VALUABLE TOOL AT OUR DISPOSAL AND SHOULD BE UTILIZED; HOWEVER; IT SHOULD NEVER TAKE THE PLACE OF MAINTAINING UTMOST SITUATIONAL AWARENESS AT ALL TIMES AND EXERCISING EFFECTIVE CREW/CRM.