Airbus A380 Undercarriage System
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Airbus A380 Undercarriage System
With such a huge airplane, I'm very surprised to learn that the A380 doesn't have any way of steering the aft body gear, apparently it's fixed straight ahead!
With a very similar geometric layout to the B747 whose the body gear steering works very well and enables the airplane to maneuver into/around some smallish places, not to mention the very small lineup allowance it has, taking into account the size of it!
The horrendous side loading stress of the A380 main struts/mounts/wheels during any ground maneuvering will surely cause structural problems, not to mention the tearing up of the runway/taxiway surface, followed by the injestion of that loose material into the low slung engines of B737 and B757 types!
Tyre sidewall deterioration is always a problem with airplane tyres, but with some of the regular deformation required to turn this very heavy airplane in service, I can see some big problems with reliability of A380 wheels, and at high speed, we all know what that can do!!
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With a very similar geometric layout to the B747 whose the body gear steering works very well and enables the airplane to maneuver into/around some smallish places, not to mention the very small lineup allowance it has, taking into account the size of it!
The horrendous side loading stress of the A380 main struts/mounts/wheels during any ground maneuvering will surely cause structural problems, not to mention the tearing up of the runway/taxiway surface, followed by the injestion of that loose material into the low slung engines of B737 and B757 types!
Tyre sidewall deterioration is always a problem with airplane tyres, but with some of the regular deformation required to turn this very heavy airplane in service, I can see some big problems with reliability of A380 wheels, and at high speed, we all know what that can do!!
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A380 MLG
A380 is equiped with the 3rd axel steerable in both Central Main Landing gear (Body LG) like the B777 MLG.
However Differently from the B777 3rd axel is not equiped of brakes.
However Differently from the B777 3rd axel is not equiped of brakes.
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Timeous post Flight Detent. I'm in the middle of designing a runway for the A380.
All the runways, taxiways and aprons taking A380s will have surfacings of asphalt (asphaltic concrete) or concrete. For accountants, these are black or white respectively. The asphalt surfacing will be at least 100mm and probably 125mm thick. In most countries (probably all of Europe, N & S America, Asia) it will have another layer of asphalt 150-250mm thick beneath it, and other materials below that. The concrete surfacings will be 350-400mm thick, and are not going to break.
The likelihood of damage to those asphalt surfacings under normal ops is fairly small. I was looking at a road engineering presentation the other day, and saw (on a steep hill carrying road trains) a horizontal force of 6 kN from the drive tyre had caused delamination of the thin asphalt surfacing layer. This layer was only 50mm thick, and had the unusual defect of being without proper bonding to the lower layer. That corresponds to a horizontal shear stress of approximately 650 kPa in the asphalt layer which eventually caused something like fatigue cracking. The asphalt didn't tear out in chunks, but eventually looked cracked and aged.
I calculated the horizontal shear stresses under medium aircraft braking, based on the earlier thread about the 747 classic and brakes, and assuming it will be stopping at 6 ft/sec/sec (don't know what deceleration maximum braking gives - any thoughts welcome). This equates to a horizontal force per tyre of 45 kN in the A380 case, assuming the same medium braking deceleration rate and at MTOW. That in turn is a horizontal shear stress of 780 kPa in a 125mm thick asphalt layer. If the asphalt is not bonded to the layer beneath, and exactly the same point on the runway gets stressed each time, the failure mode is again going to be delamination and fatigue cracking. That will only be after a few thousand movements over exactly the same spot, which is going to be rare, and that will be detected during frequent runway inspections. I would not expect to see big chunks of surfacing tearing out. If anything does happen, it would only lead to some localised surfacing patches and a general resurfacing after 5-8 years instead of the usual 8-12 years.
To get big chunks tearing out, the asphalt has to break or tear. This means shear failure, and the shear capacity of the asphalt layer is an order of magnitude above these stresses. I simply do not think it is going to tear, and that was echoed when I called a senior colleague just now, who has surfaced more than a few 747 airports with asphalt.
I know of a couple of failures where chunks of asphalt have torn out and been sucked up by aircraft in the past, but the surface was already broken and cracked and unsuited to aircraft operations. That didn't stop the airport parking planes on it, until one day someone did an engine test on one of them . . .
