Are Composite Structures Too Tough?

Spotting damage on a homebuilt is relatively easy. This wingtip damage is ready for repair. Courtesy:

Using composite materials to build an airplane has many advantages. Composites – particularly advanced materials such as carbon fiber – can weigh significantly less than aluminum. Composites can be shaped to meet any designer’s whim, and the finished surface can be extremely smooth and free of lap joints and rivet heads.

But, can composite materials be too tough for their own good? The answer is maybe. 

A fundamental aspect of aviation safety is what some might call the “A” check, but what the rest of us call the “walk around.” Before flight the prudent pilot walks all the way around the airplane looking for obvious damage, leaking fluids, low tire pressure, or any other out of the ordinary appearance of the airplane.

If a metal airplane looks fine, with no dents or cracks, or obvious corrosion it is almost certainly perfectly fine. But let’s say the fuel truck accidentally backed into the airplane. If the impact was hard enough to cause damage, there would be a dent for the pilot to spot. If the hit was just a tap and there is no dent in the skin, the airplane didn’t really lose structural integrity.

Cracks in metal are always worth a close look because, depending on the location and size of the crack, the strength of the metal may be compromised. Wrinkles in metal skin or “smoking” rivets are also signs of stress and should be investigated. Metal airframes really don’t hide a lot of problems from one looking closely with a trained eye.

Composite structures are, however, pretty resilient. You can hit a composite surface hard and the material bounces back leaving no dent. Often the paint flexes with the structural material so there is really no sign of impact. That hard hit by the fuel truck on a composite airplane may have left nothing for a pilot to see during the preflight walk around.

Could you spot composite damage on this aircraft during a walk around?

But the composite material may have hidden damage from the impact. Composite structures rely on layers of material bonded together to achieve their great strength and an impact can break the internal bonds of the structure. If the bond is broken between layers overall strength can suffer a lot. And that loss of strength can easily be hidden even from the closest visual inspection of the exterior.

The worry about undetected damage is probably pretty small in a homebuilt airplane. Most pilots of homebuilts really pamper their creations and rarely let the airplane out of their sight. Chances of a fuel truck, tug, or passing airplane smacking into a homebuilt without the owner knowing it are slim.

But in a traveling airplane – particularly an airliner – the airplane is typically surrounded by all sorts of threats on a busy ramp. If it isn’t the fuel truck, the baggage loader may hit it, or the catering truck, or even the careless limo driver crossing the ramp. Collisions of all sorts happen.

Because of that threat of hidden damage the FAA requires airplane makers using composites to prove that the structure can take the hardest possible hit that leaves no visible signs and still carry full required structural loads. That means a composite airframe must be much stronger than needed for actual flight loads in order to have sufficient redundant structure to survive internal failures that are not visible.

Various composite structures on the Boeing 787. Courtesy:

The extra material required to act as a backup to invisible damage is a frustration for composite airplane builders because it robs most, or even all, of the weight savings benefits of composites. But what other solution is possible? Nobody wants to do a walk around using an x-ray or ultrasound scanner that could detect internal bond failures or other damage in a composite structure.

One partial solution being used in some large composite airplanes are mechanical fasteners such as rivets or screws to back up the bonding of the composite layers. With mechanical fasteners in critical areas the layers of composite material can be held together and thus remain strong even if the bond between layers is broken. Again, some advantage of composite construction is gone, but the fasteners add a belt to the suspenders.

Building in enough extra strength to make up for possible hidden damage has been an issue on every FAA approved airplane from piston singles like the Cirrus to the Hawker 4000 that has a carbon fiber fuselage. But the real test comes now with Boeing’s 787 entering service. How composites perform in the hostile airline ramp environment will be interesting to watch.

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48 Responses to Are Composite Structures Too Tough?

  1. john in brisbane says:

    I am sure the tech already exists for pressure sensitive paint and other coatings – someone will incorporate it into a composite-specific paint that gives a visual indicator of previous pressure or deformation. I think they use nano-capsules of some dye that break – a bit like a piece of fruit being bruised. When that happens maybe the full weight savings of composites will be realised?

