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Alaska SNA [MLG Failure]
08-23-2023, 04:10 PM
#51
Gets Weekends Off
Joined APC: Feb 2008
Posts: 19,280
Originally Posted by NVR L8
A Trunnion failure is exactly what happened to Sun Country in LAS recently….same results.
08-23-2023, 04:15 PM
#52
Prime Minister/Moderator
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 39,324
Originally Posted by sailingfun
Why is there a good chance of a dead or degraded motor?
Originally Posted by sailingfun
Why would they go around?
But if it was gusty, and the approach or flare wasn't particularly stable to begin with a go around could get called, possibly even before impact. Once it gets called, it's probably going to happen.
I don't think the FAA will issue an AD granting 500 hours on the condition that no go arounds are performed.
08-23-2023, 04:21 PM
#53
Gets Weekends Off
Joined APC: Feb 2008
Posts: 19,280
Originally Posted by rickair7777
The photos I've seen of several events look like the motor hits the pavement. They're not necessarily designed to do that, on the guppy the gear gets there first. Normally.
You wouldn't if your side view cameras showed you exactly what happened. Assuming you had side view cameras and that you were closely monitoring them during flare and touchdown.
But if it was gusty, and the approach or flare wasn't particularly stable to begin with a go around could get called, possibly even before impact. Once it gets called, it's probably going to happen.
I don't think the FAA will issue an AD granting 500 hours on the condition that no go arounds are performed.
You wouldn't if your side view cameras showed you exactly what happened. Assuming you had side view cameras and that you were closely monitoring them during flare and touchdown.
But if it was gusty, and the approach or flare wasn't particularly stable to begin with a go around could get called, possibly even before impact. Once it gets called, it's probably going to happen.
I don't think the FAA will issue an AD granting 500 hours on the condition that no go arounds are performed.
08-23-2023, 04:30 PM
#54
Prime Minister/Moderator
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 39,324
Originally Posted by sailingfun
I just don’t see a crew executing a go around after a partial gear collapse.
But if you're unstable, catch a bad gust, or a shear that could result in a hard arrival with a go-around called properly before impact. So the same conditions that might stress the hypothetical trunnion flaw are also likely to trigger a go-around call. If it gets called, 95% it's going to happen, even if touchdown occurs (like we're trained).
Aborting an in-process go-around is not in our DNA.
08-23-2023, 05:13 PM
#55
On Reserve
Joined APC: Dec 2022
Posts: 19
Originally Posted by sailingfun
It was a very different result. The gear rotated back below 100 knots as usually happens with a Trunnion failure but did not collapse. There was no damage to the wing.
Last edited by NVR L8; 08-23-2023 at 05:20 PM. Reason: Added text
08-23-2023, 05:18 PM
#56
Gets Weekends Off
Joined APC: Feb 2008
Posts: 19,280
Originally Posted by NVR L8
That is absolutely incorrect. The strut went right up through the wing and the right engine ended up riding on the pavement. The jet was scrapped because of the extensive damage to the wing. Believe it or not, the engine was fine.
08-23-2023, 05:43 PM
#59
Ignoring; John Burke.
Joined APC: Jul 2022
Position: Wandering
Posts: 195
Sounds nearly identical
Accident: United B738 at Denver on Dec 22nd 2019, main gear collapse on landing
By Hradecky, created Monday, Dec 23rd 2019 14:16Z, last updated Friday, Sep 30th 2022 19:00Z
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.
Accident: United B738 at Denver on Dec 22nd 2019, main gear collapse on landing
By Hradecky, created Monday, Dec 23rd 2019 14:16Z, last updated Friday, Sep 30th 2022 19:00Z
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.
08-24-2023, 05:37 AM
#60
Gets Weekends Off
Joined APC: Jan 2023
Posts: 1,236
Originally Posted by sailingfun
Hard landings are way up industry wide. Most of them never make the news. United alone has lost 3 airframes recently to hard landings. It’s a discussion item at high levels in airline management. I made no specific comment on this accident. I stated major airlines. I am not on the 737 but I have a lot of time flying the aircraft. The hard landing issues are not fleet specific. The one issue in many seems to be limited experience and a perhaps checkered training background. Before you jump on that statement I have zero knowledge of the background of the pilots in this accident. It’s a bit of a sensitive subject for me because the closest I came to damaging an aircraft was a copilot that turned out to have required extensive extra training in both their aircraft checkouts. Prior to the last three years I would not have had to deal with the issue as the pilot in question would have been washed out. We need as a collective industry to stop lowering performance minimums.
I'm glad everyone is ok and I am not monday morning QB'ing the Alaska crew. Yes, I have flown the 73 and yes I've landed at SNA many times - in 3 different types of aircraft. Might it be the trunnion pin? IDK. Lets' wait 2-3 years for the final NTSB report I guess.
Last edited by Hotel Kilo; 08-24-2023 at 05:56 AM.
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