FLC HAS RWY INCURSION.

Narrative

CREW INTENSELY ENGAGED IN TURN AROUND OF LATE ARRIVING FLT WITH CREW CHANGE. TAXI OUT CHKLISTS AND PRE TKOF CHKS WERE BEING RUN WITH SHORT TAXI OUT TO END OF RWY 15. WHILE ON RAMP; CREW NOTIFIED A LIGHT ACFT TAXIING THAT COULD POSE TAXI CONFLICT AS CREW EXITED RAMP. THIS TURNED INTO A PREOCCUPATION; AS CREW DISCUSSED WHETHER OR NOT THE GND CTLR HAD ISSUED A TAXI SEQUENCE TO US. CAPT TURNED OUT OF RAMP ONTO ACTIVE RWY WHILE WATCHING LIGHT ACFT TO ASSURE CLRNC. THIS DISTR CREATED A SIT THAT SET INTO MOTION THE INCIDENT. CONTRIBUTING TO 'THE BRAIN NOT REGISTERING WHAT THE EYES WERE SEEING' WAS THAT MOST OF OUR OPS ARE AT LARGER ARPTS WHERE RAMPS HAVE EXIT/ENTRANCE TXWYS THAT JOIN WITH MAIN TXWY THAT PARALLELS THE RAMP AREAS. CONTRIBUTING TO THIS ILLUSION WAS THE ONE ACTIVE RWY WHICH WAS VERY NARROW WITH AN ACFT IN A 'TAXI BACK' TO THE END IN PROGRESS. THIS WAS THE ACFT WE WERE CONCERNED ABOUT. THE CAPT; AFTER MAKING THE TURN FROM THE RAMP TXWY B ONTO RWY 19/33; NOTICED THE RWY CTRLINE MARKINGS AND REALIZED HIS ERROR. ABOUT THAT TIME GND CTLR REALIZED SIT AND ADVISED CREW THAT THEY WERE ON AN ACTIVE RWY. I WOULD HAVE TO BELIEVE THAT WITH THE LAYOUT OF THE RAMP; THIS TYPE OF A SIT MAY BE A RECURRING SIT. THE CAPT HAD BEEN INTO BTV ON 2 OTHER TRIPS.