CAPT OF A BEECH 90 TAXIED ONTO THE ACTIVE RWY PRIOR TO BEING CLRED RESULTING IN A CITABRIA PASSING OVERHEAD FOR LNDG.

Narrative

AS SIC AND PNF ON A BEECH KING AIR; I OBTAINED IFR DEP CLRNC. AFTER COMPLETING THE RUNUP AND SYS CHKS; I CALLED GND CTL AND WAS CLRED TO TAXI TO RWY 20C. IT WAS A VERY SHORT DISTANCE TO THE HOLD SHORT POINT AT THE APCH END OF RWY 20C. WHILE TAXIING; THE CTLR ADVISED THAT WE WOULD BE #1 FOR DEP. AT THIS TIME THE PLT WAS NEARING THE HOLD SHORT LINE AND HAD SLOWED ALMOST TO A COMPLETE STOP. I DIVERTED MY ATTN INSIDE THE COCKPIT TO VERIFY THE TWR FREQ AND TO TUNE IT IN. AFTER DOING SO; I ASKED THE PLT IF HE WAS READY. HE SAID HE WAS. I THEN LOOKED OUT TO THE R FRONT TO CHK FOR TFC ON FINAL BEFORE CALLING TWR. A CITABRIA WAS PASSING DIRECTLY OVERHEAD. AT THE SAME TIME THE CTLR INFORMED US THAT IT APPEARED WE WERE ON THE RWY. INDEED WE WERE. WE WERE IN THE MIDDLE OF THE RWY; STOPPED; PERPENDICULAR TO THE CTRLINE; AS IF STILL HOLDING SHORT. I DON'T KNOW IF THE PLT REALIZED HE HAD MOVED ONTO THE RWY OR IF HE MISUNDERSTOOD THE CLRNC. I HAD NO DOUBT THAT WE WERE NOT YET CLRED ONTO THE RWY. ALTHOUGH MY INATTN TO WHAT WAS GOING ON OUTSIDE THE ACFT LASTED FOR ONLY 13 SECONDS; IT WAS ENOUGH TO CONTRIBUTE TO THIS RWY INCURSION. BETTER ATTN ON MY PART WOULD HAVE PREVENTED IT.