T28 PLT ACCIDENTALLY ENTERED RESTR AIRSPACE AFTER HAVING HIS DIRECTIONAL GYRO TUMBLE.

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VIOLATED R2501 AIRSPACE. WAS RETURNING FROM FORMATION CLINIC AT IGM. WAS FLYING WING IN FLT OF 2. HAD AGREED WITH LEAD THAT 10 MI SE OF THE TNP VOR I WOULD LEAVE ELEMENT TO HEAD TO HOME ARPT (I7A) AND LEAD WOULD HEAD HOME TO SAN DIEGO. HAD PREVIOUSLY DETERMINED HDG FROM BREAK-UP POINT B I7A WOULD KEEP ME S OF R2501. AS AGREED; BROKE WITH LEAD 10 MI SE OF TNP AND TURNED TO 270 DEGS; THE HDG OPS SHOWED FOR I7A. CALLED ZLA FOR ADVISORIES AND INPUT XPONDER CODE. CTR COULD NOT FIND ME; AND HAD ME RECYCLE XPONDER. THIS WENT ON FOR 5 MINS UNTIL CTR SAID SINCE HE COULD NOT SEE ME HE COULD NOT PROVIDE SVC. MEANWHILE; SOMETHING DID NOT FEEL RIGHT WITH NAV; TERRAIN LOOKED UNFAMILIAR. CDI FOR HSI WOULD NOT STAY CTRED (GPS WAS COUPLED TO HSI). GPS/HSI WAS COMMANDING MASSIVE COURSE CORRECTIONS; XCHKING WITH VORS SUGGESTED I WAS WAY OFF COURSE. THE SETTING SUN AND HAZE MADE IT IMPOSSIBLE TO SEE THE 2 LARGE LANDMARKS IN THE AREA (MT SAN GORGONIO AND MT SAN JACINTO). IT FINALLY DAWNED ON ME TO XCHK THE HSI TO THE COMPASS. THE HSI WAS 50-60 DEGS OFF TO THE S. (I HAD CHKED COMPASS AND HSI ON THE GND BEFORE TKOF AND THEY ALIGNED.) THE HSI ERROR RESULTED IN ME TURNING TO A TRUE HDG OF 320-330 DEGS EVEN THOUGH 270 DEGS WAS INDICATED. THIS TOOK ME INTO THE RESTR AIRSPACE. AT ALMOST 4 MI PER MIN IT DOES NOT TAKE LONG TO GET PRETTY FAR OFF COURSE. I THINK WHEN I MADE MY 60 DEG BREAK; IT PRECESSED THE HSI. SINCE I HAD BEEN FLYING WING; I HAD NOT BEEN CALIBRATING MY NAV; SO I HAD NO REF EXCEPT THE HSI. I DID A PRETTY GOOD JOB OF PLANNING TO STAY OUT OF R2501; AND WOULD HAVE USED ZLA AS BACK-UP; BUT THE COMBINATION OF HSI ERROR; ZLA NOT SEEING ME; LOW SUN OBSCURING KNOWN LANDMARKED CONSPIRED TO PUT ME IN R2501. CLRLY; HAD I XCHKED THE HSI TO THE COMPANY SOONER; I MIGHT HAVE STAYED OUT. THIS WAS A TOUGH ONE BECAUSE IN NORMAL CIRCUMSTANCES I WOULD HAVE NOTICED NAV INST PROBS EARLY ON; BUT SINCE I WAS FLYING FORMATION; I WAS NOT EVEN NAVING. THE ONLY THING I CAN THINK OF TO AVOID THIS SIT IN THE FUTURE IS TO CHK THE HSI AGAINST THE COMPASS SOONER.