TKOF WARNING HORN SOUNDS AS TKOF PROC WITHOUT FLAPS ATTEMPTED.

Narrative

COMPLETING DEICE WE WERE UNABLE TO OPEN APU BLEED AIR VALVE FOR ENG STARTS. COMPANY DISPATCHED AIR CART AND; UPON COMPLETION OF DEICING THE SECOND TIME; WE SUCCESSFULLY STARTED ENGS. THE AFTER START CHKLIST WAS COMPLETED. CAPT ELECTED TO KEEP FLAPS UP (DUE TO SNOW ACCUMULATION ON TXWYS AND LIGHT SNOW FALLING). FLAPS ARE NORMALLY PLACED TO 1 DEG OR 5 DEGS AT THIS POINT WHICH IS PART OF THE FO'S AFTER START FLOW. WE DID NOT COMPLETE THIS BEFORE TKOF CHKLIST AT THIS POINT BECAUSE WE WERE HOLDING THE FLAPS UP. AS WE SAT IN LINE FOR TKOF; WE DISCUSSED NUMEROUS REASONS AND PROBABLE CAUSES FOR NO APU BLEED AIR AVAILABLE. WHEN WE WERE NEXT FOR TKOF; THE CAPT INSTRUCTED ME TO TELL THE TWR WE NEEDED 1 MIN ON RWY (TO RUN ENGS UP TO 70 PERCENT N1 FOR 1 MIN PER ACFT MANUAL) AND WITH PARKING BRAKES SET FOR ME TO GO BACK IN THE CABIN AND CHK WINGS FOR CONTAMINATION. AS WE TAXIED INTO POS ON THE RWY; I COMPLETED THE LAST ITEMS OF THE BEFORE TKOF CHKLIST (WHICH WE REFER TO AS 'BELOW THE LINE'). I HAD NOT NOTICED AT THIS POINT WE HAD NOT COMPLETED THE BEFORE TKOF CHKLIST 'TO THE LINE.' THIS CHKLIST IS ALWAYS DONE TO THE LINE LONG BEFORE REACHING THE HOLD SHORT LINE. DURING WING CONTAMINATION INSPECTION; I STILL DID NOT NOTICE THE FLAPS WERE 'UP.' UPON RETURNING TO MY SEAT (MY LEG TO FLY) I STRAPPED IN AND WE WERE ALREADY CLRED FOR TKOF. AS THE THROTTLES WERE ADVANCED (APPROX 75 PERCENT N1) TKOF WARNING HORN SOUNDED. IMMEDIATELY SELECTED FLAPS 5 DEGS AND CAPT ELECTED TO CONTINUE TKOF (10000 FT RWY AND SNOW PACKED). HORN SILENCED IN APPROX 6 SECONDS ROLLING ABOUT 2000 FT TKOF ROLL. FLAPS GREEN LIGHT OBSERVED SHORTLY THEREAFTER. ROTATION DELAYED 25-30 KTS FOR CONCERN. CLBOUT UNEVENTFUL. REASONS: 1) FIRST TRIP AFTER 10 DAYS VACATION (FO). 2) EARLY WAKE-UP. 3) BREAK IN ROUTINE -- FO ALWAYS PLACES FLAPS 1 DEG OR 5 DEGS DURING AFTER START FLOW. 4) DISTRACTED WITH MINOR APU PROB. 5) SITUATIONAL AWARENESS. I WAS SO FOCUSED ON CONTAMINATION CHK AND BEING SOLELY RESPONSIBLE FOR MAKING THAT JUDGEMENT CALL FOR THE CAPT; I DID NOT NOTICE LEADING EDGE DEVICES OR FLAPS. IF STATIC RUN 70 PERCENT N1 HAD REQUIRED THROTTLES TO BE ADVANCED 1/4 INCH MORE; THE TKOF WARNING HORN WOULD'VE SOUNDED DURING STATIC ENG RUN-UP.