How does the Airbus use the tropopause value?

[Jason74]

Hi,

On a long dark night recently, our crew was experimenting with changing the tropopause entry on the INIT page, and checking the resultant change in optimal step altitudes. After searching our companies FCOM, there was very limited information on how this tropopause value is used, but it seems the value does indeed change FM predictions.

Do Airbus or other operators recommend changing this value during the cruise? (Our particular flight was MEL-HKG, so our departure port had a forecast tropopause of 35,000, and 9 hours later in HKG it was approx 55,000')

Thanks!

Jason

[pineteam]

We leave the tropopause value by default to 36090 feet. Our SOP does not require to change it. Yes, it will give you better predictions and more accurate recommended and max altitude. I have played with it also and did not notice very big change; and I used 2 very different values: 20 000 feet vs 50 000 feet tropopause. From what I undestand, at the tropopause, temperature stopped to decrease so performance wise it’s better to stay below it. But surely temperature is not the only factor as many aircraft fly above it. Since we do only short/medium flights on A320 in the mid lattitude (20/30 degrees north) it’s not a big deal to leave it autotune. Maybe for the long haul skippers flying along a meridian, they are changing this value. Would be interesting to have some feedback from guys who are trained about this feature.

[SW1]

The Optimum level computed from the FMGS is computed as a function of temperature amongst others things such as CI, weight, wind etc. Any temperature deviation would affect the cruising flight level. So the differing heights of the tropopause might affect the computation.

On the A330-223 with Pratt and Whitney engines- there is is a note advising not to change the default value of 30690. As always with Airbus- no explanation why.

[underfire]

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

[psychomantic]

Our SOP:
first time, enter the tropopause altitude at top of climb. After that, every hour make an update of the tropopause altitude

[Jason74]

Thanks for the replies guys!! Appreciated!

[ACMS]

Why not simply enter the TOC temp on the Init B page, that’s what we do.

Wouldn't that effectively be doing the same thing?