When I first started learning to fly, nothing made me feel more bad-ass than hoisting myself up to stand on the strut of a Cessna 172 to check the fuel tanks. I don’t know if it was the feeling of being taller than my flight instructor for once or the fact that I was power posing like Captain Morgan that made me feel so mighty, but it’s always my favorite part of the preflight check.
Throughout training, I never thought about the mechanical marvel that wing tanks were or the compounds that make them possible. It wasn’t until years later as my sister and I refueled a plane on a cross country that I started to think about the why fuel tanks are being concealed in the wing.
Let me preface the situation by saying it is so difficult to lug a fuel hose and climb up the side of an high-wing aircraft when you have absolutely no arm strength.
So as I struggled to balance the heavy hose and my body weight against the plane, I thought, “Maybe it makes sense for low-wing aircraft, but why would someone put these tanks so dang high?”
I was curious (or angry that this simple task made left me sweating before hopping back into a non-air-conditioned plane) and decided to put in some research.
Wet wings, or integral fuel tanks situated in the wing, are staple structures in many commercial aircraft. These tanks are areas inside the wing structure itself that have been sealed and allow fuel storage. It’s commonly used in larger aircraft - just look outside the next time your commuter jet is being refueled, their tanks are also in the wing.
The technique I thought was over-engineered in my training aircraft is actually well designed to eliminate the need for separate fuel bladders. This is important for three key reasons:
Efficient Weight and Balance. Useable fuel accounts for a majority of the aircraft’s weight - you want to make sure you’re getting the most out of your storage space. Bladders not only contribute extra weight and reduce the useful load, but they aren’t as space-efficient. Keep in mind, the place where that heavy fuel is situated is important because that fuel keeps aircraft centre of gravity more or less in the desired position. Integral fuel tanks are all-around more efficient to maximize maximise the structural volume of the aircraft and airplane performance.
Reduce Wing Flutter. Another factor contributing to performance; the weight of the fuel provides more rigidity to the wing than if it were hollow, thereby reducing vibration caused by airflow over the wings. While wing flutter will always be a factor, reducing the effect is ideal. Large flutter can be so hazardous that it could result in a total collapse of the wing. Storing fuel in the wings actually makes your aircraft safer!
Manageable Leaks. While fixing a leak in these tanks may be impractical, as they’re sealed from the inside, the fuel from a leak is discharged externally rather than withing the cabin of the aircraft. Imagine smelling AvGas for your entire 1.5 hour training flight! Not fun. This varies according to different aircraft manuals, but most of the time if you’re ok with wiping off occasional stains or living wiht a few drips, it’s possible to go several years without major consequences.
One issue with these tanks is that they are more difficult to fix in the event of a leak. Since the tank is part of the aircraft structure, it cannot be removed without removing the entire wing. Every rivet, bolt, nut plate, hose and tube that penetrates the metal of the wing must be sealed watertight to prevent fuel seeping around these hardware components.
On the bright side, properly sealed wet wings can last for decades. Polysulfide sealant, like that PPG and PRC Desoto create, can last for 35 years or longer!
While I still strongly dislike having to weakly refuel a high-wing aircraft, I would much rather struggle for a few minutes than share a cabin with a fuel tank. These wet tanks also give me a great reason to pose like a sea captain with an intense smolder before every flight - and for me, that is enough.
