NTSB Factual Report

On July 27, 2001, about 1000 central daylight time, an Air Tractor AT-401, N4553K, piloted by a commercial pilot, sustained minor damage when it nosed over during an emergency landing following an in-flight loss of a propeller blade tip while maneuvering near Kenesaw, Nebraska. The aerial application flight was operating under 14 CFR part 137. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot reported no injuries. The local flight departed from Roseland, Nebraska at 0700 and was performing an aerial application, at the time of the incident.

The pilot stated, "While on a actual spray run the aircraft began to shake and vibrate violently. At that time I shut the power down, climbed about 100 ft to look for a place to land. The area was rolling with pasture land and a corn field in front of me, no roads to land on in the area. Plane was gliding but loosing altitude quickly. I lined up for the corn field, lowered flaps held the aircraft just above stall speed and landed in the corn field with control. ... After a rolling out for a short while in corn the landing gear became stuck in corn and soft sandy soil. Plane stopped and went over on its back."

Approximately 12 inches of one of the propeller blade tips was found to be missing. A Federal Aviation Administration Inspector arranged to a cut away a portion of the remaining blade to include that separated surface. The cut away portion was shipped to the National Transportation Safety Board's Materials Laboratory Division. The laboratory performed an examination and produced a Materials Laboratory Factual Report No. 02-037.

That laboratory report stated that "a large portion of the fracture surface was on a 90-degree flat plane with multiple ratchet marks and crack arrest marks on this portion of the fracture surface. These features are typical of a fatigue crack. The fatigue crack originated from multiple origins at the bottom of a gouge on the flat face of the blade about 2.25 inches from the blade's leading edge. The fatigue crack extended through about two-thirds of the blade's cross-section. The remainder of the fracture surface (about one-third of the blade's cross section) was on a 45-degree slanted plane and was typical of an overstress region stemming from the fatigue crack." A section was saw cut from the fracture containing the fatigue origin. The paint was removed from the areas near the crack origin and the surface was re-examined with a scanning electron microscope (SEM). The gouge was measured using the SEM. The report further stated, "The length of the gouge was 0.6 inch (1.5 millimeters), and the depth of the gouge was 0.0055 inch (0.14 millimeters). ... Optical examination of the flat face of the blade near the gouge did not show any evidence of blending or rework in this area."

The propeller's last overhaul was completed on January 13, 1998. The operator reported that the propeller accumulated 240 hours of flight since that overhaul.

NTSB Probable Cause

The propeller blade fatigue leading to its tip separating during an aerial application. Factors were no suitable terrain for an emergency landing, high vegetation, and soft terrain encountered during the emergency landing.