WHY WE HAVE TO WARM FUEL IN AIRCRAFT ?

 

FUEL FREEZING POINT

Does it really describ the lower limit in temperature for use in aircraft operations?

Actually .. No, but the pumpability limit. 

This limit is very close to the pour point, a temperature at which the fuel, cooled without stirring, will only just still pour from a standard glass cylinder. 

However, since the pour point is difficult to determine accurately, the fuel freezing point continues to be used as a reference for low-temperature characteristics.

WHY WE HAVE TO WARM FUEL?

Fuel is a mixture of different hydrocarbons that do not all solidify at the same temperature. 

When fuel is cooled, an increasing proportion of wax crystals form in the fuel. 

The wax crystals can block fuel lines and filters, thus causing engine instability, power loss and eventually flameout, and their formation should therefore be avoided.

In Fuel Heating and Oil cooling system -- The minimum engine inlet temperature is the actual fuel freezing point with the manufacturers margin added to it.

HOW FREEZING POINT IS MEASURED?

The fuel freezing point is commonly measured with a test method is based on the observation of wax crystals completely disappearing from a warmed fuel sample that was previously frozen. The method is used widely except for Russian and other Eastern-European fuels.

JETA1:

The main focus is on JetA1, primarily available everywhere except the United States of America, and on JetA, which is used there.

FUEL MIXTURES PROBLEM

When flying between areas, mixtures of fuel with different specification fuel freezing points will occur in the tanks.

Experimental evidence has shown that fuel mixtures do not behave as ideal fluids and that the resulting fuel freezing point is commonly adversely affected.

Airbus recommends avoiding JetA -40°C mixtures in the outer, most exposed tanks.

HEAT MANAGMENT SYSTEM AS A SOLUTION

During flight:

1. Fuel temperature quickly drops below the freezing point of water,

2. Certain proportion of water is always contained in aviation fuels,

3. This water can then form ice crystals,

4. Travel to the engine inlet filters,

5. Clog Filters. 

6. To avoid this, the fuel is warmed in the oil cooling system.

7. For some engine types, a minimum fuel temperature below which takeoff

and/or flight is not permitted,

8. From the engine limited capability to warm up a water-saturated fuel flow,

9. Anti-ice additive shall then be used.