Types of Aircraft Fuel Grades

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Aviation gasoline (<abbr title="Aviation gasoline">AVGAS</abbr>) is the most commonly used fuel in piston-engine aircraft.

<abbr title="Aviation gasoline">AVGAS</abbr> is identified by an octane or performance number (grade), which describes how much compression a fuel can withstand before igniting. A higher octane rating means that the fuel is less likely to pre-ignite (or explode unexpectedly — also known as detonation) at a higher pressure and damage an aircraft's engine. Lower grades of fuel are used in lower-compression engines because these fuels ignite at a lower temperature.

Most aviation fuel contains lead. The <abbr title="Federal Aviation Administration">FAA</abbr> is currently working to eliminate lead in aviation fuel due to its potentially harmful effects on both human health and the environment — this is the fuel of aviation fuel. While the <abbr title="Federal Aviation Administration">FAA</abbr> does not directly regulate aviation fuels, it does provide initial certification approval of the aircraft with the fuel it operates on as well as oversees that aircraft operators use the correct fuel. 

If a fuel grade is unavailable the next higher grade should be used as a substitute; however, **never use Jet A in aircraft with reciprocating engines**. A color-coded system is used to help identify fuels by grade. 

_Aviation fuel color-coding system_

| Designator | Color | Engine Type |
| ---------- | ----- | ----------- |
| <abbr title="Aviation gasoline">AVGAS</abbr> 80 | Red | Reciprocating |
| <abbr title="Aviation gasoline">AVGAS</abbr> 100 | Green | Reciprocating |
| <abbr title="Aviation gasoline">AVGAS</abbr> 100LL | Blue | Reciprocating |
| Jet A | Colorless or Straw | Turbine |

There is also 91 octane <abbr title="Aviation gasoline">AVGAS</abbr>, which is a less common unleaded fuel type that is colorless. We will not cover this fuel type here.

_Note: The following image illustrations exaggerate the coloring of the fuel. The actual color will likely be less saturated and require careful examination to determine the correct fuel grade used._

What is Fuel Grade and Why Does it Matter?

_Fuel Color: Red_

This strict form of motor gasoline (MOGAS), is formulated without any ethanol or oxygenates in it because of its aimed use in low-compression aircraft engines. Although not commonly used, some certificated types and ultralights use this as it's often cheaper than 100LL. However, this fuel type is no longer in production commercially.

AVGAS 80

_Fuel Color: Green_

The standard high octane, high lead fuel for aviation piston engines. Previously the most used fuel for reciprocating engines, but now is being replaced by 100LL.

AVGAS 100

_Fuel Color: Blue_


The most common and reliable type of <abbr title="Aviation gasoline">AVGAS</abbr> is 100 octane Low Lead, also known as 100LL. This leaded fuel contains tetra-ethyl-lead (<abbr title="Tetraethyl Lead">TEL</abbr>), which is an additive used to prevent engine damage at higher power settings; it was formulated with half the lead content as a replacement for <abbr title="Aviation gasoline">AVGAS</abbr> 100, which is why it's designed as "Low Lead."

There is also 100VLL (very low lead), which is also blue in color and indistinguishable from 100LL. 100VLL <abbr title="Aviation gasoline">AVGAS</abbr> contains 19% less lead content than 100LL and is acceptable for use on all reciprocating engines as it meets the performance requirements of grades 80, 91 (not covered here), 100, and 100LL.

AVGAS 100LL (most common)

_Fuel Color: Colorless or Straw_

Jet A and Jet A1 fuels are used in turbine engines and have a very high flash point (the temperature at which there is sufficient vapor to ignite in the air). It can be found anywhere commercial aviation is present and is widely available worldwide.

Jet A/A1 fuel may also include additives that either help provides a safer, cleaner fuel burn or have anti-icing properties. The most common type of additives are Prist™ (anti-icing), static dissipative additives (chemicals that minimize the build-up of static electricity), and biocides (kills pesticides).

Jet A

Some ultralight, experimental and certificated aircraft may use other alternative fuels. This is sometimes seen in some aircraft that use Rotax engines. 

Consult with the approved aircraft operating manual to be sure which fuel grades are permissible; the proper fuel grade is also stated in the <abbr title="Airplane Flight Manual">AFM</abbr>/<abbr title="Pilot's Operating Handbook">POH</abbr>, on placards in the flight deck, and next to the filler caps. Automobile gas should NEVER be used in aircraft engines unless the aircraft has been modified with a Supplemental Type Certificate (<abbr title="Supplemental Type Certificate">STC</abbr>) issued by the Federal Aviation Administration (<abbr title="Federal Aviation Administration">FAA</abbr>).