However I can see as a result of this discussion that attention needs to be given to the bonding with the lower layers [read that as I will be giving close attention to that as soon as I've finished typing this].
The Airbus airport planning manual, in presenting its ground manoeuvring section, avoids the question:
All the runways, taxiways and aprons taking A380s will have surfacings of asphalt (asphaltic concrete) or concrete. For accountants, these are black or white respectively. The asphalt surfacing will be at least 100mm and probably 125mm thick. In most countries (probably all of Europe, N & S America, Asia) it will have another layer of asphalt 150-250mm thick beneath it, and other materials below that. The concrete surfacings will be 350-400mm thick, and are not going to break.
The likelihood of damage to those asphalt surfacings under normal ops is fairly small. I was looking at a road engineering presentation the other day, and saw (on a steep hill carrying road trains) a horizontal force of 6 kN from the drive tyre had caused delamination of the thin asphalt surfacing layer. This layer was only 50mm thick, and had the unusual defect of being without proper bonding to the lower layer. That corresponds to a horizontal shear stress of approximately 650 kPa in the asphalt layer which eventually caused something like fatigue cracking. The asphalt didn't tear out in chunks, but eventually looked cracked and aged.
I calculated the horizontal shear stresses under medium aircraft braking, based on the earlier thread about the 747 classic and brakes, and assuming it will be stopping at 6 ft/sec/sec (don't know what deceleration maximum braking gives - any thoughts welcome). This equates to a horizontal force per tyre of 45 kN in the A380 case, assuming the same medium braking deceleration rate and at MTOW. That in turn is a horizontal shear stress of 780 kPa in a 125mm thick asphalt layer. If the asphalt is not bonded to the layer beneath, and exactly the same point on the runway gets stressed each time, the failure mode is again going to be delamination and fatigue cracking. That will only be after a few thousand movements over exactly the same spot, which is going to be rare, and that will be detected during frequent runway inspections. I would not expect to see big chunks of surfacing tearing out. If anything does happen, it would only lead to some localised surfacing patches and a general resurfacing after 5-8 years instead of the usual 8-12 years.
To get big chunks tearing out, the asphalt has to break or tear. This means shear failure, and the shear capacity of the asphalt layer is an order of magnitude above these stresses. I simply do not think it is going to tear, and that was echoed when I called a senior colleague just now, who has surfaced more than a few 747 airports with asphalt.
I know of a couple of failures where chunks of asphalt have torn out and been sucked up by aircraft in the past, but the surface was already broken and cracked and unsuited to aircraft operations. That didn't stop the airport parking planes on it, until one day someone did an engine test on one of them . . .
However I can see as a result of this discussion that attention needs to be given to the bonding with the lower layers [read that as I will be giving close attention to that as soon as I've finished typing this].
The Airbus airport planning manual, in presenting its ground manoeuvring section, avoids the question:
For ease of presentation, this data has been determined from the theoretical limits imposed by the geometry of the aircraft, and where noted, provides for a normal allowance for tire slippage. As such, it reflects the turning capability of the aircraft in favorable operating circumstances. This data should only be used as guidelines for the method of determination of such parameters and for the maneuvering characteristics of this aircraft type. In the ground operating mode, varying airline practices may demand that more conservative turning procedures be adopted to avoid excessive tire wear and reduce possible maintenance problems. Airline operating techniques will vary in the level of performance, over a wide range of operating circumstances throughout the world. Variations from standard aircraft operating patterns may be necessary to satisfy physical constraints within the maneuvering area, such as adverse grades, limited area or high risk of jet blast damage. For these reasons, ground maneuvering requirements should be coordinated with the using airlines prior to layout planning.
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Off the top of my head i think the 737 MAX autobrake decel rate is about 12'/s/s- with both feet on the floor being higher (~14), and RTO being higher again (unregulated). The A330 MAX is around 6.5m/s/s...
but don't take those figures as gospel...
HJ
but don't take those figures as gospel...
HJ
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Thanks Herc Jerk. The more exciting 6.5 m/s/s maximum braking equates to a horizontal force per tyre of 162 kN in the A380 case, assuming the same maximum braking deceleration rate and at the A380 MTOW (i.e. the rejected takeoff case). This is a horizontal shear stress of 2800 kPa in a 125mm thick asphalt layer. For A380 maximum braking at MLW, the horizontal shear stress is 2000 kPa in a 125mm thick asphalt layer.
