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A380 Gear Retraction - Potential Danger?  
User currently offlineFlexo From St. Helena, joined Mar 2007, 406 posts, RR: 0
Posted (6 years 4 months ago) and read 7391 times:

As many of you will of course already know, the A380 uses a quite unique system to retract the rear main landing gear. Instead of folding inwards it gets retracted upwards into the bay.

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Now, in the past there have been a couple of incidents where the landing gear was retracted while the aircraft was on the ground. Due to the design of the inward folding main gear the aircraft's weight prevented retraction there so that it was always only the front gear that retracted then, limiting the damage.

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So what would happen to the A380 if the gear was retracted on the ground? It seems that the upward moving main gear could be retracted even on the ground. What safety measures are in place to prevent that?

26 replies: All unread, showing first 25:
 
User currently offlineGST From United Kingdom, joined Jun 2008, 934 posts, RR: 0
Reply 1, posted (6 years 3 months 4 weeks 1 day 23 hours ago) and read 7356 times:

Only the inboard gear legs retract vertically, the outer gear does infact have a conventional inwards folding mechanism. The A380 is certified to land normally & safely with only half of the main gear legs (presumably one on each side  Wow! ) deployed. Therefore if the main gear were to be retracted on the ground, the upwards retracted gear would raise up, but the outer gear would not be able to, and is still capable of taking the aircraft's weight.

User currently offlineFlexo From St. Helena, joined Mar 2007, 406 posts, RR: 0
Reply 2, posted (6 years 3 months 4 weeks 1 day 22 hours ago) and read 7361 times:



Quoting GST (Reply 1):
The A380 is certified to land normally & safely with only half of the main gear legs

Wow, I didn't know that!
Is it normal to add this amount of redundancy to civil aircraft? It seems like a huge weight penalty to carry an extra set of main gears just "to have some extra in case one fails".


User currently offlineGST From United Kingdom, joined Jun 2008, 934 posts, RR: 0
Reply 3, posted (6 years 3 months 4 weeks 1 day 22 hours ago) and read 7360 times:

Dont think of it as a weight penalty. Though the aircraft can land on two main gear legs a number of times, I'll bet it isnt designed to do that too frequently and so eventually it would lead to damage to the gear legs and the fuselage structure around them.

User currently offlineKalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 4, posted (6 years 3 months 4 weeks 1 day 20 hours ago) and read 7293 times:



Quoting Flexo (Thread starter):
ue to the design of the inward folding main gear the aircraft's weight prevented retraction there so that it was always only the front gear that retracted then, limiting the damage.

from purely mechanical perspective, it's not inward folding what ensures inability to fold while on the ground (especially for a single wheel design) - it's how that folding is implemented.
And one can easily envision something similar for vertical retraction as well - e.g. something like those retractable ballpoint pens. Part has to move out (down) before being able to retract, and that can provide adequate protection


User currently offlineFlexo From St. Helena, joined Mar 2007, 406 posts, RR: 0
Reply 5, posted (6 years 3 months 4 weeks 1 day 20 hours ago) and read 7279 times:



Quoting Kalvado (Reply 4):
And one can easily envision something similar for vertical retraction as well - e.g. something like those retractable ballpoint pens. Part has to move out (down) before being able to retract, and that can provide adequate protection

I see where you are going with that, is that part of the actual gear design of the A380 or merely a hint of what it could look like?


User currently offlineKalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 6, posted (6 years 3 months 4 weeks 1 day 19 hours ago) and read 7272 times:



Quoting Flexo (Reply 5):
, is that part of the actual gear design of the A380 or merely a hint of what it could look like?

I'm not privileged enough to see any drawings from A or B.. It's more like engineering common sense, IMHO.


User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 7, posted (6 years 3 months 4 weeks 1 day 18 hours ago) and read 7217 times:

Actually there is / was a safety issue with the landing gear during the free fall tests. The main gear would get stuck on the doors as they tried to open deploy. Even after the modifications they still got stuck. Speaking of that.....what happened to the multi part PBS special that was being done on the A380. I only saw the first 2 and no more, nor did I see anymore adverts. Did Airbus pull the plug when all the delays started..?

[Edited 2008-06-28 08:11:49]


"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineGST From United Kingdom, joined Jun 2008, 934 posts, RR: 0
Reply 8, posted (6 years 3 months 4 weeks 1 day 17 hours ago) and read 7199 times:



Quoting EMBQA (Reply 7):
The main gear would get stuck on the doors as they tried to open deploy.

I believe that was only the conventionally inward folding outer gear. And besides, they established that when the aircraft is in flight the airflow around the gear will stop it jamming when opening for some reason. Regardless, I do believe there was some fix Incorporated into the design of the undercarriage for this for the production aircraft.


