Colorado Springs, CO, USA
N326AF
DIAMOND AIRCRAFT IND INC DA 40
The flight instructor and student pilot were performing their fourth touch-and-go landing of the day when the student pilot bounced the airplane. The instructor determined that the bounce was not severe and allowed the student to remain on the controls for a second touchdown. As the airplane contacted the runway, the instructor observed the airplane’s nose wheel depart the airplane. The airplane skidded down the runway for about 120 feet on its main landing gear and nose wheel strut before coming to a stop. An examination of the nose landing gear determined that multiple fatigue cracks had originated in the forward machined pockets of the left and right sides of the nose wheel fork. The cracks propagated until the fork could no longer sustain the applied operational loads, at which point overload failure occurred. As a result of the investigation, the airplane manufacturer developed a reinforced nose wheel fork, and the operator conducted a one-time inspection of its fleet of airplanes to identify any other nose wheel forks with fatigue cracks. The operator also implemented a method to nondestructively inspect all their airplanes’ nose wheel forks for fatigue cracks on a periodic basis. Additionally, the operator evaluated and implemented ways to decrease low cycle, high intensity stress brought on by nose wheel shimmy during takeoffs and landings and published guidance outlining specific steps to take in the event of hard landings, bounces, nose wheel strikes, and excessive nose wheel shimmy or vibration.
On December 8, 2010, about 1310 Mountain Standard Time, a single-engine Diamond Aircraft DA 40, N326AF, sustained minor damage during a hard landing at the United States Air Force Academy Airfield (AFF), Colorado Spring, Colorado. Neither the student pilot nor instructor pilot were injured. The airplane was registered to and operated by Doss Aircraft Inc., of Colorado Springs, Colorado, under an exclusive contract with the United States Air Force (USAF). Visual meteorological conditions prevailed and a military flight plan was filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local flight originated from AFF about 1250. According to the instructor pilot, on the fourth touch-and-go maneuver of the day the student pilot bounced the airplane on landing. The instructor assessed that the bounce was not severe and allowed the student to remain on the controls for a second touchdown. As the airplane contacted the runway for the second time, the instructor observed the nose landing wheel depart the airplane. The airplane skidded down the runway for about 120-feet before coming to rest in an upright position. An examination of the nose landing gear revealed that the nose fork had completely fractured on the left and right sides allowing the wheel to depart the airplane. The nose fork was sent to the Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base, Ohio, for examination. The nose wheel pieces were examined on January 18, 2011. The examination determined that the failure was due to multiple crack initiation and propagation at the outboard side, inner diameter surface of both upper and lower webs in the forward machined pocket of both left and right forks until the remaining ligament could not sustain the applied operation loads, whereby final ductile overload failure ensued. The AFRL Report and the nose wheel pieces were sent to the National Transportation Safety Board Materials Laboratory for verification. The Materials laboratory confirmed the Air Force’s findings. The USAF Academy Chief of Flight Safety reported that the airplane had an airframe time of 559.9 hours at the incident. The nose wheel assembly had 2,674 cycles and was previously inspected on November 19, 2010. The airframe time from the inspection to the incident was 61.8 hours. The National Transportation Safety Board had investigated several similar incidents involving failed nose gear forks on Diamond DA-20 airplanes. The designs of the two airplane models forks were very similar. As a result of this National Transportation Safety Board investigation, the Air Force conducted a one-time inspection of its fielded fleet to identify any other nose forks that might have fatigue cracks. They implemented a method to nondestructively inspect all their airplanes’ nose wheel forks for fatigue cracks on a periodic basis. They evaluated and implemented ways to decrease low cycle, high intensity stress brought on by nose wheel shimmy during takeoffs and landings, and they published guidance outlining specific steps to take in the event of hard landings, bounces, nose wheel strikes, and excessive nose wheel shimmy or vibration.
Fatigue failure of the nose wheel fork, which resulted in separation of the nose wheel.
Source: NTSB Aviation Accident Database
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