Flight History
On 7 October 2000, a Cessna U206A Super Skywagon (registration G-BGWR) was returning to Whitchurch (Tilstock) Airfield, Shropshire, after a parachute dropping flight. The pilot, holding a Basic Commercial Pilot's Licence with 470 hours total experience (191 on type), initiated an approach to Runway 33R (concrete, 600 by 30 meters). After an uneventful touchdown, the pilot closed the throttle to idle, retracted the flaps, and applied wheel braking. The brake pedals felt unusually stiff, and the nose of the aircraft began to dip. Initially believed to be normal settling, the nose continued to sink. Before the pilot could shut down the engine, the propeller blades struck the runway surface. The nose settled onto the nose wheel, jamming it, and the aircraft skidded to a halt to the right of the centreline. The pilot exited through the normal door uninjured.
Examination of Failed Bolt
The aircraft was equipped with fixed tricycle landing gear. Inspection revealed that the 3/16-inch diameter bolt (Part No NAS 464 P5 A42) securing the nose landing gear drag brace to the fuselage had failed. The bolt, designed to be loaded in double shear, fractured through the shank at the plane of the drag brace to fork fitting interface, toward the threaded end. This allowed the nose landing gear leg to pivot forward about its upper attachments.
Metallurgical examination by the AAIB showed that the failure was preceded by the growth of fatigue cracks through the majority of the shank cross-section. One crack extended through over half the diameter from three origins on one side, comprising an initial corroded region and a brighter uncorroded region with coarse growth marks. Corrosion had destroyed marks over the initial part of the crack, but a steady increase in crack growth rate was evident. Approximately 0.5 mm axially from the major crack, a second crack had grown on the same side. A third, smaller crack had developed on the opposite side, also with an initial corroded region and a bright region that grew rapidly, suggesting earlier corrosion and possible removal and refitting in a rotated position or rotation in the assembly. Both corroded and bright regions indicated periods of accelerated growth before final ductile rupture. The small ductile rupture zone indicated relatively low loads on the bolt. The shank exhibited corrosion pitting over most of its length and light wear, with a small step at the other brace/fork fitting interface. Hardness tests showed tensile strength within specification (approximately 1145 MPa).
Previous Cases and Discussion
The aircraft manufacturer was aware of previous instances of failure of the same bolt on Cessna 206 and 207 aircraft. They attributed past occurrences to fatigue failures from bending in the bolt shank caused by high side loads on the nose gear from towing or hard landings. The manufacturer confirmed that no special inspection or life limit existed for the bolt.
Discussion of the event noted that, by design, the bolt is loaded in double shear, but the failure resulted from plain bending loads. These bending loads could arise from clearance gaps between the drag brace end fitting and the fork fitting, exacerbated by wear or high-load events. The fracture indicated a relatively low operating load given the small ductile rupture area and the long-term presence of cracks, as evidenced by corrosion. It was also possible that the bolt had been removed and replaced while already cracked.
Conclusions and Recommendation
The failure of the nose landing gear drag brace attachment bolt resulted from the growth of fatigue cracks through more than 90% of the shank cross-section under plain bending loads. The condition of the bolt shank suggested that fatigue cracks likely initiated at corrosion pits, and a small region of crack growth may have occurred during an earlier period before the bolt was removed and refitted. The steel strength was within specified limits.
The AAIB issued Safety Recommendation 2000-65, recommending that the Federal Aviation Administration and Cessna Aircraft Company consider implementing an inspection or lifing policy for the nose landing gear drag brace to fuselage fitting bolt (Part No NAS 464 P5 A42) on Cessna 206 and 207 aircraft.