History of Flight
On 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 flight operated under 14 CFR Part 121 as a 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 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 approach was stabilized, and both crewmembers reported a smooth touchdown on the runway centerline.
About 3 to 5 seconds after touchdown, after the thrust reversers had been fully deployed, the first officer felt a shudder and then a left wing down tilt. As the airplane decelerated, the first officer struggled to keep it on the centerline. The airplane came to a stop slightly left of the centerline.
The flight crew determined the left main landing gear had failed and contacted air traffic control to request fire trucks. The captain asked the lead flight attendant about any fire indication; she reported none. Emergency personnel confirmed no external fire, so passengers remained onboard until airstairs arrived. They deplaned via the main cabin door and were bused to the terminal.
Aircraft Information
Each main landing gear uses a trunnion pin connecting the main landing gear beam to the outer cylinder. The trunnion pin was made of type 4340M alloy steel, coated externally with chromium electroplate and internally with cadmium-titanium alloy electroplate.
According to UAL personnel, the pin had accumulated 23,535 total landing cycles since entering service in November 1998. The first overhaul occurred in May 2008 at 10,613 cycles; the second in December 2017 at 21,226 cycles. The airplane accumulated 2,309 cycles between the December 2017 overhaul and the accident, and 12,922 cycles between the May 2008 overhaul and the accident. A Boeing service bulletin and FAA airworthiness directive required overhaul every 10 years or 18,000 landing cycles.
Wreckage and Impact Information
Postaccident inspection found the left main landing gear aft trunnion pin fractured into two pieces. The forward section remained inside the trunnion, held by a damaged cross bolt; the aft section separated. The main landing gear beam was damaged from impact after the trunnion pin failure. Several support brackets and tie rods failed, and support fittings were damaged.
The trunnion pin fractured approximately perpendicular to its length at midspan. The fracture surfaces showed a rough, fibrous, dull gray appearance with flow lines consistent with upward progression from the bottom. A small thumbnail crack at the 12:00 position of the forward pin piece exhibited diverse colors and crack arrest marks, indicating pre-existing progressive cracking. The rest of the surface showed river patterns and chevron marks from the thumbnail crack, with limited shear lips in the upper half, consistent with overstress fracture during the accident landing.
The thumbnail crack measured 0.154 inches deep and 0.275 inches long on the outer diameter. A darker inner region measured 0.105 inches deep and 0.216 inches long.
Scanning electron microscope examination of the forward piece found the chromium electroplate adhered with no corrosion or pitting. A thin intergranular cracking layer existed below the chromium, consistent with the crack origin. Small ratchet marks were visible within this intergranular region, along with a secondary crack with sequential bands consistent with fatigue striations.
The remainder of the thumbnail crack showed fatigue striations consistent with fatigue crack propagation. No mixed-mode features were seen in the darker fatigue regions. Outside the thumbnail crack, dimpled rupture features matched the final overstress fracture.
The fracture features indicated multiple smaller crack initiation sites along the intergranular region that coalesced during early propagation, as evidenced by ratchet marks.
The initial 0.016-inch of the crack included the outer 0.006-inch chromium layer. This layer was stripped electrochemically for examination of the underlying steel away from the fracture origin. Magnetic particle inspection of the stripped aft section revealed longitudinal ladder cracks; fluorescent penetrant inspection showed a similar pattern.
Etching revealed two areas with darker contrast, consistent with localized elevated iron carbide content and overtempering, about 0.5 inches from the fracture origin and adjacent to the fracture surface, typical of previous grinding.
Additional Information
The work order for the last overhaul (December 28, 2017, by UAL) indicated the chromium layer was not stripped before cadmium plating. Postaccident testing at Boeing showed that underlying cracks could not be detected by magnetic particle inspection when chromium was present; fluorescent penetrant inspection also yielded no indications.
Goodrich, which performed the May 2008 overhaul, conducted visual inspection, temper etch, and magnetic particle inspection after stripping and machining (before shot peening). They stripped the chromium layer during that overhaul but did not inspect the bare pin.
Tests and Research
Fatigue crack growth rate analysis counted striations along the crack depth. The thumbnail crack (total depth 0.154 inches) included an initial 0.016-inch intergranular region, a 0.105-inch darker fatigue striation region, and a remaining lighter striation region. The analysis assumed each striation correlated to one landing cycle, with no comparable stresses from braking or vibrations.
The analysis found:
- Total landing cycles from intergranular initiation to crack end: 6,225 cycles.
- Cycles from initiation to end of darker portion: 4,150 cycles.
- Cycles for the portion without discoloration: 2,075 cycles.
Since the cycles since the last overhaul (2,309) were similar to the undarkened portion cycles, the crack was likely present for at least 4,150 cycles before the last overhaul.