Denver, CO, USA
N87513
BOEING 737-824
This accident occurred when the left main landing gear of a United Airlines Boeing 737-800 collapsed during the landing roll. Both flight crewmembers reported that the airplane had touched down smoothly on the runway centerline, but the first officer reported that he then felt the airplane “shudder” and tilt left wing down. The first officer further reported that, as the airplane continued to decelerate, he struggled to keep the airplane on the runway centerline. Visual meteorological conditions and a headwind were present when the airplane landed, so the weather did not play a role in the accident circumstances. Postaccident examination found that the aft trunnion pin in the left main landing gear failed during the landing due to a fatigue crack. The crack, which had grown to a depth of 0.154 inches, was large enough that stress concentration at the crack tip (from loads during the landing) caused the pin to fracture, resulting in the collapse of the left main landing gear. The fatigue crack initiated from a small intergranular region just below the external chromium electroplated layer. The size of the intergranular region was about 0.011 inches deep and 0.074 inches wide. Multiple fatigue cracks had initiated from this intergranular region. These individual cracks coalesced and propagated inward, as shown by ratchet marks and fatigue striations. Etching showed that the intergranular region where the fatigue crack initiated was located along an area exhibiting a darker visual contrast. This characteristic was consistent with over tempering and an area of localized exposure to higher temperatures relative to the alloy steel material of the pin outside this area. The most likely cause of this elevated heat input was excessive grinding performed during maintenance overhaul of the pin. Any grinding operation introduces the risk of a local microstructure change, but over-tempering indicates hard or excessive grinding involving temperatures that are high enough to change the steel material’s microstructure. According to United Airlines, the pin had accumulated 23,535 landing cycles since entering service in November 1998. The first overhaul occurred in May 2008 when the pin had accumulated 10,613 cycles; the second overhaul occurred in December 2017 when the pin had accumulated 21,226 cycles. A fatigue crack analysis showed that the thumbnail crack had been present for at least 6,225 landing cycles. Between the time of the last overhaul (December 2017) and the accident, the pin had accumulated 2,309 landing cycles, and the pin had accumulated 12,922 landing cycles between the earlier overhaul (May 2008) and the accident. Therefore, the crack was present before the December 2017 overhaul but was likely not present before the May 2008 overhaul. According to the work order for the last trunnion pin overhaul, which was completed on December 28, 2017, the chromium layer had not been stripped from the trunnion pin before the cadmium electroplating process. Postaccident testing performed at Boeing on the accident pin demonstrated that the underlying cracks could not be detected by magnetic particle inspection when the chromium electroplating layer was present. Fluorescent penetrant inspection was also performed with no indications noted. The earlier overhaul, which was completed on May 23, 2008, included grinding of the trunnion pin surface, first to remove the old chromium electroplating layer and then after the new chromium layer had been electroplated on the trunnion pin. Because grinding steps were performed during this overhaul, the over-tempering of the pin most likely occurred at that time. The investigation determined that the bare trunnion pin was not inspected after the chromium electroplating layer was stripped. According to information from the May 2008 overhaul provider, the company performs a visual inspection, temper etch, and magnetic particle inspection after stripping and machining the pin (before shot peening). The investigation could not determine why these inspections were not performed on the pin. A temper etch on the bare pin should have revealed an area of excessive grinding, which would have prevented the part from progressing through the overhaul and being placed back into service. In summary, the grinding operation on the trunnion pin that occurred during the May 2008 overhaul created heat damage and areas of over-tempering to the base alloy steel material. No inspection was performed to detect the excessive grinding, so the trunnion pin was returned to service with the crack undetected under the chromium electroplating layer, and the crack continued to grow for about 4,150 cycles, progressing to a depth of 0.105 inches. In addition, elevated temperatures during the baking steps of the May 2008 overhaul led to the initial heat tinting oxidation, which caused the initial discoloration of a portion of the crack. The nondestructive inspections of the pin during the subsequent overhaul in December 2017 revealed no indications of base metal cracking under the external chromium layer, and the overhaul processes subjected the pin to multiple elevated temperature exposures. These exposures induced additional heat tinting oxidation on the crack surface that had grown since the first overhaul. The pin was returned to service, and the fatigue crack propagated over 2,309 flight cycles (after the December 2017 overhaul) until the final fracture occurred during the accident landing due to ductile separation. The final depth of the fatigue crack was 0.154 inches. This portion of the slow growth region did not exhibit discoloration or heat tinting because there had been no elevated temperatures exposures after the December 2017 overhaul.
