ERA18FA221 · Quicksilver MXL II Sport

The private pilot and a passenger departed on a local flight and returned for landing about 15 minutes later. While the airplane was on final approach at an altitude between 100 and 200 ft, several witnesses heard a loud sound, and one of the witnesses saw the right wing fold and the airplane enter a steep descent. The airplane subsequently impacted terrain in a field near the airport.

A witnesses reported observing the airframe parachute system deploy. The inspection of the components of the airframe ballistic parachute system indicated that the system had likely deployed just before or at the time of impact. Deployment of the parachute system at that low altitude would have been insufficient to adequately arrest the airplane's descent prior to impact.

A stainless steel bolt secured the upper and lower wing cables to the outer leading edge spar of the right wing. Examination of the bolt showed that it fractured during flight due to stress corrosion cracking that extended across about 95% of the cross-section area of the bolt shank. The bolt failure allowed the leading edge spar to rotate upward relative to the trailing edge spar and fracture at the inboard cable attach point, precluding controlled flight.

The stainless-steel material used for the wing cable attachment bolt was susceptible to stress corrosion cracking when exposed to a chloride-containing environment (such as the ocean). The airplane was previously equipped with floats, and was likely operated as an amphibious airplane at some point before the pilot purchased it. In addition, the installed bolt was a different material than the AN5-30A bolt specified in the Quicksilver Assembly, Maintenance, and Parts Manual for securing the outer upper and lower wing cables to the leading edge spar of the right wing. The material of the bolt recommended in the manual would be generally resistant to stress corrosion cracking as a result of a chloride-containing environment. The maintenance records did not indicate when the stainless steel bolt was installed.

Evidence of stress corrosion cracking might not be apparent during visual inspections of the bolted joint. Also, the location of the stress corrosion cracking on the bolt would likely not have been detected during a condition inspection with the bolted joint in place. Further, the assembly, maintenance, and parts manual did not provide for a specific inspection of the bolt that secured the upper and lower wing cables to the outboard leading edge spar. However, the manual indicated that the security of all bolts should be checked every 50 hours.

Although the pilot was taking a medication that could impair his ability to perform tasks requiring judgment or motor and cognitive skills, his use of the medication did not contribute to the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset