Help with Naviation Question on FAA Knowledge Test

[eetrojan]

Help with Navigation Question on FAA Knowledge Test

Hi, wondering if somebody can help me figure out how best to fix some apparent error in my approach to a navigation question in the Sporty's test prep system.

Here's the problem, which I got wrong because I chose "63 min" rather than "59 min":

However, my measurement of true course was the same (101 deg.), and my measurement of the distance was the same (41 NM). Here are my scratch notes:

I used my E6B to determine a ground speed of 40 knots. Then, dividing distance (41 NM) by speed (40 knots), I get 1.025 hours, or 61.5 minutes. I chose 63 minutes because it's closer (delta is 1.5) than 59 minutes (delta is 2.5).

It seems that the error comes from my having determined a ground speed of 40 knots with my E6B, rather than 40.7. As you can see from the this photo, when I put the specified true airspeed of 36 knots under my red wind dot, the indicated ground speed (under the grommet) is 40. However, I have to estimate the location of the wind dot a little bit since it's supposed to be under 36 and my E6B's lowest speed is 40:

Is there any way to be more accurate with my E6B?

Thanks!

Joe

 

[poadeleted20]

No, there is not. Two issues. First, Sporty's obviously did theirs electronically, which allows more significant digits. Second, your E-6B doesn't have a speed scale below 40 knots, which means you can't accurately set the slide. That said, I don't think I've ever seen a question involving a TAS of 36 knots on a real FAA question, unless they've come up with that for Sport Pilot, where some LSA's may really fly that slow. Clearly, you know how to do it correctly, so don't let this bother you -- move on, and remember that if you see a question with 36 TAS on the real test, the answer is 59 minutes.

 

[Jim Logajan]

No, there is not. Two issues. First, Sporty's obviously did theirs electronically, which allows more significant digits. Second, your E-6B doesn't have a speed scale below 40 knots, which means you can't accurately set the slide. That said, I don't think I've ever seen a question involving a TAS of 36 knots on a real FAA question, unless they've come up with that for Sport Pilot, where some LSA's may really fly that slow. Clearly, you know how to do it correctly, so don't let this bother you -- move on, and remember that if you see a question with 36 TAS on the real test, the answer is 59 minutes.

I checked my 2010 edition of the Gleim Private Pilot and Recreational Pilot FAA Knowledge Test book and the question does appear there. So the question is definitely from the FAA and not Sport Pilot related (but possibly recreational?). I seem to recall also having some problems with that question during my own test prep studies.

Here is the discussion from Gleim:

"Answer ( B ) [59 minutes] is correct.

DISCUSSION: The distance between Majors Airport and Winnsboro Airport is 41 NM. The approximate MH heading to Winnsboro is 100 degrees. The wind produces a tailwind of approximately 6 knots. Therefore, the aircraft will travel at 42 knots (36 knots + 6 knots). 41 NM/42 NM/hr = .976 hr.

Answer (A) [55 minutes] is incorrect because the wind's effect is a tailwind of 6 knots, not 9 knots. Answer (C) [63 minutes] is incorrect because the wind's effect is a tailwind of 6 knots not 3 knots."

 

[poadeleted20]

That discussion is fine, Jim, but since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you? That's the big problem with the question.

 

[ja_user]

That discussion is fine, Jim, but since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you? That's the big problem with the question.

Which is why it may very well have been published, but may never show up in an exam...

 

[livitup]

That discussion is fine, Jim, but since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you? That's the big problem with the question.

Mine goes down to 30. I did the problem too, and came up with just more than 40 knots. I probably would have figured 41, and come up with 60 minutes, and then looking at the answers, figured that my E6-B was showing just a little less than 41 and picked B.

But worse than the groundspeed... how's that 17 degree heading correction working out for ya?

 

[Jim Logajan]

That discussion is fine, Jim, but since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you? That's the big problem with the question.

I agree with you - the problem sucks for most E-6Bs. I assume since it is in the FAA question bank it could appear on an exam. Your advice to Joe that he should just remember the answer is 59 and move on is probably what I did.

I have an E-6B that came with a 1990 era Cessna ground school kit I bought way back then that only goes down to 60 knots on the wind correction computer. It goes up to 230 knots.

But when I restarted flight training 20 years later in 2010, the E-6B I got in the Gleim ground school kit goes down to ~28 knots. It goes up to 260 knots.

Both E-6Bs have 1 knot = 1/32 inch on the speed scale. I can't remember which one I took with me to the written exam. For what it is worth, a scan of both of them side-by-side:

 

[eetrojan]

Here is the discussion from Gleim:

"Answer ( B ) [59 minutes] is correct.

DISCUSSION: The distance between Majors Airport and Winnsboro Airport is 41 NM. The approximate MH heading to Winnsboro is 100 degrees. The wind produces a tailwind of approximately 6 knots. Therefore, the aircraft will travel at 42 knots (36 knots + 6 knots). 41 NM/42 NM/hr = .976 hr.

Answer (A) [55 minutes] is incorrect because the wind's effect is a tailwind of 6 knots, not 9 knots. Answer (C) [63 minutes] is incorrect because the wind's effect is a tailwind of 6 knots not 3 knots."

Thanks Jim and all.

Given Gleim's discussion, is it more useful to go at this sort of problem by figuring out the headwind or tailwind component, rather than using a mechanical E6B to find the GS? I haven't tried to dust off my ancient trigenometry skills, but maybe I should?

