FAA Study Question: Feeling Dumb

killbilly

killbilly

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Okay, I've run into this question 2X on two different tests, and I keep getting the answer wrong, which means a) I'm doing it incorrectly or b) the answer to the question is wrong.

So I'm posting the question, and I want to see what you guys come up with. If you could tell me how you come up with the answer, I would appreciate it, because I'm apparently lacking something in my studies and feeling silly. Thanks in advance.

"On a cross-country flight, point A is crossed at 1500 hrs and the plan is to reach point B at 1530 hours. Use the following information to determine the indicated airspeed required to reach point B on schedule:"

Distance between A and B: 70NM
Forecast Wind : 310 at 15
Pressure Alt: 8,000 ft
Ambient Temp: -10C
True Course: 270

The approximate required indicated airspeed would be: ???"

I believe I've been getting it wrong because I'm running the wrong calcs on the E6B.

The way I'm reading it, I need to go 70NM in 30 minutes, which means a GS of 140kts. From there, I take the GS and correct for wind and pressure/temp, right?

Help!
 
You have the idea right. A common error that I see with my students with that type of question is that here you need to do the opposite conversions - usually you would start with your true airspeed and apply the wind correction to get ground speed. In this case you already know the ground speed, so you need to use that (put the x mark on the correct line on E6B, read the indicated speed in the center hole). If that's not helping, may be you can post your intermediate results and I'll try to see what you're doing wrong?

Your TAS should be 148 with true heading of 275?
 
milski said:
You have the idea right. A common error that I see with my students with that type of question is that here you need to do the opposite conversions - usually you would start with your true airspeed and apply the wind correction to get ground speed. In this case you already know the ground speed, so you need to use that (put the x mark on the correct line on E6B, read the indicated speed in the center hole). If that's not helping, may be you can post your intermediate results and I'll try to see what you're doing wrong?

Your TAS should be 148 with true heading of 275?
Click to expand...

I kept coming up with 154 kts. The question only asks for airspeed (I typed it VERBATIM.)

The correct answer they give is 137kts. I just don't see how they're getting it.
 
killbilly said:
I kept coming up with 154 kts. The question only asks for airspeed (I typed it VERBATIM.)

The correct answer they give is 137kts. I just don't see how they're getting it.
Click to expand...

You need to convert your TAS to CAS. That would be an approximate IAS. To find the exact IAS, you would need the IAS/CAS chart from the POH of the airplane.

It's been a while since I've picked up an E6B, so take my help for what it's worth. :) If I can find an E6B, I'll see if I can give it a go later.
 
imasaluki said:
You need to convert your TAS to CAS. That would be an approximate IAS. To find the exact IAS, you would need the IAS/CAS chart from the POH of the airplane.
Click to expand...

It's a study question, they don't give you the IAS. Just the GS and the wind, which I guess will give me the TAS. But then I still don't know where the 137 is coming from.
 
killbilly said:
It's a study question, they don't give you the IAS. Just the GS and the wind, which I guess will give me the TAS. But then I still don't know where the 137 is coming from.
Click to expand...

Convert your 154 or 148 TAS (true airspeed) to CAS (calibrated airspeed) with your E6B. What does that give you?

Edit: Looks like granlistillo got it figured out... might need to break out the instructions for the E6B if you're having trouble going from TAS to CAS/IAS.
 
granlistillo said:
yes they are looking for ias. i get 136 kts on the whiz wheel. 140 kts gs requires 152 ktas, which spins out 136 kias
Click to expand...

Okay - so what did you do to find it? I need to wrap my head around the process - everything else is arithmetic.
 
killbilly said:
Okay - so what did you do to find it? I need to wrap my head around the process - everything else is arithmetic.
Click to expand...
Ok, the question asked for ias. You know the gs required, so you need to figure out the tas required, and then solve for ias required. you need a ground speed of 140, but you have a wind at 15 knots 40 degrees off your nose. The sin of 40 degree angle will be about .65 more or less so it will be about 10 kts. Using the whiz wheel i read 12 knots, but mathematically it would be about 10. So you will need about 150 tas. Flip your whiz wheel put your temp over pa, and then find your TAS on the outer rule and read your cas/ias on the inner rule. 150 ktas = 135 very easy
 
granlistillo said:
Ok, the question asked for ias. You know the gs required, so you need to figure out the tas required, and then solve for ias required. you need a ground speed of 140, but you have a wind at 15 knots 40 degrees off your nose. The sin of 40 degree angle will be about .65 more or less so it will be about 10 kts. Using the whiz wheel i read 12 knots, but mathematically it would be about 10. So you will need about 150 tas. Flip your whiz wheel put your temp over pa, and then find your TAS on the outer rule and read your cas/ias on the inner rule. 150 ktas = 135 very easy
Click to expand...