This is still comfortably below the likely shear strength of the asphalt of 12000 kPa in summer in temperate climates. However asphalt softens in hot weather and the torsional strength will drop to possibly 4000 kPa in hot climates. On extremely hot days, the strength could drop further and the combination of rejected takeoff/very hot weather might lead to some slippage cracking. However at these temperatures, the asphalt is slightly 'self-healing' and the following aircraft will knead the cracks closed. Many airports in hot climates use modified bitumens in their asphalt to improve their torsional strength at higher temperatures.
Other large aircraft can doubtless exert the same sort of forces, yet there are no general reports of slippage and cracking problems due to heavy braking. There are occasional damage problems with aircraft making locked wheel turns on fresh soft asphalt (the DC-9/MD-80/717 series are the worst), but these are small smearing type scuff marks and not chunks ripped out. It is hard to write about these problems because in the field everyone says they are due to 'aircraft screwing around' but that explanation looks wrong in print somehow.
This is still comfortably below the likely shear strength of the asphalt of 12000 kPa in summer in temperate climates. However asphalt softens in hot weather and the torsional strength will drop to possibly 4000 kPa in hot climates. On extremely hot days, the strength could drop further and the combination of rejected takeoff/very hot weather might lead to some slippage cracking. However at these temperatures, the asphalt is slightly 'self-healing' and the following aircraft will knead the cracks closed. Many airports in hot climates use modified bitumens in their asphalt to improve their torsional strength at higher temperatures.
Other large aircraft can doubtless exert the same sort of forces, yet there are no general reports of slippage and cracking problems due to heavy braking. There are occasional damage problems with aircraft making locked wheel turns on fresh soft asphalt (the DC-9/MD-80/717 series are the worst), but these are small smearing type scuff marks and not chunks ripped out. It is hard to write about these problems because in the field everyone says they are due to 'aircraft screwing around' but that explanation looks wrong in print somehow.
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TycoPicard,
Thanks - I logged on to PPRUNE because I had just got an e-mail with these photos, and I saw your post.
I also wonder if it was a test or what the story was. Those are some skid marks if just from turning. The kinetic energy or energy of motion is dissipated between the tyres and the surfacing in the form of heat and it is this heat that dissolves, or melts, the bitumen and oils on the asphalt surface, thus creating the distinctive dark smear commonly referred to as a tyre mark, or in this case, a skid mark. Although some small particles of tyre rubber do separate from the tyre itself, the skid mark is primarily composed of bitumen. That must have been a heck of a kinetic energy level to make those marks. Still, looking at the tyre distortion (and someone in the other thread suggested the leg was bent), there is a lot of energy there.
Clearly the top of the surfacing has been torn. A computer enhanced version of one of the photographs is at http://www.kubu.net/A380_turn.jpg and you can see loose asphalt lying on the surface. Zoom in closely, (a large photo zoomed in is at http://www.kubu.net/A380_zoom.jpg) and you can see that the asphalt is being screwed out by only the (outer?) closest two tyres of the bogie. You can see one of the inside (?) tyres of the bogie and there seems to be no loose asphalt under the tyre.
The distortion of the outer tyres shown in the other photos in the other thread means that much more of the load is being taken by the one sidewall (on the inner side of the outer tyres) - pushing the asphalt out. Add to this the non-uniform distribution of stress under a tyre and the result is a very substantial increase in stress under that sidewall. Based on Morris and Emile's work, I would estimate vertical stress as high as 2800 kPa.
The sideways force on a skidding tyre depends on the coefficient of friction of the asphalt, and for new (little trafficked) dry asphalt, 0.8-1.2 is a typical range. Let's take 1.0. That gives a horizontal force per A380 tyre at MTOW of 300 kN. The horizontal stress pushing the asphalt out in front of the skidding tyre is being resisted not by the whole tyre footprint but a smaller effective area, and the horizontal shear stress is now 5952 - call it 6000 kPa at the surface of a 125mm thick asphalt layer. Much higher than under braking.
