Dreamliner already having CF problems?

Improper shimming in an aluminum structure can cause cracking in adjacent parts. Improper shimming in CF structures causes the same distress but it is expressed by de lamination of the material.

 

IMHO the only airplanes that should be plastic are in 1/72.....1/48 or 1/32 scale..............

 

+1

Carbon fiber is a major disappointment. You can't beat aluminum for aircraft.

 

Amen, As Jason said about his aircraft in another thread, it will last 100 years (aluminum).

 

I would disagree that there is something inherently wrong with using CF as a major structural medium. Done correctly, it is much stronger, stiffer and lighter than aluminum. Plus it has the added benefit of being anti-corrosive.

The polymers and plastics of today are nowhere near the quality they were even 25 years ago. In my experience, many who don't care for plastics have the old products in mind.

I will be curious to see how this all plays out, though.

 

Any material that is assembled incorrectly will be problematic. CF is a wonderful material in many many ways as is aluminum and so is wood. One must use good technique with all of them.

 

+1

As Bob said, incorrect shimming can be a problem on any aircraft, regardless of material. This would probably have happened similarly in an aluminum aircraft; just one of the things that has to be worked out in a new design.

 

How come composite aircraft have limited hull life and aluminum do not if the airframe is kept in good condition? I don't know about all of the makes and models of composite aircraft buy Cirrus and Liberty have airframes you throw away when they reach a certain amount of hours.

 

Aluminum has life limits,i.e.: the number of cycles in big tin birds, it fatigues like anything else except wood, that does not fatigue if it lives the correct environment (no moisture and fungus). The B-52's have all been refurbished with new materials to sustain life. Many aluminum transports have had skins and fittings replaced so as long as you "find and fix" they will live forever, like the B-52 and KC-135. Too bad that they haven't discovered a way to refurbish me but they have so far "stop-drilled" a few items that have prolonged my usefulness. Hats off to good docs and good medicine. However, they haven't figured any new alloys for the ol' bod. CF will work out as they determine the adjustments just as they did in aluminum.

 

Bob,

on the subject of aging, CF and other composites have only certain life spans as well, and not necessariy better than metals

i'm curious to know your knowledgable insight on that

thanks

 

Yes, but the Cirrus has a mandated life limit. I know of no unprssurized, aluminum planes that have a "life limit".

 

I can give you one that I'm sure you are very fimilar with.. How about the Boananza spar??? Initial inspection at 1500 hours and recurring inspections at 500 hour intervals. How about the spar on Beech 18's, and the spars on the Cessna twins (both pressurized and unpressurizd versions? Oh and the spar issues on King Air's isn't related to pressurization, it's just good old metal fatigue too.

These limits are considered to be "on condition". That is, when cracks appear you perform the required repair. The reason for that is pretty simple. There are major differences in how light aircraft are operated. and using up the fatigue life isn't just a matter of flight hours, it depends a lot on how the aircraft are flown and in what environment. So as not to be a burden on aircraft that don't fly a lot, or haven't been beaten up, it's possible to for them to go longer, but aluminum fatigues and the airplanes noted above are but a few examples.

Aluminum has a fatigue life limit that is very low when compared to it's yield and ultimate strength. If you were to design to respect that fatigue limit you would have a tough time getting any airplane off of the ground. I believe that aluminum is a good material for airplanes, but it isn't forever either.

Are composites better or worse? I frankly, don't know, and I'm in the industry. There are pro's and con's, and it depends on your usage, the environment you fly in, and the specific design of the aircraft in question. I think that composites can be as good or better than aluminum and, as engineering experience is accumulated, will eventually be the way of the future. We've been building airplanes using aluminum monocoque construction for about 80 years. There's a lot of experience and still we have issues with things like spars on a lot of airplanes. As experience grows with composites I think they can acutally be a lot better than aluminum, but I also think it depends a lot more on the individual design than the material itself.

 

The fatigue life of aluminum is a big reason why the US transport planes used in the ME and Afghanistan are in the process of having their main wing spars replaced with CF ones.

 

What Bonanzas are affected by the spar AD?

Why isn't mine?

Is this really an aluminum issue? Or a design issue?

Bottom line, you will never see a 30 year old 7000 hour composite airplane.

 

Interesting stuff!....

I know this is a pretty American / Boeing centric board - It seems to me that Airbus gets *slammed* if any "weaknesses" appear in their designs, but Boeing gets a free pass.

They're both new designs, pushing the envelope and faults will be found - In both - Regardless of material(s) used.........

Cheers,
Ian

 

The longevity of a material in a structure depends on too many things to list here but if one looks at the design objectives and the type of materials and how they are used one can pretty much determine the life expectancy.. I keep thinking of two prime examples of classic longevity: the B-17 and the DC-3. The materials in these aluminum these airplanes is mostly in the 2000 series of aluminum alloys and that coupled with a sound and strong structure and a low stress operational environment (discounting military action) leaves to long operational life. They were what would be called over-designed in todays terms but they subjected to a low stress flight condition. On a modern scale the 707 was subjected to high speeds, high gust loads, and stringent drag controls so the wing was very thin and flexible. To address weight reduction it was built with 7000 series alloys that were very strong for their weight BUT not as malleable as the 2000 series metals. Therefore, subjected to a violent life style, they had a shorter life. Douglas, on the other hand, built a wing that had less sweep, thicker section, and three spars. It wasn't worked as hard as the Boeing and is still in use. So, I guess it is a combination how the vehicle is designed for the materials used and how hard it is ridden.
My insight into all this is the result of living with it for 70+ years and learning all that I could. I started at Boeing in 1950 as a modification mechanic and then over the next 48 years worked in almost everything you can think of. I was an engineering training instructor for 11 years and quickly found out that to teach you must learn. i spent most of my time in engineering working in almost all disciplines,; hydraulics, structures, propulsion, environmental control, etc. I eventually ended up in engineering in Preliminary Design on the 767 and stayed there for almost 20 years, ending it on the 777 as a Technical Designer . In the middle of all that I flew with a bunch of antiquers and homebuilding nuts and helped on many projects.I helped Pete Bowers put his Fly Baby together and did the first drawings of it to present to the EAA. I flew the airplane as a biplane and monoplane and think that it is still a pleasant and friendly little airplane.
Iv'e had a lot of fun.

