PLT OF SMA SEL FAILED TO FLY ASSIGNED HDG DURING DEP DUE TO A DISTR CAUSED BY A CABIN VENT FAILURE.

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AFTER DEP I WAS ASSIGNED A 115 DEG HDG. AFTER THE TKOF ROLL AND CLBING OUT OF APPROX 1500-2000 FT I ATTEMPTED TO SET THE HDG BUG ON THE ACFT DIRECTIONAL GYROSCOPE TO 115 DEGS; AND AS I WAS SETTING THE HDG BUG THE PLT'S OVERHEAD FRESH AIR VENT PLUNGER BLEW OUT OF ITS HOLD AND THE PLUNGER AND A SERIES OF PAPERS BLEW ABOUT THE COCKPIT. THE ONLY THING I CAN FIGURE IS I MUST HAVE STOPPED THE HDG BUG ON A 150 DEG HDG AT THE TIME THE VENT BLEW OUT BECAUSE I THOUGHT I SAW MINS LATER THAT IT WAS THE ASSIGNED HDG BY THE TWR. WHEN THE TWR CALLED ME AND ASKED FOR MY ASSIGNED HDG I TOLD THEM 150 DEGS. THEY TOLD ME TO TURN TO HDG 090 DEGS. WHEN I WAS ON THE ZTL FREQ SOME MOMENTS LATER; THEY ASKED ME TO CALL THE TWR WHEN I GOT ON THE GND. WHEN I CALLED THE TWR THEY TOLD ME RADAR SEPARATION WAS LOST DUE TO THE FACT THEY DEPARTED AN ACFT AFTER US AND PUT HIM ON AN ASSIGNED HDG OF 150 DEGS. I EXPLAINED TO HIM WHAT HAPPENED AND SAID I WAS SORRY FOR ANY MIX-UP THAT MIGHT HAVE OCCURRED AND EXPLAINED ABOUT THE BLOWING PAPERS AND BLOWN OUT AIR DUCT. I FOUND OUT LATER THAT A RETAINING BUTTON AND FELT STRIP WERE BROKEN THAT HELPED HOLD THE AIR DUCT IN POS.