Problem: You install a new hose and it leaks. The old hose didn't leak so the hose must be bad.
Identify the source of the leak
What is leaking?
A hose leak usually occurs along the length of the hose or where the hose exit's the socket.
Connection leaks - leaks between the fittings - leak behind the nut.
Properly identify the source of the leak, rather than casting blame at the hose is the first step to solving the problem.
When installing a replacement hose, leaks usually occur at the connection. Two conditions make this likely:
Connecting the new hose to an old fitting, and,
When both fittings have hard mating surfaces, such as steel against steel. Especially bad is stainless against stainless.
Why fitting connections leak
Aircraft flared AN fittings seal by the mating of two flat surfaces against one another. If the two surfaces are not perfectly flat against one another, then the surfaces only make contact at the high points. Fluid then flows through the spaces. The surfaces need to be either:
1. Perfectly flat and parallel to one another, or
2. Able to deform to fill in the voids. The high points must get squished flat.
Perfectly flat and parallel surfaces don't exist. Every surface has some roughness or blemishes. But the closer to perfection the better, sealing surfaces need to be as good as possible. The sealing surface on the hose is new so it's probably in good condition, but what about the sealing surface on the male nipple? Chances are that it wasn't replaced. The male nipple is deformed as it had to be to seal the old hose.
The fact that the old hose didn't leak means that the surfaces have deformed and sealed. The deformations in the older surface are going to get in the way and prevent sealing of the new hose.
The harder the surfaces - the more likely you will have connection leaks
That's why O'rings were invented. Make one surface hard and one surface soft. The soft O'ring squishes and fills in any microscopic voids and distortions.
When sealing two metal surfaces, then make one surface softer than the other. In the past, one fitting was steel and the other softer aluminum, or both were aluminum - few leak problems. But two hard surfaces, such as steel on steel, and we have to change our assembly practices.
We must be more diligent about inspecting for leaks.
AN fitting flare surfaces must be in better condition. In the past we didn't have to replace the male nipple as the aluminum fitting on the hose sealed just fine. Now we might have to replace the male nipple if we cannot get a leak-tight seal.
We need to increase our torque value to reflect the increased torque required for steel fittings. It takes more pressure to mate steel sealing surfaces as they are harder.
When tightening the fitting, we must use two wrenches to prevent over-tightening the tapered pipe end of the male fitting.
Conical Seals
With the trend towards using steel fittings, the use of conical seals is increasing. Years ago, when must fittings were aluminum on
aluminum or steel on aluminum, conical seals were never required. A conical seal places a soft surface between our two hard sealing surfaces. This soft material deforms and fills in the voids. They are easy to use and inexpensive. They are handy to have in your tool box as when you need one, you need one now!
Chafing
With aircraft hose with firesleeve, the leak may travel between the hose and firesleeve and exit next to the fitting. Shown here is an example of a chafed hose.
Leaking avgas will stain the outside firesleeve cover of the hose.
Leaking avgas will stain the outside firesleeve cover of the hose.
Hose that has a firesesleeve jacket may not show much damage on the outside. If the fireseleeve is ripped or torn then spread it apart and inspect the hose.
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Angle Fitting Not Bent Accurately
Notice gauge mark on flare. The contact surfaces are not parallel causing high contact stress at one point and no contact 180 degrees opposite. This can occur if the angle fitting is not bent accurately.
