How to repair 787 composite fus.
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How to repair 787 composite fus.
This was touched on in JB and I'm not implying that Boeing have not solved the problem.
If, for example, a 787 has a GPU reverse into it and bounce off without doing much apparent damage, how do you know?
Plainly it would dent or crease a metal aircraft but that CFRP is tough stuff and certainly wont dent. It might leave no mark at all but still be weakened by the impact- or not!. How do you tell?
Second issue. If the a/c is punctured, how on earth do you repair it? Bearing in mind that the repair will need to be good for the remaining life of the a/c.
Looking forward to finding out!
If, for example, a 787 has a GPU reverse into it and bounce off without doing much apparent damage, how do you know?
Plainly it would dent or crease a metal aircraft but that CFRP is tough stuff and certainly wont dent. It might leave no mark at all but still be weakened by the impact- or not!. How do you tell?
Second issue. If the a/c is punctured, how on earth do you repair it? Bearing in mind that the repair will need to be good for the remaining life of the a/c.
Looking forward to finding out!
Second issue. If the a/c is punctured, how on earth do you repair it? Bearing in mind that the repair will need to be good for the remaining life of the a/c.
just put a Korkplug in. Plugs have to be konical and prefered from Red Winebottles older than 60 years. They have to be stored on full bottles just prior to be used. Not that they work better than new ones, but our mechanics insist in the older ones.
Make shure to work them into the hole from the inside of the fuselage, otherwise diff. pressure will push them out again and they donīt last for the whole airframe life.
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Having recently had a tour round the Force India F1 factory, where carbon fibre isn't in it's infancy, I have the (well, one) answer for you: Ultrasonic detection!
Basically, a part is tested with a bunch of sensors all over it, and then asked to perform it's task (in a rig a bit like how Ikea test their chairs). We saw a nice graph where the stresses on the part were shown, and any red areas indicated weaknesses. Any sign of these not in the areas on the graph where you expect them (e.g. at tolerance limits) and the part is binned.
(excuse the layman reply, but whilst i'm an avid F1 watcher, i've never laid CF or tested it myself!)
Basically, a part is tested with a bunch of sensors all over it, and then asked to perform it's task (in a rig a bit like how Ikea test their chairs). We saw a nice graph where the stresses on the part were shown, and any red areas indicated weaknesses. Any sign of these not in the areas on the graph where you expect them (e.g. at tolerance limits) and the part is binned.
(excuse the layman reply, but whilst i'm an avid F1 watcher, i've never laid CF or tested it myself!)
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Your points are valid and I was told that there is a special tool that they survey the airframe with looking for just that kind of hidden damage. It will be incumbent on ground crews to fess up when have hit something and the potential for damage exists.
I'm sure that my staement is an over simplificatioin of the process and procesure, but yes, they have made accomodations for this type of hidden damage. I would imagine walk arounds will take on a new degree of carefull inspection looking for any evidence of paint blemeish that may be the indicator of something more serious.
I'm sure that my staement is an over simplificatioin of the process and procesure, but yes, they have made accomodations for this type of hidden damage. I would imagine walk arounds will take on a new degree of carefull inspection looking for any evidence of paint blemeish that may be the indicator of something more serious.
If boeing have done their homework [& I don't doubt that they have], the walk-round should not be any worse than for a conventional aircraft. Carbon Fibre Reinforced Epoxies / Plastics - CFRE or CFRP depending on your point of view for how you want to describe them are very tough and resilient materials and will withstand a great deal of punishment, far greater than aluminium will without sustaining any visible damage, or invisible damage for that matter. When they are damaged, it's generally pretty obvious what the damage is. However, like metallic parts damage does not mean that the part is useless as the parts will retain their shape and unless catastrophically damaged, the bulk of their strength.