The maximum air temperature at Toulouse on 25/6/05 was 33 oC (if it was Toulouse); and converting that the surfacing temperature is imprecise but 60-65 oC would be a reasonable estimate. I guess that the torsional shear strength of that asphalt at that temperature and presumably at the low aircraft speed, was a bit below 6000 kPa (and I would think it not unreasonable to be so). It failed.
I look forward to learning more about the incident. If this is part of normal ops and is not some sort of test, then it's potentially got implications for airports with the A380 operating on asphalt in summer temperatures. And also for those airports that have just finished building their taxiway fillets, widening corners, and all the other works to accommodate the A380, and who used the sharp (but allowable) steering angles in their design, well this might indicate some problems. Or maybe the photos are just part of some extreme test. I sketched out the geometry of the turning aircraft and I can't see how the bogie can be skidding sideways. It's more like a minimum width runway test, with the aircraft ending up sideways. Or my wife driving. Anyway I suspect the Airbus pavement engineers will be working hard on this one, and I think I've given them the answer above. Donations in the form of Moet gratefully accepted - say a bottle per kN.
Thanks - I logged on to PPRUNE because I had just got an e-mail with these photos, and I saw your post.
I also wonder if it was a test or what the story was. Those are some skid marks if just from turning. The kinetic energy or energy of motion is dissipated between the tyres and the surfacing in the form of heat and it is this heat that dissolves, or melts, the bitumen and oils on the asphalt surface, thus creating the distinctive dark smear commonly referred to as a tyre mark, or in this case, a skid mark. Although some small particles of tyre rubber do separate from the tyre itself, the skid mark is primarily composed of bitumen. That must have been a heck of a kinetic energy level to make those marks. Still, looking at the tyre distortion (and someone in the other thread suggested the leg was bent), there is a lot of energy there.
Clearly the top of the surfacing has been torn. A computer enhanced version of one of the photographs is at http://www.kubu.net/A380_turn.jpg and you can see loose asphalt lying on the surface. Zoom in closely, (a large photo zoomed in is at http://www.kubu.net/A380_zoom.jpg) and you can see that the asphalt is being screwed out by only the (outer?) closest two tyres of the bogie. You can see one of the inside (?) tyres of the bogie and there seems to be no loose asphalt under the tyre.
The distortion of the outer tyres shown in the other photos in the other thread means that much more of the load is being taken by the one sidewall (on the inner side of the outer tyres) - pushing the asphalt out. Add to this the non-uniform distribution of stress under a tyre and the result is a very substantial increase in stress under that sidewall. Based on Morris and Emile's work, I would estimate vertical stress as high as 2800 kPa.
The sideways force on a skidding tyre depends on the coefficient of friction of the asphalt, and for new (little trafficked) dry asphalt, 0.8-1.2 is a typical range. Let's take 1.0. That gives a horizontal force per A380 tyre at MTOW of 300 kN. The horizontal stress pushing the asphalt out in front of the skidding tyre is being resisted not by the whole tyre footprint but a smaller effective area, and the horizontal shear stress is now 5952 - call it 6000 kPa at the surface of a 125mm thick asphalt layer. Much higher than under braking.
The maximum air temperature at Toulouse on 25/6/05 was 33 oC (if it was Toulouse); and converting that the surfacing temperature is imprecise but 60-65 oC would be a reasonable estimate. I guess that the torsional shear strength of that asphalt at that temperature and presumably at the low aircraft speed, was a bit below 6000 kPa (and I would think it not unreasonable to be so). It failed.
I look forward to learning more about the incident. If this is part of normal ops and is not some sort of test, then it's potentially got implications for airports with the A380 operating on asphalt in summer temperatures. And also for those airports that have just finished building their taxiway fillets, widening corners, and all the other works to accommodate the A380, and who used the sharp (but allowable) steering angles in their design, well this might indicate some problems. Or maybe the photos are just part of some extreme test. I sketched out the geometry of the turning aircraft and I can't see how the bogie can be skidding sideways. It's more like a minimum width runway test, with the aircraft ending up sideways. Or my wife driving. Anyway I suspect the Airbus pavement engineers will be working hard on this one, and I think I've given them the answer above. Donations in the form of Moet gratefully accepted - say a bottle per kN.
