Jet engines require TLC
Most pilots are first introduced to jet fuel when their instructors warn them against fuel contamination in the tanks of their piston trainers. The stories of power loss, seized engines, and crashes because of kerosene in the tanks give new pilots the impression that jet fuel must be pretty nasty stuff indeed. To an extent this is true, but only when applied to piston engines designed to run on avgas.
Characteristics of jet fuel
In the early days of turbine engine development engineers believed that these new engines would use almost anything that would burn, and theoretically that's correct. But because of the high rate of fuel consumption in turbine engines, the effect of any impurities is magnified. This fact, along with the wide range of temperatures and pressures under which jets operate, results in the modern jet engine being fairly fussy, calling for fuels with carefully controlled characteristics to function efficiently with a reasonable service life under all operating conditions.
What are some of those characteristics? First of all, jet fuel must flow easily at a wide range of temperatures. This, at first, might not seem to be a problem until you remember that the temperatures at the altitudes inhabited by jets are extremely cold — standard temperatures in the upper flight levels are minus 50 degrees Celsius. The fuel in aircraft remaining at these altitudes for long flights becomes cold-soaked, but it must still remain liquid and flow easily.
In addition to being "pourable" at extremely cold temperatures, jet fuels must be volatile enough to permit engine start in all anticipated ground conditions and have satisfactory relight characteristics in flight. This also may seem to be obvious and an easy condition to meet, but remember that ground conditions at locations such as northern Alaska can be pretty extreme in winter. The volatility of the fuel — its ability to vaporize easily, especially at low temperatures — and the degree of atomization, which depends upon volatility and fuel pressure applied, are both considerations.
Efficient combustion at all anticipated operating conditions is also required of modern jet fuels. They also must have the highest possible energy available per unit weight. These requirements directly affect the range and payload of an aircraft since, as we all know, weight is a major consideration in aircraft operations.
Jet fuels must also do minimal damage to the combustion chamber, turbine blades, and the fuel system itself. Although this might not seem to be a big problem, the sulfur and water content of fuels tends to corrode the fuel and combustion systems. When sulfur burns in air it forms sulfur dioxide. Sulfur dioxide mixed with water forms sulfurous acid and is very corrosive. Although it is impractical to completely remove the sulfur content from fuels, it is vital to keep it to a controlled minimum. Free water can be removed from fuel before use, but dissolved water cannot be effectively removed.
The fuels pumped through jet engines lubricate the fuel system components, particularly the high-pressure fuel pumps. If you've ever handled jet fuels you know that they have an oily feeling, and that's vital to the long life of system components.
Problems with jet fuel
As you might expect, jet fuels come with their own set of problems. One of the most well known of these is turbine fuel's affinity for water. Jet fuels can contain both dissolved water and water in suspension, both of which can cause problems with fuel system icing and fuel contamination. It's impossible to eliminate all water in fuel tanks. Even if a source of fuel totally devoid of water could be found, the characteristics of kerosene-type fuels would guarantee that dissolved and suspended water would soon be present. The affinity that fuel has for water varies with the composition of the fuel and the fuel temperature, but as a rule of thumb, jet fuel will be totally saturated with water when the dissolved water in parts per million (ppm) is approximately equal to the temperature in degrees Fahrenheit. A tank of fuel at 50 degrees F will contain approximately 50 ppm of dissolved water. Additional water may be suspended in the fuel. When this fuel is cooled, the water above the saturation level will come out of solution as separate and suspended water droplets.
Jet fuels are also harder to ignite than avgas. The engine-driven pumps must provide fuel at sufficient pressure to overcome the internal pressures of .he combustion chamber, and the fuel nozzles must atomize that fuel to the proper degree for combustion to occur.
Fuel system additives
Additives are required by most jet engines to make sure that the water contained in the fuel does not freeze and form ice crystals that clog filters and screens in fuel control units. Since the interface between the hydrocarbon molecules and the water in suspension in the fuel makes an almost ideal growth medium for algae, these additives — the most commonly used is the brand name Prist — are also algaecides. An overgrowth of green slime can clog fuel filters just as completely as ice crystals.
Fuel weight
Jet fuel weighs a little more than avgas — approximately 6.7 pounds per gallon at standard temperature. The colder the fuel is, the denser it is, and consequently, the more it weighs. Since the fuel control unit measures fuel solely by weight, this can be a real benefit. If you have a choice between underground fuel and that which has been sitting in a fuel truck on a hot day, take the underground fuel. Since the pump on the truck only measures volume — gallons — and your fuel control unit only measures pounds, the more pounds you can stuff into those gallons, the cheaper the fuel will be for you.
Precautions
Jet fuels do not require any different precautions other than those taken with all fuels with regard to flammability. In fact, kerosene is harder to ignite than many other fuels, and so is probably less dangerous in that regard. The anti-icing and antifungal additives in jet fuel are considered carcinogenic, so it's probably a good idea not to get more than necessary on your skin when checking fuel. And since jet fuels have such an affinity for water, it's doubly important that sumps and tanks are properly and thoroughly drained prior to each flight. If you've just had the airplane moved or fueled, however, you won't get an accurate picture of the purity of the fuel. Water stays suspended in jet fuels about four times longer than it does in avgas.
Linda Pendleton, AOPA 525616, is the curriculum development manager for Eclipse Aviation. She has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.
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