THE OWNER/PLT WAS ON A FLT, ACCOMPANIED BY TWO NEPHEWS, AGES 19 & 8 YRS OLD. AN ASSOCIATE OF THE OWNER/PLT REPORTED THAT THE 19 YR OLD NEPHEW, WHO OCCUPIED THE RIGHT FRONT SEAT, WAS A SINGLE ENG PLT, BUT HAD NO MULTI-ENG INSTRUCTION. HOWEVER, NO FAA RECORDS WERE FOUND TO VERIFY THAT THE NEPHEW WAS A PLT. ABOUT 5 MI NORTH OF THE DESTINATION (MONTGOMERY CO ARPT), 2 GROUND WITNESSES, WHO WERE BOTH PLTS, SAW A PIPER APACHE IN LEVEL FLT AT ABOUT 1000 FT AGL WITH THE RIGHT PROP FEATHERED. AFTER THEY TOOK OFF IN THEIR ACFT, THEY TUNED IN THE MONTGOMERY CO ADVISORY FREQ TO DETERMINE IF THERE WERE ANY TRANSMISSIONS FROM THE APACHE. THEY HEARD THE PLT OF AN APACHE REPORT ON DOWNWIND, BUT HEARD NO MENTION OF AN ENG PROBLEM OR AN EMERGENCY. WITNESSES ON THE ARPT & UNDER THE BASE LEG FOR RWY 14 OBSERVED THE APACHE TURN TO FINAL APCH WITH THE RIGHT PROP FEATHERED. THE ACFT THEN ROLLED INTO A RIGHT DESCENDING NOSE DOWN ATTITUDE & CRASHED IN A STEEPDESCENT. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND. DRG MED EXAM ON 8/12/82, THE PLT RPRTD 1200 HRS TOTAL TIME.