specification for fuels will give these temperatures
and the percentages of the fuel allowed to boil
For a fuel to have satisfactory handling
off to meet the desired standards.
characteristics, it must be noncorrosive and should
not clog fuel filters, even at very low temperatures.
Flash Point and Fire Point
The fuel should not produce vapor lock in the fuel
tanks or in the various fuel pumps or slugging out
The flash point is the temperature at which
of the fuel tank vents. (Slugging is the process by
which liquid fuel is carried along with vaporized
the fuel vaporizes enough to ignite with an out-
side heat source. The flash point of a fuel is an
fuel when the vapor escapes to the atmosphere.)
index of its potential safety for handling and
As far as possible, the fuel should have enough
storage. Ships require at least a 140°F flash point
of the properties of a lubricant to avoid significant
wear of the fuel-metering pumps.
for storage for safety reasons. The fire point is
the temperature where the vapors continue to burn
without an outside heat source.
Aircraft fuels must have a minimum tendency
Heat Energy Content
For aircraft engine use, it is important that the
in the efficiency of the engine results when these
fuel contain as much heat energy (thermal value)
deposits build up in the engine.
as possible, both per unit weight and per unit
volume. The thermal value is the amount of heat
expressed in calorie or British thermal units (Btu).
Only materials that will be effective when
NOTE: A calorie is the amount of heat
added in a maximum concentration of 5 percent
needed to raise the temperature of 1 gram
are considered as liquid additives. Beyond this
concentration, the material may be considered as
of water 1 degree Celsius. A Btu is the
amount of heat required to raise the
temperature of 1 pound of water 1 degree
Fahrenheit. One Btu equals 252 calories.
Gum inhibitors used in military gas turbine
fuels are the same as those used for military
Thermal value per unit of weight increases as
aviation gasolines. In aviation gasoline, gum is
almost always completely soluble and becomes
gravity increases. Energy content and density
apparent only when the gasoline is evaporated.
influence fuel selection when range or payload are
the limiting factors. This is important to under-
Both soluble and insoluble gum, especially the
insoluble form, can be expected to have serious
stand when the aircraft will be weight-limited
effects on the fuel system of the turbine engines.
rather than volume-limited.
The fuel-metering pumps, fuel pumps, and fuel
filters are likely to be seriously affected by
insoluble gum. The soluble type can be expected
to cause difficulty in the fuel system, at points
where microscopic leakage occurs and exposes
that tends to prevent it from flowing. Turbojet
The microscopic fuel leaks will usually appear at
engine fuels should be able to flow through the
fuel system and strainers under the lowest
operating temperatures to which the engine will
Certain aircraft require a minimum con-
be subjected. Fuel viscosity and density also have
centration of fuel system icing inhibitors (FSII).
considerable effect on nozzle performance,
This is put in the fuel to prevent icing in the air-
especially when varied over a wide range. The
frame fuel system, engine filter, or engine fuel
control. FSII materials are considered to be
performance is viscosity. It affects drop size, flow
dangerous before their additions to fuel;
range, and spray angle. Changes in fuel density
therefore, shipboard injection is not approved.
affect fuel flow.