What happened
On July 30, 2021, a Diamond DA40, registration HB-SDF, was performing a private VFR flight from Münster to Bad Ragaz. During the landing phase on runway 12, the aircraft approached the threshold at a speed of approximately 75 KIAS, significantly higher than the manufacturer's recommended 60 KIAS for that weight.
As the aircraft touched down within the first third of the 495-meter runway, the nose gear collapsed. This caused the aircraft to undergo a porpoising motion, leading to the nose gear strut bottoming out and the propeller making heavy contact with the runway surface. The impact caused severe damage to the propeller and the nose gear, though the pilot sustained light injuries.
The investigation
SUST examined the nose gear assembly and found that the central tube, which acts as the spring guide, had fractured. The investigation revealed that the tube suffered from significant material loss and cross-sectional reduction due to both abrasive and adhesive wear.
Technical analysis showed that a guidance washer with a flange had been installed incorrectly (backwards). This misalignment prevented the tube from being centered within the end fitting, causing it to rub against the nickel-plated steel component. Furthermore, the protective anodized layer on the tube was unusually thin and rough due to prior asymmetric corrosion, which accelerated the wear process. Additionally, one of the small elastomer elements within the shock absorber was found to be cracked and brittle.
Findings
- The primary cause of the structural failure was the fracture of the nose gear central tube caused by progressive wear.
- The wear was driven by the incorrect installation of a guidance washer, which led to metal-on-metal contact between the tube and the end fitting.
- The pilot's approach speed was approximately 19 KIAS above the manufacturer's recommended landing speed, which placed extraordinary stress on the already compromised shock absorber.
- There was no documented evidence in the aircraft's maintenance records of the required 1000-hour inspections for the nose gear strut, which were intended to check for such wear and deformation.