PLT OF AN SMT TWIN ENG ATX FRT ACFT FAILED TO STOP TURNING AFTER TKOF TO THE ASSIGNED HDG RESULTING IN LTSS WITH ANOTHER ACFT ON APCH.

Narrative

I RECEIVED A CLRNC FROM MIDWAY TWR FOR TKOF ON RWY 31C AND WAS INSTRUCTED TO; AFTER TKOF; TURN TO A HDG OF 250 DEGS AND CLB TO AN ALT OF 3000 FT. AT THIS POINT; BEFORE COMMENCING MY TKOF ROLL; THE HDG BUG WAS PLACED ON 250 DEGS. AFTER TKOF AND HAVING CLRED ALL OBSTACLES; I BEGAN THE L TURN TO 250 DEGS. XING 1200 FT; I ENGAGED THE HDG AUTOPLT AND LET THE AUTOPLT CONTINUE THE L TURN TO 250 DEGS SO MEANWHILE I COULD FOCUS ON TRANSITIONING THE ACFT FROM A TKOF CONFIGN TO A CLB CONFIGN; MAINLY SETTING PWR LEVERS; PROP LEVERS; MANAGE FUEL FLOW; EGT; FUEL PUMPS; ETC. I WAS CONSTANTLY SCANNING THE FLT INSTS TO ASSURE MYSELF THAT THE ACFT WAS CLBING AND TURNING TO 250 DEGS. AS THE AUTOPLT BEGAN THE ROLLOUT ON A HDG OF 250 DEGS; MIDWAY TWR ASKED TO CONTACT CHICAGO DEP; AND AT THIS PRECISE MOMENT IT SEEMED TO ME THAT A TURN OF 60 DEGS (FROM 310 TO 250) WAS A BIT LONG. ON INITIAL CONTACT WITH CHICAGO DEP I INDICATED THAT I WAS CLBING THROUGH MY PRESENT ALT FOR 3000 FT AND THAT I WAS ASSIGNED 250 DEGS BY THE TWR. THE CTLR ACKNOWLEDGED MY XMISSION. I IMMEDIATELY PROCEEDED TO CHK MY MAGNETIC COMPASS TO VERIFY THE HDG I WAS ON; BUT BECAUSE I'VE JUST ROLLED OUT OF A TURN; IT WAS UNRELIABLE. AT THE SAME TIME CHICAGO DEP GOT BACK ON THE FREQ AND ASKED WHAT HDG I WAS ON. I REPLIED 250 DEGS. CHICAGO NOW INDICATED THAT MY ACFT WAS HDG SSE AND THERE WAS TFC 2 MI AHEAD ON FINAL APCH AT MIDWAY. IMMEDIATELY I STARTED A R TURN TOWARDS THE W. CHICAGO ASKED AGAIN WHAT HDG I WAS SHOWING NOW; BUT BECAUSE I HAVE JUST ROLLED INTO A TURN AND WAS CONFUSED; I REPLIED: STAND BY. ONCE ESTABLISHED ON A 250 DEG HDG BY USE OF THE MAGNETIC COMPASS; DEP CTL REQUESTED AN EXPLANATION. I DID NOT EXACTLY KNOW QUITE YET WHAT WENT WRONG SO MY FIRST THOUGHT WAS AN AUTOPLT MALFUNCTION ONLY TO FIND OUT LATER THAT THE EQUIP WAS OPERATING FINE. I DISCOVERED THAT THE PROB WAS CAUSED BY A GYRO-SLAVING FAILURE. BY PRESSING THE FAST SLAVE BUTTON THE DIRECTIONAL GYRO WAS AGAIN OPERATING NORMALLY; ALTHOUGH THROUGHOUT THE REMAINING OF THE FLT WAS OFF BY 10 DEGS. CHICAGO ADVISED ME THAT MY ACFT BECAME CLOSE TO A JET TFC ON FINAL APCH AT MIDWAY BUT I DON'T KNOW BY HOW MUCH. EVEN THOUGH I HAVE CHKED MY GYRO WITH THE MAGNETIC COMPASS DURING MY PREFLT AND PRE TAXI CHKLIST; I'VE FAILED TO DOUBLE-CHK MY HDG WITH THE RWY HDG PRIOR TO BEGINNING MY TKOF ROLL. I DID SET THE HDG BUG BUT I DID NOT LOOK TO SEE IF I WAS SHOWING 310. ALSO; EVEN THOUGH THE WX WAS MARGINAL I SHOULD HAVE USED OUTSIDE REFS TO AID MY ORIENTATION. I SHOULD POINT OUT THAT I DON'T KNOW WHETHER THE FAILURE OCCURRED DURING TAXI OR AFTER LIFTOFF.