FLC MISUNDERSTOOD TAXI CLRNC AND CONTINUED TAXIING ONTO AN ACTIVE RWY.

Narrative

HOLDING SHORT (#1) OF RWY 22R; BOS TWR INSTRUCTED US TO CROSS RWY 22R AND TAXI TO RWY 22L; THEN IMMEDIATELY CANCELED THOSE INSTRUCTIONS AND TOLD US TO TAXI INTO POS AND HOLD ON RWY 22R. AS WE WERE PULLING ONTO THE RWY; TWR THEN INSTRUCTED US TO CROSS RWY 22R; AND TAXI INTO POS AND HOLD ON RWY 22L. 'YOU WILL BE GOING RIGHT OUT.' WE CROSSED RWY 22R AND WHILE TAXIING TO RWY 22L; CHKED THE WT AND BAL DATA FOR THE NEW RWY. JUST BEFORE REACHING RWY 22L; TWR GAVE US A FREQ CHANGE AND I CHKED IN WITH THE NEW FREQ TWR CTLR STATING WE WERE PULLING INTO POS AND HOLD FOR RWY 22L. TWR THEN SAID HOLD SHORT RWY 22L. BY THIS TIME WE WERE ALREADY ON THE EXTENDED OVERRUN (DISPLACED THRESHOLD) OF RWY 22L AND I SAW A BEECH COMMUTER GO OVER US AND LAND IN THE NORMAL TOUCHDOWN ZONE OF RWY 22L. I ACKNOWLEDGED THE HOLD SHORT CLRNC AND IMMEDIATELY TWR CAME BACK WITH CLRNC ONTO THE RWY AND HOLD. THIS WHOLE SEQUENCE OF EVENTS TOOK PLACE IN ABOUT 1 MIN. THERE WERE NO SUBSEQUENT RADIO TRANSMISSIONS TO INDICATE THAT ANYONE HAD NOTICED A CONFLICT. I THINK THE MAIN CAUSE WAS A LATE FREQ CHANGE TO THE NEW (22L) RWY TWR FREQ. HAD IT BEEN GIVEN WITH THE RWY CHANGE INSTRUCTIONS; THERE MIGHT HAVE BEEN TIME TO CLARIFY CONFLICTING INSTRUCTIONS. SUPPLEMENTAL INFO FROM ACN 315471: THE COCKPIT ENVIRONMENT WAS BUSY CHKING THE TKOF DATA FOR A NEW RWY AND MAKING UPDATES TO THE FMC TO REFLECT THE NEW RWY. I AM NOT SURE ANYONE NOTICED HOW CLOSE AND INSIDIOUS THIS SCENARIO WAS BECAUSE OF THE RAPID CHANGE IN RWY INSTRUCTIONS AND THE FACT THE RWYS' CTLRS WERE ON SEPARATE FREQS. THIS HINDERED OUR ABILITY TO HAVE BETTER SITUATIONAL AWARENESS OF ARRIVING ACFT. I DID NOT MAKE A VISUAL CHK TO SEE IF AN ACFT WAS ON SHORT FINAL AS I WAS THINKING ABOUT GETTING MY ACFT READY TO TKOF IN A SHORT PERIOD OF TIME.