From an old Airbus Fast magazine
Honeywell Aerospace
Airbus General FAQs
1.What is the value of the Tropopause entry on the Init page of the FMS and should the entry be changed from the default -56/36090 during preflight?
The FMS uses air density calculations for it's predictions. The slope on a graph created by plotting a line from the current aircraft sensed temperature and altitude to the Tropopause is key to these predictions.
he definitions of the atmosphere's layers follow.The ICAO Standard day is a surface temperature of 15 degrees Centigrade and a 2 degree decrease in temperature per 1000 feet, up to 36090 feet, above which the temperature is a constant minus 56 degrees centigrade.
The Tropopause the line separating the Troposphere from the Stratosphere.
The Stratosphere therefore by definition is a minus 56 degrees Centigrade until reaching approximately 50 miles where the Ionosphere begins.
Therefore, the Tropopause is the transition between the Troposphere and the Stratosphere.
There is variation to this standard day and this is known as the "Delta T". In the summer in mid latitudes I have observed a plus 16 degrees Delta T on rare occasions. This means that at 36090 feet the temperature may be as much as 16 degrees warmer and the Tropopause may not occur until an altitude of 44090 feet.
Recommendation: If you do not know the altitude of the tropopause for a flight, leave it alone. Leave the default entry. Your predictions may be slightly affected by this. I observe that most airlines have the policy that if this is unknown, then they do not change it. On some Airbus aircraft, the actual tropopause may exceed the entry limit, therefore do not enter 440 for 44090. The system will erroneously use the 420 feet for predictions and this will surely cause erroneous predictions.
2. Should the Tropopause entry be changed during flight?
There is no precedent for changing the tropopause entry during flight.
Recommendation: If one insists on modifying the default number, do it once before flight, and then use an altitude that is a computed average for the flight
Last edited by underfire; 24th June 2018 at 13:59.