Wrong ISA temp

[Driver170 42]

Hey i have provided a pict below so you guys can do some calculations and prove me wrong that infact its working just fine.

 

I am trying to learn about ISA which i still don't know why its used? something to do with performance maybe? can't see sense of it, anyway i calculated my ISA at FL140 

 

FL140 - 14 X 2 = 28 then minus 15 = -13 c ISA so why is my FMC showing 2 c ?

 

 

 https://www.dropbox.com/s/663rewbm5hvr6c2/2014-8-30_11-15-38-684.BMP?dl=0

[pschlute 37]

The FMC is showing ISA DEV (ISA Deviation) at +2.  This agrees with ISA -13 versus SAT -11

[Driver170 42]

Someone feels stupid. Just waiting for kyle to come along and say human error haha!

[pschlute 37]

Someone feels stupid. Just waiting for kyle to come along and say human error haha!

 

We have all done it at one time or another :rolleyes:

[freddy_ 9]

I am trying to learn about ISA which i still don't know why its used? something to do with performance maybe? can't see sense of it, anyway i calculated my ISA at FL140

 

Why is it used?  I found this post in the Tech/Ops forum on the Airliners.net web page.

 

I still don't fully understand ISA DEV, but this post does clear up some of the questions.

 

Temperature Deviation

 

(I apologise this is not a clickable link; but for some reason I am unable to get the "insert link" option working when posting.)

[scandinavian13 5,192]

Someone feels stupid. Just waiting for kyle to come along and say human error haha!

 

Human error!

(Wouldn't want to disappoint!)

 

...but you're definitely not the only one who doesn't quite understand it, which is why I made this:

 

 

 

I still don't fully understand ISA DEV, but this post does clear up some of the questions.

 

See above.

 

...but also, that thread won't help you too much.

 

Basically, the plane is designed on a perfect atmosphere: a (sea level) temperature of 15 and a (sea level) pressure of 29.92.  ISA DEV is simply a concept that adjusts for the difference - or deviation - from that temperature of 15, while accommodating the temperature lapse (or decrease) as altitude increases.

 

Instead of looking at temperatures at my cruise altitude and having to figure out the difference between "normal" or "hotter/colder than normal," ISA DEV makes it immediately apparent: +5 means it's 5 degrees hotter than a "standard" day; -5 means it's 5 degrees colder.

 

Why is this important?  Hotter than standard generally means worse performance; colder generally means better.  Giving this data to the FMC can help it give you more accurate predictions.

[Spin737 102]

 

 

Instead of looking at temperatures at my cruise altitude and having to figure out the difference between "normal" or "hotter/colder than normal," ISA DEV makes it immediately apparent: +5 means it's 5 degrees hotter than a "standard" day; -5 means it's 5 degrees colder.



Why is this important? Hotter than standard generally means worse performance; colder generally means better. Giving this data to the FMC can help it give you more accurate predictions.

 

This is good.

 

ISA deviation is more intuitive for knowing how well the plane will perform. Is it PLUS or MINUS?

 

Years ago I was flying Sabre 65 from Saint Pete to San Jose. There were some storms enroute (the same the took out the AA at LIT) with tops to FL430. We tried top them by climbing to FL450, but the ISA DEV was something like +20, IIRC. Well, the Sabre didn't like that and couldn't get to FL450. Over the gulf and to LAS we went for a now required fuel stop.

 

That's how I learned about ISA DEV.

 

Now, I mostly look at the OPT ALT and reports of turbulence and cruise winds to decide what alt to use.