  2. John Patson says:

    Electronics have changed a lot and the mobile x-ray or ultrasound scanner could now be a small hand-held device, as opposed to the suitcase of just a few years ago.
    Prudent construction workers and DIYers have, for the last 30 years, run a cable detector over a wall before drilling or gouging.
    Deaths from drilling into cables are not rare. Even road diggers now routinely use ground radars before getting the compressor roaring.
    It could be that prudent pilots will, in the future, run a composite bond break detector over their planes as a matter of routine.

    • John in Brisbane says:

      How long before there is a phone app for that? :-) After all, there is one already to use your phone as a rudimentary Geiger counter!

      • Mac says:

        Hawker Beechcraft uses a water aided ultrasound device to inspect the carbon fiber fuselages of the Premier and Hawker 4000 business jets. The water jet transmits the ultrasound waves as the device moves very slowly over the entire fuselage after it has come out of the autoclave. The water aided ultrasound inspection machine is so sensitive that very small voids in the bond can be detected and repaired before the fuselage moves on. The device examines only a tiny area of the structure at any one time so the entire process takes a very long time.

        This inspection process is very expensive, but because of its accuracy allows all involved to have confidence that the bonding is solid in all parts of the fuselage. However, this type of inspection is far too costly for a piston single such as the Cirrus or Cessna 400.

        Can an “inspection” process of sufficient reliability be created for use on composite airplanes in the field? Probably. But it’s not here yet.

        Mac Mc

        • B. Johnson says:

          Affordable field deployable hand held ultrasonic inspection units, typically using pulse echo technology, have been in use in the industry for a long time. If damage is suspected, that can be verified on the ramp quickly. Pulse echo requires only one side access and looks for reflections of sound energy from the back of the structure or from embedded defects. Here is a link to a typical unit.

  3. Hallo Mac,
    nobody recognise what will happen in a crash !Years ago we had a railroad accident:
    Many people died because of the horrible GFK-debris unlike metall the debris where
    “Splitter” sharp like hell,because in the train was made mostly from modern GFK !
    This problem is also clandestinely dicussed by sailplanemakers and – owners.

    • Herbert Kilian says:

      I own a German sailplane made mostly of carbon fiber. There’s nothing ‘clandestine’ about the risks of especially carbon fiber shards to the pilot in accidents. All glider manufacturers select fiberglass or kevlar with epoxy in the matrix to build the cockpit. Carbon is heavily used over the rest of the plane. Pilots die in glider accidents from the impact force but not really from being spiked by pieces of aircraft.
      That might be very different in a Cirrus or Dreamliner. Our cockpits are small and located entirely in front of the wing structure.


      • JimC says:

        Herbert, I suspect there is a slight language barrier here, although Hubertus’ command of the English language is very good. I do not presume to speak for Hubertus, but I interpreted “clandestine” to mean “private” or “discreet,” rather than “in secret” or “deceptive.”

        Composite builders study how a crash might cause their structures to splinter. The science behind those hazards is fairly well understood but we don’t know everything yet.

        In the 1920s and 1930s, Bernie Pietenpol wrapped the wooden longerons of his airplanes in fabric… wood is nature’s original composite… everything old is new again.


  4. Gordon Arnaut says:

    Interesting article, Mac…

    We have to be careful when fastening composites because they are plastic, so they will tend to creep, acting like a quasi-fluid over time…especially if subjected to point loads…

    That is why you need hard points in the fiber matrix, in order to spread the load over a bigger area…

    I am not a fan of composites in commercial aviation…and I work in the industry…it will take some years before the material is fully understood in this application…

    I think there is a lot of talk in the media about weight savings and how that would translate to better fuel efficiency…

    But let’s take an engineer’s pencil to this…the biggest single weight item on a commercial airplane is the fuel load…

    For example a B747-400 ER (extended range) is going to carry nearly half its max takeoff weight as fuel…about 430,000 lb out of about 910,000 MTOW…

