Understanding Aviation Fuels

The aircraft fuel sold and used at most FBOs and airports is 100LL (low lead) avgas or Jet-A. Let’s take a moment to examine the differences between various aviation fuels. Information courtesy of stackexchange.com.

Most aviation fuels fall into two separate families — kerosene or gasoline. The difference between the families mainly has to do with the molecular weight (and thus boiling point/vapor pressure) of the fuel components, in order from heaviest to lightest. The differences in fuels within each family are related to refining, blending, and additives, as well as process controls imposed on the fuel.

Jet-A Fuel and the kerosene family

The aviation kerosene family includes one of the most popular and widely-used jet fuels known as Jet-A. Jet-A is a high-purity, kerosene-based fuel and used in jet turbine engines and manufactured under a specific ASTM International petroleum standard (D1655), with specific physical properties.

Jet-A is suited for turbine engines, but can also be burned in other compression-ignition piston engines like diesel can. There are a wide variety of “aero diesel” engines available from several manufacturers.

Fuel not meeting the Jet-A specification is generally recycled for other purposes, for example it may be used in blending kerosene for heating, or blended into diesel for trucks.

Other grades of kerosene are used for motor fuels, cooking fuel, heating, lighting, and other uses, depending on their refining and purity.

Avgas and the gasoline family

Aviation gasoline, commonly known as Avgas, actually comes in several grades, all manufactured to an ASTM International standard (D910), with specific physical properties and specific permitted and required additives, such as Tetra-Ethyl Lead — TEL. 100LL is the most widely available aviation fuel because it works in the widest range of engines.

The grade or octane rating of aviation gasoline is identified by colored dyes added to the blend. The colors used in the United States are:

• Green: AvGas 100 (100/130 Octane)
  
• Blue: AvGas 100LL (100/130 Octane). This was formulated as a replacement for AvGas 100, with half the lead, thus the name 100LL for Low Lead.
• Red: AvGas 80 (80/87 Octane)
  

The difference between aviation and automotive fuels

Why don’t we burn less expensive diesel or other fuel in a jet aircraft? Purity and confidence. The additional controls imposed on Jet-A fuel by the associated ASTM standard ensures the production of a fuel with well-known properties.

When you fuel a plane with Jet-A, you can be confident the fuel won't freeze in the tanks at altitude (at least not unless you cold-soak it below -40 degrees). You also know the fuel will be "clean" and won't gunk up fuel filters or leave deposits inside the combustion core of the engine that can cause problems later.

On the other hand we don't burn Jet-A in an 18-wheel truck because it would be prohibitively expensive — highway diesel doesn't have to meet the same strict tolerances as jet fuel, and there’s no reason to burn a more tightly specified—and thus more expensive — fuel when a less expensive one is readily available.

Commonly used aviation fuels

100LL — The common "avgas" found at most GA airports and used by most piston-engine aircraft. Similar to a super-premium gasoline in formulation — it contains a lot of high-octane alkylate 7md with a small amount of tetra-ethyl lead (TEL) added to it.

87UL/E0 — A tightly specified form of motor gasoline (mogas) without any ethanol or oxygenates in it because many aircraft fuel systems aren't compatible with ethanol used in low-compression aviation piston engines mounted in airframes certified or STCed for operation on motor gasoline. Many light (SLSA) and ultralight aircraft, as well as some certificated types, can use this fuel as it can be cheaper than 100LL, though it’s rarer to find it on the ramp. Similar fuels are called 82UL or 85UL.

Jet-A/JP-5/JP-8 -- It is a highly refined kerosene jet fuel, or turbine fuel in some contexts. Can be burned both in turbine engines and in diesel piston engines, Spark-ignition engines can't use it though. Additive packages such as Prist™ or generic fuel system icing inhibitor (FSII), static dissipative additives, biocides, and other additives are commonly found in this fuel. It can be found anywhere commercial aviation is present, and thus is available worldwide.

The future of aviation fuel

Research is ongoing in the U.S. to develop an unleaded fuel with sufficient octane rating to be used in high-compression piston engines. Several refiners have been working on high-performance blended fuels based on alkylate base stocks with proprietary additive packages to meet this need. Types include 94UL, G100UL, 100SF, and 91/96UL.

Source: stackexchange.com, a question and answer website for pilots, mechanics and aviation enthusiasts.

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