H25 CREW AT PHL TAXIING FOR DEP; RECEIVING PROGRESSIVE INSTRUCTIONS; MANAGE TO INADVERTENTLY CROSS A RWY WITHOUT CLRNC.

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I WAS COPLT ON THIS HAWKER ON NOV/XA/01 AT PHL. WE WERE ON THE E SIDE OF THE FIELD. OUR TAXI INSTRUCTIONS WERE TO 'TAXI TO RWY 35 HOLD SHORT RWY 8.' THEN THE CTLR CLARIFIED AND SAID; TAXI RWY 35 TXWY A AND TXWY D HOLD SHORT RWY 8.' APCHING RWY 8 WE WERE CLRED TO TAXI RWY 35 VIA TXWY D. NEARING TXWY K WE WERE TOLD TO HOLD POS. THE CTLR SOUNDED HURRIED OR PANICKED. APPARENTLY THERE WAS SOME CONFUSION ON THE FIELD TO THE W OF OUR POS. WHEN WE STOPPED WE WERE AT THE INTXN OF TXWY K AND TXWY D. THE NOSE OF THE ACFT WAS POINTED TO THE R OR SW SO WE COULD SEE THE CONFUSION. A SMALL ACFT (PIPER) WAS TAXIING UNDER THE WING OF AN ACR HVY JET (ON TXWY N; I BELIEVE). EVERYONE ON THE GND FREQ WAS CONSUMED WITH THE EVENT ON THE FIELD. THE FREQ WAS CONGESTED WITH INSTRUCTIONS FROM THE CTLR AND COMMENTS FROM PLTS. AFTER THE COMMOTION HAD CEASED WE WERE TOLD TO 'CONTINUE OUR TAXI TO RWY 35.' WE CROSSED RWY 35 AT TXWY K; TURNED L ON TXWY E. AT THAT POINT WE REALIZED OUR MISTAKE AND HELD SHORT RWY 9L/27R AT TXWY E. THE CTLR CONFIRMED WE WERE HOLDING SHORT OF RWY 9L/27R. FINALLY WE WERE CLRED TO 'CROSS RWY 9L/27R AND TAXI RWY 35.' AT THE TIME OF THE OCCURRENCE (RWY XING) ALL CHKLISTS WERE COMPLETE. CONTRIBUTING FACTORS WERE; OF COURSE; DISTRS. IT'S NOT EVERY DAY YOU SEE A GA PLANE TAXI UNDER A HVY JET. THIS WAS MY FIRST TIME IN PHL SO I WAS NOT FAMILIAR. ALSO; TXWY O HAD BEEN EXTENDED AFTER TXWY K; SO THE CAPT OUT OF HABIT MAY HAVE BEEN USED TO XING RWY 9L/27R AT TXWY E.