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On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point.

For example: at 36,000 feet the standard outside air temperature is -56.5°C (-69.7°F) and is well below Jet-A's freezing point of -40°C/F.

What measures are put in place on aircraft to mitigate the risk of the fuel in the tanks freezing and preventing the engines from operating?

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Much like water, exposing fuel to below freezing temperatures does not instantly make it freeze. It takes some amount of time, and as long as the fuel is not exposed to below freezing temperatures long enough to make it freeze, it won't be a problem.

How much time? That depends on a number of factors like:

  • Type of fuel
    • Jet A (common in the US) has a freezing point of -40 C
    • Jet A1 (common outside the US) has a freezing point of -47 C
    • Jet B (common for some military aircraft and very cold airports) has a freezing point of -60 C
  • Initial fuel temperature
    • If you fuel in the middle east during the middle of summer, the fuel will be a lot warmer than when you fuel in Siberia in the middle of winter. Warmer fuel will take longer to freeze.
  • Outside air temperature
    • Much like trying to freeze water to form an ice cube, the colder the air around it, the faster that it will freeze.
  • Aircraft speed
    • The faster that the airplane goes, the more friction there is as the air flows over the wing. This heats up the air close to the wing and makes it take longer for the fuel to freeze (see previous point). The temperature at the wing is known as TAT:
    • For example, at an actual temperature of -56.5 C (SAT) and Mach 0.72 the TAT is -34 C, but at Mach 0.80 the TAT is -29 C.
    • For more information about SAT -vs- TAT, see my answer about Air Data Computers.
  • Quantity of fuel
    • Much like a small pond will freeze before a large lake, in order to get fuel to freeze you need to lower the temperature of all of it.
    • The more that there is, the longer it will take.
  • Aircraft design
    • Some aircraft run hydraulic lines close to the fuel so that the heat will help slow down the freezing.
    • Some aircraft constantly pump fuel to keep it moving and mixing if it tends to freeze in one part of the tank before the rest
    • Some aircraft use fuel to cool the engine oil using a heat exchanger, and return the warmed fuel to the tanks
    • Many other design factors

In general, fuel will not freeze in the typical amount of time that an aircraft is aloft. If particular routes pose a problem, there are several options that the operator and pilot have:

  • Use a fuel with a lower freezing point
  • Fly at a lower altitude where it is not as cold
  • Choose a route with warmer temperatures
  • Increase the speed of the airplane
  • Carry extra fuel
  • Transfer fuel in a way to keep fuel warmer. Typically this is done by transferring fuel from the relatively warm fuselage tanks to the wing tanks which are exposed to more of the cold air.
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    $\begingroup$ sometimes you get water in the tank and if that freezes you have ice crystals at the bottom of the tank where it may cause havok $\endgroup$
    – ratchet freak
    Commented Mar 13, 2014 at 3:01
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    $\begingroup$ @ratchetfreak That's what fuel heaters and/or prist is for! $\endgroup$
    – Lnafziger
    Commented Mar 13, 2014 at 3:02
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    $\begingroup$ use Prist as an additive to prevent freezing $\endgroup$
    – user11459
    Commented Sep 30, 2015 at 4:22
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    $\begingroup$ @larryw Prist does not keep fuel from freezing. It keeps the water that is suspended in the fuel from freezing. $\endgroup$
    – Lnafziger
    Commented Sep 30, 2015 at 15:05
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    $\begingroup$ In addition to the excellent points, I'd mention outside air pressure. Yes, it's cold at high altitudes, but not like it being cold at sea level; it's cold because there is an insufficient amount of air to conduct heat into the object being measured (or doing the measuring). By the same token, that air isn't conducting heat away from the aircraft as fast as cold air at sea level would be. So while the wings are in -40*C air, they aren't equalizing to that temperature by conduction as fast as they would be if parked at McMurdo Station Antarctica. $\endgroup$
    – KeithS
    Commented Oct 1, 2015 at 0:47
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Most Jet engine fuel systems have means to heat the fuel to prevent it from freezing. Typical heat exchange methods followed are by employing hot air or hot oil taken from the engine.

Reference: FAA Aviation Maintenance Technician Handbook, Chapter 14: Aircraft Fuel System

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  • $\begingroup$ or putting the AC's heatsink under the tank $\endgroup$
    – ratchet freak
    Commented Mar 12, 2014 at 9:12
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    $\begingroup$ If I recall correctly, the couple of A380s that suffered engine failures back in 2009 or 2010 did so because the fuel/oil heat exchange manifold iced over. $\endgroup$
    – StallSpin
    Commented Mar 12, 2014 at 16:12
  • $\begingroup$ @ratchetfreak: Not generally a good idea, given that most of the flights flown by any given type of jet (unless it's a type specifically specialised for cold-weather operations) are not ones where the fuel would be at risk of freezing. $\endgroup$
    – Vikki
    Commented Oct 16, 2018 at 3:43
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Since the airplane is in motion, so is the fuel. That principle alone keeps it from freezing. As the fuel agitates, the molecules are prevented from freezing because of motion. This only works to a certain degree.

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  • $\begingroup$ Can you add some sources for that? I would think that it might not be enough movement to affect it very much. $\endgroup$
    – dalearn
    Commented May 9, 2018 at 12:12

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