FAA Study Question: Feeling Dumb

FutrLftSeat82 said:
When you originally posted this question, the first thing I went for was my Sporty's E6B...except I was in the same boat as you...not knowing or seeing how to get the IAS. Maybe if you get a moment in the near future, you could explain how you got this answer using only your Sporty's E6B (don't have a conventional). Thanks in advance...!!
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Well...here's the deal. The question wants IAS, and I get the right NUMBER on the Sporty's E6B, but it's showing as CAS. I don't know why.

Here's what you do. Go to the REQ TAS Function, and enter wind direction, speed and course, then GS. Press enter and you get 150.98 TAS.

Now arrow over to the REQ CAS function. Enter pressure alt, temp and TAS (which will automatically carry over from before) and you'll get a CAS of 136.5 - round up to 137.

Now - I'm GUESSING HERE - but at these speeds either CAS and IAS aren't that far off from each other and the question is assuming that, or since they don't give us a table to adjust, CAS and IAS are interchangeable here. It's not clear to me why this is, but the numbers did crunch correctly.
 
killbilly said:
Now - I'm GUESSING HERE - but at these speeds either CAS and IAS aren't that far off from each other and the question is assuming that, or since they don't give us a table to adjust, CAS and IAS are interchangeable here. It's not clear to me why this is, but the numbers did crunch correctly.
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Yes for the FAA question they are interchangeable. However, technically the question should have asked for CAS. There is no way that the e6b can know what the position and installation errors are for your particular aircraft, or speed for that matter. That is why you see CAS printed on the e6b slide rule face.
Are you familiar with the acronym ICE T Good?
 
killbilly said:
Well...here's the deal. The question wants IAS, and I get the right NUMBER on the Sporty's E6B, but it's showing as CAS. I don't know why.

Here's what you do. Go to the REQ TAS Function, and enter wind direction, speed and course, then GS. Press enter and you get 150.98 TAS.

Now arrow over to the REQ CAS function. Enter pressure alt, temp and TAS (which will automatically carry over from before) and you'll get a CAS of 136.5 - round up to 137.

Now - I'm GUESSING HERE - but at these speeds either CAS and IAS aren't that far off from each other and the question is assuming that, or since they don't give us a table to adjust, CAS and IAS are interchangeable here. It's not clear to me why this is, but the numbers did crunch correctly.
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I think I've got it! Thank you for taking the time to explain it! :D I also did not realize that CAS and IAS were interchangeable as far as FAA testing was concerned. But granlistillo makes a good point with "the question" not knowing was factors are influencing your actual CAS.
 
FutrLftSeat82 said:
I think I've got it! Thank you for taking the time to explain it! :D I also did not realize that CAS and IAS were interchangeable as far as FAA testing was concerned. But granlistillo makes a good point with "the question" not knowing was factors are influencing your actual CAS.
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I did NOT say IAS and CAS were interchangeable as far as FAA testing goes. They are NOT. I was only pointing out to the question he said he typed verbatim, and in since no other info was provided IAS and CAS was the same. You solve for CAS on your flight computer. Don't want to be picky but there is a difference.
 
killbilly said:
No. What is it?
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ICE Tea Good

Indicated: read off your ASI (basically the difference between static and dynamic pressure)

Calibrated (rectified in europe) Indicated corrected for instrument error and installation, position etc. (biggest factor is that the pitot tube is not always aligned with the relative wind at all AOA, and you have to apply a trigonometric correction. You get this info from your POF). Think of this as Indicated if there was no error.

Equivalent: This number is an engineering number that takes into account compressibility. Not a factor at all unless you are high and fast. Below 300 kias there is no correction. A pilot does not figure out Equivalent airspeed but rather a correction factor to apply to TAS.

TRue TAS is Equivalent or CAS corrected for PA and temperature

G Groundspeed Correct for wind, tells speed over the ground.
 
granlistillo said:
I did NOT say IAS and CAS were interchangeable as far as FAA testing goes. They are NOT. I was only pointing out to the question he said he typed verbatim, and in since no other info was provided IAS and CAS was the same. You solve for CAS on your flight computer. Don't want to be picky but there is a difference.
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I stand corrected, sir. For THIS question, the IAS and CAS were the same because of a lack of needed information...correct? My apologies for the misinterpretation. In cases like this picky is necessary, because I would not want to be the reason some information was passed on incorrectly. Have a good night...
 
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