    So if we can reduce fuel burn by 10 percent with more efficient engines then we are going to shave about 43,000 lb of fuel weight for the same range…

    The second biggest weight is the wing…which on the same airplane is about 84,000 lb…the “body system” which is engineerspeak for the bare fuse without gear or seats etc is about 54,000 lb…

    So the airframe is about 140,000 lb and a 10 percent weight saving here is only 14,000 lb… less than 1/3 of what we get with more efficient engines…

    And that’s if this weight savings actually materializes…I am still waiting to see definitive numbers on the 787…which has been plagued by problems btw…

    Btw, just the landing gear system on the 747 is 28,000 lb…more than half the fuse weight…so some savings here could pay off…you can see some sample weights for various airplanes here…

    Personally I think it is far from certain that the industry will move fully to composites…we’ll see how the 787 experiment works out…I think Boeing is doing this for business reasons and their new focus on outsourcing…rather than any real aeronautical advantage…we shall see…

  5. Gordon Arnaut says:

    Sorry I have to correct myself on the fuselage numbers as I was looking at the wrong column…the fuselage on a 747 is 68,000 lb…also should add the at 12,000 lb…and the landing gear is 32,000 lb…

    Just the cabin furnishings and equipment is 48,000 lb…That’s more than 100 lb per seat…why not build some lightweight seats using composites…?

  6. Gordon Arnaut says:

    Sorry again…that’s the tail that weighs 12,000 lb…

  7. Tom Curl says:

    There are other ways to detect impending failure, such as embedded strain guages, like are used in new bridges, and large roofs subject to snow loads.

    There is also the possibility of simply embedding thin wires into the material so that damage which stretches or breaks them sets off an alert.

    Composites have value in aircraft construction, we just need to learn different, and more advanced technologies, for maintaining them.

  8. Bob Wilson says:

    Perhaps a simple and light weight answer is a ‘smart structure.’ Lay one or more optical fibers along critical stress paths to detect abnormal strain and vibrations. For example, use a looped thread on the bottom of the lift spar; another on the top; a drag spar loop; landing gear; and engine mounts. A low power, laser, beam splitter and recombined to separate optical detectors so the interference pattern pulses would provide a pattern for normal operation. Abnormal patterns would indicate a fault (“check airframe light” reduce to maneuvering speed) and require more extensive tools determine why the strain pattern has changed.

    Bob Wilson

    • john in brisbane says:

      Oh you’re dead right. I am sure that we are heading towards aircraft being like organisms with afferent and efferent nervous systems designed in. Every part will be constantly taking stock of itself and the exterior will be a pressure sensitive skin. When you consider that RFID tags which incorporate a radio are getting so cheap that they are about to take over from bar codes, this stuff is not far off.

  9. Brian E. Evans says:

    I am now retired from that industry, but for close to 40 years I repaired glass fiber and advanced composite fiber sailplanes and motor gliders.
    As others have said here, advanced composite structures deflect, then snap back like a steel spring, when subject to abnormal loads. But like a steel spring, if you over do it, the structure fails. The problem is that the surrounding structure has not reached that point of no return, so it snaps back into place taking the damaged structure with it. The experienced eye can spot this surface anomaly, very often appearing as no more than a wrinkle or fine crack in the surface or a spalling of the gelcoat.
    The average Pilot or mechanic is not programmed to look for this kind of damage.

    He is looking for a groove or a dent in the structure, but when the nature of the damage on an advanced composite structure is pointed out. It does not take long for that person to be sufficiently aware, and to start asking the right kind of questions.
    I think part of the answer is for manufacturers of advanced composite aircraft to show their customers different levels of damage with the implications of each level.
    Brian Evans.

  10. bdk says:

    Ultrasonic inspection using a couplant gel (A-scan) can be done on the spot and is relatively inexpensive. It does however require some training to operate. The water method mentioned is called a C-scan, but is normally done in a factory to check new parts for voids and disbonds.

    Adhesion between skin and honeycomb cores can be performed with a sonic bond tester, also requiring specialised training.

