ISA deviation

[neil_1821]

I'm currently studying for exams upcoming in feb and i'm going through questions to try and get it all stuck into my head firmly, since i didnt do much in the past week of christmas my brain doesnt seem to function anymore (too much drink) and i cant find the answer i need anywhere, so i was wondering if anyone could post up how to find out ISA deviation at a given FL and also the formula for NAM and NGM.

Thanks

[jonjon]

Hi,

Not sure this is what you're asking for but for the ISA deviation, take the Temperature given and the Flight Level, lets say -45°c and FL350
What you do is this:
Surface T° is 15°c decreasing 2°/1000ft, ISA T° at FL350 is therefore 35x2=70° - 15°= -55°c
So in this example we are ISA +10°c.

For NAM/NGM:
NAM= TAS*(NGM/GS), just remember that NAM/TAS=NGM/GS

There is also NGM = NAM +/- [(Wind speed*Time)/60]

Voilà

I hope this is what you asked for
J

[mcgoo]

The formulas can also be found in the CAP.

[neil_1821]

yeah that's pretty much what i was looking for, supposing though that you dont have the temperature at the FL, how do you go about calculating what the temperature is up there?

[jonjon]

you're always given a starting point in the questions, if nothing is mentionned you assume ISA conditions and you start from 15°c at ground level and decrease every 1000ft.
Now if the temperature is let's say -23°c @ 22000ft and you have to work out the t° @ FL390, you start from -23°c and substract 2°c /1000ft up to the right level.
Bare in mind not to go lower than -56.5°c (if in the Gen Nav or Instrumentations exams for example), i.e. if they tell you the t° @ FL250 is -40°c and ask for t° @ FL390, it's going to be -56.5°c, not less :ugh: :ugh: . I remember a couple of questions like that during these exams
Mcgoo,
They have removed the formulas for NGM in the new CAP edition, I know as I have it in front of me for my Flight Planning exam
Happy New Year!

[mcgoo]

Mcgoo,
They have removed the formulas for NGM in the new CAP edition, I know as I have it in front of me for my Flight Planning exam
Happy New Year!

The formula is no longer given, instead it is offered in text on SECTION 4 page 18 para 5.2 (d)

[jonjon]

hahaha
Well spotted!!
You obviously pay a lot more attention than me!!

[mcgoo]

Its worth getting to know the CAP well, you can often find answers to exam questions that do not refer you to the CAP.

[neil_1821]

thanks for the help, i'll be sure to have a read through the cap, i would never of spotted that if someone didnt point it out, I'll have to find the question where you needed to calculate the isa deviation, i'm sure it didnt give you a start temp, maybe it's the drink:ugh: i better lay off it for a while.

Thanks for the help