Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat My Starred Topics | Profile | New Topic | Forum Index | Help | Search
 ISA - International Standard Atmosphere
 Skystar From Australia, joined Jan 2000, 1363 posts, RR: 2Posted Sat Oct 13 2001 11:03:26 UTC (14 years 7 months 2 weeks 5 days 6 hours ago) and read 32767 times:

 Hi, I thought I had a good idea about ISA, until I read this statement in an A340 Flight Manual (re. FLEX Thrust). According to this, ISA+15=18°C I'm perplexed, isn't ISA 15°C at sea level? Secondly, what is the definition of ISA - eg, in ISA+15, what does the+15 refer to (I assume temperature) Here's a link that I found useful, http://www.aero.usyd.edu.au/aero/atmos/atmtab.html Cheers, Justin
 OPNLguy From , joined Dec 1969, posts, RR: Reply 1, posted Sat Oct 13 2001 22:16:12 UTC (14 years 7 months 2 weeks 4 days 19 hours ago) and read 32767 times:

 A couple of sites you might find helpful... http://www.ae.su.oz.au/aero/atmos/ISA.html http://www.booty.demon.co.uk/metinfo/isa.htm 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, 2748 posts, RR: 46 Reply 2, posted Sun Oct 14 2001 11:22:08 UTC (14 years 7 months 2 weeks 4 days 6 hours ago) and read 32767 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 Sun Oct 14 2001 13:46:06 UTC (14 years 7 months 2 weeks 4 days 3 hours ago) and read 32767 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 Cheers, Justin
 Top Of Page Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat Forum Index

Reply To This Topic ISA - International Standard Atmosphere
• Tech/Ops related posts only!
• Not Tech/Ops related? Use the other forums
• No adverts of any kind. This includes web pages.
• No hostile language or criticizing of others.
• Do not post copyright protected material.
• Use relevant and describing topics.
• DETAILED RULES

 Similar topics: More similar topics...
International Route Applications posted Sat Jan 27 2007 23:30:54 by Worldjet777
GE90-115 On Standard 773/772ER posted Thu Jan 18 2007 20:30:25 by Kaitak
Standard Passanger Weights? posted Mon Nov 20 2006 18:35:30 by VIflyer
New Exhaust Standard For Airplane Engines posted Fri Apr 7 2006 16:06:24 by Azstagecoach
Tcas Not Standard? posted Tue Dec 27 2005 15:43:18 by Chiguire
Is The Standard MD80 CAT II Or CAT III ( A / B ) posted Mon Feb 28 2005 21:51:31 by PW2040
ISA Altitude And Speed Chart Help posted Tue Jan 11 2005 16:26:56 by Delta07
Is This Hte Standard For Crew Rest On Airbus? posted Thu Dec 16 2004 19:33:00 by Mirrodie
Ryanair/ryan International 738 posted Mon Dec 6 2004 05:01:58 by COAMiG29
Standard Weights Per Passenger Per Class posted Fri Oct 1 2004 00:43:50 by SunriseValley
Is Mach 0.85 Becoming The Industry Standard? posted Thu May 30 2013 21:58:54 by A380900
Honolulu International Airport Runway Question posted Tue Jan 1 2013 08:35:54 by JetBlueGuy2006
Standard Calls Question posted Wed Nov 21 2012 08:01:52 by Novice
Anchorage International Airport Infield Parking posted Mon Aug 20 2012 19:25:08 by tzadik
Typical Pilot Week For AA International posted Mon Jun 18 2012 13:39:11 by N62NA
AOA Indicator:Make It Standard! posted Thu Sep 8 2011 14:15:24 by Chamonix
Icao Standard Speeds posted Sun Jul 17 2011 03:55:10 by smartt1982
Does 95 Kg (210 Lbs) Apply To International? posted Wed Jun 22 2011 06:53:40 by pnd100
Do International Flights Require Assigned Seating? posted Tue May 10 2011 12:55:22 by Valorien
Turning International Flights V. Domestic Flights posted Tue Apr 26 2011 15:41:06 by Valorien