Selecting the correct fuel for an aircraft is one of the most safety-critical tasks performed on the ground. A fueling error — putting Jet-A into a piston engine or AVGAS into a turbine — can be catastrophic, and such accidents have occurred in general aviation with fatal results. The FAA and fuel industry have developed a layered system of fuel types, color-coding, container markings, and nozzle sizing specifically to make misfueling as difficult as possible. Every Aviation Maintenance Technician (AMT) must understand these standards at a level deep enough to prevent errors and to recognize when something is wrong before the aircraft ever leaves the ramp.
This article covers the two primary families of aviation fuel — aviation gasoline (AVGAS) and aviation turbine fuel (Jet fuel) — along with their grades, colors, and the physical and procedural safeguards built into the system.
Aviation Gasoline (AVGAS)
AVGAS is formulated specifically for reciprocating (piston) aircraft engines. Unlike automotive gasoline, AVGAS must meet stringent specifications for volatility, energy content, and — critically — resistance to detonation under the high compression and manifold pressures used in aircraft engines. Detonation resistance is expressed as an octane or performance number rating.
AVGAS Grades and Colors
The grade designation system for AVGAS uses a number that reflects its antiknock rating. The FAA Weight and Balance Handbook and the Aviation Maintenance Handbooks reference the following grades in common use:
- 100LL (Low Lead) — BLUE: This is by far the most widely available AVGAS in the United States. The "100" indicates a lean-mixture octane rating of 100, and "LL" means it contains a reduced amount of tetraethyllead (TEL) compared to the older Grade 100. It is dyed blue to distinguish it visually from other fuels. Most piston aircraft operating today are certified to use 100LL.
- Grade 100 — GREEN: Higher lead content than 100LL, same octane rating. Grade 100 (green) is rarely found at U.S. airports today but was once standard. Its color is green.
- Grade 80 — RED: A lower-octane AVGAS (80/87 octane rating) formerly used in lower-compression engines. Dyed red. Grade 80 is essentially obsolete in the United States but may appear on older type certificates and legacy documents.
- Unleaded AVGAS (G100UL / UL94): The industry is actively developing and certifying unleaded replacements for 100LL. UL94 (clear/purple-tinted in some formulations) is approved for use in engines with lower compression ratios under specific STCs. G100UL is a 100-octane unleaded alternative in the FAA approval pipeline. AMTs should follow current aircraft type certificate data sheets and applicable STCs to determine acceptability.
A key rule: never use a lower-grade AVGAS than the engine requires. Using a fuel with an octane rating below the engine's minimum can cause detonation — uncontrolled combustion that can destroy pistons, connecting rods, and cylinder heads within minutes. Using a higher grade than required is generally acceptable from a detonation standpoint, but lead fouling of spark plugs may increase.
Aviation Turbine Fuels (Jet Fuel)
Turbine engines — turbojets, turbofans, turboprops, and turboshafts — burn a kerosene-based fuel that is chemically and physically very different from AVGAS. Jet fuels are defined by their flash point, freeze point, energy density, and other properties rather than by an octane rating.
Jet Fuel Grades and Colors
- Jet-A — CLEAR to STRAW (pale yellow): The standard commercial and general aviation turbine fuel in the United States. Jet-A has a freeze point of –40 °C (–40 °F) and a flash point of at least 38 °C (100 °F). Its color is clear to straw — it looks like pale water or light tea. Jet-A is the fuel you will find at virtually every U.S. FBO with turbine service.
- Jet-A1 — CLEAR to STRAW: Similar to Jet-A but with a lower freeze point of –47 °C. Jet-A1 is the international standard and is used extensively outside North America. Visually identical to Jet-A.
- Jet-B — CLEAR to STRAW (sometimes light purple): A wide-cut gasoline/kerosene blend with much higher volatility and a freeze point of approximately –50 °C. Because of its volatility and fire hazard, Jet-B use is largely confined to extremely cold-weather military and remote operations in Canada and Alaska. Jet-B is not interchangeable with Jet-A without specific aircraft approval.
- JP-8: The primary military equivalent of Jet-A1, with additives for corrosion inhibition and lubricity. Clear to straw in color. Civilian AMTs working on military contract aircraft should confirm fuel spec applicability.
