B737 CREW GOES AROUND BELOW 300 FT ON FINAL WHEN ACFT PULLS ONTO THE LNDG RWY AT THE APCH END TXWY WITHOUT CLRNC. TWR WAS UNAWARE OF THE PROB AND THOUGHT GAR WAS DUE TO PRIOR LNDG TFC DELAYING EXIT FROM RWY.

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I WAS COMMANDING OFFICER OF ACR FLT ABC ON JUL/AB/99. ON APCH TO PHL RWY 27R; AT APPROX 500 FT; WE NOTICED A TAXIING ACFT APCHING THE RWY FROM THE TERMINAL SIDE (WE BELIEVE TXWY N). THE ACFT NEVER STOPPED SHORT OF THE RWY AND PROCEEDED VERY SLOWLY ACROSS THE RWY. WHEN HIS NOSEWHEEL FIRST TOUCHED THE RWY; WE WERE APPROX 300 FT; AT 100 FT AGL; WE WENT AROUND; THE ACFT WAS APPROX IN THE MIDDLE OF THE RWY; STILL CREEPING. WHEN I CALLED THE TWR ON THE PHONE AFTER LNDG; THE INDIVIDUAL I TALKED TO; (I DID NOT GET HIS NAME) THOUGHT WE WENT AROUND FOR A COMPANY ACFT THAT LANDED IN FRONT OF US; AND MISSED THE HIGH SPD TXWY. I TOLD HIM THAT ACFT WAS NOT FACTOR; AND THE REASON FOR THE GAR WAS AN ACFT XING THE RWY. HE SAID THE TWR HAD NO IDEA WHY WE WENT AROUND; WHICH BRINGS UP A LOT OF QUESTIONS. 1) WAS THIS ACFT CLRED TO CROSS RWY 27R? 2) DID NO ONE IN THE TWR NOTICE A CONFLICT? 3) WHY WOULD ANY COMMANDING OFFICER CROSS THE RWY IN THE FACE OF VERY CLOSE ONCOMING TFC; REGARDLESS OF HIS CLRNC? IT WAS HAZY AND THE SUN WAS BEHIND THE XING ACFT; SO WE COULD NOT TELL WHAT AIRLINE HE WAS. WE THINK IT WAS A B737 OR AIRBUS 319/320. THE TWR MAN I TALKED TO ON THE PHONE SAID THERE WAS NO WAY TO FIND OUT. (RIGHT!) I CALLED THE TWR AGAIN THE FOLLOWING MORNING AND TALKED TO MR XYZ. HE SAID HE WOULD LOOK INTO IT. THIS EVENT DID NOT SIT WELL WITH ME; IF THE WX HAD BEEN IMC; WE MAY HAVE HAD A VERY SERIOUS CONFLICT ON THE RWY.