Event Description
While descending through approximately 8,000 feet mean sea level, a twin-engine Piper PA-30 developed a sudden, violent airframe vibration. The pilot responded by shutting down the right engine. Following the shutdown, the pilot observed that a portion of one propeller tip was missing.
Metallurgical Examination
A metallurgical examination of the propeller blade determined that the tip loss was the result of a fatigue fracture. This fracture originated from a dent located approximately 0.2 inch from the leading edge of the blade. The dent measured about 0.07 inch wide in the chordwise direction and approximately 0.03 inch deep. The dent was covered with flat black paint, obscuring the underlying defect.
Historical Maintenance
Review of the propeller log revealed that the propeller had been repeatedly dressed and painted on historical occasions. The practice of dressing and painting may have concealed the dent, which was a stress concentration point. The relevant standards for such repairs are outlined in FAA Advisory Circular 43.13-1B/2A, 'Aircraft Inspection, Repair & Alterations.' Section 4, 'Repair of Metal Propellers,' paragraph 8-73(a), 'Flaws in Edges,' instructs mechanics to exercise care to remove the deepest point of the injury and also remove any raised metal around the edges of the injury. The dent in this instance was not properly addressed during prior maintenance actions.