The best way (IMHO) is to think of ISA as a baseline atmosphere. At sea level, it's 15C and a pressure of 29.92. Naturally, as you increase in altitude *above* sea level, the temperature will decrease, as will the pressure. If it's 15C at sea level, the temperature at, say, 30,000 feet will be lower than 15C, but the temperature at 30,000 will still be ISA if the sea level tempt is 15C. In other words, the actual temperature of ISA varies with, and is corrected for, altitude.
Wherever you see figures for ISA plus or minus something, it's telling you that the temperature is that many degrees warmer/colder than ISA. For example, ISA+10 means that it's 10 degrees warmer than ISA. As far as translating that into an actual temperature, one must know the altitude involved.
We use ISA as a more general consideration in flight planning, i.e. the amount of deviation in the airmass as a whole, since the aircraft will be operating at various altitudes on climbs/descents, as well as at specific crusing altitudes. Colder air, like ISA, or ISA-nn, is better than operating at ISA+nn as far as fuel consumption goes.
Sabenapilot From Belgium, joined Feb 2000, 2739 posts, RR: 45
Reply 2, posted (13 years 9 months 3 weeks 1 day 21 hours ago) and read 32100 times:
HI there Skystar,
OPNLguy is absolutely correct!
Wherever you see figures for ISA plus or minus something, it's telling you that the actual temperature of the atmosphere is that many degrees warmer or colder than the international standard atmosphere.
It is a misconception to believe ISA stands for 15°C in all cases, it merely stands for a standard temperature at any given altitude.
At sea level this is indeed 15°C, but for instance at 10,000ft ISA is -5°C.
Normally we consider a drop in temperature of about 2°C per 1000ft (in fact it is 1.96°C but anyway) so you can easily find for yourself the ISA temperature at any altitude if you remember ISA at MSL is 15°C!
(All you have to do is to deduct 2°C from 15°C for each 1000ft of altitude above MSL.)
When the Airbus manual tells you:
ISA + 15°C = 18°C
then this is a correct statement, if you know how to read it.
What they are trying to tell you is that at the given atitude, the temperature is 18°C, which is 15°C more then in the Standard atmosphere.
The altitude they are talking about here is 6000ft BTW, which can easily be found from what I explained here above.
Mind you ISA + 15°C is already a very tropical condition; it means it's 30°C at MSL!!!
Skystar From Australia, joined Jan 2000, 1363 posts, RR: 2
Reply 3, posted (13 years 9 months 3 weeks 1 day 19 hours ago) and read 32095 times:
Thanks for all your responses,
When I started reading FAA Certification notes on the A340 & CFM56, it started to all fall into place. ISA+15 happens to be the flat rating temperature for almost all CFM56 engines, which is 30°C at SL - if only I could replicate this accurately in Flight Simulator (you have a max RPM of 104.2% on the CFM56-5C4, but it's displayed on the ECAM as 100.3% - so all the numbers I use have to be multiplied.)
Still, it's all useful to learn, and I'm sure others will have learnt from this. Now I need to get some accurate charts for a A340-313X