    None of this is new- these tools (and qualified operators) have been available for decades.

    All this technoloy has been well proven over many years on aircraft like the F-16 and F-18. These aircraft see much higher stress and operate under more extreme conditions. Anyone that thinks the 787 is an experiment has been deceived by the media.

    Any bonded joint that also requires bolting (or “chicken rivets”) is in fact a poorly designed joint. Bonds rely on surface area for their strength, and small overlaps can carry tremendous loads. Composites have very poor bearing strength however, so bolted joints tend to be very heavy because you have to make the laminate so thick. The individual composite fibers just don’t have a lot of strength in compression when a bolt bears on them.

    If any of you are this afraid of bonding, I hope you never fly in a wooden airplane! BTW, most of the composite analysis laminate theory tools have their basis on work done by the Plywood industry.

    • B. Johnson says:

      Bonding is the utopia in composite structures relegated to experimentals, military aircraft, UAV’s and smaller aircraft. But you won’t get a purely bonded joint in a primary structure certified by the FAA for transport aircraft. Too many un-inspectable variables like poor bond prep, poor adhesion, etc. The addition of “Chicken fasteners” is the only way to prevent damage growth if there is a failure. Otherwise the bond can unzip. Perhaps with 250 souls on board a better name would be “prudence fasteners”.

  11. bdk says:

    With respect to composite damage, I believe the FAA requires that any significant structural damage will generally be visible to the naked eye. Metallic airliners already have heavy landing and over-G inspections in the books, composite airliners are no different. The static and durability testing required of large aircraft require that these areas be identified ahead of time.

    Composite fibers overloaded in compression to the point of failure will buckle, which broomsticks the fibers. It is like cracking a piece of wooden dowel and then trying to get it to fit exactly back into place. The fibers don’t all break in the exact same place (like a green stick fracture) so they won’t fit back together. Disbonds tend to manifest themselves as visible blisters on the surface, again, easily seen.

  12. Dov Elyada says:

    You don’t have to have a collision with a service truck to get a composite delamination damage. From personal experience — aircraft operating from bush-type airstrips are often impacted by pebbles thrown up by the tires or propeller stream during takeoff or landing. (That may happen also on paved runways if they are not regularly swept.) With enough energy and a strategic impact point, a pebble projectile may cause serious or even critical delamination damage. In the 1980s I witnessed a series of experiments that demonstrated this effect. Hence, homebuilts are by no means exempt from this worry.

    • bdk says:

      How do you define “serious or even critical delamination damage”? Would this go unnoticed during preflight?

      How many composite airplanes have fallen out of the sky due to this kind of damage?

      On the flipside, how many metallic airplanes have fallen out of the sky due to metal fatigue?

      • Mac says:

        The answer is that so few composite airframes have been built we don’t have a long history yet. The certification standard for failure of critical structure that would cause the loss of the airplane is one in a billion flights. That’s billion flights, not billion hours. A big number.

        As for airplanes crashing due to metal fatigue the rate is extremely low. Even in the most famous fatigue failure of the 737 in Hawaii the airplane made it to a runway even though there were injuries and fatalities. In the early days of pressurized airplanes the record was not so good with the Comet being the most prominent example of fatigue that was not understood at the time. The industry has made great strides in identifying metal behavior over a long time, but as with most advances in aviation there were failures and fatalities along the way to our present excellent record.

        Mac Mc

        • bdk says:

          I respectfully disagree. There are plenty of transport category airframes and large components out there to give a very good statistical sampling. C-17s have been built exclusively with composite horizontal stabilizers since 1999. There are literally thousands of F-15, F-16 and F-18 airframes that have been through numerous overhauls where these composite primary structures have been inspected and evaluated for not only inherent failures but for induced damage.

          My point about metal fatigue is that new examples are being identified daily and many fleets have been taken out of service because of it. Aging aircraft are a focus area for the FAA, and everything we have learned in that arena is being applied to the development and testing of composites. While engineering is never perfect, the 787 isn’t likely to suffer from any more growing pains than a comparable metallic aircraft. There are always unknown unknowns in aircraft design and analysis, but the purpose of development and service testing is to identify these items before a revenue paying passenger ever sets foot on a new aircraft design.

