On June 20, 2007, an Air Tractor AT-301 (registration N23667) was involved in an aviation accident near Fergus Falls, MN. Investigators recorded the probable cause as: The propeller blade failed as a result of a fatigue crack, with features typical of impact of a rotating propeller blade with a foreign object. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
An agricultural aircraft sustained substantial damage after a forced landing caused by severe engine vibration. Investigation revealed a propeller blade fatigue crack from foreign object impact.
Incident Overview
An agricultural spray airplane experienced a severe engine vibration during cruise flight at about 500 feet above ground level. The pilot reported that the engine and airframe began to vibrate violently, and the windshield became covered with oil. A forced landing was executed in a field, but upon touchdown, the main landing gear broke off and the airplane flipped over, resulting in substantial damage.
Post-Accident Examination
Inspection of the aircraft revealed that one propeller blade was missing approximately 9 inches of its tip. The fractured blade section was sent to the National Transportation Safety Board's Materials Laboratory for analysis. Examination of the fracture surface determined that the propeller blade failed due to a fatigue crack. The features of the fracture were consistent with impact of a rotating propeller blade with a foreign object.
Maintenance History
Aircraft maintenance records indicated that the propeller blade had been repaired and overhauled on October 21, 2003. Overhaul documentation stated that no discrepancies were found during visual and non-destructive testing of the propeller blades and hub. The aircraft operator reported purchasing the propeller blade from another operator as a "0" time blade since major overhaul. Since installation on the accident airplane, the propeller had accumulated approximately 30 hours of flight time. The operator also confirmed that the accident airplane had not experienced a propeller strike during those 30 hours of operation prior to the accident flight.
Probable cause
The propeller blade failed as a result of a fatigue crack, with features typical of impact of a rotating propeller blade with a foreign object.