Piston-engine aviation fuel is specified by ASTM D910 (Aviation Gasoline) and ASTM D7547 (unleaded Avgas). Wrong fuel grade causes engine damage or engine failure.
Current grades
| Grade | Colour | Lead | Use | Spec |
|---|---|---|---|---|
| Avgas 100LL | blue | low (~0.56 g/L TEL) | Worldwide standard for piston engines | ASTM D910 |
| Avgas 100 | green | high | Largely phased out | ASTM D910 |
| Avgas UL91 | clear/yellow | unleaded | Engines with STC (e.g. Rotax 912, some Lycoming) | ASTM D7547 / EI 5.13 |
| Avgas 94UL | clear | unleaded | Rare; older engines with STC | ASTM D7547 |
| Jet A / A-1 | clear | n/a (kerosene) | ONLY turbine / diesel engines | ASTM D1655 |
AVGAS 100LL — blue colour
The standard aviation fuel AVGAS 100LL has a characteristic blue colour (added dye to avoid confusion with other grades). The colour must always be visually verified in the fuel sample check — a different colour indicates the wrong grade or contamination.
Octane / anti-knock properties
The octane rating or fuel grade describes the anti-knock rating / properties of a fuel:
- Higher octane = higher knock resistance = the fuel burns in an ordered way without uncontrolled explosion.
- AVGAS 100LL has a Motor Octane Number of ~100 (lean) and 130 (rich).
Knocking arises from uncontrolled explosions in the fuel-air mixture — typically at high compression, lean mixture, or low octane. Knocking creates mechanical shocks on the piston and can destroy the engine.
Mogas / auto fuel — generally NOT permitted
Automotive fuel (Mogas, Super plus RON98) must generally not be used as aviation fuel — except in the exceptions explicitly stated in the flight manual:
- Some aircraft have an STC for Mogas — e.g. older Cessna 150 with Petersen Mogas STC.
- Some Rotax engines (Rotax 912) are explicitly cleared for Mogas in the Type Certificate.
- Without STC or AFM approval: Mogas is prohibited — risk of vapour lock (higher volatility), detonation (lower octane / no anti-knock additives), or fuel-system damage.
Mixing prohibitions
- Avgas 100LL must not be mistaken for Jet A (wrong nozzle = engine damage). In Europe Avgas hoses are red or yellow; Jet black.
- Avgas and auto fuel (Mogas) must not be mixed unless AFM/STC explicitly allows.
Tank selection with multiple tanks
For an aircraft with separate tanks (left/right, or main/aux), the tank specified in the AFM must be selected for take-off and landing. Reason:
- Tanks may have different supply lines, pumps, or valves.
- Critical phases require maximum supply pressure and flow.
- AFM is binding — typically "BOTH" on the C172 (both simultaneously) or "LEFT/RIGHT specific" on PA-28.
Alternate air
Alternate air is a procedure on carburetted or injected engines where air is taken NOT through the normal filtered inlet, but from inside the engine cowling:
- Use: with blocked air filter (icing, dirt, insects) or icing of the air inlet.
- The cowling air is warmer and unfiltered — not recommended for standard operation (wear).
- On carburetted engines carb heat serves the equivalent function; on injected engines it is the alternate-air valve.
Refuelling practice tip
Before refuelling: connect grounding (static dissipation), compare the grade on hose and AFM specification, water check (see §3.3) after refuelling.