Incident Overview
On December 8, 2010, at approximately 1310 Mountain Standard Time, a single-engine Diamond Aircraft DA 40, registration N326AF, experienced a hard landing at the United States Air Force Academy Airfield (AFF) in Colorado Springs, Colorado. The aircraft, operated by Doss Aircraft Inc. under an exclusive contract with the United States Air Force (USAF), was on a local instructional flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a military flight plan had been filed. The flight originated from AFF at about 1250. Neither the student pilot nor the instructor pilot was injured.
According to the instructor pilot, during the fourth touch-and-go maneuver of the day, the student pilot bounced the aircraft on landing. The instructor assessed the bounce as not severe and allowed the student to remain on the controls for a second touchdown. Upon the second runway contact, the instructor observed the nose landing wheel depart the aircraft. The airplane skidded down the runway for approximately 120 feet before coming to rest upright. The aircraft sustained minor damage.
Examination Findings
An examination of the nose landing gear revealed that the nose fork had completely fractured on both the left and right sides, allowing the wheel to separate. The nose fork was sent to the Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base, Ohio, for analysis. On January 18, 2011, AFRL determined that the failure resulted from 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 operational loads, leading to final ductile overload failure.
The AFRL report and nose wheel pieces were subsequently sent to the National Transportation Safety Board (NTSB) Materials Laboratory for verification. The NTSB Materials Laboratory confirmed the Air Force’s findings.
The USAF Academy Chief of Flight Safety reported that the aircraft had an airframe time of 559.9 hours at the time of the incident. The nose wheel assembly had 2,674 cycles and had been previously inspected on November 19, 2010. The airframe time from the inspection to the incident was 61.8 hours.
Background and Actions
The NTSB had investigated several similar incidents involving failed nose gear forks on Diamond DA-20 airplanes, noting that the fork designs of the two aircraft models were very similar. As a result of this investigation, the U.S. Air Force conducted a one-time inspection of its fielded fleet to identify any other nose forks with potential fatigue cracks. The Air Force also implemented a method for periodic nondestructive inspection of all its airplanes’ nose wheel forks for fatigue cracks. Additionally, they evaluated and implemented ways to decrease low-cycle, high-intensity stress induced 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.