THE AIRCRAFT CRASHED INTO HINCH MT AT THE 2700 FOOT LEVEL DURING CRUISE FLT IN IMC WEATHER. THE PILOT WAS INFORMED DURING THE INITIAL AND SUBSEQUENT BRIEFINGS THAT VFR FLT WAS NOT RECOMMENDED FOR HIS ROUTE. THE FIRST LEG OF THE FLIGHT FROM NEW ORLEANS TO TUSCALOOSA ENDED WITH A LANDING UNDER IFR CONDITIONS. THE PILOT TOLD THE FSS PERSONNEL HE WAS GOING BACK TO MERIDIAN, MS BUT INSTEAD CONTINUED ON TOWARD HIS ORIGINAL DESTINATION OF MASON WV. THE PILOT RECEIVED FATAL INJURIES FROM THE CRASH IN TN BUT HIS SON, A PASSENGER, SURVIVED. THE SON SAID THAT HIS FATHER WAS INSTRUMENT RATED BUT HAD NOT FILED AN IFR FLIGHT PLAN BECAUSE OF A KNOWN MALFUNCTIONING DIRECTIONAL GYRO. A STOP HAD BEEN MADE ON A PREVIOUS TRIP TO FIX THE DG BUT THE PROBLEM WAS NOT RESOLVED; FIRE DAMAGE PRECLUDED POSTACCIDENT DETERMINATION OF CAUSE OF THE MALFUNCTION.