Last edited by OverRun; 15th Jul 2005 at 12:10.
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Get your facts right folks...
A380 tyres withstand extreme handling tests: Airbus
London (15Jul05, 12:01 GMT, 174 words)
Airbus is claiming that the undercarriage and tyres on its A380 aircraft have performed as expected after being subjected to extreme ground-handling tests last month.
Images show that the tests resulted in deformation and damage to the aircraft’s Michelin tyres but that they performed “at and above” expected levels during the abusive ground-handling tests on 25 June.
The A380 main undercarriage comprises two four-wheel under-wing bogies and, behind them, two six-wheel fuselage-mounted bogies. The rear axle of the six-wheel bogie is normally steerable during push-back and taxiing.
But unless electrical power is supplied to the aircraft during towing – by the aircraft or from generators on the tow-tractor – the axle remains locked.
Airbus states that the undercarriage tests were designed to take the undercarriage and tyres “way beyond the limit of normal operations” and adds: “[The tests] were the equivalent of the structural static airframe tests to destruction.
“Although these tests were designed to test up to maximum deformation and beyond, the gear did exactly what it is supposed to do.”
Source: Air Transport Intelligence news
A380 tyres withstand extreme handling tests: Airbus
London (15Jul05, 12:01 GMT, 174 words)
Airbus is claiming that the undercarriage and tyres on its A380 aircraft have performed as expected after being subjected to extreme ground-handling tests last month.
Images show that the tests resulted in deformation and damage to the aircraft’s Michelin tyres but that they performed “at and above” expected levels during the abusive ground-handling tests on 25 June.
The A380 main undercarriage comprises two four-wheel under-wing bogies and, behind them, two six-wheel fuselage-mounted bogies. The rear axle of the six-wheel bogie is normally steerable during push-back and taxiing.
But unless electrical power is supplied to the aircraft during towing – by the aircraft or from generators on the tow-tractor – the axle remains locked.
Airbus states that the undercarriage tests were designed to take the undercarriage and tyres “way beyond the limit of normal operations” and adds: “[The tests] were the equivalent of the structural static airframe tests to destruction.
“Although these tests were designed to test up to maximum deformation and beyond, the gear did exactly what it is supposed to do.”
Source: Air Transport Intelligence news
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I hope r304ndy meant his post for the other thread. thefacts reported this 3 days ago, as MarkD noted, in the other thread as the news unfolded:
Towing the aircraft laterally at MTOW would do the sort of damage we saw in the photos, and it fits the pattern of skid marks. The above analysis though has been very useful because it has thrown some light of the phenomenon of groove breakage [on grooved runways] which was being puzzled over in another forum, and we now have a better model to understand what is causing it and how to test for it.
The test is a normal part of the ground testing of the aircraft, and the results exceeded expectations - the gear is tested to failure. All is fine. The test consists of towing the aircraft, at maximum weight, forward, backward, in normal turns, in tight turns, and laterally.
Towing the aircraft laterally at MTOW would do the sort of damage we saw in the photos, and it fits the pattern of skid marks. The above analysis though has been very useful because it has thrown some light of the phenomenon of groove breakage [on grooved runways] which was being puzzled over in another forum, and we now have a better model to understand what is causing it and how to test for it.
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OK R304ndy....
But just one not so small update to your info.....
The two fuselage mounted bogies are fitted with three fixed axles each, some 6 wheels in total. The steerable axle with an extra two unbraked wheels is in addition to the fixed ones, a total of eight wheels on each bogie.
Cheers, FD
But just one not so small update to your info.....
The two fuselage mounted bogies are fitted with three fixed axles each, some 6 wheels in total. The steerable axle with an extra two unbraked wheels is in addition to the fixed ones, a total of eight wheels on each bogie.
Cheers, FD
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The two fuselage mounted bogies are fitted with three fixed axles each, some 6 wheels in total. The steerable axle with an extra two unbraked wheels is in addition to the fixed ones, a total of eight wheels on each bogie.
http://images.airliners.net/open.fil...=862819&size=L
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The test was performed with the steerable axles locked to maximum the strain on the system.
In some respects it looks like the asphalt let go before the tyres.
In some respects it looks like the asphalt let go before the tyres.
