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BA 787 Nosewheel collapse @ LHR

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BA 787 Nosewheel collapse @ LHR

Old 23rd Jun 2021, 09:56
  #101 (permalink)  
 
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Stupid question from SLF - is there not a sensor on the landing gear that prevents retraction while there is weight on it?
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Old 23rd Jun 2021, 10:32
  #102 (permalink)  
 
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Yes, but it can be overridden for the purposes of function checks. Hence why steel ground lock pins are fitted to prevent the gear from moving.
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Old 23rd Jun 2021, 10:38
  #103 (permalink)  
 
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See post #62 which I think suggests that the 'Weight on Wheels' interlock would have been disabled as part of the test.
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Old 23rd Jun 2021, 10:52
  #104 (permalink)  
 
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In all of 40+ years in maintenance I have put a parked aircraft in air mode perhaps as many times, the electronic wizards nearly daily for auto pilot checks with hydraulics on, older airframes, but if I had anything to do with it I preferred with a tug and tow bar attached and all gears pinned and chocked, but none of these required the gear selected up!
Used the lock solenoid over-ride button on the gear selector panel, perhaps 20 times mostly for unlock indication faults, again hydraulics not needed.
Aircraft are routinely pushed back and towed with only the steering lock-out pin fitted; though I recall on B727's incidents when this was not enough to stop a tow bar shear pin event, if the FE selected hydraulics or started engine motoring; so it begs the question if this was a maintenance caused incident:- what was going on?
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Old 24th Jun 2021, 08:35
  #105 (permalink)  
 
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As a matter of interest, how do gear lock pins work?

Do they operate an electric switch; preventing fluid flow? Do they operate an hydraulic valve; preventing fluid flow? Do they prevent movement of a side stay, which in turn prevents hydraulic pressure being applied to the main jacks?* Or do they physically withstand the full force of 3000psi hydraulic pressure?

*Airbus A320 main gear locks consist of a sleeve which prevents retraction of a side-stay ram. So they must withstand full hydraulic force. The nose gear is a pin, if I recall.
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Old 24th Jun 2021, 08:46
  #106 (permalink)  
 
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Good explanation, with pics.... posting #63
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Old 24th Jun 2021, 09:06
  #107 (permalink)  
 
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So... there is a hydraulic valve kept shut until the side stay moves? By locking the side stay - which presumably has much less pressure fed to it - a small pin can prevent activation of the main jack?
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Old 24th Jun 2021, 10:13
  #108 (permalink)  
 
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The Lock pins physically prevent the side brace/drag brace from moving. The side/drag brace forms a geometric lock that prevents the gear collapsing even without hydraulic pressure applied. The A320 uses sleeves around the down lock actuator that prevents the side stay from moving.
When the gear is selected 'up' on the ground, hydraulic pressure is still applied to the actuators but they are prevented from moving by the pins and/or sleeves.
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Old 24th Jun 2021, 14:53
  #109 (permalink)  
 
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Uplinker

"Do they operate an electric switch; preventing fluid flow? Do they operate an hydraulic valve; preventing fluid flow? Do they prevent movement of a side stay, which in turn prevents hydraulic pressure being applied to the main jacks?* Or do they physically withstand the full force of 3000psi hydraulic pressure."

None of the above.

See the response in post #63 to the previous poster who asked the same question.
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Old 24th Jun 2021, 22:13
  #110 (permalink)  
 
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It was mentioned earlier in the thread, but to prevent rehashing the issue, the hollow pivot bolt necks down internally and can accept the lockpin fairly snugly.

But - the 777 has this exact same configuration, with a hollow pivot bolt right next to the lockpin location. (Sorry, can't find a good photo online.) I'm curious why this has not been an enough of an endemic issue on the 777 to issue an AD. There have certainly been inadvertent 777 nose gear retractions that you can find on Google.

Last edited by slacktide; 24th Jun 2021 at 23:21.
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Old 25th Jun 2021, 02:02
  #111 (permalink)  
 
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Image posted on Reddit by @jonsky7

Last edited by Redlands; 25th Jun 2021 at 03:16.
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Old 25th Jun 2021, 06:53
  #112 (permalink)  
 
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slacktide

"It was mentioned earlier in the thread, but to prevent rehashing the issue, the hollow pivot bolt necks down internally and can accept the lockpin fairly snugly.

But - the 777 has this exact same configuration, with a hollow pivot bolt right next to the lockpin location. (Sorry, can't find a good photo online.) I'm curious why this has not been an enough of an endemic issue on the 777 to issue an AD. There have certainly been inadvertent 777 nose gear retractions that you can find on Google."

The gear pin is a bog-standard pip pin that any engineer from the last 50 years or more would recognise. It is indeed the same part number for the 777 and 787:



Judging from the photo of the nose bay, I suspect that a "snug fit" inside the (larger diameter) pivot bolt would only occur if the pin were inserted up to and including the shoulder.

It would (should?) then be obvious to the user that the pips were not engaged with anything and that the pin could be pulled out without having to depress the button to release it.

While I agree that it's not 100.00% Murphy-proof, any engineer or ground handler who doesn't understand something so basic is in the wrong job.

(with the caveat that the "pin in the wrong hole" scenario just a theory so far and has yet to be confirmed by the AAIB)
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Old 25th Jun 2021, 09:56
  #113 (permalink)  
 
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The 'offending stepped hole' and the mod.
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Old 25th Jun 2021, 12:08
  #114 (permalink)  
 
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A modified version.
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Old 25th Jun 2021, 12:54
  #115 (permalink)  
 
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Being a fan of that Murphy guy:
- The upside of the RBF flag on this pin is: it lowers the chance of going airborne with a pin locking down the gear.
- The downside of it is: a cursory look will confirm that a pin is in place, when it is in fact in the hole next door, and above happens.
Doing a traffic circuit with gear locked down might be the cheaper option .
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Old 25th Jun 2021, 15:47
  #116 (permalink)  
 
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Question from the peanut gallery

How much leeway is there in pinning/unpinning landing gear between operators? Might some pin the gear only when necessary (towing, maintenance) and others pin the gear and leave it until pushback and uncoupling the tug?

How might this situation be exacerbated by new hiring to fill vacancies created by Covid?
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Old 25th Jun 2021, 23:52
  #117 (permalink)  
 
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I have to say, the incorrect reinforced looking hole does look like it should be the one to be used. The correct one looks quite insignificant in comparison! I can’t believe that it was not marked properly. Very poor engineering from Boeing…. 🤭
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Old 26th Jun 2021, 09:46
  #118 (permalink)  
 
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BDAttitude

Yes. But a 788 departing at max weight still has to burn off/dump 55 tonnes before it's light enough to land. (77W about 100 tonnes)
so it's a pretty big circuit!
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Old 26th Jun 2021, 10:05
  #119 (permalink)  
 
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EEngr

From my experience the pins are only used for maintenance tests as required or for long term parking. Even towing between stands these days seems to be operator choice. Some insist, some just want the nose pin, some directly mandate that they're not used for towing at all to avoid any unpleastness. The risk of them being left in out ways the risk of a gear collapse during towing.
If you are only ever working with one operator then there is no real excuse for not knowing their procedure. If you are working multiple airlines and aircraft types then it is more likely that an incorrect procedure could be used.
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Old 30th Jun 2021, 13:42
  #120 (permalink)  
 
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Deepinsider:
Yes and quite expensive.. But not as expensive as having the NLG retract at the gate
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