RWY INCURSION. UNAUTH ENTRY INTO TXWY AT THRESHOLD OF RWY.

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AT APPROX XA40 LCL TIME ON AUG/XX/94 I CONTACTED THE CTL TWR AT MFE AND WAS INFORMED THAT I HAD TAXIED ONTO RWY 18 WITHOUT PROPER AUTH. ALTHOUGH IN RETROSPECT I REALIZE MY ACTIONS MAY NOT HAVE BEEN PROPER; AT THE TIME OF THE INCIDENT I BELIEVED I WAS PROPERLY AUTHORIZED AND THAT MY ACTIONS WERE IN COMPLIANCE WITH FAA REGS. THIS INCIDENT WAS NOT THE RESULT OF AN OVERSIGHT OR INTENTIONAL DISREGARD FOR PROC. MY ACTION WAS FULLY CONSCIOUS AND REASONED. I WAS PARKED ON THE RAMP IN FRONT OF FBO; A NON MOVEMENT AREA E OF RWY 18 AT MFE. AFTER RECEIVING ATIS I BEGAN TO TAXI TOWARD THE N AND CONTACTED GND CTLR REQUESTING A VFR DEP HDG 315 DEGS; CLBING TO 3000 FT. THE CTLR TOLD ME TO TAXI TO RWY 13 WHICH WAS THE ACTIVE RWY. I REQUESTED TO DEPART FROM RWY 18 SINCE THE WIND WAS 160 DEGS AND THIS WOULD FACILITATE A QUICKER TURN TO MY DESIRED HDG. THE CTLR THEN TOLD ME TO TAXI TO RWY 18. I STOPPED AT THE NORTHERN MOST E/W TXWY N OF RWY 18 AND CONDUCTED MY RUN UP. AFTER COMPLETING THE RUN UP I SWITCHED TO TWR. FOR SEVERAL SECONDS THE TWR WAS OCCUPIED IN COMS WITH OTHER ACFT DEPARTING AND APCHING RWY 13. RWY 18 HAS A DISPLACED THRESHOLD S OF THE E/W TXWY I WAS FACING. THE AREA AHEAD OF ME AT THAT POINT IS MARKED AS A TXWY ON THE GND AND IS DEPICTED AS A NON RWY AREA ON THE ARPT DIAGRAMS ASSOCIATED WITH NOAA INST APCH PROCS CHARTS FOR MFE. (ENCLOSURE 1). I REASONED THAT I HAD BEEN CLRED INTO THIS AREA WHEN TOLD TO TAXI TO RWY 13; THAT RWY 18 WAS NOT IN USE; THAT THE AREA IMMEDIATELY AHEAD AND TO MY L WAS NOT RWY 18 AND THAT I COULD THEREFORE CONTINUE MY TAXI. I INSURED THE FINAL APCH AND BASE LEG WERE CLR; THEN PULLED FORWARD AND TO THE L BEHIND THE RWY 18 THRESHOLD WHILE CONTACTING THE TWR. THIS IS WHERE THE ALLEGED INCURSION OCCURRED. MY REASONING WAS DEFINITELY SLANTED TOWARD A SOLUTION THAT WOULD EXPEDITE MY DEP SINCE I HAD BEGUN A NEW JOB ONLY 2 DAYS EARLIER; FLYING FOR A PART 135 OP; AND I FELT PRESSED TO STAY ON SCHEDULE. THE MOST SIGNIFICANT FACTORS THAT LED TO MY DECIDING THE WAY I DID WERE THE FIRST INSTRUCTIONS FROM GND WHICH AUTHORIZED ME TO TAXI THROUGH THIS AREA; THE RWY AND TXWY MARKINGS AND THE FACT THAT THE ARPT DIAGRAM SHOWS THIS AREA AS NON RWY. BY CONTRAST THE ARPT DIAGRAM FOR RWY 14 AT STINSON (SSF); MY HOME BASE; DEPICTS THE AREA BEHIND THE DISPLACED THRESHOLD; N OF RWY 14 AS A RWY AREA WHICH INTERSECTS THE E/W TXWY (ENCLOSURE 2). IN THIS CASE THERE IS NO QUESTION THAT TAXIING ON THE E/W TXWY WOULD MEAN XING RWY 14. TO INSURE THERE IS NO FUTURE CONFUSION SIMILAR TO THAT WHICH OCCURRED IN THIS INCIDENT I MAKE 3 RECOMMENDATIONS. ANY TIME A TKOF RWY IS CHANGED; HOLD SHORT INSTRUCTIONS SHOULD BE ISSUED; ESPECIALLY WHEN THE PREVIOUS TAXI INSTRUCTIONS AUTHORIZED XING THE NEWLY ASSIGNED TKOF RWY. IN A CASE WHERE THE AREA BEHIND A DISPLACED THRESHOLD INTERSECTS A TXWY AND IT IS INTENDED TO BE PROTECTED AS RWY AREA; IT SHOULD BE MORE CLRLY MARKED TO INDICATE THIS. FINALLY; ALL AREAS ON THE ARPT SURFACE THAT ARE TO BE TREATED AS RWY FOR GND OPS PURPOSES SHOULD BE DEPICTED AS RWY ON THE ARPT DIAGRAM OF ALL FLT INFO PUBS.