Recently the FAA issued a warning to operators of jets and turboprops not to substitute K-1 kerosene (used for home heating, among other things) for Jet-A. The FAA had discovered a parachuting operation using K-1 in its fleet of turboprops and warned other turbine operators not to do the same.
The kerosene used in this case may work for a considerable amount of time until one gets a load of bad fuel, the FAA reported. The manufacturing quality control system used to produce K-1 kerosene is far less stringent than for Jet-A aviation fuel.
The use of kerosene in certified aircraft is a violation of the aircrafts type certificate, but warbird aficionados have wondered what differences, legalities aside, would come of using kerosene instead of Jet-A. The recent influx of relatively cheap Eastern European ex-military aircraft is fueling much of the debate, since most are licensed as experimental planes. Using different fuel certainly qualifies as an education about things unknown, and isnt that what the experimental category is all about?
Cheap, of course, is a relative term when talking about fighter jets. However, even the well-heeled may balk at the appetite some military aircraft have for fuel. Consider the MiG 21, which burns 450 to 650 gallons per hour at cruise and, for the adventurous who want to flip on the afterburner, 67 gallons per minute.
Obviously saving a few cents per gallon can really add up.
The characteristics of both kerosenes are determined by specifications set forth by the American Society for Testing and Materials. Early jets were fueled by kerosene, but as the turbines became more specialized, the standards for refining and handling Jet-A became more stringent.
There are four main factors to consider in determining turbine fuel choice: burn characteristics, lubricity, corrosive properties and fuel purity.
From a refining standpoint, K-1 and Jet-A are very similar. When it comes to burn characteristics such as heat of combustion and volatility, the two are indistinguishable.
But the two are also very different. Jet-A is formulated to withstand lower temperatures. While K-1 will begin to form wax crystals at -30 degrees C, Jet-A will safely will go down to -40.
That difference becomes important during extended high-altitude flight, because the wax crystals can clog filters, pumps or fuel lines and cut off engine power. For the purposes of most civil pilots flying ex-warbirds, the temperature variation is not likely to be significant because the planes are capable of only a relatively short time aloft.
Lubricity, or the ability of the fuel to lubricate parts, is substantially different between the two fuels. Lubricity is largely determined by sulfur content in civil fuels. In K-1, sulfur is limited to a maximum of 0.04 percent by mass. Fuel containing sulfur produces sulfur dioxide when burned, and limiting sulfur in heating oil is one way to reduce such emissions.
By contrast, Jet-A may contain up to 0.3 percent sulfur by mass – more than 10 times what youd find in K-1. The extra sulfur helps turbine engines by keeping the fuel pumps lubricated. Substitute K-1 and start budgeting money to replace fuel pumps more often.
The corrosive properties of the two fuels are also different. Both are measured by a copper strip corrosion test in which a copper strip is exposed to the fuel and then the amount of tarnish is compared to a set of standards. Jet-A is measured by two hours of exposure, while K-1 is measured at three. Despite the shorter exposure in the Jet-A standard, the strip still must show less corrosion than in the K-1 test.
Burning K-1 in a jet may therefore increase corrosion in the tanks and fuel lines.
Fuel purity is more of a crap shoot. Jet-A and K-1 share many of the same standards for water and particulate contamination, but keep in mind that those standards exist for wholesalers buying from suppliers. Jet-A has further standards that stipulate how the fuel will be stored at the airport and handled by delivery trucks. K-1 may be handled in much less stringent circumstances.
The two fuels also vary in other ways. Jet-A may contain antioxidants and additives for anti-icing and electrical conductivity. Whether those factors will affect jet performance depends largely on the vintage of the aircraft. Newer engines and fuel systems will have been designed for a more refined fuel. Older and more primitive designs will be more tolerant of substandard fuel.
The operator of an airplane licensed as experimental has substantial leeway to use different fuels. Blindly substituting one for another may have operational ramifications far beyond the cost of fuel per hour.