Specific Fuel Consumption (SFC) is the amount of fuel used per unit of thrust (for jets) and unit of power (for props) in one unit of time. SFC varies with the ambient conditions. SFC is best at a minimum and one of the factors it is dependent upon include:

Temperature. In cold air, at a particular altitude, the RPM may be reduced to provide the power required because:

(a) The power available from the engine is greater.
(b) The power required to propel the airframe in cold air is less.

The colder the temperature, therefore, the lower will be the SFC because the power absorbed (wasted) at the lower temperature is reduced and the thermal efficiency is increased.

• As temperature increases, the air becomes less dense. Consequently, engine efficiency decreases and SFC increases. If 1º on ISA gives +0.2% fuel flow increase per unit of thrust, 10º will give an increase of 2%.