HISTORY OF FLIGHTOn December 22, 2019, about 1859 mountain standard time, United Airlines (UAL) flight 2429, a Boeing 737-800, N87513, experienced a left main gear collapse during the landing roll on runway 17R at Denver International Airport (DEN), Denver, Colorado. The 171 passengers and 7 crewmembers aboard were not injured. The airplane was substantially damaged. The airplane was operating under Title 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from Newark Liberty International Airport (EWR), Newark, New Jersey. The first officer was the pilot flying, and the captain was the pilot monitoring. Both flight crewmembers stated that takeoff, climb, and cruise were normal. The first officer stated that he performed a visual approach to runway 17R using flaps at 30°, a reference landing speed of 145 knots, and a target speed of 151 knots. The first officer also stated the approach was stabilized, and both flight crewmembers reported that the airplane touched down smoothly on the runway centerline. The first officer further stated that, about 3 to 5 seconds after touchdown and after the thrust reversers had been fully deployed, he felt the airplane “shudder” and then tilt left wing down. The first officer recalled that, as the airplane continued to decelerate, he struggled to keep the airplane on the runway centerline. The airplane came to a stop slightly to the left of the centerline. Afterward, the flight crew determined that the left main landing gear had failed, and the crew contacted air traffic control to report the situation and request that fire trucks be dispatched. The captain then contacted the lead flight attendant to find out if there was any indication of a fire, and she responded that she saw no fire. Emergency response personnel confirmed that there was no fire outside the airplane, so the passengers remained on the airplane until the airstairs arrived. The passengers and crewmembers then deplaned via the main cabin door and were transported via buses to the terminal. AIRCRAFT INFORMATIONEach main landing gear has a trunnion pin, which connects the main landing gear beam to the outer cylinder. The trunnion pin was made of type 4340M alloy steel, which was then coated externally with a chromium electroplated layer and internally with a cadmium-titanium alloy electroplated layer. According to UAL personnel, the pin had accumulated 23,535 total landing cycles since the time that the pin entered service in November 1998. The first overhaul occurred in May 2008 when the pin had accumulated 10,613 landing cycles; the second overhaul occurred in December 2017 when the pin had accumulated 21,226 landing cycles. The airplane had accumulated 2,309 landing cycles between the December 2007 overhaul and the accident and 12,922 landing cycles between the May 2008 overhaul and the accident. According to a Boeing service bulletin and the corresponding Federal Aviation Administration airworthiness directive, the trunnion pin was to be overhauled every 10 years or 18,000 landing cycles. AIRPORT INFORMATIONEach main landing gear has a trunnion pin, which connects the main landing gear beam to the outer cylinder. The trunnion pin was made of type 4340M alloy steel, which was then coated externally with a chromium electroplated layer and internally with a cadmium-titanium alloy electroplated layer. According to UAL personnel, the pin had accumulated 23,535 total landing cycles since the time that the pin entered service in November 1998. The first overhaul occurred in May 2008 when the pin had accumulated 10,613 landing cycles; the second overhaul occurred in December 2017 when the pin had accumulated 21,226 landing cycles. The airplane had accumulated 2,309 landing cycles between the December 2007 overhaul and the accident and 12,922 landing cycles between the May 2008 overhaul and the accident. According to a Boeing service bulletin and the corresponding Federal Aviation Administration airworthiness directive, the trunnion pin was to be overhauled every 10 years or 18,000 landing cycles. WRECKAGE AND IMPACT INFORMATIONPostaccident inspection found that the left main landing gear aft trunnion pin had fractured into two pieces. The forward section of the aft trunnion pin was inside the trunnion; it was held in place by a damaged but intact cross bolt. The aft section of the pin had separated and was located with the beam. The main landing gear beam sustained damage as a result of impact with the main landing gear after the trunnion pin failure. Several support brackets and tie rods also failed, and several support fittings were damaged. The trunnion pin had fractured approximately perpendicular to the length of the pin at midspan. Initial examination of the fracture surface showed that most of the pin fracture faces exhibited a rough, fibrous surface with a dull gray luster. This surface exhibited flow lines consistent with a fracture that progressed from the bottom upward and around the pin cavity. The flow lines emanated from a small thumbnail crack located approximately at the 12:00 position of the forward pin piece fracture surface. This thumbnail crack exhibited diverse colors and crack arrest marks. These features were consistent with pre-existing progressive cracking in this section of the pin. The rest of the fracture surface exhibited mostly river patterns and chevron marks that emanated from the small thumbnail crack. Some limited shear lips were observed in the upper one-half of the fracture along the outer diameter surface. These features were consistent with subsequent overstress fracture during the accident landing. The thumbnail crack dimensions, which were measured with a digital optical microscope, were 0.154 inches deep and 0.275 inches long (the linear length on the outer diameter). A darker region inside the thumbnail crack measured 