A BELL 206 CLRED TO PASS ACROSS RWY 10 BEHIND A DEPARTING DC9 MISJUDGES THE ACCELERATION OF THE DC9 REQUIRING A TURN TO PARALLEL THE RWY UNTIL THE DC9 PASSED. 200 FT SEPARATION IS ESTIMATED.

Narrative

A DC9 HAD BEEN CLRED FOR DEP ON RWY 10. I HAD BEEN CLRED FROM THE N SIDE OF RWY 10 TO THE PARALLEL TXWY. I WAS THEN CLRED TO DEPART TO THE S ACROSS RWY BUT TO PASS BEHIND THE DC9. IT APPEARED TO ME THAT THE DC9 WAS ACCELERATING RAPIDLY ENOUGH TO ALLOW ME TO CONTINUE SBOUND AND PASS BEHIND AND ABOVE THE PATH OF THE DEPARTING DC9. THE ACFT WAS ACCELERATING MUCH SLOWER THAN I'D ANTICIPATED AND IT BECAME APPARENT I WOULD NOT BE ABLE TO CONTINUE MY DEP PATH AND PASS BEHIND THE DC9. TWR CALLED TO CONFIRM I WAS PASSING BEHIND THE DC9. BY THAT TIME I'D MADE A TURN PARALLEL TO THE DEP PATH OF THE DC9 ABOUT 200-300 FT FROM RWY 10 UNTIL THE DEPARTING ACFT PASSED ME; THEN I TURNED S ON COURSE. THE CLRNC FROM TWR WAS CLR AND APPROPRIATE. I MISJUDGED THE RELATIVE SPD OF THE DC9 AND MADE MY DEP TIMING TOO SOON. TWR CONFIRMED TWICE (CLRNC AND SUBSEQUENT) THAT I HAD THE ACFT IN SIGHT. MY ERROR WAS IN MISJUDGING THE GND SPD OF THE DEPARTING ACFT; CAUSING ME TO SELECT AN ALTERNATE PATH PARALLEL TO THE DEPARTING ACFT UNTIL I COULD INSURE APPROPRIATE SEPARATION.