          One last thing, tens of thousands of hours have been spent developing documentation for the 787 which sets inspection requirements, damage limits and repair procedures. If you bump into a 787 with a baggage loader, rest assured the inspection and repair procedures are already in the book!

          • B. Johnson says:

            Add to that list nearly 1000 777s with their composite empennages, and all A300, A320, A330s, A340s, A380 empennages. The A380 also has a composite tailcone.

      • Dov Elyada says:

        Let me describe what I’ve seen.
        A carbon fiber composite panel is subjected to a moderate in-plane uniform compressive stress in a loading machine. A BB pellet is shot at the panel normal to its surface. The pellet energy is such that would hardly make a visible dent. The panel then literally explodes with a big bang. Analysis turns up an dynamic interaction phenomenon between internal delamination and local buckling of the detached laminae, enhancing each other in a positive feedback cycle.
        Now suppose such a phenomenon occurs due to a fast pebble striking a compressed primary structure component during landing.
        But let me make myself clear. I don’t mean to say there’s a great likelihood of such a phenomenon occuring outside the lab — compressed panels are mostly on the upper wing surfaces, quite protected from pebbles. Also, a non-simultaneous impact and compressive stress would probably cause a milder reaction. Neither do I imply that composite primary structures are inherently unsafe or that their certification is an oversight. I trust that over the many years since that experiment engineers have found sufficient solutions.
        To put things in proportion, all I wanted to note is that the possibility of delamination damage is not confined to airliners suffering service-truck collisions and that people who fly and maintain homebuilts and other light aircraft should be aware of it as well.

  13. Evangelos Bakas says:

    Another angle worth approaching the problem of detecting damage to airplanes, is to promote a culture of reporting any contact, or activity suspect of causing damage to an airplane, without the fear of persecution. This type of approach has done wonders for airlines that do not persecute their pilots or mechanics when they report an incident, ie: a flap overspeed, that might have caused an invisible damage, that will hurt someone else down the line.

  14. Bill Berson says:

    I think you should consider writing an article about damage detection and inspection of composite structure.
    I wrote an article about this subject that was published a few years ago in the EAA Technical Counselor newsletter. The article described my method of looking at condensed dew on the wings to find main spar disbonds on my G109. The spars and inner structure are visible under some early morning and evening conditions as dew forms. A disbond might be visible.
    Unfortunately, not many read the TC newsletter.

    • Brian Evans. says:

      Some 25 years ago, I did a lecture for FAA inspectors at Embry Riddle in Daytona Beach Florida on finding hidden damage in glass fiber aircraft where I used a Grob G109 as the subject aircraft.
      I will see if I can dig out those old notes.


  15. Bob Hartunian says:

    Damage tolerance of composite structures has been the weak point in the technology. The first epoxy resin systems (Narmco 5208) had low elongation and resistance to impact energy. Compression strength after impact was severely reduced. Impact damage could produce subsurface delamination or crack propogation invisible at the surface. The only way to pick it up was by ultrsonic inspection which is expensive/time consuming. Resin formulators improved toughness by adding agents which provided higher elongaation and increased the resistance of laminate to onset of delamination. All aircraft primary structure now uses toughened systems and some combine “health monitoring” features to signal if damage happens and it’s extent.
    Composite (carbon) structures have been flying since 1977, first on military aircraft to prove worthiness and gain service experience, and then gradually on commercial transports as the level of confidence was established. The design and manufacturing abilities and material systems are well proven with years of field experience. B-2 bombers are flying since 1989 with less toughened epoxy and T-38 wings have been in service even longer. So although it has taken close to 40 years to acquire the confidence levels and cost reductions to allow composite transports to fly passengers, the 787 and others will incorporate these materials because they save fuel costs.

    • bdk says:

      There has ben a DC-10 flying with an all composite vertical stabilizer since 1984. To the best of my knowledge it is still in commercial service.

