C402 PLT HAD A TRACK DEV IN N90 CLASS E.

Narrative

I TOOK OFF FROM WHITE PLAINES ARPT; NY; IFR ON THE WESTCHESTER 1 DEP. SOMETIME BTWN TAXIING FROM THE FBO AND TKOF ROLE; MY HSI COMPASS SWUNG ABOUT 145 DEGS OFF OF THE CORRECT HEADING. THIS CAUSED ME TO TURN TO THE WRONG HEADING ON THE DEP PROC PUTTING ME IN THE PATH OF ANOTHER ACFT THAT TOOK OFF FROM WHITE PLAINS BEHIND ME. NEW YORK APCH SAW THE DEV AND IMMEDIATELY ISSUED COMMANDS TO BOTH ACFT TO CORRECT THE SIT. I WAS FLYING A C402B. I WAS THE SOLE PLT. THE ACFT HAD PLT'S AND COPLT'S INSTRUMENTATION AND A GPS. WHILE TAXIING TO THE RWY I SET THE COPLT'S DIRECTION GYROSCOPE TO MATCH MY HSI HEADING. WHEN THE ACFT IS MOVING; THE HSI IS NORMALLY THE MOST ACCURATE COMPASS. I WAS NOT ON A TXWY THAT PARALLELED MY DEP RWY AT THE TIME; SO DID NOT VERIFY IT WAS THE CORRECT HEADING. JUST PRIOR TO TKOF; I SET THE HEADING BUG ON THE HSI TO A HEADING OF 295 DEGS. I NOTICED THAT IT WOULD REQUIRE ME TO MAKE A L 270 DEG TURN AND I THOUGHT THAT WAS ODD; SO RECHKED THE ASSIGNED HEADING ON THE DEP PROC CHART AND CONFIRMED IT. ON TKOF; AT ABOUT 300 FT; I RECHKED THE ASSIGNED HEADING QUICKLY AGAINST THE DEP PROC; BECAUSE A L 270 DEG TURN DIDN'T SEEM RIGHT. I ENTERED THE CLOUDS; I NEVER MADE THE CONNECTION THAT MY COMPASS WAS OFF. I SHOULD HAVE MADE ABOUT A 45 DEG L TURN BUT INSTEAD STARTED MY 270 DEG L TURN. AFTER MY TURN; THE CTLR ASKED ME WHAT MY HEADING WAS. I SAID 295 DEGS. BUT IN ACTUAL FACT; I THINK I WAS HEADING E. HE THEN ASKED WHAT ALT I WAS. I SAID 2200 FT AND CLBING. HE THEN TOLD THE KING AIR THAT HAD TAKEN OFF BEHIND ME TO LEVEL OFF IMMEDIATELY. I WAS IN THE OVCST CLOUDS; SO I DIDN'T SEE HIM. THE CTLR SPOKE TO ME AGAIN; AND I REALIZED THAT I HAD A COMPASS PROB AND I TOLD HIM SO. I TRIED TO FIGURE OUT WHICH WAY I WAS ACTUALLY GOING. I WAS CLBING; SO MY MAGNETIC COMPASS WAS NOT STABLE ENOUGH TO READ. I LOOKED AT THE GPS AND IT SHOWED ME HEADING TOWARDS THE E. THE GPS DOES NOT SHOW MY MAGNETIC HEADING; IT ONLY SHOWS GND TRACK INFO. I RESET MY HSI TO THE GPS HEADING AND FOLLOWED THE CTLR'S DIRECTIONS FROM THERE. AFTER FLYING STRAIGHT FOR A MIN OR TWO; I WAS ABLE TO RESET THE HSI COMPASS CARD TO WITHIN 10 DEGS OF THE ACTUAL HEADING AND FLY THE REST OF THE FLT NORMALLY. I FAILED TO DO A LINEUP CHK WHEN I GOT ON THE RWY FOR TKOF AND VERIFY THE HSI COMPASS WAS ACCURATE. I'M NOT TAKING OFF IFR WITHOUT CHKING HSI HEADING AGAIN.