United B738 at Denver on Dec 22nd 2019, main gear collapse on landing

Last Update: September 30, 2022 / 19:00:56 GMT/Zulu time

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Incident Facts

Date of incident
Dec 22, 2019



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-800

ICAO Type Designator

Airport ICAO Code

A United Boeing 737-800, registration N87513 performing flight UA-2429 from Newark,NJ (USA) to Denver,CO (USA) with 171 passengers and 7 crew, landed on Denver's runway 17R when sparks became visible from the left main gear. The crew requested emergency services to attend to the aircraft advising they believed they had blown a tyre, or maybe a gear. The aircraft became disabled on the runway resting on the left hand engine cowl, nose gear and right hand main gear. The left main gear, although appearing to have been down, didn't support the aircraft.

Passengers reported as soon as the aircraft touched down sparks became visible from the left main gear. They later disembarked via the right hand front door and stairs.

The airline reported the aircraft experienced a mechanical issue on landing, all passengers were bussed to the terminal.

The FAA reported the gear collapsed on landing, there were no injuries, the aircraft sustained unknown damage.

The aircraft was towed off the runway the following day with the left hand wing being supported by a flat bed semi truck.

On Apr 4th 2020 the NTSB reported the occurrence was rated an accident, the NTSB is investigating. The aircraft suffered a left main gear collapse during the landing roll on Denver's runway 17R.

A possibly related occurrence happened earlier this year, see Accident: Ryanair B738 at Frankfurt on Jan 29th 2019, gear strut penetrates wing during retraction.

On Sep 14th 2021 the NTSB released their preliminary report stating briefly (with passenger and crew numbers in error):

On December 22, 2019, at about 7:00 PM MST, United Airlines flight 2429, a Boeing 737-800, N87513, experienced a left main gear collapse during landing roll on runway 17R at Denver International Airport (KDEN), Denver, Colorado. There were no injuries to the 165 passengers and 6 crew members onboard. The airplane was substantially damaged. The airplane was operating under 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from Newark Liberty International Airport (KEWR), Newark, New Jersey.

On Sep 30th 2022 the NTSB released their final report concluding the probable causes of the accident were:

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.

The NTSB analysed:

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.

Related NOTAMs:
!DEN 12/527 DEN TWY M2, M4, M5, M6, M7, M9, TWY EC BTN TWY M AND TWY P, TWY M10 BTN TWY M AND RWY 17R/35L, TWY A BTN TWY M AND RWY 17R/35L CLSD 1912230625-1912250600
!DEN 12/526 DEN RWY 17R/35L CLSD 1912230605-1912250600
!DEN 12/523 DEN RWY 17R/35L CLSD 1912230341-1912231900
!DEN 12/522 DEN RWY 17R/35L CLSD 1912230208-1912230800

KDEN 230253Z 24007KT 10SM BKN240 07/M12 A3005 RMK AO2 SLP154 T00671117 53002=
KDEN 230153Z 18008KT 10SM BKN220 07/M13 A3005 RMK AO2 SLP155 T00671128=
KDEN 230053Z 17012KT 10SM BKN240 07/M14 A3004 RMK AO2 SLP153 T00721139=
KDEN 222353Z 15010KT 10SM FEW150 BKN220 07/M12 A3004 RMK AO2 SLP156 T00721122 10194 20072 53002=
KDEN 222253Z 12006KT 10SM SCT160 BKN220 12/M10 A3004 RMK AO2 SLP141 T01221100=
KDEN 222153Z 06004KT 10SM SCT150 BKN220 17/M16 A3004 RMK AO2 SLP129 T01671156=
KDEN 222053Z 04003KT 10SM FEW150 BKN220 19/M19 A3004 RMK AO2 SLP127 T01891189 56018=
Aircraft Registration Data
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Incident Facts

Date of incident
Dec 22, 2019



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-800

ICAO Type Designator

Airport ICAO Code

This article is published under license from Avherald.com. © of text by Avherald.com.
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