Greetings all,I've seen references to ISA DEV in this forum, and also see this on the FMC. What exactly is it, and how is it used? Is it only for pre-flight, or does it also apply during cruise and/or descent?I know it has something to do with temperature deviations, but any help here would be appreciated.(edit): I also use ActiveSky2012. How can that program be used to help in any of this?Many thanks,Ron

Edited January 28, 201214 yr by RPRIEVER

ISA - International Standard Atmosphere.http://en.wikipedia.org/wiki/International_Standard_Atmosphere

It is necessary to present performance data at temperatures other than the ISA temperature for all flight levels within the performance spectrum envelope. If this were to be attempted for the actual or forecast temps, it would usually be impracticable. To overcome the presentation difficulty and still retain the coverage or range required, it is necessary to use ISA deviation. This is simply the algebraic difference between the actual or forecast temps and the ISA temps for the flight level under consideration.

ISA DEViation is the disparity between the ISA table's predicted value and the current/actual temperature at flight levels. Usually expressed in +/-. As aircraft performance tables are written based on STANDARD atmospheric conditions- the ISA DEViation will allow you to apply the appropriate/necessary factor to determine the aeroplane's performance on a particular day. Immeasurably important when hot and heavy.BTW the performance affected is throughout the envelope- from startup to shutdown.

Edited January 28, 201214 yr by lasnubes

Ron,When you make fmc pre-flight at Perf Init page, you have in right side T/C Oat and ISA dev. T/C OAT means top of climb OAT temperature, if you have active sky, with flightplan loaded, just take a look of the top of climb nearest fix temperature and insert that temp to the line T/C OAT, automatically the ISA dev will change. You are done, if the ISA dev is hotter, at ND you will see the T/C point walk a bit in front of the route, the reason is a hotter air has a penalty in a/c performance.Exemple: you plan to flight at FL370 that has a normal temp. of -56°C (that is the ISA for FL370) and at active sky in the vicinitys of T/C shows -50°C. Just put -50 (dont forget the minus) in T/C OAT and ISA dev will show 6°C hotter than normal temp for FL370.

Greetings all,I've seen references to ISA DEV in this forum, and also see this on the FMC. What exactly is it, and how is it used? Is it only for pre-flight, or does it also apply during cruise and/or descent?I know it has something to do with temperature deviations, but any help here would be appreciated.(edit): I also use ActiveSky2012. How can that program be used to help in any of this?Many thanks,Ron