What happened
On August 25, 2021, a Robin DR 400/180R, registration HB-KHO, was performing glider towing operations at Bern-Belp Airport. During the third towing flight of the day, the pilot was returning to the grass runway 34 GLD after the glider had released.
As the aircraft approached the runway, the ground speed was approximately 136 km/h, which was higher than the manufacturer's recommended landing speeds. The aircraft touched down in a three-point position but immediately bounced. This initiated a series of five successive ground impacts, known as porpoising, with each impact becoming increasingly severe. During the final contact, the nose gear collapsed, causing the propeller to strike the ground. The pilot was able to exit the aircraft without injury, though the aircraft sustained damage to the nose gear, propeller, engine, cowling, and fuselage.
The investigation
Investigators examined the aircraft's nose gear assembly, maintenance records, and video footage from airport security cameras. The investigation focused on the structural integrity of the nose gear pivot and the assembly of the shimmy damping system. Metallurgical analysis was performed on the pivot and the broken retaining plate.
Findings
- The investigation established that the nose gear pivot had suffered a fatigue fracture at the weld area, which had already spread across 125 degrees of the tube circumference.
- The nose gear assembly had been incorrectly configured; the required Belleville washer was missing and had been replaced by a flat washer not specified in the manufacturer's maintenance manual.
- This improper assembly prevented the necessary preload from being achieved and compromised the effectiveness of the shimmy damping system.
- Poor material properties, including high sulfur content and improper annealing in the weld, contributed to the fatigue failure.
- The aircraft's approach speed was higher than the limits defined in the Aircraft Flight Manual, leading to an unstable approach and an insufficient flare.
- The use of grass runways, which are inherently uneven, likely contributed to the mechanical stresses on the landing gear.