BFO96LA015

The pilot reported that the airplane was in cruise flight at 5,500 feet when it began to vibrate severely. He stated that the subsequent oscillations in the pitch and yaw axis were so severe that he shut off the engine. The pilot made a forced landing in a soft field. During the landing roll, the nose gear and left main landing gear separated. Postaccident examination revealed that the outboard portion of a propeller blade had separated in flight. Metallurgical examination of the remaining propeller blade revealed a fracture with features indicative of fatigue cracking. The fatigue cracking features extended through about 80% of the cross section of the blade. The origin area of the fatigue cracking showed presence of corrosion damage. The airplane was in storage about 22 years. AD69-09-03 required all 76EM series propellers to be modified and a 'K' stamped after the serial number. The 'K' modification thinned and retuned the entire blade for improved vibratory characteristics. This modification lowered the stresses in the minimum power operating range. The AD also required that the tachometer be placarded to avoid continuous operation between 2150-2350 RPM. The propeller showed no 'K' stamp afer the serial number, and the pilot stated that he knew of no placard that existed in the airplane.

HISTORY OF FLIGHT On October 30, 1995, at 1830 eastern standard time, a Piper PA-28-180, N477SP, collided with the terrain during an emergency landing near Lebanon, Virginia. The private pilot, the sole occupant, was uninjured. The airplane was destroyed. No flight plan had been filed and night visual meteorological conditions (VMC) prevailed. The flight was conducted under 14 CFR Part 91. The flight originated from Virginia Highlands Airport, Abingdon, Virginia, at approximately 1820 eastern standard time, with an intended destination of Tazewell County Airport, Richlands, Virginia.

The pilot stated that he had just leveled off at five thousand five hundred feet when the airplane began to vibrate severely. He reported that the airplane's attitude was oscillating in the pitch and yaw axis to such an extreme that he immediately turned off the engine. The pilot stated that it was a dark night, but he was in VMC, and he found a pasture in which to make the emergency landing. He indicated that when the airplane touched down on the soft grassy terrain, the nose and left main landing gears separated. Post impact fire destroyed the airplane.

AIRCRAFT INFORMATION The airplane was equipped with the Lycoming O-360-A3A, 180 horsepower engine constructed with a hollow crank-shaft. Attached to the engine was a Sensenich propeller, Model No. 76EM8-0-60, S/N 7139. The owner stated that the airplane was in storage from 1973 until he purchased it in 1995. On May 1, 1995, the airplane was inspected by a certificated Airframe and Powerplant mechanic and was determined to be in airworthy condition.

WRECKAGE AND IMPACT INFORMATION Post accident examination revealed that the outboard portion of one blade on the propeller had separated, and could not be found in the vicinity of the wreckage. The remainder of the propeller was removed, and was submitted to the National Transportation Safety Board's (NTSB) metallurgical laboratory for evaluation.

TEST AND RESEARCH Inspection of the propeller showed no "K" stamp after the serial number and the pilot stated that he knew of no placard that existed in the airplane showing that continuous operations at power settings between 2150-2350 RPM were to be avoided. Metallurgical examination of the propeller showed that the fracture in the propeller exhibited fatigue cracking features, which extended through about 80 percent of the cross section of the blade. The NTSB metallurgist reported that they stripped the paint from the propeller "...in the vicinity of the origin area. Examination revealed the presence of corrosion damage [in] localized areas, including an area directly adjacent to the origin area.

ADDITIONAL INFORMATION In 1969, the Sensenich Propeller Manufacturing Company, Inc. issued an Airworthiness Directive (AD), numbered 69-09-03. This AD pertained to the 76EM8 propeller installed on Lycoming O-360 type engines. The AD required all 76EM series propellers to be modified and a "K" stamped after the serial number. The "K" modification thinned and retuned the entire blade for improved vibratory characteristics. According to the manufacturer, this modification lowered the stresses in the minimum power operating range and narrowed the high stress RPM range to 2200 and 2300 on the hollow crankshaft engine. The AD also required that the tachometer be placarded to avoid continuous operation between 2150-2350 RPM.