AN SA227 ABORTS ITS TKOF WHEN A T34 IS NOTED ON TKOF ROLL; OPPOSITE DIRECTION; SAME RWY AT SAN MARCOS; HYI; TX. BOTH ACFT XMITTED TKOF INTENTIONS; NEITHER ONE HEARD THE OTHER.

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ON JAN/XA/00 AT SAN MARCOS; TX; I WAS ENGAGED IN THE FLT TESTING OF A NEW STC FOR THE INSTALLATION OF BRAKES MANUFACTURED BY XYZ ON AN SA-227 WHILE THE ACFT WAS TEMPORARILY IN THE EXPERIMENTAL CATEGORY. TESTING PARAMETERS REQUIRED THAT THE ACFT BE FLOWN ON RWY 12 ILS TO 50 FT ABOVE THE RWY THRESHOLD; LANDED AND STOPPED WITHIN A SPECIFIC DISTANCE. SAN MARCOS IS AN UNCONTROLLED FIELD AND I BROADCASTED ALL ACFT MOVEMENTS BOTH ON THE GND AND IN THE AIR ON THE CTAF FREQ (123.05). PRIOR TO START WE HAD CALCULATED AND CAME TO THE CONCLUSION THAT WE WOULD UTILIZE RWY 26 FOR TKOF. I TAXIED DOWN TO RWY 35 ENROUTE TO RWY 26 AND STATED MY ACTIONS ON CTAF. I STOPPED SHORT OF RWY 26 AND PERFORMED ALL NECESSARY CHECKS. I TAXIED INTO POS AND STATED MY TKOF AND INTENTIONS ON CTAF. A VISUAL CHECK INDICATED A CLR RWY AND I STARTED MY TKOF ROLL. FULL PWR WAS IMMEDIATELY UTILIZED AS TKOF WAS NEAR MGTOW. AT APPROX 100 KTS AND ABOUT 2000 FT INTO MY TKOF ROLL I SAW ANOTHER ACFT ON OPPOSITE END OF THE RWY; RAPIDLY INCREASING IN SIZE. I IMMEDIATELY PULLED THE ENGS INTO FULL REVERSE AND APPLIED MAX BRAKING. I WAS MOVING SLOWLY AND NEARLY AT A FULL STOP WHEN THE OTHER ACFT; A T34; FLEW OVERHEAD AT VERY CLOSE RANGE. I CONTACTED THE T34 AND ASKED HIM WHY HE DID NOT BROADCAST HIS INTENTIONS. HE RESPONDED THAT HE DID AND ASKED ME WHY I HAD NOT BROADCAST MINE; AS HE DID NOT HEAR MY XMISSIONS. I STATED THAT I DID; AND THAT I HAD WITNESSES TO THAT FACT. I WAS PROPERLY SET UP AND BROADCASTING ON FREQ. CONVERSATION WITH ONE OF THE PRINCIPALS AT THE COMPANY INDICATED THAT THEY COULD HEAR THE T34 ON THE END OF RWY 8 (THE PRINCIPAL HAD A HANDHELD RECEIVER). HE STATED THAT MY BROADCASTS BROKE UP AS I APPROACHED RWY 35 DURING TAXI; WHICH AFTER THAT POINT HE COULD NOT HEAR ANY FURTHER XMISSIONS FROM ME. INTERESTINGLY ENOUGH; AFTER TALKING TO TEST PERSONNEL THAT WERE LOCATED NEAR THE MIDPOINT OF THE RWY; THEY STATED THAT THEY COULD ONLY HEAR MY XMISSIONS AND THEY NEVER HEARD ANYTHING FROM THE T34. ONE OTHER CONTRIBUTING FACTOR IS THAT THERE IS SMALL RISE IN THE MIDDLE OF THE ARPT THAT OBSCURES THE VIEW OF THE OPPOSITE END OF THE RWY. THAT IS WHY I WAS NOT ABLE TO SEE THE OTHER ACFT UNTIL I WAS FURTHER DOWN THE RWY. THIS RISE MIGHT ALSO AFFECT THE ABILITY TO XMIT/RECEIVE ON THE CTAF WHILE ON THE GND. THERE SHOULD BE A NOTAM POSTED AT THIS FIELD INDICATING RADIO XMISSIONS/RECEPTION ANOMALIES PRESENT AT THIS ARPT. ACFT MANAGEMENT MIGHT WANT TO CONSIDER THE INSTALLATION OF A LOW WATTAGE RADIO REPEATER AT THE CTR OF THE FIELD TO ALLEVIATE THIS PROB. CALLBACK CONVERSATION WITH THE RPTR REVEALED THE FOLLOWING INFO: RPTR FELT THAT SOMETHING SHOULD BE DONE ABOUT THE SIT BUT HADN'T KNOWN WHERE TO START.