Soldier Bar, ID, USA
N7372Q
Cessna 206
After a smooth landing and rollout, the pilot noticed the attitude of the airplane changing as the nose pitched down. Subsequently, the nose gear collapsed and the aircraft slid to a stop in an upright position. The aircraft sustained substantial damage to the fuselage structure where the nose gear drag link connects to the casting. There was also damage to the lower front cowling and the propeller blades as a result of impacting the runway surface. The nose gear yoke and nose gear strut assemblies were sent to the NTSB Materials Laboratory Division for examination. The examination revealed that the nose gear failed in overstress, consistent with the nose gear yoke moving to the aft and to the right relative to the nose gear strut. It was reported that incorrect nose gear yoke attachment bolts and nuts were installed incorrectly and that one of the bolts was fractured, consistent with overstress in shear. It was also reported that a wedge-shaped spacer was not installed between the nose gear strut and the nose gear yoke, which would affect the loading of the four yoke attachment bolts. Also noted was the presence of various cracks around the attachment bolt through holes. The fracture surfaces were consistent with fatigue.
On June 17, 2003, approximately 0930 mountain daylight time, a Cessna 206 single-engine airplane, N7372Q, sustained substantial damage after the nose gear collapsed during landing roll at the Soldier Bar United States Forest Service Airstrip, located 45 nautical miles west of Salmon, Idaho. The airplane was registered to Wilderness Aircraft II LLC, of McCall, Idaho, and operated by McCall Aviation, of McCall, Idaho. The commercial pilot and another company pilot, who was undergoing training, were not injured. Visual meteorological conditions prevailed for the 14 CFR Part 91 company instructional flight, and a flight plan was not filed. The flight departed the McCall Municipal Airport at 0900. In a telephone interview with the NTSB investigator-in-charge, the pilot reported that after a smooth landing and during the landing rollout, he noticed the attitude of the airplane changing as the nose of the aircraft pitched down and his view of the horizon changed. The pilot stated that he then realized that the nose gear had collapsed, and subsequently the airplane slid to a stop in an upright position. There was no post-crash fire and both pilots exited the airplane without injury. The pilot reported the occurrence to the Federal Aviation Administration (FAA). After repairs were made and an FAA ferry permit was issued, the aircraft was flown back to the company's base of operations in McCall without incident. An FAA inspector, who traveled to the company's headquarters, reported that the airplane had sustained substantial damage to the fuselage structure where the nose landing gear drag link connects to a casting. There was also damage to the lower front cowling and the propeller blades as a result of impacting the runway surface. The nose gear yoke and nose gear strut assemblies were sent to the NTSB Materials Laboratory Division, Washington, D.C., for examination (see Attachment #1). A Senior Metallurgist reported that multiple fractures were observed in the nose gear linkage pieces, each consistent with overstress fracture. The four nose gear yoke attachment bolts were assembled with the threaded end down and with two spacers under the head. The Cessna 206 parts catalog specifies that the bolts of a correctly assembled nose gear are installed with the threaded end up with one washer under the head. It was also noted that the Cessna catalog specifies AN5-17A bolts for the forward two bolts and AN5-21A bolts for the aft two bolts for the heavy duty nose gear. The three bolts which were intact were consistent with the AN5-17 bolt, and none consistent with the AN5-21 bolt. It was observed that one of the attachment bolts was fractured, consistent with overstress fracture in shear. It was reported that a nut thread remnant observed on one of the yoke attachment bolts had a microstructure of ferrite and pearlite consistent with low-carbon steel. According to the Cessna 206 parts catalog, the nut is specified as a NAS679A5 nut, a corrosion-resistant steel. Wear marks were observed on the upper side of the nose gear yoke, which corresponded to the lower surface of the nose strut. The Cessna 206 parts catalog specifies that a correctly assembled nose gear has a wedge-shaped spacer with a rectangular footprint located between the lower end of the nose strut and the upper surface of the nose yoke. No evidence of a wear mark corresponding to this shape was found on the upper side of the yoke. It was further noted that cracks were observed in the nose gear yoke around the forward attachment bolt through holes. On the lower surface of the left forward hole, a crack was observed at the forward side of the hole. On the lower surface of the right forward hole, four cracks were observed at the forward side of the hole, and a crack was observed at the aft side of the hole. At the upper surface of the left forward hole, one crack was observed at the forward side of the hole and one crack was observed at the aft side of the hole. One of the cracks in the nose gear yoke around the right forward attachment bolt through hole was gaped open. The fracture surfaces were consistent with fatigue.
The overload failure of the nose landing gear assembly during the landing roll, due to an improper maintenance installation. A factor was the rough/uneven terrain condition.
Source: NTSB Aviation Accident Database
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