The Critical Danger: Jet-A into a Piston Engine
Jet fuel has a much lower octane rating than AVGAS — roughly equivalent to 15–20 octane — and its energy content per volume is slightly lower. If Jet-A is introduced into a piston aircraft, the engine will typically run rough or not at all during the takeoff roll, but in some cases it may run long enough to get airborne before failing catastrophically. Because Jet-A is clear to straw and 100LL is blue, a visual check of the sump sample can reveal a misfueling: the blue color of 100LL should be evident in a proper sample. A clear or yellowish sample from a piston aircraft's sump following fueling is a warning sign.
Conversely, introducing AVGAS into a turbine engine causes different problems: AVGAS contains TEL, which deposits lead oxides on turbine blades and fuel system components, causing damage and potentially voiding the engine's type certificate approval. Even small amounts of AVGAS contamination in a jet fuel load must be treated as a serious maintenance event requiring engine inspection per the engine manufacturer's maintenance manual.
Physical Safeguards Against Misfueling
The industry uses nozzle and port sizing to create a physical barrier against misfueling:
- AVGAS nozzles are smaller in diameter (typically 1 inch / 25 mm) so they will physically fit into the smaller AVGAS filler ports on piston aircraft.
- Jet fuel nozzles (deadman nozzles) are larger in diameter and will not fit into the standard AVGAS filler port of a piston aircraft. This is intentional — it is a go/no-go physical check.
- Truck and placard color-coding: Jet-A fuel trucks and placards are typically marked in black with white lettering. AVGAS trucks and dispensers are marked with the fuel color and grade (e.g., blue for 100LL). The AMT should verify both the aircraft's placard and the fuel truck's markings before opening any filler cap.
Fuel Sampling and Contamination Checks
Before flight, an AMT or pilot performs a fuel sample check by draining a small quantity from each sump point into a clear sampler. The checks are: color (correct for grade), clarity (no haze or cloudiness that could indicate water contamination), and absence of sediment. Water is denser than AVGAS and will sink to the bottom of a sample as a clear bead or bubble. Water in Jet-A can be harder to detect visually because Jet-A itself can appear clear — technicians may use water-detecting paste on a dipstick for fuel tanks where visual sampling is insufficient.
Key Numbers and Rules
- 100LL: antiknock index 100, dyed blue, low lead
- Grade 100: antiknock index 100, dyed green, higher lead — rare today
- Grade 80: antiknock index 80/87, dyed red — obsolete
- Jet-A freeze point: –40 °C; flash point: minimum 38 °C (100 °F)
- Jet-A1 freeze point: –47 °C (international standard)
- AVGAS filler nozzle: ~1 inch diameter (smaller); jet nozzle: larger — a jet nozzle cannot fit into a piston filler port
- Always verify fuel type against the aircraft's fuel placard, Type Certificate Data Sheet (TCDS), and Pilot's Operating Handbook (POH) approved fuel list before fueling
- Any cross-contamination event (AVGAS into turbine or Jet-A into piston) must be reported and the aircraft grounded until inspected per the manufacturer's maintenance manual
Memory Aid
Use the phrase "Blue is for pistons, Black is for turbines" as a quick ramp-side reminder:
- Blue — 100LL, AVGAS, piston engines, blue color, smaller nozzle
- Black — Jet-A placard/truck markings, turbine engines, larger nozzle, clear/straw fuel color
This simple two-color contrast helps prevent the most dangerous misfueling scenario: grabbing the wrong nozzle on a busy ramp.
Common Test Traps
- Confusing Grade 100 (green) with 100LL (blue): Both have an antiknock rating of 100, but they differ in lead content and color. The AMT exam may ask about color for a specific grade — memorize blue = 100LL, green = 100, red = 80.
- Assuming Jet-A and Jet-A1 are identical: They are very similar but differ in freeze point (–40 °C vs. –47 °C). Substitution requires aircraft manufacturer approval — they are not universally interchangeable.
- Believing AVGAS is safe in a turbine "in an emergency": It is not. Lead from AVGAS causes irreversible damage to turbine hot-section components. There is no approved emergency exception for most civilian turbine aircraft.
- Missing the water check in Jet-A: Because Jet-A can be clear, some technicians assume a clear sample is clean. Water can dissolve into Jet-A under warm conditions and then fall out of solution when cooled — always use proper water-detection methods in addition to visual inspection.
- Ignoring the TCDS or POH approved fuel list: An aircraft may be approved for one grade but not another, even if both are AVGAS. Operating on an unapproved fuel grade can constitute a maintenance release error and violates airworthiness requirements under 14 CFR Part 43.