DEV FROM CLRNC - PLT DID AN INCOMPLETE PREFLT AND DID NOT CHK HIS HSI SO HIS HDG WAS 30 DEGS OFF.

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I WAS GIVEN CLRNC 'AFTER TKOF L TURN TO 260 DEGS. AFTER COMPLETING TURN TO 260 DEG HDG ON MY HSI; THE CTLR CALLED AND TOLD ME TO MAKE A SHARP L AND ASKED FOR MY HDG. HE SAID I WAS FLYING 30 DEGS PLUS OFF. HE GAVE ME AN ADDITIONAL VECTOR WHICH I TURNED TO WHICH WAS ALSO 30 DEG PLUS OFF. I COMPARED THE 2 HSI; THEY AGREED TO WITHIN 10 DEGS; BUT THE OVERHEAD COMPASS WAS IN AGREEMENT WITH THE 30 DEG OFF MENTIONED BY THE CTLR. I THEN SWITCHED TO USING THE COMPASS FOR NAV AND CANCELED MY IFR FLT PLAN SINCE IT WAS VMC AND ALSO LOOKED AT THE GND FOR VISUAL CONFIRMATION SINCE I WAS FAMILIAR WITH THE AREA. AFTER CHKING VARIOUS SYS I DETERMINED THAT THE A AND P WHO HAD WORKED ON THE PLANE THE PREVIOUS NIGHT HAD TURNED OFF THE SLAVE MODE ON BOTH HSI'S. WHEN DOING THE PREFLT CHK SINCE BOTH HSI'S ROUGHLY AGREED WITH EACH OTHER AND WERE IN THE APPROX CORRECT DIRECTION I DID NOT LOOK AT THE SLAVE SWITCH. WHILE TAXIING ACROSS ARPT FOR TKOF WITH MANY TURNS AND WITH ALMOST 180 DEGS REQUIRED AFTER TKOF; THE HSI'S MUST HAVE DRIFTED BY 30 DEGS OR MORE. IN FUTURE SLAVE SWITCHES WILL BE PART OF MY PREFLT AND DIFFERENCES BTWN HSI'S AND THE OVERHEAD COMPASS WILL BE MORE CLOSELY MONITORED.