PLT OF AN CPR SMT TWIN INADVERTENTLY ENTERED RESTR AIRSPACE DUE TO AN ERROR IN HIS VOR NAV RECEIVER. HE WAS NOT ON A FLT PLAN.

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WHILE FLYING FROM PHOENIX DEER VALLEY TO YUMA; I INTERCEPTED THE GBN 247 DEG RADIAL APPROX 33 MI FROM GBN. AS I APCHED MOHAK INTXN WITH THE CDI CTRED (GBN 247 DEG RADIAL); I STARTED A VFR DSCNT AND CONTACTED YUMA APCH WITH ATIS INFO. THE CTLR INFORMED ME I WAS 3 MI INTO A RESTR AIRSPACE; AND TO IMMEDIATELY CORRECT WITH A L TURN. I DID THIS AND THEN WAS TOLD I WAS XING OUT OF THE RESTR AIRSPACE ABOUT 1 MIN LATER. I COULDN'T BELIEVE THIS WAS HAPPENING -- AS THE CDI WAS CTRED ON GBN 247 DEGS. I THEN SWITCHED TO BZA 075 DEG RADIAL INBOUND 255 DEGS AND SAW THE CDI MOVE JUST ABOUT FULL SCALE L -- INDICATING TO ME THAT INDEED I WAS N OF COURSE. ON FURTHER INSPECTION IT HAS BEEN DETERMINED THAT WHILE A VOT CHK WAS DONE ON JUN/XA/99; AND WELL WITHIN LIMITS -- THE VOR WAS INDEED MALFUNCTIONING WITH AN ERROR OF UP TO 8 DEGS. MAINT HAS BEEN APPRISED OF THIS MALFUNCTION AND HAS TAKEN STEPS TO CORRECT THE NAV RECEIVER. WHILE HAVING FLOWN FOR 32 YRS; I HAVE NEVER SEEN SUCH AN ERROR IN A VOR. IN THE FUTURE ALL VOR USAGE WILL BE BACKED UP BY GPS. IT IS IMPERATIVE TO NOTE THIS WAS NEITHER INTENTIONAL; NOR DELIBERATE; AS I HAVE NEVER HAD A VIOLATION OF FARS IN 32 YRS OF PROFESSIONAL FLYING.