Boeing 737-890 N516AS accident near Santa Ana, California

Probable cause

Maintenance personnel’s excessive grinding of the left main landing gear’s aft trunnion pin during machining, which imparted heat damage to the base metal and led to the fatigue cracking that caused the pin to fracture during landing.

NTSB analysis

This accident occurred when the left main landing gear (MLG) of an Alaska Airlines Boeing 737-800 collapsed during landing. Postaccident examination revealed that the aft trunnion pin in the left MLG failed during landing due to a fatigue fracture. The fracture initiated from a small intergranular region below the external chromium electroplated layer. The fatigue crack, which had propagated to a depth of 0.144 inches, was large enough to cause the remaining material to fracture in tensile or upward-bending overstress during landing, resulting in the collapse of the left MLG.

Further examination of the aft trunnion pin revealed that the intergranular region where the fatigue crack initiated was located along an area with a darker visual contrast following temper etch and metallographic inspections. This region also showed an elevated reading from a Barkhausen noise inspection. The elevated readings and area of visual contrast were consistent with the area being exposed to higher temperatures becoming softer than the surrounding material. The heat exposure most likely occurred as part of the excessive grinding of the surface, performed during the maintenance overhaul of the pin that occurred on July 5, 2018, as part of the overhaul of the left MLG assembly. This area was subject to grinding to first remove the original chromium layer, and then a second grinding after the new chromium layer was applied to bring the trunnion pin into specified dimensions.

The over-tempered region and the surrounding material created an area of intergranular fracture within the material in the area where grinding had occurred. The investigation considered whether the formation of this area could have occurred during the baking and stress relief processes following aggressive grinding during the chromium removal step of the overhaul. However, after removing the chromium electroplating layer, the temper etch inspections did not detect any grinding burns. Because a temper etch inspection cannot be used after the chromium electroplating layer was reapplied, the grinding burn likely occurred during machining after the new chromium layer electroplating process.

At the time of the left MLG assembly overhaul, the landing gear assembly had accumulated 11,116 landing cycles. The overhauled left MLG assembly was installed on the accident airplane on July 17, 2018. Afterward, the left MLG, including the trunnion pin, accumulated an additional 4,710 landing cycles.

A fatigue crack analysis performed on the aft trunnion pin fracture surface showed that the fatigue crack had been present for at least 797 landing cycles. Therefore, the crack was not present when the pin was overhauled in July 2018 but had likely developed later as a result of the grinding performed during the overhaul. Because most of the nondestructive inspection techniques used to detect damage to plated trunnion pins typically rely on identifying cracks, techniques such as magnetic particle inspection and fluorescent penetrant inspection would have been ineffective before crack initiation. However, results of this examination and previous NTSB investigations demonstrate that even relatively mild heat exposure from grinding and/or machining during overhaul can lead to cracking, which can lead to fatigue crack growth and failed landing gear components, as occurred in this accident.

Text from the NTSB. Open the NTSB docket for DCA23FA417 for the full record.