Summary
On December 4, 2016, at approximately 1453 central daylight time, N161SY, an Embraer ERJ170 200L operated by SkyWest Airlines, experienced an uncommanded retraction of the nose landing gear during the landing rollout at San Antonio International Airport (KSAT), San Antonio, Texas. The aircraft had originally been scheduled to land at General Mariano Escobedo Int'l Airport (MMTY) in Monterrey, Mexico, but diverted after encountering a landing gear warning. The flight was conducted under Part 121 in visual meteorological conditions.
History of Flight
After departure from George Bush Intercontinental Airport (KIAH) in Houston, Texas, the flight crew heard a loud "thud/pop" just aft of the flight deck. No warnings or indications appeared on the flight deck, so the crew continued to MMTY. During approach, when the landing gear was extended, the crew received warning indications regarding the landing gear position and a "LDG GEAR LEVER DISAGREE" message. The crew executed a missed approach, retracted the landing gear per the quick reference handbook (QRH) procedures, and decided to divert to KSAT. Upon entering US airspace, they declared an emergency and performed a low pass by the control tower, which confirmed that the landing gear appeared down. After touchdown on runway 4, during the rollout, the nose gear retracted without command as the aircraft slowed to a stop. The crew and passengers evacuated via the aft cabin doors using the evacuation slides.
Aircraft and Damage
The aircraft, an Embraer ERJ 170-200 LR with registration N161SY and serial number 17000569, was manufactured and delivered in 2016. It had accumulated 1,310 hours and 740 cycles. The nose landing gear downlock springs were installed during manufacture and had the same time and cycles. The airplane sustained minor damage, including scraping to the lower fuselage forward of the nose gear bay, the nose gear doors, and the underside of the right-hand engine nacelle. During recovery, a failed downlock spring was found lodged in a lightening hole in the nose gear yoke, obstructing downlock operation.
Injuries
One passenger reported a minor injury, which occurred during the evacuation.
Investigation Findings
The NTSB examined the nose landing gear downlock springs. The right spring was intact; the left spring was found in two pieces. The left spring had separated in the ninth coil from the lower end due to progressive crack formation, with no evidence of overstress. Visual inspection revealed small openings on the spring wire surface, resulting in surface roughness measurements above Embraer’s allowable limit. Cross-sections showed intergranular cracking at these openings, associated with a white etching microstructure consistent with retained austenite, while the base material was martensite as specified. The amount of retained austenite in a circumferential surface layer exceeded Embraer’s allowable limit per AMS 5678. Computed tomography imaging was inconclusive.
The manufacturer provided information on additional NLG and MLG downlock spring failures; this incident was the only one resulting in an uncommanded gear retraction on landing. Manufacturing records identified a suspect batch of raw material common to multiple failures, with higher than allowable retained austenite. The aircraft manufacturer conducted quality audits throughout the supply chain.
Probable Cause
The investigation did not state a probable cause; however, the findings indicated that the failure of the left downlock spring due to progressive crack formation from material anomalies led to the uncommanded retraction.