Chapter 3 - Deicing/Anti-icing Fluids

Lowest Operational Use Temperature of Types I, II, III, & IV Fluids

49.  Just as an aircraft has a specific operating envelope within which it is approved to be operated, de/anti-icing fluids are also tested and qualified for operation within a specific temperature envelope.

50.  The qualification of de/anti-icing fluids, also called freezing point depressants (FPD), is a complex and thorough process, which evaluates a multitude of fluid properties and characteristics. The one of particular interest in this case is the lowest operational use temperature (LOUT). The LOUT is fluid concentration specific. The fluid concentration may change if the fluid is subjected to sustained heating.

51.  The LOUT for a given fluid is the higher of:

  • The lowest temperature at which the fluid meets the aerodynamic acceptance test for a given aircraft type, or
     
  • The actual freezing point of the fluid plus its freezing point buffer of 
    10°C, for a Type I fluid, and 7°C for a Type II or IV fluid.

Note:  Manufacturers state that a fluid must not be used when the outside air temperature or skin temperature is below the LOUT of the fluid.

52.  An example of establishing an LOUT. Lets take as an example a Type I fluid that has met the aerodynamics acceptance test down to -45°C.

The reported freezing point of the fluid (as measured by the Deicing Operator) is -43°C. The OAT is -39°C.

Can this fluid be used to de-ice the aircraft under these conditions?

The LOUT for a given fluid is the higher of:

  • The lowest temperature at which the fluid meets the aerodynamic acceptance test for a given aircraft type, in this case -45°C; or
     
  • The actual freezing point of the fluid plus a freezing point buffer of 10°C, in this case -43°C + 10°C = -33°C.

The LOUT is -33°C and since the OAT is -39°C, this fluid as is, can't be used.