This is the part that I don't know how to calculate. I'm looking at my E6B (I have both wheeled and electronic versions) and trying to figure that out. I don't think I've covered this in my studies yet, which is why I'm struggling. Please forgive my ignorance.

With the wind 40 deg. off my nose, (which I did figure out) I don't know where to look to calculate the sin - what's more embarassing, I guess, is that I didn't know to calculate the sin to figure everything else out.

Or am I now making this more complicated than it is?
 
killbilly said:
This is the part that I don't know how to calculate. I'm looking at my E6B (I have both wheeled and electronic versions) and trying to figure that out. I don't think I've covered this in my studies yet, which is why I'm struggling. Please forgive my ignorance.

With the wind 40 deg. off my nose, (which I did figure out) I don't know where to look to calculate the sin - what's more embarassing, I guess, is that I didn't know to calculate the sin to figure everything else out.

Or am I now making this more complicated than it is?
Click to expand...

perhaps i was making the answer a little to complicated.

The windside of your e6b, does the trigonometry for you. If you understand trig, you can use the sin or cosin to do a quick calculations for you. if you arent a trig guy you will need to use the windside of the computer. Either way in the problem that 15 knot wind will give you a 10 kt headwind component. So your TAS speed will have to be 10 faster than your GS. Now that you know your TAS, you need to go to the slide rule face or use an electronic computer and convert your TAS (using pressure altitude and temperature) into CAS/IAS. In this case it would ias because no correction info is given in the problem.

You need to read the instructions on calculating IAS when given TAS, PA, and Temp
 
granlistillo said:
perhaps i was making the answer a little to complicated.

The windside of your e6b, does the trigonometry for you. If you understand trig, you can use the sin or cosin to do a quick calculations for you. if you arent a trig guy you will need to use the windside of the computer. Either way in the problem that 15 knot wind will give you a 10 kt headwind component. So your TAS speed will have to be 10 faster than your GS. Now that you know your TAS, you need to go to the slide rule face or use an electronic computer and convert your TAS (using pressure altitude and temperature) into CAS/IAS. In this case it would ias because no correction info is given in the problem.

You need to read the instructions on calculating IAS when given TAS, PA, and Temp
Click to expand...

I got it now! Thank you so much!!!!

It just took me a bit to figure it out on both devices. I really appreciate the help - I have a much better understanding now.

It's a little faster on the whizwheel than the electronic one, but more accurate on the electronic one. Trying to become proficient with both.
 
no probs,
I find that doing wind problems in the head are quicker and just as accurate as the computers. Very useful if you pick up atis and want to know what the crosswind component will be. If you learn the unit in Trig, it is a very very useful tool in aerodynamics and especially in navigation.
 
E6BAV8R said:
What were the possible answers? I got 133 IAS.
Click to expand...

136.5 kts was the exact answer, 137 was the accepted answer - it was a flashcard thing.

After setting this aside for a while, I came back to it - I had worked it out on the whizwheel, but I was still trying to figure out how to do it on the electronic E6B that came with my ground DVDs. (Sporty's) Turns out it has built-in functions for exactly this kind of problem, I just hadn't sorted it out in the manual yet.
 
granlistillo said:
no probs,
I find that doing wind problems in the head are quicker and just as accurate as the computers. Very useful if you pick up atis and want to know what the crosswind component will be. If you learn the unit in Trig, it is a very very useful tool in aerodynamics and especially in navigation.
Click to expand...

My whizwheel has a crosswind component chart on it, but I've not really had to use it yet.

Thanks again, very much, for the patient explanations. I'm not much of a mathematician.
 
killbilly said:
136.5 kts was the exact answer, 137 was the accepted answer - it was a flashcard thing.

After setting this aside for a while, I came back to it - I had worked it out on the whizwheel, but I was still trying to figure out how to do it on the electronic E6B that came with my ground DVDs. (Sporty's) Turns out it has built-in functions for exactly this kind of problem, I just hadn't sorted it out in the manual yet.
Click to expand...

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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