User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 9, posted (6 years 3 months 4 weeks 1 day 17 hours ago) and read 7187 times:



Quoting Kalvado (Reply 4):
from purely mechanical perspective, it's not inward folding what ensures inability to fold while on the ground (especially for a single wheel design) - it's how that folding is implemented.

Friction is the reason that the inward folding gear can not be retracted on the ground. The retract actuator can not over come the friction between the ground and the tires.


User currently offlineSmeg From United Kingdom, joined Feb 2008, 159 posts, RR: 0
Reply 10, posted (6 years 3 months 4 weeks 1 day 15 hours ago) and read 7109 times:



Quoting GST (Reply 8):
I believe that was only the conventionally inward folding outer gear. And besides, they established that when the aircraft is in flight the airflow around the gear will stop it jamming when opening for some reason. Regardless, I do believe there was some fix Incorporated into the design of the undercarriage for this for the production aircraft.

Absolutely correct!

From what I remember, the initial static test failed on the "folding" gear. The weight of the gear was supposed to force the doors open, but there was a point where the tyres got hung up and stuck on the door. The initial fix for the static test was to simply place a shedload of grease on the inside of the landing bay door so that the tyre would still be able to force the door open!  Wow! (I kid you not, and it worked like a charm)

Then once it was established that that it was only a very minor change that was needed, from what I remember, they "tweaked" the inside of the door.

As GST stated, they were confident however that once airborne, the airflow would ensure that the landing gear would drop under gravity even without the "tweaks"

Also I think that the "upward" moving gear (rather than the folding gear) have to move down first before they can move up, thus preventing them from being retracted on the ground.


User currently offlineKalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 11, posted (6 years 3 months 4 weeks 1 day 14 hours ago) and read 7107 times:



Quoting 474218 (Reply 9):
Friction is the reason that the inward folding gear can not be retracted on the ground.

well, it's a design feature of making friction work for you, not the idea of inward folding itself. Just think of aircraft standing on half-folded gear. Friction probably wouldn't prevent further folding. It's still same friction, though.

My point is that it's not retraction direction that keeps it safe, but engineering implementation properly using forces in a system (e.g. friction and/or aircraft weight).


User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 12, posted (6 years 3 months 4 weeks 1 day 11 hours ago) and read 7035 times:



Quoting GST (Reply 1):
The A380 is certified to land normally & safely with only half of the main gear legs (presumably one on each side ) deployed.

I'm quite certain that the 747 is certified as such as well. I definitely remember seeing a video of a United 747 (744/) - in the old orange colour scheme - performing an emergency landing with the wing gears extended only. I seem to remember it may have been at SFO. Does anyone else know of this, or is it a figment of my imagination? I have Googled without success to find the video.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineFr8mech From United States of America, joined Sep 2005, 5499 posts, RR: 14
Reply 13, posted (6 years 3 months 4 weeks 1 day 11 hours ago) and read 7035 times:



Quoting Flexo (Thread starter):
quite unique system to retract the rear main landing gear. Instead of folding inwards it gets retracted upwards into the bay.

Not unique. B747 body gear retracts forward.

Not sure about the A380, but the B747 body will retract on the ground...and given the correct weight distribution, the aircraft will promptly sit on its tail, if the nose doesn't retract.

Does the A380 body gear sit forward of aft of the wing gears? If the body gear sits aft, then its possible that the A380 would also sit on its butt.



When seconds count...the police are minutes away.
User currently offlineGST From United Kingdom, joined Jun 2008, 934 posts, RR: 0
Reply 14, posted (6 years 3 months 4 weeks 1 day 11 hours ago) and read 7019 times:



Quoting Fr8mech (Reply 13):
Does the A380 body gear sit forward of aft of the wing gears? If the body gear sits aft, then its possible that the A380 would also sit on its butt.

It also sits aft of the wing gear on the A380.


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 15, posted (6 years 3 months 4 weeks 1 day 2 hours ago) and read 6881 times:

What would be the pitch of the A380 while on Ground.would 1deg nose down pitch be accurate.
regds
MEL



Think of the brighter side!
User currently offlineMoo From Falkland Islands, joined May 2007, 4021 posts, RR: 4
Reply 16, posted (6 years 3 months 4 weeks 23 hours ago) and read 6827 times:



Quoting EMBQA (Reply 7):
The main gear would get stuck on the doors as they tried to open deploy. Even after the modifications they still got stuck.

The modifications were only temporary - they did fix the problem later on, before the aircraft went into service.

