Narrative:

I was holding short of runway 4L at bos. Tower control instructed us to position and hold on runway 4L. I taxied and position the aircraft on runway 4L. It was the first officer's leg; so I xferred the controls over to him once positioned on the runway. After I xferred the controls; I went on the PA to make a brief passenger announcement. As I was briefing the passenger; I heard a takeoff clearance. I noticed the first officer advancing the power levers and the aircraft started rolling. I quickly finished the passenger brief and turned on the external lights. As the aircraft was rolling; tower instructed us to stop the roll. I immediately took control of the aircraft and came to a stop. The aircraft speed at the time I stopped was somewhere below 60 KTS. Tower instructed us to make a 180 degree turn to repos and hold on runway 4L. I then requested to exit off the runway instead. I exited off and held short of runway 4L once again. I then informed tower we were ready for departure. We were then instructed to position and hold runway 4L. I positioned the aircraft on the runway and xferred the controls to the first officer. A departure clearance was given which I repeated and we departed boston. As we were on the initial takeoff roll; I believe there was an aircraft about to cross the departure end of the runway. This runway incursion was prevented when I was asked to stop the takeoff roll by tower control. Looking back; I am not sure if the departure clearance was given to us. When I was briefing the passenger; my attention was on the passenger. As I noticed the first officer advancing the power levers; I assume that he received a takeoff clearance and that's why he started applying takeoff power and initiated the takeoff roll. Supplemental information from acn 699063: on runway 4L; while crossing the hold short line; the captain implied that he was 'going to the back;' in other words to talk on the PA to brief the passenger. The captain was taxiing onto the runway while talking on the PA. Upon stopping the aircraft on the runway; he stated 'your brakes -- your controls.' I then stated 'my controls.' at that time it was my understanding that we were cleared for takeoff and I advanced the power levers to initiate takeoff roll. The captain turned on the necessary exterior lights; and stated 'exterior lights on; before takeoff checklist complete.' after reaching 40 KTS during takeoff roll on runway 4L; the tower controller stated; 'call sign; emergency. Stop on runway.'

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Original NASA ASRS Text

Title: B1900 FLT CREW HAS RWY INCURSION ON RWY 4L BOS.

Narrative: I WAS HOLDING SHORT OF RWY 4L AT BOS. TWR CTL INSTRUCTED US TO POS AND HOLD ON RWY 4L. I TAXIED AND POS THE ACFT ON RWY 4L. IT WAS THE FO'S LEG; SO I XFERRED THE CTLS OVER TO HIM ONCE POSITIONED ON THE RWY. AFTER I XFERRED THE CTLS; I WENT ON THE PA TO MAKE A BRIEF PAX ANNOUNCEMENT. AS I WAS BRIEFING THE PAX; I HEARD A TKOF CLRNC. I NOTICED THE FO ADVANCING THE PWR LEVERS AND THE ACFT STARTED ROLLING. I QUICKLY FINISHED THE PAX BRIEF AND TURNED ON THE EXTERNAL LIGHTS. AS THE ACFT WAS ROLLING; TWR INSTRUCTED US TO STOP THE ROLL. I IMMEDIATELY TOOK CTL OF THE ACFT AND CAME TO A STOP. THE ACFT SPD AT THE TIME I STOPPED WAS SOMEWHERE BELOW 60 KTS. TWR INSTRUCTED US TO MAKE A 180 DEG TURN TO REPOS AND HOLD ON RWY 4L. I THEN REQUESTED TO EXIT OFF THE RWY INSTEAD. I EXITED OFF AND HELD SHORT OF RWY 4L ONCE AGAIN. I THEN INFORMED TWR WE WERE READY FOR DEP. WE WERE THEN INSTRUCTED TO POS AND HOLD RWY 4L. I POSITIONED THE ACFT ON THE RWY AND XFERRED THE CTLS TO THE FO. A DEP CLRNC WAS GIVEN WHICH I REPEATED AND WE DEPARTED BOSTON. AS WE WERE ON THE INITIAL TKOF ROLL; I BELIEVE THERE WAS AN ACFT ABOUT TO CROSS THE DEP END OF THE RWY. THIS RWY INCURSION WAS PREVENTED WHEN I WAS ASKED TO STOP THE TKOF ROLL BY TWR CTL. LOOKING BACK; I AM NOT SURE IF THE DEP CLRNC WAS GIVEN TO US. WHEN I WAS BRIEFING THE PAX; MY ATTN WAS ON THE PAX. AS I NOTICED THE FO ADVANCING THE PWR LEVERS; I ASSUME THAT HE RECEIVED A TKOF CLRNC AND THAT'S WHY HE STARTED APPLYING TKOF PWR AND INITIATED THE TKOF ROLL. SUPPLEMENTAL INFO FROM ACN 699063: ON RWY 4L; WHILE XING THE HOLD SHORT LINE; THE CAPT IMPLIED THAT HE WAS 'GOING TO THE BACK;' IN OTHER WORDS TO TALK ON THE PA TO BRIEF THE PAX. THE CAPT WAS TAXIING ONTO THE RWY WHILE TALKING ON THE PA. UPON STOPPING THE ACFT ON THE RWY; HE STATED 'YOUR BRAKES -- YOUR CTLS.' I THEN STATED 'MY CTLS.' AT THAT TIME IT WAS MY UNDERSTANDING THAT WE WERE CLRED FOR TKOF AND I ADVANCED THE PWR LEVERS TO INITIATE TKOF ROLL. THE CAPT TURNED ON THE NECESSARY EXTERIOR LIGHTS; AND STATED 'EXTERIOR LIGHTS ON; BEFORE TKOF CHKLIST COMPLETE.' AFTER REACHING 40 KTS DURING TKOF ROLL ON RWY 4L; THE TWR CTLR STATED; 'CALL SIGN; EMER. STOP ON RWY.'

Data retrieved from NASA's ASRS site as of January 2009 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.