WITNESSES REPORTED SEEING THE HELICOPTER CIRCLE TWICE OVER A RESIDENCE AT AN ALT OF APRX 50' AGL. UPON INITIATING THE THIRD TURN THE HELICOPTER ENTERED AN ABRUPT 90 DEG BANK AND NOSED DOWN TO IMPACT WITH THE TERRAIN. EXAM OF THE HELICOPTER FAILED TO REVEAL ANY FAILURE/MALFUNCTION. POSTMORTEM EXAM OF THE PLT REVEALED SEVERE ARTHERIOSCLEROTIC HEART DISEASE WHICH MOST PROBABLY RESULTED IN A CATASTROPHIC MYOCARDIAL EVENT JUST PRIOR TO THE ACCIDENT. THE PLT HAD A HISTORY OF HEART DISEASE DATING PRIOR TO OCT, 1981. IN SEPT, 1984, THE PLT WAS DENIED A CLASS II MEDICAL BASED ON HIS MEDICAL HISTORY. IN JAN, 1985, THE NEWLY APPOINTED FAS ISSUED A SPECIAL ISSUANCE CLASS II WITHOUT FURTHER MEDICAL EXAMS.AFTER FURTHER MEDICAL EXAMS IN APRIL, 1986, THE PLT WAS ISSUED AN UNRESTRICTED CLASS II MEDICAL IN MAY, 1986. AT THIS TIME THE PLT WAS ADVISED TO REPORT ANY CHANGES IN HIS MEDICAL CONDITION. DURING THAT SAME MONTH, THE PLT WAS HOSPITALIZED FOR A MYOCARDIAL INFARCTION. THIS EVENT WAS NOT REPORTED TO THE FAS AND THE PLT CONTINUED FLYING.