need help with an instrument question

[aslan1982]

Hi can anyone help with these two questions. i just cant get the right answer

1.
fl310 cas 290kts coat = -48c what is the mach
ans = .78m

2.
f330 cas 340kts iaot = -19 what is the mach
ans = .975

if someone would be so kind to run through how to go about these that would be great

thanks in advance

[md-100]

dont want to write so check Equivalent airspeed - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Equivalent_airspeed) and follow the info

bye

[babymike737]

Easy Peasy!!!!!:eek:

[Pugilistic Animus]

don't you have a flight computer?

[LA931]

If you have a flight computer is much easier, but if you want to do it with your brain, here´s how it goes. I´ll show you an example with the first one:

First I want to know how fast is Mach 1 at that temperature ( I say temperature because Mach only depends on it).

Mach 1 = 661.5 kts x [square root (absolute ambient temperature/ 273 kelvin)]
Mach 1 = 661.5 kts x [square root (273-48/273)]
Mach 1 = 661.5 kts x [square root (0.824)]
Mach 1 = 661.5 kts x 0.9078
Mach 1 = 600.5 kts

Now, what's your speed, 290 CAS? So we can use a simple flight computer to determine your TAS or you can do approximately 2% of your CAS every thousand feet.
2% x 31 (FL310) = 62%

TAS = 290 kts x 1.62
TAS = 469.8 kts

And finally your Mach number is:

M = TAS/Speed of sound
M = 469.8 kts / 600.5 kts

M = 0.78

The others are the same. Remember that if you are using Total Air Temperature instead of Static Air Temperature you have to take into account the temperature rise, which again you can get it with a flight computer or you can use the simple formula T Rise = 30 x Mach ( This formula isn't very exact).

Cheers

[Rainboe]

3333333333333333333333

[LA931]

You´re right Rainboe. I made a mistake in the formula and the speed of sound at that temperature ( -48C) is exactly 584.7 Knots.
Now the fact that I got .78 as an answer was a complete coincidence.
Now that I have the correct speed of sound I cannot get the .78. Even using a flight computer to get the TAS I get a mach number of .82!!

[Sepp]

LA 931:

You're likely misreading (or more accurately, I suppose, mis-applying) the outer scale: 45 - 6 -7 -8 to use 480 TAS rather than 45 POINT 6 -7 -8 to use 458 TAS when reading Mach in the final step :)

[Pugilistic Animus]

Aslan1982,
[and I'm not picking on you]

The fact this question has been asked suggest to me that you need to delve more into the significance of that type of calculation ---

---on a CR3 it's two step process first find the TAS after alighning the appropriate windows for Press. Alt and T then use that to find Mach number in the other window after aligning the TAS:O. and T in another window---


However, the parameters entered as well as the possible reasons for should be well understood far beyond the test question--for lack of understanding of a high speed high altitude environment---is a very dangerous situation---perhaps you need to delve a little deeper in to the theory--then ALL {pilot} calculations in aviation become very simple..because if you're ever in an extreme situation---these fact may help---but they definitely help to keep you out of that extreme situation in the first place----Mach is much much more than a test question-or a simple number that you get right for the exam----

it's compressibility, flutter, divergence, stalling, stability controllability, and overall structural survivability et al.

PA