water there is no growth. Remove any free water and
Microorganisms in jet fuel can cause severe corro-
sion damage to metal aircraft fuel tanks. Organic acids,
or other byproducts produced by the growth of fungi or
bacteria, react chemically with certain matter contained
within the fuel to penetrate tank coatings. Once the
coating is penetrated, the metal tank is attacked.
Microbiological growth causes fouling of aircraft
fuel system filters and erratic operation of fuel-quantity
probes. Microbiological contamination is more preva-
lent in tropical and semitropical climates because of the
more favorable temperature and higher humidity. The
presence of microbiological growth in fuel being deliv-
ered to an aircraft is a reliable indication of failure of
the fuel system cleanup equipment and personnel.
The fuel from an aircraft suspected of having
microbiological contamination must not be defueled
into a clean system. Once a fuel system is contami-
nated with microbiological growth, the organisms
continue to multiply unless the system is thoroughly
An emulsion is a liquid suspended in other liquids.
There are two types of emulsions; water-in-fuel and
fuel-in-water (inverse) emulsions.
The water-in-fuel emulsion is the most common
of emulsions found by fuel handlers. It appears as a
light-to-heavy cloud in the fuel. (See the second and
third bottles of fuel shown in fig. 3-2.) This type of
emulsion may break down and settle to the bottom of
the sample container at any time ranging from a few
minutes to a week, depending on the nature of the
Surfactant is a contraction of the words surface
active agent. A surface active agent is a substance that
causes a marked reduction in the interracial tension of
liquids. A surfactant in fuel causes the fuel and water to
mix more easily and become much harder to separate.
Surfactants disperse both water and dirt in fuel and in
some cases form very stable emulsions or slimes.
The surfactants that appear in jet fuels are usually
the sulfonates or naphthenates of sodium. These can be
present as naturally occurring materials in the crude oil
or as residual refinery treating materials. Refinery proc-
essing must be such that it removes all traces of these
materials, or poor quality fuel results.
Many other materials are also surface active.
The list includes common household detergents, clean-
ing compounds used to clean fuel storage tanks and
earner vehicles, greases used to lubricate valves, and
corrosion inhibitors used in petroleum products to re-
duce rust in pipelines and tanks.
Surfactants in jet fuel can be a major problem.
These materials accumulate and concentrate in the coa-
lescer elements of filter/separators, destroying the abil-
ity of the elements to coalesce and remove water from
fuel. Concentrations of less than 1 ppm of a surfactant
in jet fuel have been known to cause malfunctioning of
coalescer elements. Elements so affected pass free
water and suspended particulate matter.
Surfactants are also associated with microbiologi-
cal slime growths. It is not necessary that surfactants be
present for microorganisms to flourish, but they pro-
mote luxuriant growth by aiding the mixing and emul-
sifying of fuel and water. Microorganisms need free
water to multiply and grow. Surfactants help them to
The problem with surfactants is that they quite
often are not detected in jet fuels until after they have
poisoned filter/separators, which in turn have al-
lowed water and/or slime to be delivered to aircraft.
There are laboratory tests for surfactants in fuel, but
as yet there are no accurate field tests. However, a
surfactant problem can usually be detected by one or
more of the following observations:
1. Dark, red-brown, or black water in filter/ sepa-
rator sump drains, refueler sump drains, or pipeline
2. Excess quantities of dirt and/or free water in the
fuel at dispensing points or downstream of filter/sepa-
3. Storage tanks not yielding a clear, bright fuel
after prescribed settling times
4. Dark or black water and/or slime in drawoffs
from storage tank bottoms
5. Triggering of fuel monitors in delivery systems,
No two cases of surfactant contamination in fuel
systems are exactly alike. However, some general
measures can be used to correct and control this type
of contamination. Some of these procedures are as
1. Change monitor fuses.
2. Change falter/separator elements and clean out