# How is engine fuel kept from freezing? [duplicate]

The following table shows the operating temperature of Jet fuels (Source http://www.skybrary.aero/)

I know a lot of flights will be cancelled during bad weather conditions including the cold. How is the Engine fuel kept from freezing when the flight is operated at the mentioned fuel freezing temperature?

• -40°C is very cold (-40°F in imperial units). You'll have different problems at that point. Commented Sep 24, 2015 at 11:10
• @ratchetfreak the temperature at 10000m is well below that. Commented Sep 24, 2015 at 14:59
• To expand on the above commenet, under the International Standard Atmosphere with a sea-level air temperature of 15°C, air temp at altitude can drop as low as -57°C. Commented Sep 24, 2015 at 16:41
• Commented Sep 24, 2015 at 21:38
• @anaximander It can get much colder at altitude than that. I'm currently at 43,000 ft. and it is -61C. However, this question appears to be asking about temperatures on the ground and there is another question which covers in flight fuel freezing. Commented Sep 24, 2015 at 23:02

In regions and weather conditions where this is likely to be a problem, other fuels are used instead of Jet-A.

For example, Jet-B has a freezing point of -60C and is used during the winter in some parts of Russia and other very cold places.

There are multiple ways for preventing fuel freezing in aircraft.

• In large aircraft, the fuel tanks have heating systems.
• Fuels with low freezing point, like Jet A-1 can be used. In really cold conditions (like Canada, Alska, Russia etc) Jet B (with freezing point -60$^{\circ}$) can be used, though this has higher flammability. The military equivalent of Jet B is the JP-4.
• An anti-freezing compound, called the the Fuel System Icing Inhibitor (FSII) is added to the fuel to prevent icing.

The problem in aircraft is not the fuel itself freezing, but the free water in aviation fuels freezing. All aviation fuels contain some (small) amount of water, which cannot be extracted. At (high) flight altitudes, the fuel cools and its capacity to retain dissolved water is reduced.

Some of the dissolved water separates out as discrete water that can form into ice crystals, blocking filters or form supercooled liquid which can crystallize and form ice crystals when it comes into contact with fuel lines.

FSII dissolves sparingly in fuel, but more easily in water. Any water present will extract FSII from the fuel; the additive then acts to reduce the freezing point of the free water, preventing the formation of solid ice crystals which could block the fuel supply system.

The FSII is usually ethylene glycol monomethyl ether (EGME) or diethylene glycol monomethyl ether (DiEGME). These fuel icing inhibitor additives are mandatory in all military aircraft fuels (like JP8) and are optional in world-wide commercial aviation fuels depending on route, flight length, and season.

• Following your last sentence (and just out of curiosity), does it mean that the at the same time and at the same airport, two flights can get a Jet A with and without EGME, respectively?
– yo'
Commented Sep 24, 2015 at 12:35
• @yo' Usually the FSII are not added in case of large aircraft having fuel heating systems, though they can be added (for small aircraft, especially) while fuelling. Commented Sep 24, 2015 at 12:39
• @yo' At some airports the FSI is premixed, while at others it can be mixed while fueling so is optional. Commented Sep 24, 2015 at 12:42
• Ok thanks. That's interesting (for me as a layman, I mean...)
– yo'
Commented Sep 24, 2015 at 12:50