On June 14, 1993, about 1253 central daylight time, an Enstrom, F-28C, helicopter, N51717, received substantial damage when it impacted a barn and the ground near Buckner, Missouri during an aerial observation flight for the operator, Missouri Public Service. The commercial pilot, and one other passenger were both seriously injured. The flight was operating in visual meteorological conditions under 14 CFR Part 91.

An interview with the pilot/owner of N51717 revealed that the flight was carrying a passenger, an employee of Missouri Public Service, for the purpose of inspecting power lines for damage. The pilot left Grain Valley, Missouri (3GV),picked up the passenger at Oakgrove, and then proceeded to fly segments of power lines before refueling at Lexington, Missouri airport. Upon departing, the flight headed southbound to inspect the Sibly-Blue Springs power line.

Approximately 45 to 50 minutes into the flight between Sibly and Blue Springs, the pilot and passenger reported hearing a "clunk"/"pop" and felt a jolt. The pilot stated he started an immediate auto rotation, lowering the collective to stabilize the aircraft, and made a left turn to clear the power lines and try to position the aircraft into the wind. With no place for a safe run on landing, the pilot made a hard emergency landing with a nose left attitude. The helicopter slid next to a corrugated metal and pole shed. Immediately upon landing, the pilot noticed that a fire had started in the engine area. The pilot and passenger were both able to evacuate the aircraft before it and the nearby pole shed were completely engulfed in flames.

Initial investigation found the forward portion of the aircraft, including the cabin, seats, instrument panel, and lower fuselage were consumed in the fire.

The tail rotor drive shaft and tail rotor gear box were checked and found the tail rotor to rotate freely and control of the blades to be satisfactory. However, the tail rotor drive shaft and shaft coupling were found separated. Metallurgical examination found the fracture features were consistent with a ductile overstress torsional separation and there was not any evidence of preexisting fracture areas.

The engine, viewed under the engine cooling shroud, revealed that the crankcase was damaged above the No. 2 cylinder in the area of the cam followers. An engine tear down showed the left crankcase half had a hole in the top and bottom by the No. 2 cylinder. A section of the case that contains the cam followers was loose, and upon its removal it was noted that the No. 2 cylinder connecting rod was imbedded in the skirt of the cylinder and had hit the crankcase.

The cylinder was removed and it was noted that portions of the large end of the rod were missing. The No. 1 cylinder was removed and a portion of a connecting rod bolt and a section of the connecting rod cap was found in the piston behind the connecting rod pin. Upon removal of the No. 4 cylinder, it was noted that a portion of a connecting rod bolt with a nut threaded on it and a connecting rod bolt nut had been wedged between the piston and cylinder walls.

The crankcase halves exhibited gouges and scars. The right crankcase half at No. 1 had an imprint of a nut in the aluminum. The cam sustained strike damage between lobes and in line with the No. 2 connecting rod.

The connecting rod bolt nut (item "c", figure 4 of NTSB Metallurgist\s Factual Report), disclosed the nut "was grossly deformed as a result of mechanical damage on the washer face. Even though the nut was ovalized, the internal threads appeared to be free of substantial damage and were not stripped. Examination of the connecting rod bolt pieces revealed fracture features typical of over stress separations.

The connecting rod cap was fractured adjacent to the cut-out for the connecting rod bolt head. The severe damage to the fracture from exposure to the fire obliterated the fracture features and prevented a positive determination of the fracture mode. The connecting rod bolts and cam follower contained over stress separations. The underside of the nut for a connecting rod bolt was heavily peened, resulting in distortion of the nut.

According to Bill Taylor from Enstrom Helicopter, the nose will turn to the left in the event of an engine failure. Both the pilot and the passenger's interview statement indicate the tail turned to the right after hearing the "clunk", and feeling the "jolt" in flight.

Aircraft records and the pilot report show the engine with 1520 to 1530 hours SMOH (since major overhaul). Recommended TBO (time between overhaul) for this engine from the manufacturer is 1500 hours TBO.

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