Synopsis
On 12 August 2012 at 1915 UTC, a Jabiru UL-430 microlight aircraft, registration G-BYIM, experienced a collapse of its right main undercarriage shortly after landing on Runway 29 at Ince Airfield, Liverpool. The pilot, who was the sole occupant, was not injured. The accident resulted in damage to the main undercarriage, nose leg, propeller, and right wing.
History of the Flight
The pilot reported a normal approach and landing following a 20-minute uneventful flight. Approximately 10 to 20 meters after touchdown, the right main undercarriage collapsed, followed by the nose leg. The right wingtip and propeller blade struck the ground. The wind was calm and the grass runway was described as slightly soft.
Aircraft Information
The Jabiru UL-430 is a high-wing two-seat microlight with a tricycle undercarriage, powered by a single Jabiru 2200A piston engine. The aircraft was manufactured in 1999 (serial PFA 274A-13397). At the time of the accident, the aircraft had been re-sprayed in October 2010 without removal of the old paint. The maximum permitted empty weight is 248 kg; however, after the accident, the empty weight was measured at 276.5 kg, with 8 kg of non-essential items on board.
Examination Findings
Examination of the failed forward outboard attachment bolt using microscopic and scanning electron microscope techniques revealed that the bolt thread remained intact but contained remnants of the nut thread that had been pulled off. Damage to the cadmium coating indicated that the nut had bottomed out on the bolt thread. Energy Dispersive X-ray analysis indicated the bolt was low alloy steel and the nut was made of a softer steel alloy. The rear attachment bolt was bent but remained intact. The nose leg mounting bolts had pulled out of the fibreglass structure.
Maintenance History
A worksheet from the Permit renewal inspection in August 2010 noted the undercarriage attachment to the fuselage was loose, which was corrected by adding washers and tightening nuts. The same inspector performed the November 2011 renewal and detected no movement. There was no record of torque checks on the undercarriage bolts or reference to Service Bulletins JSB 008-1 and JSB 025-2. The owner stated he was unaware of these bulletins and the relevant LAA Type Acceptance Data Sheet.
Safety Action
Following the investigation, the Light Aircraft Association mandated a 500-hour life for undercarriage attachment bolts on all Jabiru aircraft, required replacement of 5/16" (AN5) bolts with 3/8" (AN6) bolts, and reminded owners and inspectors to regularly check bolt torque and reweigh aircraft after painting.
Probable Cause
The right main undercarriage collapsed as a result of the nut having been pulled off the forward outboard attachment bolt.