Incident Overview
While in cruise flight at 5,500 feet, the pilot reported that the airplane began to vibrate severely. Pitch and yaw axis oscillations became so intense that the pilot shut off the engine and executed a forced landing in a soft field. During the landing roll, the nose gear and left main landing gear separated.
Postaccident Examination
Examination of the propeller revealed that the outboard portion of one blade had separated in flight. Metallurgical analysis of the remaining blade showed a fracture with features indicative of fatigue cracking, which extended through approximately 80% of the blade's cross section. The origin area of the fatigue cracking exhibited corrosion damage.
Maintenance History
The airplane had been in storage for about 22 years. Airworthiness Directive (AD) 69-09-03 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, lowering stresses in the minimum power operating range. The AD also required that the tachometer be placarded to avoid continuous operation between 2150 and 2350 RPM. The propeller in question showed no "K" stamp after the serial number, and the pilot stated that he was unaware of any placard in the airplane.