Quicksilver Strut Fitting Safety Recommendation
Failed strut eyebolt
On April 8, 2010, the pilot of N439MM, a strut-braced Quicksilver Sport 2S, initiated a turn to land at an airport in Canton, Texas, when he heard a pop and lost vertical control of the aircraft. The pilot indicated he still had directional control. He tried to deploy a BRS parachute, but it didn’t fully deploy. Seconds later he impacted some trees; the aircraft took out a 6-inch-thick branch and fell through the limbs to the ground. The pilot seat was within inches of the ground.
The 70-year-old pilot wasn’t injured, but the aircraft was substantially damaged. An investigation found that there was continuity of all flight controls, but a wing lift strut attach fitting had separated from the eyebolt. The FAA investigation revealed that the assembly instructions hadn’t been followed correctly, and the FAA recommended the customer base be notified of this potentially dangerous situation. The manual indicates the eyebolt must have a minimum of 5/8-inch thread engagement with 3-1/2 threads exposed. The failed fitting had only 5/32-inch thread engagement, and approximately 16 threads were exposed. The thread engagement area was a quarter of the minimum needed.
The author of an FAA memorandum stated that a “witness hole” for verification of thread engagement is an industry standard and should be included in this design. Presumably the builder had deviated from the instructions in an attempt to adjust the washout of the wing or to correct an out-of-trim condition. Owners and operators of aircraft using this type of strut hardware should consult the appropriate assembly or repair manual and inspect the aircraft to ensure correct assembly. The contact info for Quicksilver aircraft is at www.QuicksilverAircraft.com.
The following response was issued by a representative of the engineering department at Quicksilver Mfg. Inc.:
“This instance was related to builder error in not following the guidance in the assembly instructions. The assembly instructions provided in the Sport 2S manual to properly install and adjust the lift strut fittings are clear and simple. Very little, if any, adjustment from the typical thread exposure is actually needed to achieve the specified angle for the washout. Should any more than about 6 threads be exposed on the eyebolt, one should adjust it to have more thread engagement. Rather than threading out the eyebolt at the trailing edge, one can thread in the eyebolt at the leading edge. Witness holes may be a possible design feature, but are by no means an industry standard. A minimum of thread engagement or a maximum of thread exposure are common references for assembly. Just for reference, turnbuckles are one of the most common adjustable threaded assemblies found in a wide spectrum of aircraft. While some manufacturers of AN turnbuckle hardware may provide witness holes, for the most part, they do not. The simple practice to observe a maximum of 3 threads exposed is followed.”