 

Here is a link to the Bonanza spar AD.

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/1B4E8DED6C75A36B86256A09005D18F7?OpenDocument

You can look down on the affected aircraft section and figure out if it applies to your aircraft. After Beech found the problem they made a change to production aircraft and afer a specific S/N the AD does not apply.

As is well know in the industry, aluminum has fatigue issues. If you require too much of it you will have a great airplane in terms of performance, but it won't last that long. Most airplanes are pretty conservative in their design, but it is not uncommon for fatigue to rear it's ugly head. As Bob noted, some older airplanes were very conservative and for all intents and purposes, have an infinite fatigue life, but those are actually the exceptions. Modern design techniques have been refined to the point where a cyclic life can be defined and that life is usually pretty long. Unless the design is perfect (as in the proverbial Decon's shay) something has to go first. Sooner or later, as airplanes get old you will start finding fatigue issues.

Also as tin airplanes get old you start to see a lot of things like loose rivets that need to be drilled out and properly replaced or things can unzip. One big advantage of composites is that the fatigue life of the material is closer to the ultimate strength, which translates into a higher useful material capability.

There are already 30 year old composite airplanes out there. The oldest Glasairs and Lancairs are approaching that age and are doing fine. I doubt that there are a lot of 7,000 hour homebuilts out there, but somebody could easily prove me wrong. I don't doubt that there will be 7,000 hr composite airplanes out there very soon. Look at how fast the the 787 will build hours and 7,000 will be a cake walk.

Still, the perfect aircraft material has yet to be invented. The perfect airplane material is easy and inexpensive to join (and those joints need to be aerodynamically clean), can be formed into complex aerodynamic shapes, has exceptional fatigue life, is strong, light weight and inexpensive to to begin with, and doesn't corrode or degrade in life over time. Right now it doesn't exist. If you can find and develop something better than the current generation of aluminums or composites, the world will beat a path to your door.

 

Any machine is going to need to be worked on a kept up to date.

Is it cheaper to fix a composite airframe or trash it and buy new?

 

Nobody knows.

It depends on where and what is the life limiting area and what the nature of the life limiting degradation is. That's true for metal planes too. There were a lot of Cessna 421's and 402's that were used hard flying freight, and when the spar problems came along a lot got scrapped. The cost of the spar mod was so high that it wasn't worth it to fix them. The airplanes that had cracking were pretty well beaten on airplanes. They were constantly flown at (or above) gross weight, in all kinds of rough conditions and they flat wore out. Unfortunately the hourly limit that they established was applied to planes flown much more gently and it really hurt the owners who carefully flew and maintained their airplanes. I doubt that many well maintained and owner flown 421's would ever have had the cracking issue.

Relize too that what is a "design life limit" for airplanes like the Cirrus isn't going to be a grounding order. What will likely happen is that, as airplanes like the Cirrus age and approch that life limit, there will be assessments of the strength and life of the airframes. If areas of delamination are found, or cracking occurrs, the life, or mandated repairs will be adjsted. They will also develop means of inspection (like ultrasound) to help define the life and insure airworthiness. They will take test samples from airplanes that are damaged or lost due to other issues, and adjust the life accordingly. Note how your owners organization gets involved with issues like the spar cracking, and these groups have an intrest in preserving the value of the airplanes of their owners. There will be ongoing efforts to assess the life and insure the FAA doesn't slam their resale value and impose a grounding at a specific hour point. I just don't see that happening.

 

I have heard too many times that the Airbuses are throw away airplanes when they reach the level of maintenance/ airframe value. The cost of elex rework plus airframe work is not worth the value of the airplane. Just what I have heard.

 

I know Gulfstreams, or at least the G-II, have an airframe life limit. I know this because a friend of mine used to operate one which hit the life limit.

What happened? Gulfstream came up with an inspection program, they did the inspection, and continued to operate. That G-II has probably been melted down by now, but it wasn't a result of the life limit.

I will also point out that they started wrangling with Gulfstream about 1,000 hours before the life limit (I believe it was a 20,000 hour limit).

 

That would have been from a Boeing fan .

Seriously Airbus (Sud, etc.) have been given a hard time since the Caravelle ... with Douglas effectively stealing the design (

) and along came the DC-9. I've quoted Wikipedia but my reference should really be the Concorde book I have at home, but I cannot remember it's correct name, where the author made the stealing or at least getting all the research they needed reference.

Don't get me wrong, I'm happy to fly in Boeing's but the problem we have is really with modern aircraft pushing the boundaries due to cost and over use (IMO) of clever computers ... and we all need to remember that the wonderful 747 had issues when it first arrived on the scene, but like a 911 Porsche the design has been refined constantly since then.
Pete

 

The book is simply called "Concorde", by Frederic Beniada and Michel Fraile.

Pete

 

What I've found, and being in this industry for 35 years, is that if you lock up two different groups of engineers in two different places, and give them the same requirements, and shut them off from the outside world, what pops out at the other end looks pretty darn similar. There's a lot of logic in designing airplanes and engines, and while somebody sometimes gets clever, there isn't a lot of room for big differences. That's why most airplanes look a heck of a lot alike, once you optimize everything there isn't much room for variation.