0.105 inches deep and 0.216 inches long. Examination of a section encompassing the forward piece of the thumbnail crack with a scanning electron microscope found that the chromium electroplated layer had generally remained adhered with no indications of corrosion or pitting. A thin layer of intergranular cracking was present below the chromium electroplating layer. This intergranular cracking area was consistent with the location of the thumbnail crack origin. A series of small ratchet marks were observed within this intergranular region. The intergranular cracking area also exhibited a secondary crack that propagated perpendicular to the fracture surface and had sequential band features consistent with fatigue striations. The remainder of the thumbnail crack exhibited fatigue striations consistent with fatigue crack propagation. There was no indication of mixed-mode features, such as dimpled rupture or intergranular cracking, within the fatigue regions of the darker portions of the thumbnail crack. Outside the thumbnail crack, dimpled rupture features were observed, which were consistent with the final fracture of the overstress fracture that occurred during the accident landing. Overall, the fracture features of the thumbnail crack were consistent with multiple smaller crack initiation sites along the intergranular region. These smaller cracks had coalesced during early crack propagation, as evidenced by the ratchet marks in that portion of the fatigue crack. The initial 0.016-inch portion of the crack consisted of the outer 0.006-inch chromium electroplated layer. This layer of the pin was stripped electrochemically to examine the alloy steel under the chromium in areas away from the fracture initiation point. The stripped aft section of the pin was examined using magnetic particle inspection. This nondestructive inspection revealed longitudinal ladder cracks. A subsequent fluorescent penetrant inspection revealed a similar cracking pattern. The stripped surfaces were then etched. Two areas exhibited darker contrast on the surface, which was consistent with localized elevated iron carbide content and localized over tempering. An area that was about 0.5 inches from the fracture origin and an area that was adjacent to the fracture surface had marks that were consistent with heat exposure and were typical of a previous grinding operation. ADDITIONAL INFORMATIONAccording to the work order for the last trunnion pin overhaul, which was completed by UAL on December 28, 2017, the chromium layer had not been stripped from the trunnion pin before the cadmium electroplating process. Postaccident testing performed at Boeing on the accident pin demonstrated that the underlying cracks could not be detected by magnetic particle inspection when the chromium electroplating layer was present. Fluorescent penetrant inspection was also performed with no indications noted. According to information from Goodrich, which performed the May 2008 overhaul, the company conducts a visual inspection, temper etch, and magnetic particle inspection after stripping and machining the trunnion pin (before shot peening). The company stripped the chromium electroplating layer as part of the overhaul of the accident pin, but the bare pin was not inspected. TESTS AND RESEARCHTo determine how long the fatigue crack had been present, a fatigue crack growth rate analysis was performed by counting striations at intervals along the crack depth. As noted previously, the thumbnail crack exhibited mostly striations, consistent with fatigue crack propagation. This fatigue cracking occurred before the accident, and its total depth was 0.154 inches. The initial 0.016-inch portion of the crack included an internal region with intergranular fracture. The 0.105-inch portion of the crack exhibited fatigue striations and a darker color. The remainder of crack exhibited fatigue striations without discoloration. A field emission scanning electron microscope was used to count multiple areas exhibiting striations and measure the distance of the counted striations. This fatigue growth rate analysis assumed that each counted striation (a feature consistent with crack growth) correlated directly to a landing cycle with no other comparable stresses (such as those from braking or vibrations). The analysis also assumed that the sampled regions represented regions of fatigue crack growth at each crack depth, the fracture surface was flat, and the fatigue striation spacing increased with crack depth. The fatigue growth rate analysis found the following: The total number of landing cycles from the intergranular initiation site to the end of the crack was 6,225 cycles. The total number of landing cycles from crack initiation to the end of the darker portion of the crack was 4,150 cycles. Therefore, the number of landing cycles at the end of the crack (the portion without discoloration) was 2,075 cycles. The number of landing cycles since the last overhaul in December 2017 (2,309 cycles) was similar to the number of cycles for the portion of the fatigue crack without discoloration. Because the number of cycles counted in the portion without discoloration was likely related to the number of landings since the last overhaul, the crack had likely been present for at least 4,150 cycles before the last overhaul.
Maintenance personnel’s excessive grinding of the left main landing gear’s aft trunnion pin during its initial overhaul, which caused heat damage to the base metal and led to the fatigue crack that caused the pin to fail during the accident flight. Contributing to the accident was the failure of maintenance personnel to detect the excessive grinding during the initial overhaul and the fatigue crack during the subsequent overhaul.
Source: NTSB Aviation Accident Database
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