Aircraft Fuel Systems — Page 316, Lesson 394

I want to walk you through the additives that go into jet fuel, and then the big problem of water in the fuel, and finally a phenomenon called waxing. We're in the middle of the fuel chapter, so let's pick up with the last of the fuel additives. The first one is HITEC, which is a lubricity agent. Now, "lubricity" just means the ability of a fluid to lubricate. Jet fuel on its own isn't a great lubricant, so a lubricity agent is added to reduce wear in the fuel system components — specifically things like the pumps and the fuel control unit. Those are moving parts that rely on the fuel to keep them from grinding against each other. Next is a static dissipater additive. This one partially eliminates the hazards of static electricity. Here's the problem: when fuel moves rapidly through modern high flow rate fuel transfer systems, it generates static electricity. That's a real hazard, especially during refuelling and defuelling. The additive helps dissipate that static charge so it doesn't build up to a dangerous level. Then we have corrosion inhibitors. These protect ferrous metals — that means iron-based metals — in the fuel handling systems, such as pipelines and storage tanks, from corrosion. There's an interesting side note here: certain of these corrosion inhibitors appear to improve the lubricating qualities, the lubricity, of some gas turbine fuels. So you get a two-for-one effect with some of them. Finally, metal de-activators. These suppress the catalytic effect which some metals, particularly copper, have on fuel oxidation. In plain terms, copper can act as a catalyst — it speeds up the chemical reaction of the fuel oxidizing, which is a form of degradation. The metal de-activator stops that catalytic reaction from happening. Now, let's move to water in the fuel. This is a big one. Water is always present in fuel. The amount varies according to the efficiency of the manufacturer's quality control and the preventive measures taken during storage and transfer. So you can't eliminate it entirely, but you can manage it. Once the fuel is in the aircraft tanks, there are further measures to minimize water accretion — that's the build-up of water. The first measure is water drains. If the fuel can be allowed to settle after replenishment, then the water droplets — being heavier than the fuel — will fall to the bottom of the tank. Then they can be drained off through the water drain valve. That's why you see fuel samples taken from the bottom of the tanks before flight. The second measure is a fuel heater. This is provided in turbine engine aircraft fuel systems to prevent water in the fuel from freezing and blocking the fuel filters. Here's the mechanism: in gas turbine engine systems, the fuel is passed through a heat exchanger that utilizes hot compressor delivery air. The purpose is to remove any ice crystals which may have formed while the fuel was exposed to the very low temperatures experienced at high altitudes. So the hot air from the compressor warms the fuel and melts the ice crystals before they can clog the filters. Some systems also utilize a fuel cooled oil cooler. This is a clever piece of engineering — it uses the hot engine oil to warm the fuel, and in doing so, it also cools the oil. So you're solving two problems at once: warming the fuel to prevent ice, and cooling the oil that's doing the lubricating. The third measure is atmosphere exclusion. Once the fuel is in the aircraft fuel tanks, the main source of water contamination is the atmosphere that remains within the tank. The air inside the tank carries moisture. If the tanks are topped up to full, then the atmosphere is excluded together with the moisture it contains, thus minimizing the likelihood that the fuel will be contaminated. But — and this is important — caution is required here. Filling up the tanks may prove an embarrassment the next day if the ambient temperature rises. Why? Because the volume of the fuel in the tank will increase as it warms, and there's a danger that it may spill out of the vent system. So you fill the tanks full, the temperature goes up overnight, the fuel expands, and it spills out the vents. That's the embarrassment. There's also a performance penalty. Filling the fuel tanks may make the aircraft too heavy to take off with the required traffic load, and some defuelling may be required. So you have to balance the benefit of excluding atmosphere against the weight penalty and the expansion risk. Now, the last topic: waxing. Waxing is the depositing of heavy hydrocarbons from the fuel at low temperatures. The deposits take the form of paraffin wax crystals. These crystals can clog the fuel filter and interfere with the operation of the fuel control unit. So it's a mechanical blockage problem. The effects of waxing can be minimized in two ways. First, the refinery keeps the levels of heavy hydrocarbons low — so it's a fuel quality issue at the source. Second, the inclusion of a fuel heater in the engine fuel system — the same heater we talked about for ice prevention also helps with waxing, because warming the fuel keeps those heavy hydrocarbons from crystallizing. So to tie it together: we have additives to protect the fuel system from wear, static, corrosion, and oxidation. Then we have water, which is always present and managed through drains, heaters, and atmosphere exclusion. And finally waxing, which is a low-temperature problem managed by fuel quality and heating. Each of these is about protecting the fuel system components and ensuring the fuel flows cleanly to the engine.