      “A 737 R/H horizontal stabilizer was one of the oldest medium primary structures built using composite materials. The structure was built as part of the NASA ACEE program using carbon reinforced graphite epoxy composites, was certified by the FAA and entered service in August 1984. The structure was in service for the following 18 years and was retired in 2002 after accumulating 48000 flights.”

  16. B. Johnson says:

    The composite material on the 787 is not your run of the mill glider or experimental material, or even Premier or Cirrus material. The epoxy matrix used on that aircraft is the toughest in the industry and is well suited for the abuse of every day airline use. It is not new. It is the same material that has been used on the 777 empennage for over 15 years. Check out the video. The similar amount of energy on an aluminum panel would punch a hole. The toughness of the resin prevents or minimizes hidden damage. As was said earlier, the FAA requires the structure to handle barely visible damage.

    Also Airbus has been flying composite empennages from the A300 on without any issues. And no, the composite material was not the issue on the one A300 that lost the vertical fin.

    Other advantages afforded by the composite materials are larger windows without increased weight and lower cabin altitudes for a more comfortable ride. These do not save weight, but instead adds performance to the aircraft that will differentiate it from the competition. Also, long term maintenance will be less because corrosion does not occur in the composites. Older airliners are always fighting the corrosion problem.

    One thing you will not find on a commercial aircraft primary structure is purely bonded joints on primary structure. There are not any adequate methods of ensuring proper bonding once the bond is complete. Plus bonds typically do not have crack arresting features in them. Any bonds, co-bonds, or co-cured joints do get fasteners through them to prevent peel and to arrest dis-bonds. The low level of risk assumed on GA aircraft or experimental aircraft using bonding joints is just too high to be acceptable for large commercial aircraft.

    Crashworthiness is also an advantage of the composite. Energy absorption in carbon fiber structures occurs through crushing, rather than yielding, but significantly, fire will not burn through a composite panel like an aluminum structure. The matrix may burn, but that is only 40% of the material. The carbon fiber is formed at 2000F and can withstand an open flame well beyond the melt temp of aluminum, acting as a fire barrier longer. Flame retardant in the matrix also minimizes the burning of the matrix.

    Damage tolerance of a properly designed composite structure will set the 787 and A350 apart from the rest of the fleet.

  17. Eugene says:

    It is the best discussion on the subject, I’ve ever read before. Thank you folks for new knowledge and fresh look on the things. I’ve met with the problem many years ago while flying and maintaining the rasin aifraime two seat amphibian homebuilt.

  18. Bob Hartunian says:

    When I worked at Douglas Aircraft on ACEE Program, we built Upper Aft Rudders for DC-10 as secondary structure and then the Vertical Stabilizer as primary structure. Lockheed built the Inboard Aileron for L-1011 and Boeing made spoilers for 727. All components flew successfully and started the gradual conversion away from metals to more composites. This is a very slow, conservative process because of the passenger lives at risk on commercial transports and much engineering effort and money went into developing the analysis tools for design and manufacturing technology to keep costs down and reliability high. When we were making design or manufacturing choices, one of the basic questions we asked ourselves was- “If you made this choice, would you put your wife and kids on this plane?” That above all governed the decisions.
    Bob H

  19. Mitch Velickovich says:

    Mac. I noticed no one is talking about ramp temperature limitations imposed on composite aircrafts. Most of GA production aircraft (both experimental and certified) have a safe operational limitation if the internal spar reaches over 161f. Which is easely attainable if on a ramp in some worldwide locations.

    • Mac says:

      Hi Mitch,

      Some certified composite airplanes survived extreme temperature and humidity tests that hte FAA beleives represent the worst nature can deliver and have no high temp operating limits. Other airplanes, such as some of the Diamond models, have certified temp limits and a thermometer device to show if the spar temp, for example, is above the limit.

