A PC12 FATIGUED PLT EXPERIENCED A STALL WARNING FOLLOWING A PITOT INPUT FAILURE TO THE ACFT'S ADC.

Narrative

BEING VECTORED TO ILS RWY 34R (WX) 4000 FT BROKEN; VISIBILITY 2 NM. I WAS ASSIGNED HDG 070 DEGS. I JUST GOT STABLE AT 8000 FT PER TWR. ACFT WAS ON AUTOPLT HDG AND ALT PRESELECT MODE. I WAS CHKING FREQ; ATIS; AND DECISION HT FOR THE ILS WHEN THE AUTOPLT DISCONNECTED. ANGLE OF ATTACK INDICATOR WAS SLOW AT THE BOTTOM (1.3 VERSUS AT MID RANGE; STICK SHAKER AT 91 KTS +8 CLEAN). STICK SHAKER DISCONNECTED THE AUTOPLT WHILE AIRSPD INDICATOR SHOWED 170 KTS. I INITIATED RECOVERY AT 7500 FT MSL. TWR ADVISED I WAS LOW; THEN ASSIGNED ME 6500 FT. THE REST OF THE APCH WAS HAND FLOWN. WHOLE EVENT TOOK PLACE VMC WITH RWY IN SIGHT. CAUSE: MAINT TECHNICIANS FOUND A BROKEN PITOT LINE GOING TO THE ADC; IE; PRESSURIZATION BEGAN DRIVING AIRSPD. CONTRIBUTING FACTORS: ATC HAD ASSIGNED ILS RWY 34L THEN AT MY REQUEST CLRED ME TO ILS RWY 34R; IE; DIVERTING MY ATTN TO SETTING UP FOR A NEW APCH CLOSE IN. HUMAN FACTOR: FATIGUE -- THIS WAS LAST LEG OF THE NIGHT (5.5 HRS FLOWN IN LAST 12 HRS). PERFORMANCE: IT TOOK 4 OR 5 SECONDS AND 500 FT TO RECOGNIZE THE PROB THEN RESPOND WITH APPROPRIATE COURSE OF ACTION. FATIGUE MAY HAVE BEEN A FACTOR IN RESPONSE TIME.