Incident Overview
On August 27, 2015, at approximately 1405 mountain standard time, a Cirrus SR22, registration N915TD, experienced a nose landing gear collapse during the landing roll at Scottsdale Airport (SDL) in Scottsdale, Arizona. The airplane was operated by Scottsdale Executive Flight Training as a 14 Code of Federal Regulations Part 91 instructional cross-country flight. It had departed from Flagstaff, Arizona, around 1300 and was destined for SDL. Visual meteorological conditions prevailed, and no flight plan was filed.
Occupants and Damage
The certified flight instructor (CFI), the pilot receiving instruction, and one passenger were not injured. The airplane received minor damage.
Landing Sequence
According to the CFI, the pilot receiving instruction performed the landing. The main landing gear touched down first, and the nose gear was slowly lowered onto the runway. The CFI reported feeling a shimmy in the nose gear at that moment. The pilot then applied back pressure on the control stick to lift weight off the nose gear. When the nose gear came down a second time, the shimmy appeared to have stopped, and the aircraft seemed level on all three wheels. However, after a few seconds, the nose slowly began to fall forward until the nose landing gear completely collapsed.
Post-Accident Examination
The nose landing gear consists of a main strut tube and two gusset tubes near the top portion of the main strut tube. The landing gear was observed fractured through the strut tube adjacent to the forward edge of the gusset tube attachment welds.
The National Transportation Safety Board Materials Laboratory examined the nose landing gear strut and determined that the failure was the result of high stress fatigue cracking due to sideways bending from one side. No mechanical or metallurgical anomalies were noted.
Manufacturer Actions
Following the incident, Cirrus Design Corporation issued multiple service advisories and bulletins:
- On March 7, 2016, Service Advisory Letter SA 16-03 required visual inspection of the nose landing gear strut assembly welds whenever the engine cowling is removed. Cracks had been discovered at the welds between the strut tube and the left-hand (LH) and right-hand (RH) gusset tubes. Aircraft with cracks were prohibited from flight until the strut assembly was replaced.
- On April 12, 2016, Service Advisory Letter SA 16-05 provided aircraft towing guidance to prevent damage from non-standard or abusive tug operation, including avoiding abrupt stops/starts and towing at angles greater than 45° or speeds above 15 mph.
- On the same date, Service Bulletin SB2X-32-22 (considered mandatory) was issued for inspection of all nose landing gear for cracks at the gusset welds. A post-shimmy inspection was added to the Aircraft Maintenance Manual (AMM). Revisions followed on July 14, 2017 (SB2X-32-22R1) and January 5, 2018 (SB2X-32-22R2), requiring compliance every 50 hours until termination action.
- On July 14, 2017, Service Bulletin SB2X-32-23 (mandatory) addressed nose wheel shimmy reduction, noting that shimmy may exist on aircraft with Beringer wheels. It provided instructions for adjusting nose tire pressure and rotational force of the nose wheel fork. Revision 1 was issued on January 5, 2018.
- Also on July 14, 2017, Service Advisory SA17-08 revealed that cracks had been found on some nose landing gear strut assemblies at the fillet welds, leading to collapse. The advisory defined nose wheel shimmy as a lateral oscillation or wobble causing a shaking feeling in the cabin, normally encountered during landing roll-out. It recommended lowering tire pressure to 40–50 psi. Revision 1 (January 5, 2018) updated the pressure to 30–35 psi.
Cirrus also incorporated specific emphasis on discouraging shimmy during landing and actions to take if shimmy occurs into their pilot training program (Landing Standardization Course). Maintenance guidance after a shimmy event was also provided.
To increase strength, the thickness of the main strut tube was analyzed and changed from 0.125-inch to 0.156-inch. This design change was applied to all new and replacement nose landing gear.
Probable Cause
The NTSB Materials Laboratory determined that the failure was due to high stress fatigue cracking from sideways bending. No probable cause was explicitly stated by the investigation.