Having used CFRE materials in the oil industry to protect sensitive electronics inside high pressure/high temperature oil production pipelines I can vouch that they are very robust and are both damage resistant and resistant to chemical attack. As an example, on one of my applications, a 2.5mm thick sleeve was more than capable of withstanding an external pressure in excess of 2200 psi, plus additional imposed bending and other stresses without collapse. The pressure difference on the 787 can't exceed 15psi, so the skin thickness needed to support this pressure difference isn't much.
Conversely, if there is any damage visible on the aircraft skin, then the odds are that something has gone dramatically (catastrophically) wrong on the apron which will be difficult to cover up.
As I said earlier, assuming Boeing have done their work, then any part of the aircraft's load bearing structure which is damage sensitive will be buried well within the skin of the aircraft.
In the fullness of time when more details of the construction of the 787 become known, I think that it will be comparable with the human body in that there will be a load bearing structure with an attached aerodynamic surface structure in the same way that our skin is hung over our skeleton.
And in the same way that we slap an elastoplast over a graze or a small cut, I wouldn't really be surprised if we see duct tape [or gaffer tape if you prefer] as an approved temporary repair method for lightly damaged panels. The odds are that it won't be visible as it will be on the inside held in place by the differential pressure. There is also the option to use self healing CFRP which contains micropockets of resin & hardener that when the part is overstressed, cracked or punctured {on a pretty small scale of course} the pockets are ruptured, mix and cure to seal the breach.
Now as far as the GPU is concerned, the damage to it will probably be fairly evident by the fuselage shaped dent in the cover. That might be a bit of a clue that something untoward has taken place.
Having used CFRE materials in the oil industry to protect sensitive electronics inside high pressure/high temperature oil production pipelines I can vouch that they are very robust and are both damage resistant and resistant to chemical attack. As an example, on one of my applications, a 2.5mm thick sleeve was more than capable of withstanding an external pressure in excess of 2200 psi, plus additional imposed bending and other stresses without collapse. The pressure difference on the 787 can't exceed 15psi, so the skin thickness needed to support this pressure difference isn't much.
Conversely, if there is any damage visible on the aircraft skin, then the odds are that something has gone dramatically (catastrophically) wrong on the apron which will be difficult to cover up.
As I said earlier, assuming Boeing have done their work, then any part of the aircraft's load bearing structure which is damage sensitive will be buried well within the skin of the aircraft.
In the fullness of time when more details of the construction of the 787 become known, I think that it will be comparable with the human body in that there will be a load bearing structure with an attached aerodynamic surface structure in the same way that our skin is hung over our skeleton.
And in the same way that we slap an elastoplast over a graze or a small cut, I wouldn't really be surprised if we see duct tape [or gaffer tape if you prefer] as an approved temporary repair method for lightly damaged panels. The odds are that it won't be visible as it will be on the inside held in place by the differential pressure. There is also the option to use self healing CFRP which contains micropockets of resin & hardener that when the part is overstressed, cracked or punctured {on a pretty small scale of course} the pockets are ruptured, mix and cure to seal the breach.
Now as far as the GPU is concerned, the damage to it will probably be fairly evident by the fuselage shaped dent in the cover. That might be a bit of a clue that something untoward has taken place.
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I think, but can not yet be certain (because I have not seen a 787 SRM), that the 787 SRM will provide for:
1. Field Repairs using titanium parts attached over damaged composite structure with mechanical fasteners (like a scab patch on aluminum structure), and
2. Maintenance Base Repairs using composite bonding processes with all of the BAC Process Spec controls that assure that proper environmental controls, equipment, instrument calibrations, operator skills, QC checks (and double checks, etc.), material specs, and testing and lots of record keeping all are done.
The scab patches are ugly and do not need to last the life of the arframe, only to the next C or D Check where skin panel replacement and/or proper flush patches are done. The same will be true of the 787 process.
1. Field Repairs using titanium parts attached over damaged composite structure with mechanical fasteners (like a scab patch on aluminum structure), and
2. Maintenance Base Repairs using composite bonding processes with all of the BAC Process Spec controls that assure that proper environmental controls, equipment, instrument calibrations, operator skills, QC checks (and double checks, etc.), material specs, and testing and lots of record keeping all are done.
