FLC OF AN LTT ACR ACFT TAXIED ONTO A NON CTLED ARPT RWY THAT WAS IN USE RESULTING IN THE OTHER ACFT TAKING OFF OVER THE RPTR'S ACFT.

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AFTER BROADCASTING OUR INTENTIONS; WE TAXIED ON TO RWY TO BACK-TAXI TO OTHER END (34). THERE WAS ANOTHER ACFT THAT HAD TAKEN OFF THAT WAS AIRBORNE ABOUT 2/3 THE WAY DOWN THE RWY 500-600 FT ABOVE US. THERE WAS NO EVASIVE ACTION REQUIRED AT ALL!! I CALLED ON FREQ 6-7 TIMES BEFORE THE ACFT RESPONDED. HE SAID THAT HE XMITTED HIS TKOF. WE DID NOT HEAR THIS AND WE HAD RADIOS ON THROUGH START OF THE ENG IN THE BLOCKS; FLAG OUT; AND TAXI. THE OTHER ACFT ALSO SAID HE DID NOT HEAR US ANNOUNCE OUR INTENTIONS TO BACK TAXI. I DON'T KNOW IF THERE IS A DEAD SPOT WHEN XMITTING FROM ONE END TO THE OTHER. I KNOW THAT WE MADE A CALL TO ALERT THE OTHER ACFT IN THE AREA. WE ALSO BOTH CHKED EACH WAY VISUALLY AND VERBALLY CALLED IT CLR BOTH WAYS. THE RWY HAS A SMALL OVER HILL SLOPE PLUS TREES AND A SMALL CITY AT THE S END. PLUS MY SUN VISOR WAS HANGING DOWN; WHICH MAY HAVE BLOCKED SEEING THE ACFT ALSO THE SKY WAS BLUE AND THE ACFT THAT TOOK OFF HAD A DARK UNDERSIDE (MAYBE OIL EXHAUST) WITH LIGHT COLORED WINGS. I DID NOT NOTICE A LNDG LIGHT ON. AS I STATED I DID CALL THE OTHER ACFT AND HE AND I BOTH AGREED NO EVASIVE ACTION WAS NEEDED WITH HIS NOSE HIGH; I DON'T THINK WE SAW US. MY COPLT AND I HAVE NO SOLUTION; BUT WE WILL MAKE MORE THAN ONE CALL FROM NOW ON.