Figure 2-1. Vaporization Characteristics
Table 2-2. AVGAS Identification
100/130 High Lead
100/130 Low Lead
2-4. Contamination of Fuels. There are several forms
of contamination in aviation fuels. The higher the
viscosity of the fuel, the greater is its ability to hold
contaminants in suspension. For this reason, jet fuels
having a high viscosity are more likely to have
contaminants. Table 2-3 shows visual contaminant
characteristics. Water, solids, and microbial growths are
the principal types of contamination.
Water. Either fresh or salt water may be present
in fuel, and either may be present as dissolved or free
Dissolved water. Dissolved water is water
that has been absorbed by the fuel. It cannot be seen and
cannot be separated out of the fuel by either filtration or
mechanical means. The danger of dissolved water is that
it settles out as free water when the fuel is cooled to a
temperature lower than that at which the water dissolved.
Such a cooling of fuel is likely at high altitudes. Once
freed, all the dangers of free water are present.
Free water. Free water can be removed
from fuel by adequate filtering. It can be seen in the fuel
as a cloud, an emulsion, droplets or, in large amounts, as
water on the bottom of a tank, sample container, or
filter/separator. Free water, either fresh or salty, can
freeze in the aircraft fuel system, can make certain
aircraft instruments malfunction, and can corrode the
components of the aircraft fuel system. (Salt water is
more corrosive than fresh water.) Ice in an aircraft fuel
system can make the engines fail.
Solids. Sediment from tanks, pipes, hoses,
pumps, people, and the air contaminates fuel. The most
common elements of the sediment found in aviation fuels
are bits of rust, paint, metal, rubber, dust, and sand.
Sediment is classified by particle size as shown in figure
Coarse sediment. Particles classified as
coarse are 10 microns in size or larger (25,400 microns =
1 inch). Coarse sediment settles out of fuel easily, and it
can also be removed by adequate filtering. Particles of
coarse sediment clog nozzle screens, other fine screens
dangerously, the fuel orifices of aircraft engines. Particles
of this size also get wedged in sliding valve clearances
and valve shoulders where they cause excessive wear in
the fuel controls and fuel-metering equipment.
Fine sediment. Particles classified as fine
are smaller than 10 microns in size. Removing fine
sediment by settling or filtering is effective only to a
limited degree; the particles can, however, be centrifuged
out in a rotating chamber. Fine sediment accumulates in
fuel controls and forms a dark shellaclike surface on the
sliding valves. It can also form a sludge-like material that
Particles of fine sediment are not visible to the naked eye,
but they do scatter light. This light-scattering property
makes them show up as point flashes of light or as a
slight haze in the fuel.