The scab patches are ugly and do not need to last the life of the arframe, only to the next C or D Check where skin panel replacement and/or proper flush patches are done. The same will be true of the 787 process.
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The scab patches are ugly and do not need to last the life of the arframe, only to the next C or D Check where skin panel replacement and/or proper flush patches are done. The same will be true of the 787 process.
Skin panel replacement??? What skin panel? The tube is one oven baked delacy.. I am sure they have some process in mind but it must be an epoxy temperature related process. I am betting that it's impact resistance qualities are far greater than aluminium but long term degeneration is a serious issue. New tech..I claim ignorance but seriously doubt a scab patch is the answer, nor flush patches, nor doublers or triplers.
The origional integrity of the composite material will need to be repaired with a composite "layup", probably a hot repair to bond with the existing structure.
Ground equipment hitting aircraft happens quite often and unfortunately is not reported on many occations.
It would be nice to think that all the people working near aircraft were highly trained and responsible people,who would report any incidents, but the reality is that if you pay peanuts, you get monkeys. Also if someone is likley to loose their job then a cover up is going to happen.
The worst case I've seen is of a Viscount aileron being bent when hit by a belt loader. The loaders involved only came and told the line maintenance guys (me) after it had taken off. Fortunately it arrived safely.
So Boeing had better make sure that when the structure is hit and no mark is left, the residual strength is going to last until the deepest NDT inspection has taken place.
I imagine that all the design and testing has taken this into account, but the fail safe structure of the 707 horizontal stabilizer was supposed to take care of all damage between inspection intervals. A Dan-Air cargo crew found out that this system is not infalible and paid with their lives.
Further, when doing a walk round, if there is a crack in the paint, then if the aircraft is fairly new I suppose you would assume something has happened. But what happens on an older aircraft when the paint is tired and starting to crack of its own accord. How do you know what the cause is?
I think there will be a lot to learn about these primary structures when they get on the line.
Safe flying.
It would be nice to think that all the people working near aircraft were highly trained and responsible people,who would report any incidents, but the reality is that if you pay peanuts, you get monkeys. Also if someone is likley to loose their job then a cover up is going to happen.
The worst case I've seen is of a Viscount aileron being bent when hit by a belt loader. The loaders involved only came and told the line maintenance guys (me) after it had taken off. Fortunately it arrived safely.
So Boeing had better make sure that when the structure is hit and no mark is left, the residual strength is going to last until the deepest NDT inspection has taken place.
I imagine that all the design and testing has taken this into account, but the fail safe structure of the 707 horizontal stabilizer was supposed to take care of all damage between inspection intervals. A Dan-Air cargo crew found out that this system is not infalible and paid with their lives.
Further, when doing a walk round, if there is a crack in the paint, then if the aircraft is fairly new I suppose you would assume something has happened. But what happens on an older aircraft when the paint is tired and starting to crack of its own accord. How do you know what the cause is?
I think there will be a lot to learn about these primary structures when they get on the line.
Safe flying.
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Well I'll go back to my earlier post and say again that there is a "tool" that will do a farly quick inspection for skin damage. Not to say that any of the other posters are not correct, just that Boeing has addressed the problem at this hour.
How do I know this? It was discussed by a Boeing maintenance instructor some time back and unfortunetly I have forgotten a good part of the details at this hour. Give me another week and I'll tell you the latest on this subject
How do I know this? It was discussed by a Boeing maintenance instructor some time back and unfortunetly I have forgotten a good part of the details at this hour. Give me another week and I'll tell you the latest on this subject
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To quote Spooky 2 "It will be incumbent on ground crews to fess up when have hit something and the potential for damage exists."
