Chapter 2 Engine Fuel and  Fuel Metering  Systems Fuel System Requirements The engine fuel system must supply fuel to the engine’s  fuel metering device under all conditions of ground and air  operation. It must function properly at constantly changing  altitudes and in any climate. The most common fuels are  AVGAS for reciprocating engines and Jet A for turbine  engines. AVGAS is generally either 80 (red) or 100LL (blue)  octane. The LL stands for low lead although it contains four  times the lead of 80 octane AVGAS. Jet A is a kerosene-based  fuel that is clear to straw in color. 