Incident Details
On November 7, 2015, at 1234 Pacific standard time, a Cirrus SR22T, registration N999VX, sustained minor damage during the landing roll at Paso Robles Airport, Paso Robles, California. The airplane was registered to PHD Ventures Inc. and operated under Part 91. The flight instructor, student pilot, and three passengers were not injured. Visual meteorological conditions prevailed; no flight plan was filed.
According to the flight instructor, the student pilot was flying the airplane. The airplane touched down normally on the main gear. The student pilot then lowered the nose of the airplane, and the nose landing gear collapsed.
Post-incident Examination
Post-incident examination revealed that the nose landing gear had separated. The nose landing gear consists of a main strut tube and two gusset tubes near the top portion of the main strut tube. The separation involved a crack beginning at the edge of the side gusset tubes weld to the main strut tube. Prior to this incident, a similar event had occurred (NTSB Incident WPR15IA252), and following this event, additional incidents occurred, including one in Japan.
The NTSB Materials Laboratory examined the nose landing gear strut and determined that the failure of the landing gear was the result of high stress fatigue cracking due to sideways bending from one side. No mechanical or metallurgical anomalies were noted with the landing gear.
Service Actions by Cirrus
On March 7, 2016, Cirrus Design Corporation issued Service Advisory Letter SA 16-03, which denoted that cracks had been discovered on the nose landing gear strut assembly at the welds between the strut tube and the LH and RH gusset tubes. The advisory required a visual inspection of the welds every time the engine cowling is removed, and if cracks are found, the aircraft is prohibited from flight until the strut assembly is replaced.
Cirrus Design Corporation performed structural testing of the nose landing gear. Based on data from the NTSB metallurgy lab and a video of the Japan incident airplane experiencing nose landing gear shimmy 6 months before collapse, Cirrus explored two methods of producing side loads: through taxi and towing, and through shimmy. Flight testing showed that significant side loads would develop during a shimmy event.
On April 12, 2016, Service Bulletin SB2X-32-22 was released to inspect all nose landing gear in the field for cracks. Additionally, Cirrus added a post-shimmy inspection to the Aircraft Maintenance Manual. On April 12, 2016, Service Advisory SA16-05 offered aircraft towing guidance, including not stopping/starting abruptly when the tow bar is at an angle greater than 45°, and towing speeds not exceeding 15 mph.
On July 14, 2017, Service Bulletin SB2X-32-22R1 was issued, considered mandatory, updating compliance and instructions. On January 5, 2018, Revision 2 was issued. On July 14, 2017, Service Bulletin SB2X-32-23 regarding nose wheel shimmy reduction was issued, with instructions for tire pressure adjustment and force required to rotate the nose wheel fork. Revision 1 was issued on January 5, 2018.
On July 14, 2017, Service Advisory SA17-08 revealed cracks at fillet welds leading to collapse, with each involved aircraft having a history of excessive nose wheel shimmy. It defined shimmy as a lateral oscillation during landing rollout. Tire pressure adjustment to 40-50 psi was recommended. On January 5, 2018, Revision 1 adjusted tire pressure to 30-35 psi.
Cirrus Aircraft incorporated specific emphasis on discouraging shimmy in pilot training and maintenance guidance. To increase weld strength, the main strut tube wall thickness was increased from 0.125-inch to 0.156-inch, resulting in a 3-5% increase in local stress levels in static analysis. This design change has been made for all new and replacement gear.