The
density and heat of combustion of aviation gasoline is specified.
However, other specification, such as distillation range, greatly
limit any variability in these measurements. The freezing point
of the gasoline is specified to prevent the formation of solid
hydrocarbons during prolonged cold soak at altitude. The formation
of solids would of course jeopardize fuel flow and prevent full
fuel availability. Avgas must also be stable in storage and
under a variety of conditions. A primary form of deterioration
is the formation of "gum" through oxidation and polymerization
of fuel molecules. These can deposit on fuel system components
and cause serious problems. Therefore gum formation is strictly
limited in the specifications. The addition of anti-oxidants
is required and results in good to excellent storage stability.
The sulfur content of avgas is limited to a very small amount.
This is needed because sulfur can cause a deterioration in the
anti-knock performance of the lead additive. Sulfur also contributes
to corrosion of fuel system and engine components. Fuel corrosiveness
is tested by a cooper strip corrosion test. Avgas is also tested
for water reaction. In this test, samples of fuel and water
are mixed and resulting changes are noted. This is used to detect
the presence of high octane, and water soluble components such
as alcohols in the fuel. These are not permitted and can be
detected by both volume change and phase separation in the test
sample.