A British company is working on something to make the blemished paint job a lot more obvious and difficult to miss, so that the more advanced ultasonic testers can be targeted usefully. I think it adds new meaning to the phrase "Bleeding paint job"
Bruisable Composites
A British company is working on something to make the blemished paint job a lot more obvious and difficult to miss, so that the more advanced ultasonic testers can be targeted usefully. I think it adds new meaning to the phrase "Bleeding paint job"
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So Boeing had better make sure that when the structure is hit and no mark is left, the residual strength is going to last until the deepest NDT inspection has taken place.
I'm not sure how firm this commitment was, possibly as firm as date of the first flight...
However, localised areas could be weakened by external damage and then fail under pressurisation loads, could they not?
This could give rise to destructive decompression.
This could give rise to destructive decompression.
Good Old Mythbusters had a try at shooting out a window and puncturing a pressurised fuselage of an aircraft by gunfire in the hope of proving explosive decompression was an actual phenomenon. Needless to say as there is only a few psi difference in pressure between inside and out, there isn't enough force to drive crack propogation in either a window or more importantly, the fuselage skin. So explosive decompression was judged as "busted".
If it came to the crunch that a crack created by impact with ground equipment was big enough to have a large enough force applied across it by means of the pressure difference, then the crack would certainly be big enough to be obvious.
The CFRP in a 787 is also probably a damn sight more reistant to cracking than the aluminium in a conventional aircraft. I understand from industry publications that much of the skin panels in the 787 are composed of three dimensional woven fabric, rather than uni-directional roving. Hence the finished panels have fibres running perpendicular to each edge and surface which significantly enhances its resistance to transverse cutting or cracking, whereas the metallic skin, despite years of metalurgical research into microalloying to avoid cracking can still in unfortunate circumstances prove to be susceptible to fatigue and stress corrosion cracking in any direction.
Last edited by Donkey497; 26th Jun 2009 at 21:30.
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I understand from industry publications that much of the skin panels in the 787 are composed of three dimensional woven fabric, rather than uni-directional roving. Hence the finished panels have fibres running perpendicular to each edge and surface which significantly enhances its resistance to transverse cutting or cracking
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Well I'll go back to my earlier post and say again that there is a "tool" that will do a farly quick inspection for skin damage. Not to say that any of the other posters are not correct, just that Boeing has addressed the problem at this hour.
How do I know this? It was discussed by a Boeing maintenance instructor some time back and unfortunetly I have forgotten a good part of the details at this hour. Give me another week and I'll tell you the latest on this subject
How do I know this? It was discussed by a Boeing maintenance instructor some time back and unfortunetly I have forgotten a good part of the details at this hour. Give me another week and I'll tell you the latest on this subject
The bigger problem is that the environmental loads ( allthough monitored by FDR dumps) do not fit into the program as I presume. From one airframe to the next the stress an airframe endures is not nearly the same. Do you expect Boeing to mandate NDT of the complete airframe after severe turbulence.
Fact is composite materials degrade within where as aluminum starts to fail in a manner that is visible to the naked eye far before it fails.
Lap seam failures of older Boeing aircraft were a result of sealant removal with metallic objects that scarred the skin. We learned from this.
Just saying that some costly lessons are sure to be experienced in the future. New technology is great...But there is allways a learning curve.
Composite structures for aerospace applications are designed with the assumption that the structure has already sustained "Barely Visible Impact Damage". BVID is usually defined as damage that can be seen visually from approximatley 5 ft away. The structure must be able to sustain ultimate load with BVID and no growth of damage through the fatigue life of the aircraft.
Damage that's greater than BVID obviously must be repaired, but again, the structure is designed to sustain limit load with visible damage and two inspection intervals.
As for repairs, composite materials isn't all that difficult to repair, just different. Generally the damaged area is cut /scarfed out and replacement plies bonded in place.
Damage that's greater than BVID obviously must be repaired, but again, the structure is designed to sustain limit load with visible damage and two inspection intervals.
As for repairs, composite materials isn't all that difficult to repair, just different. Generally the damaged area is cut /scarfed out and replacement plies bonded in place.
