Narrative:

Left the gate with a taxi clearance to runway 9 outer taxiway bravo, S1 hold short runway 9 overrun. At south taxiway we held behind an air carrier Y when the line of aircraft in front started to move onto B. We were then cleared to follow the air carrier Y hold short of the 4L overrun. This was lead back. As we taxied around B taxiway we saw the 9 hold short line and the 9 overrun markings. I taxied around this looking for the 4L hold short line or the 4L overrun. At this time an air carrier Z large transport was on approach to what I thought was 4R. Seconds later it appeared he was headed right for us. Thinking I had crossed onto 4L, I immediately added power to get to S1 where I knew the aircraft would be safe.

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Original NASA ASRS Text

Title: ACR LGT UNAUTH RWY CROSSING AT BOS.

Narrative: LEFT THE GATE WITH A TAXI CLRNC TO RWY 9 OUTER TXWY BRAVO, S1 HOLD SHORT RWY 9 OVERRUN. AT S TXWY WE HELD BEHIND AN ACR Y WHEN THE LINE OF ACFT IN FRONT STARTED TO MOVE ONTO B. WE WERE THEN CLRED TO FOLLOW THE ACR Y HOLD SHORT OF THE 4L OVERRUN. THIS WAS LEAD BACK. AS WE TAXIED AROUND B TXWY WE SAW THE 9 HOLD SHORT LINE AND THE 9 OVERRUN MARKINGS. I TAXIED AROUND THIS LOOKING FOR THE 4L HOLD SHORT LINE OR THE 4L OVERRUN. AT THIS TIME AN ACR Z LGT WAS ON APCH TO WHAT I THOUGHT WAS 4R. SECS LATER IT APPEARED HE WAS HEADED RIGHT FOR US. THINKING I HAD CROSSED ONTO 4L, I IMMEDIATELY ADDED PWR TO GET TO S1 WHERE I KNEW THE ACFT WOULD BE SAFE.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.