THE PLT STATED THAT THE FLT PATH OF THE HELICOPTER WAS PERPENDICULAR TO POWER LINES THAT WERE AHEAD. THE FLIGHT CONTINUED AND HE FURTHER STATED THAT HE HEARD A LOUD EXPLOSION & THE ENG QUIT. HE PULLED THE COLLECTIVE CONTROL & CLEARED THE POWER LINES THEN LOWERED IT, & PRIOR TO IMPACT WITH THE GROUND, PULLED THE COLLECTIVE CONTROL TO CUSHION THE IMPACT. THE HELICOPTER LANDED HARD, BOUNCED, TOUCHED DOWN, ROLLED OVER ON THE LT SIDE, THEN NOSED OVER & CAME TO REST INVERTED. A SMALL FIRE WAS EXTINGUISHED BY SEVERAL INDIVIDUALS WHO RESPONDED TO THE ACCIDENT SITE. THE ENG WAS REMOVED & SENT TO THE MANUFACTURER'S FACILITY FOR EXAMINATION. EXAM OF THE ENG REVEALED THAT A MOUNTING BOSS ON THE AIR DIFFUSER FAILED DUE TO FATIGUE. THE LINE ATTACHED TO THE BOSS WAS THE INTERSTAGE BLEED AIR LINE. ALSO, A POWER TURBINE BLADE FAILED DUE TO FATIGUE. REVIEW OF THE ENG LOGBOOKS REVEALED THAT THIS ENG WAS REMOVED FROM ANOTHER HELICOPTER & INSTALLED IN THIS HELICOPTER 6 DAYS PRIOR TO THE ACCIDENT. THE ENG HAD ACCUMULATED ABOUT 10 HRS SINCE THEN & 703 HRS SINCE MAJOR OVERHAUL. ACCORDING TO THE ENG DAILY INSPECTION CHECKLIST, THE AIR DIFFUSER EXTERNAL VANE BRAZEMENTS SHOULD BE CHECKED FOR CRACKS.