--by James Williams, FAA Safety Briefing Associate Editor
Creepy as it might be, “bugs” and their cousins might be a more apt analogy for an airplane than a bird. For starters, a modern airplane’s fuselage functions as both a skin and a skeleton, a feature that resembles an arthropod more closely than members of the avian family.
One of the key features of arthropods is their exoskeleton, which combines the protective attributes of a skin with the structural attributes of a skeleton. This feature relates to an aircraft’s skin, which is actually a structural load bearing part.
Monowhat?
Modern airplanes are built using a method called monocoque construction. This method uses stressed skin as the main structural component. To help visualize this, think of a soda can. The skin sits around two bulkheads, or formers (the top and bottom of the can), providing a surprisingly strong unit when undamaged. Key advantages of monocoque construction include high strength, light weight, and increased internal volume potential. An important goal with aircraft design is to have the lightest airplane that can hold the most stuff (e.g., people and cargo) while being rugged enough to withstand the rigors of flight.
Monocoque construction does have a few drawbacks. Even small dents or dings can potentially weaken the structure. You can run your own experiment to prove this point. Take any empty soda can (undented) and apply a downward force on the top. You’ll be surprised by how much force the can will withstand. Now make a slight dent in the can and watch how little force is required to crush it. That’s a perfect example of the Achilles heel of monocoque construction.
To counter this problem, manufacturers use a method called semi-monocoque construction, which incorporates reinforcing stringers that run longitudinally between the bulkheads and formers. This method allows some of the stress to be transferred from the skin to the structural reinforcement. It makes the structure more robust but adds weight and complexity to the finished product.
Composite Composition
We often talk about composite materials as futuristic or high tech, but that’s not really true in many cases. In its most basic definition, a composite is a combination of two or more different materials, in which all individual properties of the material are preserved. The best example is decidedly low tech: concrete. Concrete is a combination of cement and small rocks and stones (called aggregate). These are the two requirements for a composite material: a matrix or binder (the cement) and a reinforcement (the aggregate). The reinforcement makes up most of the volume and carries most of the load, while the matrix holds the reinforcement together and allows it to be shaped. In aviation, we use things like fiberglass and carbon fiber that follow the same principle. A glass fiber, sometimes woven into fabric, is laid down as the reinforcement and then a resin or glue is applied as the matrix. This is usually done in several layers to provide strength.
Shaping is actually one of the key advantages of composites. It’s far easier to create smooth, rounded, or complex shapes with composites than with traditional materials. Weight can be another advantage of composites but that depends on the material. While carbon fiber can have dramatic weight savings over metal construction, fiberglass generally does not.
The use of composites has slowly grown in general aviation. It started with small, non-structural parts like wing tip fairings and wheel pants but has progressed to the whole aircraft. We usually see fiberglass in GA because the cost is substantially lower than carbon fiber. Fiberglass also allows manufacturers to try designs that would be very difficult or impossible to build in metal.
Taking Care to Keep Yourself in the Air
Regardless of material, an airplane’s fuselage is more akin to the exoskeleton of an arthropod than a vertebrate body. Birds have a skin that is important, but it isn’t a structural member. When combined with muscle tissue, birds’ skin provides some padding around their “structure” and offers some protection when “dented or dinged.” But airplanes and arthropods wear their skeleton on the outside. This is why spotting potential damage during preflight is so critical.
What to look for varies depending on the material. Metal aircraft can be easier to inspect because metal deforms from impact force. For example, if an Aviation Maintenance Technician (AMT) drops a tool while working on the aircraft, you will see a dent. Significant dents, dings, and punctures should be referred to an AMT to be evaluated. When in doubt, err towards caution. As with the soda can example, the actual dent might not look all that bad, but it could cause significant risk by compromising the monocoque.
In the case of composites, it’s possible for the material to absorb an impact without showing damage or to bounce back into shape after impact. Unfortunately, such “invisible” damage can cause delamination between the layers of fiber, or cracks in the matrix that can weaken the structure. This is why you need to have an AMT with composite experience evaluate any impact. The appearance of a whitish area in a composite may indicate delamination below.
With composites, there are also possible issues with extreme heat damaging the resin. Much depends on the particular resin system used for the aircraft, but some resins can weaken above 150ºF. While that seems out of reach, remember that aircraft are often parked in warm climates where sun, asphalt, and dark paint can combine to push temperatures to 220ºF. The simple fix is to paint composite structures white, which helps keep temperatures below 140ºF. Other sources of heat damage include exhaust leaks and minor fires that are quickly extinguished — like those from overheated brakes or electrical faults. Any potential thermal damage should be evaluated carefully before returning the aircraft to service.
Final Thoughts on the Fuselage
The fuselage doesn’t attract the attention of an airplane’s avionics or its engine. It’s not analyzed as closely as its wings or preflighted as intricately as its empennage. It is an aircraft’s skeleton. It plays a critical role without much fanfare. But like any skeleton, its faults and failures can be at best disabling, and at worst crippling. Understanding at least a little about it will help you detect any flaws before they become real problems.
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James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.