What happened
On May 17, 2015, a privately owned Hoffmann H-36 Dimona, registration JA2406, was conducting a training flight at Fukushima Sky Park. The flight was operated by a trainee pilot in the left seat with an instructor in the right seat. After completing various air-work maneuvers, the aircraft returned for landing on Runway 32.
During the landing roll, the trainee partially retracted the spoiler lever, which simultaneously applied the wheel brakes. While the aircraft initially maintained a straight path despite two slight bounces, a gust of wind from the left caused the left wing to lift. This shift in weight increased the load on the right main wheel. As the aircraft began to veer toward the right, the trainee applied full braking and left rudder, and the instructor attempted similar corrective actions. However, the aircraft could not be stabilized and veered off the runway, eventually catching its main wheels in a parallel ditch.
The investigation
The JTSB investigation examined the aircraft's mechanical condition, the meteorological data, and the flight crew's actions. Investigators reviewed the aircraft's braking system, finding no mechanical failures, leaks, or imbalances in the brake oil or tire pressure. The investigation also looked into the flight manual and the crew's previous flight habits, noting that the club members often used aggressive braking to minimize landing distances.
Findings
- Asymmetric Braking: A gust from the left during the landing roll increased the load on the right main wheel. Because the brakes are linked to the spoiler lever, this caused the right brake to be applied more effectively than the left, contributing to the rightward veer.
- Pilot Hesitation: The trainee pilot, influenced by a previous observation of a different aircraft entering a ground loop, hesitated to apply left-side control corrections. This lack of appropriate response to the crosswind allowed the deviation to worsen.
- Directional Control: The tail wheel had not firmly established contact with the runway when the deviation occurred, meaning the steering mechanism was not yet fully effective for directional control.
- Structural Damage: The impact with the ditch caused the bolts securing the main landing gear to rupture, rotating the gear 90 degrees backward. This caused the wheel fairings to strike the underside of the wing, and the subsequent nose-down motion resulted in the wooden propeller blades breaking upon ground contact.
Safety action
Following the accident, the Fukushima Motor Glider Club Sendai branch implemented several prevention measures, including guidelines to apply brakes only after sufficient speed reduction, instructions to temporarily reset the spoiler lever during touchdown if approaching with spoilers extended, and improved incident reporting and briefing procedures.