Quoting EMBQA (Reply 7):
Speaking of that.....what happened to the multi part PBS special that was being done on the A380. I only saw the first 2 and no more, nor did I see anymore adverts. Did Airbus pull the plug when all the delays started..?

I guess not, since the first airing of it was after the first delay. Its still shown periodically here on the Discovery Channel.


User currently offlineCeph From Singapore, joined Jun 2007, 141 posts, RR: 0
Reply 17, posted (6 years 3 months 4 weeks 20 hours ago) and read 6784 times:



Quoting GST (Reply 8):
I believe that was only the conventionally inward folding outer gear. And besides, they established that when the aircraft is in flight the airflow around the gear will stop it jamming when opening for some reason. Regardless, I do believe there was some fix Incorporated into the design of the undercarriage for this for the production aircraft.

I believe that it was a Teflon coating on a ramp inside the gear door that solved the problem.


User currently offlineFilton From United Kingdom, joined May 2006, 51 posts, RR: 0
Reply 18, posted (6 years 3 months 1 week 6 days 2 hours ago) and read 6116 times:

The A380, like all Airbus aircraft, has a solenoid that locks the gear handle in the down position while on the ground to prevent inadvertent retraction. It only unlocks after several inputs indicate that the aircraft is flying.

User currently offlineA10WARTHOG From United States of America, joined Jul 2004, 325 posts, RR: 0
Reply 19, posted (6 years 3 months 1 week 5 days 16 hours ago) and read 6040 times:



Quoting Filton (Reply 18):
The A380, like all Airbus aircraft, has a solenoid that locks the gear handle in the down position while on the ground to prevent inadvertent retraction. It only unlocks after several inputs indicate that the aircraft is flying.

ERJ have the same thing, but you can overide the feature. Is that the same on Airbus?


User currently offlineGST From United Kingdom, joined Jun 2008, 934 posts, RR: 0
Reply 20, posted (6 years 3 months 1 week 5 days 15 hours ago) and read 6014 times:



Quoting A10WARTHOG (Reply 19):

It would have to be as aircraft are required to undergo periodic gear retraction tests on jacks. You can bet the sensors wont classify that as flight!


User currently offlineFilton From United Kingdom, joined May 2006, 51 posts, RR: 0
Reply 21, posted (6 years 3 months 1 week 5 days 2 hours ago) and read 5881 times:



Quoting A10WARTHOG (Reply 19):
you can overide the feature. Is that the same on Airbus?

There is a procedure going through the MCDU to test the gear lever, but the gear will still stay down. When it's on jacks there is another procedure to raise the gear but it is fairly straight forward as the sensors indicate wieght off wheels.


User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 22, posted (6 years 3 months 1 week 5 days 2 hours ago) and read 5880 times:



Quoting GST (Reply 20):
It would have to be as aircraft are required to undergo periodic gear retraction tests on jacks. You can bet the sensors wont classify that as flight!

This reminds me of a gear retraction test we once did on a 767. The plane was all jacked up ready to go with power on. Gradually, we noticed a burning smell, and shortly thereafter, smoke emanating from the drain mast.

Apparently, the amount of power required by the heating element to prevent the mast freezing includes the cooling effect of the slip stream, which is not present in the hangar!


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Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 23, posted (6 years 3 months 1 week 5 days 1 hour ago) and read 5869 times:



Quoting JetMech (Reply 22):
This reminds me of a gear retraction test we once did on a 767. The plane was all jacked up ready to go with power on. Gradually, we noticed a burning smell, and shortly thereafter, smoke emanating from the drain mast.

Apparently, the amount of power required by the heating element to prevent the mast freezing includes the cooling effect of the slip stream, which is not present in the hangar!

Shouldn't there be an Electrical switching from 115vAc to 28vDc when jacked up by the Air-Gnd sense to avoid excessive current flow to the Heating elements to avoid overheat & personell injury...aka B737.

regds
MEL



Think of the brighter side!
User currently offlineAnthob From United Kingdom, joined Jul 2005, 18 posts, RR: 0
Reply 24, posted (6 years 3 months 1 week 4 days 23 hours ago) and read 5839 times:

The drag brace which is the mechanical linkage going forward (may be rearwards on the A380) from the main leg is hinged in the middle. This hinge is over centre with the gear locked down so with the weight on the wheels you would have to overcome the weight to break the over centre lock. Purely a mechanical safeguard. Also if you are on the ground doing anything with the gear you should have the downlock pins in.

25 A10WARTHOG : It might not count it has a flight, but the gear will retract normally, since the weight on wheel would have an air signal. ERJ you could pin the gea
26 JetMech : G'day Mel, I'm really not too sure on this one, as I never really had the inclination to learn about any of the electrical distribution systems. All
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