History of the Flight
On 2 May 2013 at 1800 UTC, a Jabiru UL (registration G-BZEN) was completing a training flight at Wickenby Aerodrome in Lincolnshire. As the aircraft touched down, the pilot heard a loud bang and felt the left wing drop. Initially suspecting a tyre burst, the pilot applied right stick to lift the weight off the left mainwheel tyre. The nose pitched up and the left wing dropped further, leading the pilot to realize the problem was more than a burst tyre. Despite various control inputs to maintain directional control, the airspeed decayed and the wing dropped again. The pilot shut down the engine to avoid propeller damage and engine shock-loading, but was unable to keep the aircraft on the runway. The aircraft veered onto the grass and came to a stop, canted to the left with its nose and right wheel on the ground. The left landing gear leg had collapsed and twisted forward underneath the fuselage. The pilot and passenger were uninjured and vacated the aircraft.
Aircraft Information
The Jabiru UL-430 is a high-wing two-seat microlight aircraft equipped with a fixed tricycle landing gear. The main landing gear (MLG) consists of separate left and right cantilever spring legs. Each leg is secured to the underside of the fuselage by one inboard and two outboard attachment bolts.
Engineering Findings
Examination of the collapsed left landing gear revealed that all three 5/16 inch diameter (AN5-1) mounting bolts had failed. The inner bolt was bent, while the outer bolts were both bent and fractured. The remains of all three bolts showed general wear and distress. It is probable that one of the outboard bolts failed first, leading to overload and distortion of the remaining two bolts.
The aircraft was fitted with the original design 5/16 inch diameter landing gear mounting bolts rather than the recommended 3/8 inch diameter bolts specified in Jabiru Service Bulletin (JSB) 008-1, issued on 31 March 2005. JSB 008-1 recommended the introduction of larger diameter mounting bolts using a proprietary Jabiru modification kit. Additionally, JSB 052-2 (issued 7 May 2009) advised a 500-hour life for the 3/8 inch bolts. G-BZEN had not had these service bulletins applied, though they were not mandatory at the time of the incident. Following this and a previous similar incident, the Light Aircraft Association began action to mandate both bulletins via an Airworthiness Information Leaflet.