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BA 787 Nosewheel collapse @ LHR

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BA 787 Nosewheel collapse @ LHR

Old 18th Jun 2021, 22:06
  #41 (permalink)  
 
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dixi188

Boeing did a lot of dumb things during the 787 development, and one of the dumbest was - in order to encourage 'out of the box' thinking and solutions - they took a blank sheet of paper design outlook without bothering to reference the many decades of "lessons learned" experience. While there are certainly gains to be made with 'out of the box thinking', they way they did it resulting in countless cases of 'what the were they thinking?' design errors. A few well known examples: A structural design error that resulted in a last minute delay of six months in first flight - the design error was known and well understood 'trap' in composite design (the military side of Boeing knew all about it but no one on the 787 bothered to contact them). The L/R fire system hookups were not Murphy proofed - resulting in several aircraft being delivered with the systems switched. The APU Controller (APUC) was designed by a group that had minimal experience with turbine engine control systems - resulting in an APU that was horribly unreliable at EIS - and for years after.
I was involved in a design audit of the APUC about five years ago (at which time they were on the tenth major redesign of the APUC) and I literally had a hard time believing what I was hearing. Some of the design decisions simply defied any sort of logic. Just one example - most APUs use either aircraft power or a dedicated alternator to power the APUC. Modern turbofan FADEC systems have redundancy - a dedicated alternator normally powers the FADEC, but if the alternator fails, the FADEC will switch over to aircraft power for the remainder of the flight (not normally dispatchable that way). So the APUC design team decided to use both a dedicated alternator and aircraft power - but the way they did it didn't provide redundancy, the APUC needed both! Loose the alternator or aircraft power, and the APU shutdown! WTF?

Many of the managers who oversaw the 787 development then took their management technics to the KC-46 development. We all know how well the turned out...
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Old 18th Jun 2021, 22:17
  #42 (permalink)  
 
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To be fair to Boeing, they could be forgiven for not foreseeing that anybody would think sticking a pin through the middle of a hollow pivot bolt (if indeed that's what happened) was going to stop anything moving ...
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Old 18th Jun 2021, 22:59
  #43 (permalink)  
 
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John4321

Because for getting on 18 months many of us who do this type of work (Engineers), are currently stood down/redundant/unemployed/left the industry so not as many manhours available to do the work.

Also, mods like this tend to get done on a planned check in the hangar. If the aircraft is not flying as much and using hours it isn't reaching the hours limit for the check, so checks are getting further apart on a calendar basis. Also, if the aircraft had been put into parking/storage mode at some point, usually no maintenance is carried out apart from those requirements under the parking check regime.

Having said that, if it is a simple mod, no reason it can't be done on the line with a bit of planning, which is probably what will happen now if this is the root cause!
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Old 18th Jun 2021, 23:31
  #44 (permalink)  
 
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SQ had a nose gear retraction on an A330 at the gate a few years ago during a maintenance check. Another possible explanation would be damage from a hard landing and the structure fails under the stress of loading. This is less likely as FDM would flag up a landing sufficient to cause the required damage and an inspection would be carried out.

https://www.straitstimes.com/singapo...etracts-during
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Old 19th Jun 2021, 01:54
  #45 (permalink)  
 
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Being picky, the AD says do it within the times specific and they are specified in the RB, the RB says 36 months from the issuance of ISS 01 of said RB, that was the 12 March 2019, so should really say three years from March 2019.

So think whoever wrote the safety alert may wish to go re-visit the dates. Not that it would have changed much if indeed this was the root cause of the incident.

Last edited by Station Zero; 19th Jun 2021 at 02:50.
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Old 19th Jun 2021, 04:22
  #46 (permalink)  
 
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Aircraft sitting on a gate ready for dispatch do not normally have gear down locks fitted. They are usually only required for testing/maintenance and towing.
The fact that the nosegear has tried to retract on ground points to either a cmc test being run (or other maint or retraction selection) without downlocks fitted, or some serious system malfunction that may cause this to happen. Unless a tractor/tug has tried to drive off in a hurry without disconnecting.
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Old 19th Jun 2021, 06:28
  #47 (permalink)  
 
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Ngineer

"Aircraft sitting on a gate ready for dispatch do not normally have gear down locks fitted. They are usually only required for testing/maintenance and towing."

The aircraft had indeed been towed onto the gate.

See earlier posts.
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Old 19th Jun 2021, 06:42
  #48 (permalink)  
 
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Stupid question, but don't A/C have 'weight on wheels' switches anymore to prevent undercarriage retraction on the ground ?
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Old 19th Jun 2021, 07:44
  #49 (permalink)  
 
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Originally Posted by DaveReidUK View Post
To be fair to Boeing, they could be forgiven for not foreseeing that anybody would think sticking a pin through the middle of a hollow pivot bolt (if indeed that's what happened) was going to stop anything moving ...
Iím not so sure. That task is repeated regularly around the World 24/7/365; if it is possible to do it wrong, it will happen. Thatís the point of good mechanical design: to make it very hard to have parts in the wrong configuration.

