*
In the stratosphere, SAT increases / wind decreases
-Left Hand (LH) drift angle corresponds to wind from the right The aircraft is flying towards a high-pressure zone, i.e. it is flying uphill,
*
In the troposphere, SAT increases / wind decreases
*
In the stratosphere, SAT decreases / wind increases
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
The opposite phenomenon prevail in the Southern Hemisphere.
Wind velocity increases below the tropopause and decreases above the tropopause by approximately 5% per 1000 ft except in jet stream zones. Near the tropopause, the wind velocity is maximum.
In order account for the isobaric slope, the aircraft should be given a bonus when flying uphill (LH drift angle in Northern Hemisphere, RH drift angle in Southern hemisphere) and a penalty when the aircraft is flying downhill (RH drift angle in Northern hemisphere, LH drift angle in Southern hemisphere).
.SR.FU
The correction is applied on the (or . ) as follows:
SR FU
. .SR . ..FU..2
. .=.. .=.1.107×10 ×TAS×sin(LAT)× tan(DA)
SR FU
..CORR ..CORR
where TAS is the true airspeed in knots DA is the drift angle LAT is the latitude
Figure B11 - Example of SR deviation correction
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
In practice, drift (track-heading), temperature (SAT) and wind observations (direction/speed) allow to consider:
.
Pressure patterns (high and lows)
.
Wind barbs (direction/speed / FL)
.
Tropopause height
.
Stratospheric lapse rate
.
Temperature trends around tropopause
.
Jetstream core locations
.
Turbulence
In any case the aircraft must be stabilized (Flight Path Acceleration, Vertical Velocity).
Whilst carrying out an aircraft performance monitoring audit, one would refrain from taking stabilized cruise performance readings if the pressure system is changing rapidly or when drift angles are greater or equal to 5 degrees. Very often, a positive .T can be observed (. 10° C in horizontal flight) when passing through the tropopause from the troposphere to the stratosphere. This temperature increase is even more noticeable when the tropopause slope angle is steep and therefore when wind velocity is highest at the point where the tropopause is passed through.
The equation in Figure B11 is valid only for high-altitude winds; less-than ideal conditions like topographic effects (mountain waves) or strong curvature for the isobars > 5° drift would lead to erroneous results.
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