      Over the years the FAA has gained enough experience with composites to allow colored paint to be used. At first composite airplanes could only be painted white with very specific, and small, areas of trim color. More recently Cirrus and some others have gained approval to use colors other than white. The concern is, of course, that darker colors will absorb more heat energy from sunlight and perhaps weaken the composite.

      But, temperture and humidity changes remain a challenge for composite structures and are just additional reasons why the potential weight savings of composites cannot be fully achieved in the real world of operating airplanes in all possible conditions.

      Mac Mc

      • bdk says:

        Most 250F autoclave cured composites have a service temperature of 180F. Anything lower tends to be non-structural fairings.

        There are plenty of military aircraft painted dark gray (or black).

        I would be more concerned with homebuilts that utilize a wet layup/room temperature cure build process. The issue with temperature has to do with cross-linking within the resins. Higher temperature cure materials have a higher glass transition temperature and are thus stronger and more stable at elevated temperatures.

        • B. Johnson says:

          Good point bdk. And the 787 used 350F autoclave cured composites. Just another example of how knowledge of composites for GA aircraft do not translate to composites for airliners.

          The service temp testing referred to at 180F is done at hot and wet conditions, taking into account the effects of moisture.

          The BMI matrix composites used in the F-22 and F-35 are good to 400F service temperatures. Mach 2 is hotter than a summer day at mojave

  20. Mitch Velickovich says:

    I own an experimental giles 202 which is 100% carbon fiber. My one method of inspection at every condition inspection (annual) is the tap method. If any debonding exists a nice void thud sound will reveal. This unfortunatelly is not practical methos in large corporate or commercial operations but it is the most basic and reliable form of inspection available not to mention how cheap your required tool is….(quarter coin)

  21. Pingback: Questions about Composites | High Altitude Flying Club

  22. Scott Jackson says:

    At the risk of derailing this excellent thread, I will point out the advantage of a lower-cabin altitude that composite airframes allow comes at the often-overlooked penalty of the weight of the compressed air, on the order of several tons for a 747, IIRC.

    • Dov Elyada says:

      Myself, I do not IIRC things like that. I estimate:

      Suppose the 747 cabin length is 50 meters and its diameter is 6 meters. Than its volume is 50*pi*6^2/4=1414 cubic meters. The density of air at standard atmosphere MSL is 1.225 kg/m^3, so the weight of all the air in the cabin at MSL is just 1732 kg. Even if as much as a quarter of that air is added to keep the cabin altitude lower than in a comparable metal structure, the weight penalty would be just 433 kg — a far cry from several tons.

      Although the cabin dimensions I chose for the calculation, and consequently the final result, are only rough estimates, the conclusion is nevertheless valid.

      Sorry for using SI units, it’s kind of religion.

  23. Bob Hartunian says:

    There is a big difference between ambient temp epoxies, as used on homebuilts, and 350F cure systems, as used on primary structure for transports, in the Tg or softening temp. A RT cure epoxy will have a Tg around 160F, which can be increasd another 20F by postcure at 200F, but most homebuilt structures are too big for a kitchen oven. The 350F systems have a Tg around 325F, far above any thermal exposure from sitting on tarmac. Moisture saturation will reduce the hot/wet properties but with still plenty of structural margin.
    You don’t want to use BMIs on transport primary structure because of low damage tolerance/compression after impact. BMIs are used judiciously in hot areas above the working temps of epoxies. Some military vehicles use BMIs for exposures around 375- 400F and polyimides for needs around 550F. As temps increase, so does the possibility of thermal oxidation which degrades resin strength.

    Coin tapping for defects is an old technique used before ultrasonics were developed and is very limited in sensitivity and can not record a structure’s original internal signature for later comparison with service time. It’s fine for homebuilts in looking for delams but too simplistic for transport reliability. The best systems employ a rastering water squirter which couples an ultrasonic signal to the part and receives the emerging signal as a recording of attenuation thru the part. It is really effective and all primary structure is inspected this way.
    Bob H

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  25. Sam Craig says:

    The Hawker 4000 and Premier 1A are changing the world as we speak because they offer amazing performance and speed. They are leading the private jet industry in terms of composite materials.

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