Aircraft Hose Assembly

If you squeeze rubber hose into the hose fitting, it bulges out somewhere else. Controlling where the bulge occurs is the key to success. Lets take a material that is just the opposite to rubber, cork. Cork has a Poisson's ratio of close to 0.0. Meaning that if you squeeze cork it doesn't bulge. This allows you to squeeze a wine cork back into the bottle whereas rubber would bulge outward and cannot be pushed back into the bottle without tapering the rubber. Both rubber and cork are used as gasket material. In fact, Lycoming rocker cover gaskets are either made from cork or silicone rubber. With a cork gasket you can torque the rocker box screws tight; whereas with a silicone rubber gasket if you tighten the screws too tight the rubber flows away from the stress and bulges out at the closest free surface.

If you don't control the bulge, you get flaps, leaks, hose liner tearing, and just trouble installing the fitting. The key to controlling the bulge is following the manufacturer's assembly instructions. If you aren't going to do that then a little understanding of a rubber property called Poisson's ratio might help.

Contrary to popular belief, rubber doesn't compress. When you squeeze the fitting nipple onto the hose, the rubber doesn't compress into a smaller volume. What rubber does when squeezed is to change shape. You can say that rubber runs away from the stress. This property of rubber is a lot like water - it will change shape to fit its surroundings. There is no permanent distortion. Technically, rubber has a poisson's (pronounced "pwa-son") ratio close to the upper limit of 0.5 meaning that it is incompressible.

When you screw the nipple into the socket the rubber must move away from the nipple to make room. There must be a free surface somewhere for the rubber to go. One free surface is at the end of the nipple where it forms a bulge. It is this bulge that gives us problems and that we need to control. We control the bulge with a mandrel and by allowing another free surface at the other end of the socket.

In the picture to the left, the blue arrow is our bulge we need to control. The red arrow is where we have left a small gap for the rubber to flow into. By providing a small gap at the other end, we reduce the size of the bulge at the end of the fitting.

A mandrel is used, shown in the picture to the left, to compress the bulge that forms at the end of the nipple. *1 This reduces the pressure against the nipple and makes it easier to thread the nipple onto the socket. The mandrel also prevents the nipple from cutting into the bulge rather than moving it out of the way. For the mandrel to spread out the bulge, it's diameter must be as large as possible and still slide into the nipple. One problem with aftermarket mandrels is that their diameter is too small. If you can wiggle the mandrel in the nipple, then it's diameter is too small.

We also allow a gap between the end of the hose and the socket for the rubber to flow into. This gap is shown in the picture to the left. If no gap is left then all of the rubber bulges out at the other end. This increases friction and makes assembly difficult, the nipple can cut into the bulge and make a flap, the bulge can partially block flow thru the hose, and a bulge can create enough friction during assembly that the rubber liner seizes to the nipple, tears free and winds around the nipple. This is the "snap-crackle-pop" sounds you hear when screwing the fitting together. I usually get a call about the "defective hose".

So how much gap should you leave and how do you create the gap? It depends. Nominally, back the socket off 1/4 turn. But here is where the manufacturer's instructions become important. Assembling the socket onto the hose is the most critical part. This is where the mistakes usually occur.

Shown to the left is a "bulge gage". This is used to make sure the bulge is not too large. Every aircraft hose of this type that leaves the factory is bulge gaged checked. The bulge gage also gives us feedback on our gap we left at the other end of the socket. There is a tolerance for this gap and we can work within the tolerance to make the gap smaller or larger to control our bulge.

So the gap at the socket end is critical to controlling the bulge. But you cannot form the proper gap unless the cut end of the hose is clean and true. This little gage to the left is an angle gage. The hose cut needs to be square within 5 degrees.

The person who installed this fitting stopped before he was finished. Why? The fitting became too tough to turn. The job isn't done when the going gets tough. The job is done when the hose is properly assembled, tested, and inspected. Since none of these were accomplished, hoses like this should not be installed and if found on the aircraft should be immediately removed.

1. A bulge is a structure that can be compressed, whereas rubber is a material that cannot. When the mandrel "compresses" the bulge it spreads the rubber forward along the length of the mandrel.

2. Why are aftermarket mandrel often made undersize? My theory is that for -8 and above they need to be undersize to remove old fittings. The nipple on larger fittings distort during their years in service and are no longer round. When a properly sized mandrel is inserted into the old fitting it gets stuck. An undersize mandrel allows you to remove and reuse this distorted fitting. Now you have too problems, a nipple that is oval shaped and a mandrel who's diameter is too small. When you reuse old parts and use off-sized mandrels to build an aircraft part you are asking for problems.