Cuby's Structural Failure In Extreme Turbulence
Several friends were at Mullaghmore Airfield, the base for a flying club mainly concerned with microlight aircraft.
Several friends were at Mullaghmore Airfield, the base for a flying club mainly concerned with microlight aircraft. The weather was initially unsuitable for flying, with a surface wind of 25 mph, gusting to thirty mph from the north.
By 1700 the wind had dropped to fifteen mph and some of the group prepared their various aircraft for flying. These comprised a flexwing microlight, an AX 3 conventional microlight, and the accident aircraft, a Cuby II which did not conform to the UK definition of a microlight and was registered as a light aircraft under the auspices of the PFA.
The owner of the Cuby, together with the holder of an ATPL, were to fly the aircraft. The owner held a PPL limited to PIC of microlight aircraft.
The three aircraft took off at 1730 with the intention of routing to the north and over the Giant's Causeway. The surviving pilots estimate the wind at their altitude to have been 20 to 25 mph from the north. As they approached the Causeway, the CFI in one aircraft commented to his pilot that, although the conditions were good in their present position, they would be much more severe just inland from the cliff face. the AFI in the second aircraft transmitted to the crew in the Cuby and asked them if they were over the sea. He received a reply in the negative, but the Cuby was then seen to turn right and descend towards the cliffs behind the Causeway. Nothing more was heard from it.
Witnesses saw the aircraft in a descending spiral to the ground; the CFI saw the Cuby at 800 feet with one of its wings 'folded'. Some witnesses also stated that they saw one of the wings folded up about a half to a third along its length. One airborne witness stated that she could see the underside of the left wing as the Cuby 'flipped up'. Two witnesses walking along the cliff top state that the wind was strong.
Both the other aircraft turned towards the area of the crash, but both experienced extreme turbulence. The two crew of the crashed aircraft had sustained fatal injuries.
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Research has shown that a turbulent zone can extend to some three times the height of an obstacle and that the turbulence can be felt up to a distance of ten times the height of the obstacle downwind of it. In the vicinity of the crash site, the cliffs extend to a height of 200 to 230 feet, therefore turbulence could have been present up to a height of about 700 feet extending 700 metres downwind.
This Cuby II aircraft had been imported to the UK in the early 1990s as a kit from its Canadian manufacturer. It was the only Cuby II kit in the UK. The Canadian kit manufacturer was later dissolved and few examples were completed in North America. Production of a derivative aircraft, the Eurocub, has continued in Hungary, with a number of design changes. According to the current manufacturer, these include changes to the wing design.
Both wings had been severely damaged in the impact but detailed examination showed that there had been an upward failure of the right hand outer wing panel in flight. This had resulted from a failure of both the main wing spar and the rear spar just inboard of the attachment point of the wing strut. It appeared to have been the main spar that failed first, twisting into the plane of the wing and then failing in bending. Evidence shows that when the right wing suffered its instability failure, the left wing was close to a similar collapse.
Although the workmanship in the Cuby appeared to be of a good standard, a number of experienced aircraft engineers commented on the design of the wing structure. One observation was that, with the main wing spar not occupying the full depth of wing, the leading edge structure would make little contribution to the torsional stiffness of the wing and to the stability of the main spar. Another observation concerned the wing ribs, which were of simple 'trapezoidal' construction and appeared to have low in plane stiffness.