Thinking about it, we donít know how many times the pin has been put in the wrong hole as you wouldnít find out about it unless a) you ran a systems test that actually would retract the gear on the ground without a pin or b) the person who removed it noticed that it wasnít right. The setup has to be foolproof enough that a short person who is not that familiar with the type can reach in on a dark night and put the pin in the right place, and I donít think the current arrangement passes this test, hence the AD...
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Old 19th Jun 2021, 07:50
  #50 (permalink)  
 
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Flypro

Yes but they can normally be overridden, for testing etc.
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Old 19th Jun 2021, 07:50
  #51 (permalink)  
 
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Merely SLF, albeit an enthusiast of 50+ years standing, but how would a weight on wheels switch prevent collapse due to (say) hydraulic pressure dropping over time when engines not running?
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Old 19th Jun 2021, 07:54
  #52 (permalink)  
 
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Yes they do, but they also have extensive built in test systems (BITE) that enable engineers to simulate the in air condition and thereby override the squat switches. No need to go round fitting slugs to the prox switches when you do this.
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Old 19th Jun 2021, 08:25
  #53 (permalink)  
 
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Airbanda

"Merely SLF, albeit an enthusiast of 50+ years standing, but how would a weight on wheels switch prevent collapse due to (say) hydraulic pressure dropping over time when engines not running?"

Once a gear is down and locked, there is no need for hydraulic pressure to be maintained on the extension jacks. Geometry is sufficient.
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Old 19th Jun 2021, 08:38
  #54 (permalink)  
 
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tdracer

I don't work on 787s very often, but speaking to people who do all the time say that the aircraft was designed with no input from the engineers who maintain the thing on a day-to-day basis. Jobs that would likely take 1-2 hours on previous types, can often take double that.
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Old 19th Jun 2021, 09:34
  #55 (permalink)  
 
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DaveReidUK

You are missing the point, with the aircraft on gate or in the hangar (ie stationary with no tow in progress) there is no point for having the Downlock pin fitted. The gear is down and locked. And the pins are removed after the tow, they do not take off with them fitted you know.

To say that this incident solely happened as the pin was fitted in the wrong point, without further explanation as to why the gear actually retracted, is a very incomplete explanation of this incident.

Obviously the aircraft had been towed, or did taxi onto gate. But are you saying that the tow commenced without gear pins fitted correctly, and the NLG collapsed with the tow bar and tractor connected during the tow? I see no evidence of this.

I am rated on the B787, and yes I have towed them onto gate many times. The first thing you do after you tow onto gate is remove the gear pins. The 787 is a very complex aircraft, you cannot second guess it, or take any shortcuts when carrying out any maint activity. And that is all I will say without speculating as to what actually happened here.

Last edited by Ngineer; 19th Jun 2021 at 10:03.
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Old 19th Jun 2021, 10:12
  #56 (permalink)  
 
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Ngineer

"Obviously the aircraft had been towed, or did taxi onto gate. But are you saying that the tow commenced without gear pins fitted correctly, and the NLG collapsed with the tow bar and tractor connected during the tow? I see no evidence of this."

The AAIB are on the scene. It might be best to leave the investigation, analysis and explanation to them.

My comment was purely factual - the aircraft didn't taxi onto the gate, it was towed. As to whether or not that will turn out to be relevant to what happened subsequently, I'm not about to try and guess.
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Old 19th Jun 2021, 13:19
  #57 (permalink)  
 
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The AAIB are on the scene. It might be best to leave the investigation, analysis and explanation to them.

Quite so. But the very purpose of this forum, is to discuss Rumours. We can all wait a year for the
full facts, but our interest and entertainment is what we read above, isn't it?
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Old 19th Jun 2021, 16:27
  #58 (permalink)  
 
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Yep, system safety at its finest. They would have the "AD" out quicker if there was a dead body under that fuselage !
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Old 19th Jun 2021, 19:19
  #59 (permalink)  
 
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Not 787 rated but I guess the design of the gear lock pins is pretty universal. Do they depressurize the hydraulics in the retract lines or do they physically prevent the down lock from moving (with hyd. pressure on)?

What kind of test would the engineers possibly do with crew and passengers onboard that would pressurize the landing gear retract cycle?
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Old 19th Jun 2021, 19:31
  #60 (permalink)  
 
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It was preparing for a cargo flight there were no passengers on board.
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