Fuel system icing inhibitor (FSII) is an additive to
aviation fuel
Aviation fuels are petroleum-based fuels, or petroleum and synthetic fuel blends, used to power aircraft. They have more stringent requirements than fuels used for ground use, such as heating and road transport, and contain additives to enhanc ...
s that prevents the formation of ice in fuel lines. FSII is sometimes referred to by the registered,
genericized trademark
A generic trademark, also known as a genericized trademark or proprietary eponym, is a trademark or brand name that, because of its popularity or significance, has become the generic term for, or synonymous with, a general class of products or ...
Prist. Jet fuel can contain a small amount of dissolved
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
that does not appear in droplet form. As an aircraft gains altitude, the temperature drops and the fuel's capacity to hold water is diminished. Dissolved water can separate out and could become a serious problem if it freezes in fuel lines or filters, blocking the flow of fuel and shutting down an
engine
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
.
Chemical composition
Chemically, FSII is an almost pure (99.9%)
ethylene glycol monomethyl ether
2-Methoxyethanol, or methyl cellosolve, is an organic compound with formula that is used mainly as a solvent. It is a clear, colorless liquid with an ether-like odor. It is in a class of solvents known as glycol ethers which are notable for the ...
(EGMME, 2-methoxy ethanol, APISOLVE 76,
CAS number
A CAS Registry Number (also referred to as CAS RN or informally CAS Number) is a unique identification number assigned by the Chemical Abstracts Service (CAS), US to every chemical substance described in the open scientific literature. It inclu ...
); or since 1994,
diethylene glycol monomethyl ether (DEGMME, 2-(2-methoxy ethoxy) ethanol, APITOL 120, methyl carbitol,
CAS number
A CAS Registry Number (also referred to as CAS RN or informally CAS Number) is a unique identification number assigned by the Chemical Abstracts Service (CAS), US to every chemical substance described in the open scientific literature. It inclu ...
).
Prior to 1994, Prist was regulated under the MIL-I-27686E standard, which specified use of EGMME, but subsequently came under the MIL-DTL-85470B, with use of less hazardous DEGMME with higher
flash point
The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". (EN 60079-10-1)
The fl ...
.
FSII was thought to retard the growth of microorganisms eventually present in the fuel, mostly ''
Cladosporium resinae
''Amorphotheca resinae'' is an ascomycete fungus of the family Amorphothecaceae which is known to thrive in environments containing alkanes (and water), like aviation fuel, from which it derives its trivial name 'kerosene fungus'. As such it ...
'' fungi and ''
Pseudomonas aeruginosa
''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerugi ...
'' bacteria, known as "hydrocarbon utilizing microorganisms" or "HUM bugs", which live in the water-fuel interface of the water droplets, form dark, gel-like mats, and cause
microbial corrosion Microbial corrosion, also called microbiologically influenced corrosion (MIC), microbially induced corrosion (MIC) or biocorrosion, is "corrosion affected by the presence or activity (or both) of microorganisms in biofilms on the surface of the cor ...
to plastic and rubber parts. But has since been removed from labeling.
EGMME had been certified as a
pesticide
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
by the EPA, but as the requirement changes raised the certification costs, DEGMME has no official pesticide certification. DEGMME is a potent
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
, and at high concentrations can damage fuel bladders and filters. Long-term storage of FSII-fuel mixtures is therefore not recommended.
Anhydrous
isopropyl alcohol
Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the simple ...
is sometimes used as an alternative.
Purpose
FSII is an agent that is mixed with
jet fuel
Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...
as it is pumped into the aircraft. The mixture of FSII must be between 0.10% and 0.15% by volume for the additive to work correctly, and the FSII must be distributed evenly throughout the fuel. Simply adding FSII after the fuel has been pumped is therefore not sufficient. As aircraft climbs after takeoff, the temperature drops, and any dissolved water will separate out from the fuel. FSII dissolves itself in water preferentially over the jet fuel, where it then serves to depress the
freezing point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
of water to -43 °C. Since the freezing point of jet fuel itself is usually in this region, the formation of ice is now a minimal concern.
Large aircraft do not require FSII as they are usually equipped with electric fuel line heaters or fuel/ oil intercoolers that keep the fuel at an appropriate temperature to prevent icing. However, if the fuel heaters are inoperable, the aircraft may be still be declared fit to fly, if FSII is added to the fuel.
Storage and dispensing
It is extremely important to store FSII properly. Drums containing FSII must be kept clean and dry, since the additive is
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
and can absorb water directly from moisture in the air. Since some brands of FSII are highly toxic, a crew member must wear gloves when handling it undiluted. Many
FBOs allow FSII injection to be turned on or off so that one fuel truck can service planes that do require FSII as well as planes that don't. Line crew, however, must be able to deliver FSII when it is needed.
References
{{Reflist
Aviation fuels