Learning objective

You can place avgas grades (colour, lead, octane/knock resistance) and say when mogas is even a candidate: AFM/STC only. Tank selection for take-off/landing is AFM, not “always BOTH like a C172” as a universal.

60 seconds

100LL is blue. Wrong grade or Jet A in a piston engine wrecks it. Octane = knock resistance. Mogas only with approval (vapour pressure, ethanol, octane). Check colour in the sample. Alternate air belongs with induction, not as a fuel grade.

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.

Exam knowledge

100LL blue; avgas 100 was green; UL91 clear/yellow; Jet A not in the Otto without approval.
Octane / grade = anti-knock.
Mogas only AFM or STC.
Take-off/landing tank: AFM (BOTH/LEFT are examples, not universal limits).
Colour and water: sample — details in water contamination.

Check yourself

1. Colour of AVGAS 100LL, and why the dye?

A) Green — unleaded avgas ASTM D7547
B) Clear/yellow — to tell it from Jet A-1
C) Blue — to tell grades apart; check it in the fuel sample
D) Red — EU hose coding

2. When may mogas be used as aircraft fuel?

A) Only if the AFM or an STC (or the type certificate, e.g. some Rotax 912) expressly allows it
B) Whenever you can buy ROZ 98
C) Always 50/50 with 100LL
D) Only in turbine/diesel engines instead of Jet A

C — 100LL is blue. Green was avgas 100; clear/yellow is UL91.
A — mogas only with AFM/STC. Do not mix without approval.