FTW99LA025

The propeller blade separated during cruise flight and the aircraft started 'shaking violently.' The pilot pulled the mixture control to idle-cut-off and put the aircraft into a steep climb to stop the propeller. The pilot then diverted to an airport where he landed safely. The remainder of the blade was sent to the NTSB Materials Laboratory in Washington, D.C. A detailed examination of the fractured surface revealed crack arrest positions characteristic of fatigue cracking. Examination of the fatigue crack with a scanning electron microscope (SEM) revealed that corrosion pitting on the camber side of the blade was the origin of the fatigue crack. The propeller was last overhauled on November 16, 1989, and the last annual inspection was completed on January 1, 1998.

On July 27, 1998, approximately 0950 central daylight time, a Mooney M20C airplane, N9744M, sustained substantial damage following a partial propeller blade separation during cruise flight near the Wilbarger County Airport near Vernon, Texas. The commercial pilot, sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and a flight plan was not filed for the Title 14 CFR Part 91 personal flight. The flight originated from the Front Range Airport near Denver, Colorado, at 0415 and was destined for Denton, Texas. The aircraft was owned by a private individual and leased to Flight Training Center, Inc., of Watkins, Colorado.

In the enclosed NTSB Form 6120.1/2 Pilot/Operator Aircraft Accident Report, the pilot stated that approximately four hours into the flight the "aircraft began to shake violently." The pilot added that he immediately "pulled the mixture and began a steep climb to stop the engine." The pilot then extended the landing gear and executed an emergency landing at the Wilbarger County Airport.

Examination of the airplane by an aircraft mechanic revealed that the airframe's engine support structure sustained structural damage. Additionally the starter, exhaust, and alternator were found loose. Examination of the propeller revealed that one blade tip was missing outboard from approximately the 30 inch station.

The morning of the flight, the pilot performed a preflight inspection of the airplane, which included the propeller. During the dark hours of the morning, the pilot taxied to the runway via a taxiway covered with asphalt debris and gravel left by construction at the airport. The owners of the accident airplane stated that they mentioned the potential of foreign object damage (FOD) hazard to the airport manager the day before the flight. When the owners of the airplane received word of the propeller separation, they photographed the debris on the taxiway.

The blade was cut approximately seven inches inboard of the fracture to facilitate shipment to the NTSB Materials Laboratory in Washington, D.C. A detailed examination of the fractured surface with the aid of a bench binocular microscope showed crack arrest positions, which are characteristics of fatigue cracking. Examination of the fatigue crack with a scanning electron microscope (SEM) revealed that corrosion pitting on the camber side of the blade was the origin of the fatigue crack. There was no indication on the section of propeller blade sent to the NTSB Materials Laboratory that the origin of the blade fatigue failure initiated from FOD. See the enclosed NTSB Materials Laboratory Factual Report for further details of the examination.

Maintenance records revealed that the last propeller overhaul was completed on November 16, 1989. The recommended time between overhaul for the propeller assembly is 5 years or 2,000 hours, whichever comes first. The estimated total time of the propeller at the time of overhaul was 15,590 hours. The last annual inspection was completed on January 1, 1998.

The NTSB and the FAA were not notified of this occurrence until November 13, 1998.