What happened
On April 7, 2012, a Grob G109B motor glider, registered JA109B, was conducting a familiarization flight at the Fujikawa Glider Strip in Shizuoka Prefecture. The flight, operated by the Fuji Glider Club, carried a pilot and one passenger. During a series of touch-and-go training maneuvers, the aircraft approached runway 18 with a flight path slightly higher than standard due to fluctuating southerly winds.
As the pilot initiated the flare, the aircraft experienced a rapid sink rate, likely caused by a loss of lift following a reduction in airspeed. Upon touchdown, the aircraft's right main landing gear suffered a structural failure. The impact caused the glider to tilt sharply to the right, resulting in the propeller blades striking the runway. The aircraft veered approximately 70 meters from the touchdown point before exiting the runway and coming to a halt on the grass. There were no injuries to the occupants, though the aircraft sustained minor damage.
The investigation
An investigation by the Japan Transport Safety Board (JTSB) focused on the mechanical failure of the landing gear assembly. Investigators examined the mounting bolts and nuts of the right main landing gear and utilized material analysis from the National Institute for Materials Science. The analysis focused on the threads of the inboard and outboard mounting nuts to determine why the gear assembly failed under the vertical loads of touchdown.
Findings
- The right main landing gear failed because the threads of the inboard mounting nuts stripped, causing them to slip off the bolts. This shifted the entire load to the outboard mount, which subsequently failed when the forward outboard nut also stripped.
- The investigation determined that the nuts used were not the parts specified by the manufacturer and possessed approximately 25% lower tensile strength than required.
- The sudden vertical force applied during touchdown exceeded the reduced tensile strength of these non-specification nuts, causing the gear to swivel 90 degrees aft on the remaining bolt.
- It could not be determined exactly when or how these incorrect, lower-strength nuts were installed on the aircraft.