ON 5/1/86, THE PLT HAD AN ACOUSTIC TUMOR REMOVED FM HIS RGT EAR WHICH INCLUDED THE REMOVAL OF THE 8TH NERVE (BALANCE NERVE). HIS ATTENDING PHYSICIAN SUGGESTED APRX A 1 YR PERIOD OF RECUPERATION DUE TO EQUILIBRIUM PROBLEMS ASSOCIATED WITH THIS TYPE OF SURGERY. ON 7/12/86, HE HAD A 1ST CLASS MEDICAL EXAM & RECEIVED A MEDICAL CERTIFICATE WHICH WAS VALID FOR FLT TEST ONLY. ON 8/28/86, WHILE TAKING OFF AT NGT, THE ACFT COLLIDED WITH RISING TERRAIN APRX 1.5 MI FM THE END OF THE RWY AT AN ELEV OF ABOUT 5600'. THE ARPT ELEV WAS 5586'. A WITNESS STATED THAT HE SAW THE ACFT TAKEOFF, BUT IT DID NOT CLIMB AFTER TAKEOFF. AN INVESTIGATION REVEALED THAT THE ACDNT OCCURRED IN AN AREA THAT WAS ISOLATED FM GND LGTS. A POST CRASH EXAM REVEALED NO EVIDENCE OF A MECHANICAL FAILURE OR MALFUNCTION OF THE ACFT STRUCTURE, FLT CONTROLS, ENG OR PROPELLERS. ACCORDING TO THE PLT'S LOG BOOK, HIS LAST NGT FLT WAS ON 3/22/85. HE HAD BEEN ON DUTY FOR APRX 14 HRS WHEN THE ACDNT OCCURRED.