Using Alternative Fuels

Many engine failures occur due to fuel contamination. These accidents have been traced to:
- Inadequate preflight inspection by the pilot
- Servicing aircraft with improperly filtered fuel from small tanks or drums 
- Storing aircraft with partially filled fuel tanks
- Lack of proper maintenance 

Water in the sump is hazardous because in cold weather the water can freeze and block fuel lines. In warm weather, it can flow into the carburetor and stop the engine. If water is
present in the sump, more water in the fuel tanks is probable, and they should be drained until there is no evidence of water. Never take off until all water and contaminants have been removed from the engine fuel system.

Fuel should be drained from the fuel strainer quick drain and from each fuel tank sump into a transparent container and then checked for dirt and water. Contaminates are often heavier than aviation fuel so it will sink to the bottom of the mixture, making it easier to identify.  

After leaving the fuel tank and before it enters the carburetor, the fuel passes through a strainer that removes any moisture and other sediments in the system. Since these contaminants are heavier than aviation fuel, they settle in a sump at the bottom of the strainer assembly. A sump is a low point in a fuel system and/or fuel tank. The fuel system may contain a sump, a fuel strainer, and fuel tank drains, which may be collocated.

When the fuel strainer is being drained, water in the tank may not appear until all the fuel has been drained from the lines leading to the tank. This indicates that water remains in the tank and is not forcing the fuel out of the fuel lines leading to the fuel strainer. Therefore, drain enough fuel from the fuel strainer to be certain that
fuel is being drained from the tank. The amount depends on the length of the fuel line from the tank to the drain. If water or other contaminants are found in the first sample, drain further samples until no trace appears.

Water may also remain in the fuel tanks after the drainage from the fuel strainer has ceased to show any trace of water. This residual water can be removed only by draining the fuel tank sump drains. Water is the principal fuel contaminant. Suspended water droplets in the fuel can be identified by a cloudy appearance of the fuel, or by the clear separation of water from the colored fuel, which occurs after the water has settled to the bottom of the tank. As a safety measure, the fuel sumps should be drained before every flight during the preflight inspection.

Fuel tanks should be filled after each flight or after the last flight of the day to prevent moisture condensation within the tank. To prevent fuel contamination, avoid refueling from cans and drums.

In remote areas or in emergency situations, there may be no alternative to refueling from sources with inadequate anticontamination systems. While a chamois skin and funnel may be the only possible means of filtering fuel, using them is hazardous. Remember, the use of a chamois does not always ensure decontaminated fuel. Worn-out chamois do not filter water; neither will a new, clean chamois that is already water-wet or damp. Most imitation chamois skins do not filter water. 

Fuel Contamination

Refueling aircraft has inherent risks primarily due to static electricity build-up. Static electricity is formed by the friction of air passing over the surfaces of an aircraft in flight and by the flow of fuel through the hose and nozzle during refueling.

Nylon, Dacron, or wool clothing is especially prone to accumulate and discharge static electricity from the person to the funnel or nozzle. **To guard against the possibility of static electricity igniting fuel fumes, a ground wire should be attached to the aircraft before the fuel cap is removed from the tank.** Because both the aircraft and refueler have different static charges, bonding both components to each other is critical.

If fueling from drums or cans is necessary, proper bonding and grounding connections are important. Drums should be placed near grounding posts, and the following sequence of
connections observed:
1. Drum to ground
2. Ground to aircraft
3. Drum to aircraft or nozzle to aircraft before removing the fuel cap

When disconnecting, reverse the order.

Best Practices When Fueling Aircraft

FAA-H-8083-25B: Pilot's Handbook of Aeronautical Knowledge, pg. 7-19; 7-26 — 7-29

FAA Programs & Initiatives: Aviation Gasoline

Business Aircraft Jet Center: Understanding Aviation Fuels

Only use the <abbr title="Aviation gasoline">AVGAS</abbr> approved by the manufacturer. Use of a lower fuel grade than that specified by the aircraft manufacturer can cause detonation and ultimately lead to engine failure.

If a fuel grade is unavailable the next higher grade should be used as a substitute; however, **never use Jet A in aircraft with reciprocating engines.**

The aircraft fuel strainer should be drained before each flight and after every refuel. Fuel samples should be drained and checked visually for water and contaminants.

Contaminates are often heavier than aviation fuel so it will sink to the bottom of the mixture, making it easier to identify.

Consult with the approved aircraft operating manual to be sure which fuel grades are permissible; the proper fuel grade is also stated in the <abbr title="Airplane Flight Manual">AFM</abbr>/<abbr title="Pilot's Operating Handbook">POH</abbr>, on placards in the flight deck, and next to the filler caps.

To guard against the possibility of static electricity igniting fuel fumes, a ground wire should be attached to the aircraft before the fuel cap is removed from the tank.

Sump the fuel adequately enough to allow clearing of the fuel lines and give enough time for contaminants to show if any exist in the mixture.

Types of Aircraft Fuel Grades

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