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.
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.
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.