Temperature -56.5 °C +15 °C
Figure B12 - ISA Temperature model
Indeed, in the real world, at a given FL, the temperature changes continuously. Engine efficiency depends on the difference between the fuel temperature (in fuel tanks, the temperature is fixed) and the outside air temperature (static temperature, SAT). In cruise, if the SAT increases, engine thrust decreases and vice-versa. The autothrust corrects this in order to maintain the pressure altitude. Recordings should be performed in a zone where the SAT is forecasted stable.
Figure B13 - Stable Temperature zone
For aircraft performance monitoring purposes, the autothrust beingdisengaged, the SAT variation should be limited to 1°C during the actualdata recording leg.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
In order to verify the influence of the temperature gradient on aircraft performance, the following should be considered. Temperature gradients also modify the slope of the isobaric surfaces. For example, low-pressure areas are cold compared to high-pressure areas but the colder the low pressure, the steeper the isobaric surface slope.
Figure B14 - Illustration of the isobaric slope due to the temperature
In order to compensate for the modified isobaric slope, the aircraft will be given a bonus or a penalty depending on the temperature gradient, and as follows:
. .SR ..3 CL 1
. .= 9.4×10 ×(0.25× FL .11.5)× ×.SAT×
SR C TAS
..CORR D
A graphic example is given in following Figure B15.
Figure B15 - Example of graphical result
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
Note: The usefulness of these isobaric slope corrections is, in fact, rather questionable since the theoretical assumptions are usually not applicable to the real atmosphere. What we are looking for is the change in potential energy represented:
.
by the slope of the flight path, and / or
.
by the change of geopotential altitude
However, when performing an assessment of this slope through the observed drift, and/or temperature trend, only the conditions between, earth’s surface and flight altitude are relevant ; this applies for both the assessment of the pressure-related slope as well as for a temperature related slope. There is presently no system which is capable of sensing flight path slope with the required accuracy (better than 0.002°).
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