Payson, AZ, USA
N7810T
CESSNA 172A
The pilot reported that, as the airplane touched down, the nose gear separated and the airplane veered to the left before coming to rest on the runway. During the event the right wing was substantially damaged. Postaccident examination of the separated nose landing gear fork indicated that the alloy used for forging the part was 2014 series aluminum, which is susceptible to stress corrosion cracking (SCC) given the proper environment and a continuous stress state. A Safety Board metallurgist concluded that the fracture characteristics were consistent with SCC, with slow crack growth over time. The crack appeared to have originated from a point on the surface of the bolt hole bore and propagated upwards and outwards. The last annual inspection of the airplane was conducted about two weeks prior to the accident, although the investigation was unable to determine if the crack would have been visible to the unaided eye during a routine inspection.
On October 3, 2009, at 0900 mountain standard time, a Cessna 172A, N7810T, was landing at Payson Airport, Payson, Arizona, when the nose landing gear fork separated during landing and the pilot lost control of the airplane. The student pilot was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. The student pilot, who was also the registered owner of the airplane, was not injured. The airplane was substantially damaged. Visual meteorological conditions prevailed and no flight plan was filed. The flight departed from Payson Airport about 0800. According to the student pilot, he was conducting his second or third landing. As the airplane touched down, the nose gear separated, the airplane veered to the left, and then impacted the runway. The right wing sustained substantial damage. The student pilot indicated that the previous owner of the airplane had landed on unimproved terrain quite frequently. The nose landing gear fork was submitted to the National Transportation Safety Board Materials Laboratory for further examination. According to the metallurgist, the part was manufactured using 2014 series aluminum, and the fracture surface had two distinct regions. One portion of the fracture was a relatively flat area with evidence of corrosion, and the remainder of the fracture consisted of slanted irregular surfaces with a shiny crystalline appearance. The metallurgist indicated that the appearance of corrosion was consistent with slow crack growth over time. Scanning electron microscope examination of the corrosion crack revealed no fracture features consistent with fatigue. Further examination revealed that the propagation of the crack appeared to be intergranular with cracks branching into adjacent grains. Although no single origin was positively identified, the metallurgist indicated that the crack appeared to have originated from a point on the surface of the bolt hole bore and propagated upwards and outwards. It was not possible to determine if the crack would have been visible to the unaided eye during a routine inspection.
The stress corrosion cracking of the nose gear landing fork, which resulted in the fork's failure during landing.
Source: NTSB Aviation Accident Database
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