Pressure Testing Your Fuel Tank By James Goss      During the last year or so I have come across an important procedure that I think we all should do on a regular schedule as part of our airplane maintenance. Over the years when I installed a fuel tank in a model I would assemble it and try to put a little pressure in it by blowing into it to see if it would hold the pressure. This method seemed to work ok at the time but there is a much better way to make sure your fuel tank is airtight and will not leak. The reason I am so interested in the subject of leaking fuel tanks is because of the type flying I am doing at this time. I like to hover close to the ground and to do this the engine must not fail. If you are using a pumped engine such as the os .91 a small leak in the tank will not hurt anything because this type engine does not depend on feedback pressure from the muffler. Any other engine that requires tank pressure will normally quit running if the tank looses pressure. For most 2-stroke engines the tank pressure is between .2 and 1.5 psi from idle to max rpm. The 1.5-psi is not much to start with, so if you loose any of the tank pressure at all the engine will drop in rpm, especially if you are in a hover with the nose pointed straight up.      The best way to check your tank is to pressurize it with a known pressure amount. I use a small hand pump that can put positive pressure into the tank or a vacuum as well. I prefer the positive pressure because this is what the tank is exposed to in actual use. A vacuum may tend to pull a crack together whereas positive pressure will tend to open the crack to some degree. I also use a small pressure gauge that reads from 0 to 30 pounds of pressure. When I assemble the tank for the first time I always use compressed air to clean out the tank. In the past I have found several tanks that had plastic shavings in them from their molds. These cuttings ended up in my carburetor and had engine failure. (Where was that in-line filter?) After the tank is assembled I connect the pump and gauge set and pump 5 pounds of pressure into the tank. I let the tank set for about ten minutes and allow the pressure to exhaust; I then repeat the process again. If I really want to test it I leave the charged tank over night, it will not leak down any if the tank is sealed. It is not good enough to pressurize the tank and just listen for the air to escape; you need a pressure gauge so you can see the slightest amount of pressure change. The next best thing would be to charge the tank and put it under water so you can see bubbles if it is leaking. This works ok if the tank is not inside your plane, but you do not know how much pressure the tank has in it. Knowing the exact amount of pressure in your tank is the key to really knowing what is going on.  Using the pressure gauge will allow you to check the tank any time and anywhere, even at the field.      So remember that tanks having pressure leaks can cause a lot of problems for your plane and they can be hard to pinpoint without a way of testing such as this. After I set up my fuel tank pressure tester I went back and checked several tanks that I had used in the past with engines that had troubles. To my surprise I found four tanks that would not hold 5-psi of pressure for a period of one minute. From now on if I have engine trouble I will let this be the first check on my list. It is a simple test even at the field. Just get access to your fuel lines and connect the pump and pressure gauge. Set the pressure to 5 pounds and see if it holds for a few minutes. Most leaks I found were around the stopper. If you have access to your tank while it is charged to 5 lbs you can also squeeze the tank. This will surge the pressure up and down and a peak pressure of about 6 pounds really makes for a good test.      You might think that if the fuel tank only receives a maximum of 1.5 psi on a 2-stroke, even if the tank had a leak at 5-psi, it may not leak at 1.5-psi. Chances are that if it does leak at any pressure you are going to have trouble in the future. The idea of this test is to prevent a major engine failure before it happens by testing on a regular basis. If you charge the tank to 5 psi and after 5 minutes it has leaked down to 4.8 psi, even this tiny amount tells you that something is wrong and an inquire is in order.      If you are using a YS engine you had better perform this test for sure. This type engine places about 6 pounds of pressure in the tank. I have not measured the tank pressure that a YS engine develops, but I was told that it was in this range. The next time I run my YS 120 FZ engine I will test it and let you know the precise amount. I know for a fact that a great planes fuel tank will not withstand the feedback pressure of my 1.20 engine. While hovering about 30 feet off the ground the 12-ounce great planes tank exploded from excess pressure and instigated a crash. You may remember about two years back Great Planes had a massive fuel tank call back. Their tanks would always split down the front seam after only one flight. I had several to do that and several of my friends also had that to happen. So just because a tank is new doesn't mean it is ok. Always check your new tanks under controlled conditions before it is installed into your expensive plane. Your engine is no better than its fuel tank. If you are going to spend two, three, or four hundred dollars for an engine, spend a few bucks for an outfit to pressure check the fuel tank. This time will be well spent and worth the effort. Also check the system every few weeks during flying season because leaks can occur at any time and it only takes three minutes of your time. By doing it on a regular basis a fault will show up as a tiny leak before it becomes a major problem.      Here is a handy tip to help insure against pressure leaks on your tanks, especially if the tank is installed out of site. Even if the lines coming out of your fuel tank have barbed connectors or if they are straight brass pipe, use this method. Place small nylon tie wraps around each tank line for added protection against leaks. This is really needed on YS engines, but will improve any system. These nylon tie wraps will work better that the fuel line spring clamps such as Great Planes and Dubro offers. Update      Since I started this article I have now had an opportunity to check my YS-120 4-stroke to see what the maximum pressure is that the tank must withstand. I did the check last Wednesday (November 28, 2001) at the Eastaboga field. I first checked the pressure that the engine produced without it running. To do this I connected the pressure gauge on the engine side of the check valve. Each turn of the prop generated a peak pressure pulse of 2-psi. With the gauge connected on the tank side of the check valve I started to run the engine. It builds up pressure fast and in about 15 seconds with the engine at max rpm the gauge showed almost 8 pounds of pressure on the tank. This is more than I was expecting and now I am really concerned about my fuel line connections being able to handle this pressure. The slightest nick or cut on a fuel line can induce a leak in the system. This high pressure is not only acting on the fuel tank, but also on all your fuel lines and fittings that you might be using. So to pressure test this type engine you would need to charge the tank to at least 10-psi minimum and maybe 12 pounds maximum. I wish all fuel tanks came with barbed connectors because straight brass pipe offers little resistance to the fuel line coming off. I am searching for 1/8 inch barbed brass pipe at this time and if I find any I will let you know.