MDT CROSSED RWY HOLD LINE AND WAS NARROWLY MISSED BY A JET THAT WAS ROTATING ON RWY 18.

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ON THE EVENING OF NOV/TUE/93; WE WERE ON OUR THIRD LEG OF A 4-LEG TRIP INTO DCA. WE WERE BEING VECTORED BY APCH CTL FOR RWY 21. CTLR KEPT US UP AT 3000 FT UNTIL ABOUT 4 MI FROM THE RWY; WHEN HE GAVE US ANOTHER VECTOR FOR SEPARATION FOR AN ACFT THAT WAS LNDG ON RWY 18. HE WAS 4 MI OUT AND WE WERE SUPPOSED TO LAND BEHIND HIM. WE TOLD THE CTLR THAT IT WASN'T GOING TO WORK SINCE WE WERE ONLY NOW ABOUT 2 1/2 FROM THE RWY. SO HE TOLD US TO GAR AND ENTER A R DOWNWIND FOR RWY 15. WE DID SO AND THE TWR CLRED US TO LAND ON RWY 15. UPON LNDG THE TWR TOLD US TO CLR THE RWY ON TXWY MIKE AND HOLD SHORT OF RWY 18. NOT BEING REAL FAMILIAR WITH THE TXWY LAYOUT OF TXWY MIKE AND WHERE RWY 18 LAY IN REF TO MIKE; WE MADE THE R TURN ON MIKE. LITTLE DID WE REALIZE THAT THE TWR HAD CLRED A JET FOR TKOF ON RWY 18 AND TXWY MIKE WAS SO SHORT THAT WE MISSED THE HOLD SHORT LINE FOR RWY 18 SINCE THE TXWY WAS ONLY ABOUT 50 FT LONG. TWR YELLED AT US TO 'HOLD SHORT; HOLD SHORT.' RIGHT THEN THE JET WAS ROTATING AT INTXN MIKE. I FEEL THAT THE TWR SHOULD HAVE MADE IT MORE CLR INITIALLY TO CLR THE RWY AND STOP FOR DEPARTING TFC SINCE THE TXWY IS SO SHORT. I MYSELF HAD NEVER BEEN ON IT. AFTER THE JET LIFTED OFF; WE WERE TOLD TO CROSS RWY 18 AND CONTACT GND. WE DID AND NOTHING WAS SAID ABOUT THE SIT THAT OCCURRED. IT IS VERY EASY TO CROSS THE HOLD SHORT LINE FOR 18 SINCE THE CLR HOLD SHORT FOR RWY 15 IS VERY EASY TO MISS AND THERE IS NOT MUCH ROOM BTWN THE 2 LINES. WE THOUGHT WE HAD CROSSED THE FIRST LINE (CLR LINE FOR RWY 15) WHEN WE HAD REALLY CROSSED THE HOLD SHORT LINE FOR RWY 18. I FEEL THAT SOME OF THE CONTRIBUTING FACTORS WERE NOT KNOWING HOW SHORT TXWY MIKE WAS AND THE CTLR NOT STRESSING TO US TO USE CAUTION SINCE AN ACFT HAD BEEN CLRED FOR TKOF.