 

[gprellwitz]

Thanks Jim and all.

Given Gleim's discussion, is it more useful to go at this sort of problem by figuring out the headwind or tailwind component, rather than using a mechanical E6B to find the GS? I haven't tried to dust off my ancient trigenometry skills, but maybe I should?

To be honest, if you understand the concepts, as it seems you do, as long as you can score more than the 70% minimum passing score on the knowledge exam you're good to go. If you're ever to the point in planning out a real flight where these levels of precision are important, you're going to be applying some very conservative assumptions and fudge factors on top of it anyway. You would NOT (or at least should not) be saying to yourself "I can stretch this flight to that further airport because I have 41 and not 38 knots ground speed!" The only place I can think of where this level of precision would be of use would be a competition of some sort. There are just too many unknowns in the real world.

 

[poadeleted20]

Thanks Jim and all.

Given Gleim's discussion, is it more useful to go at this sort of problem by figuring out the headwind or tailwind component, rather than using a mechanical E6B to find the GS? I haven't tried to dust off my ancient trigenometry skills, but maybe I should?

Those trig skills are useless without trig tables. Dunno if they'd let you take those into the test room -- might be numerically coded answers. Like I said -- either memorize this answer or bring an electronic device.

 

[MDeitch1976]

What about drawing out a wind triangle?


Sent from my iPad using Tapatalk HD

 

[poadeleted20]

What about drawing out a wind triangle?

Works fine if your instructor taught you how to do that with a plotter, paper, and pencil, but that's not a technique we teach much.

 

[MAKG1]

That discussion is fine, Jim, but since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you? That's the big problem with the question.

If you double the wind speed and TAS, you'll also double the GS, and get it on-scale. Or if that E-6B handles jet speeds (as it seems most do), you can multiply by 10 instead (easier).

No slide rule is intended to specify order of magnitude. And when used precisely, they should get 3 significant figures.

 

[MDeitch1976]

Sent from my iPad using Tapatalk HD

Image loaded upside down

I know I cheated and used drafting paper. However, this can be done with just a plotter. And, you can even take it a step further and calculate wind correction angle.

This is in your Pilots Handbook of Aeronautical Knowledge.

 

[MDeitch1976]

Works fine if your instructor taught you how to do that with a plotter, paper, and pencil, but that's not a technique we teach much.

None of my instructors taught this to me. It was in the PHAK, so I tried it a few times.
Just another tool, if someone cares to know.


Sent from my iPad using Tapatalk HD

 

[eetrojan]

The proctor has pre-blessed my HP 32SII which has trig functions so, in theory, I should be able to figure this out and also do it during the test if required.

I will explore the wind triangle approach too. It hasn't been taught to me, but I have run across it.

Thanks All!

Joe

 

[eetrojan]

I like'de wind triangle. The 6 NM tailwind is easy to see. Thanks again.

 

[MDeitch1976]

I like'de wind triangle. The 6 NM tailwind is easy to see. Thanks again.

I have never been asked to use it, or needed it. But, that unusual problem you had triggered the thought. It really is not difficult at all to learn. Basically, the E6B is a very handy tool, and the electronic flight bags as well.


Sent from my iPad using Tapatalk HD

 

[dmspilot]

since 36 knots TAS is off the scale of any standard mechanical E-6B, just how is one supposed to determine the tailwind is 6 knots other than with an electronic device or bringing the trig tables with you?

Double the true airspeed and wind speed, calculate groundspeed, then cut it in half. Angles stay the same.

I've always brought simple scientific calculators with trig functions on them to written tests, never had a problem.

 

[flyingron]

I took groundschool from a university physics professor. He said we all should understand vector math being college students and taught us winds that way. Never even saw an E6.

 

[MAKG1]

I took groundschool from a university physics professor. He said we all should understand vector math being college students and taught us winds that way. Never even saw an E6.

So, if you're in the air in significant winds at altitude, and want to estimate a time, you would solve the Law of Cosines? Or do a conversion to plane Cartesian vectors, so the much simpler vector math, and then reconvert to the polar form? While you can do it correctly that way, it's error prone and complex.

That's a lot of keypunches to do what an E-6B does in three much simpler steps (usually with the steps written right on the face of the device).

 

[bobmrg]

These time-speed-distance knowledge test questions bug the hell our of me. I know that the student's understanding of the procedure is required, but to my mind they give the false impression that figuring out your ETA to a gnat's eyelash is essential...and it's not. An air carrier is deemed to be "on time" if it lands within 15 minutes of its published arrival time, so what's the big deal if someone in a 172 is off by ten minutes? We all know that the carefully prepared flight log becomes semi-useless as soon as something unexpected pops up while departing the area.

I've been asked "How do I adjust my airspeed so as to make my ETA?" Sheesh!

Bob Gardner

 

[poadeleted20]

I've been asked "How do I adjust my airspeed so as to make my ETA?" Sheesh!

Well, there are situations where that's an issue -- like if you're in the middle of a large strike package and you have a TOT (time over target) which must be made +/- 60 seconds or you may have the previous plane's bombs go off in your face or your bombs go off in the next crews' faces. Other than that...

 

[flyingron]

So, if you're in the air in significant winds at altitude, and want to estimate a time, you would solve the Law of Cosines? Or do a conversion to plane Cartesian vectors, so the much simpler vector math, and then reconvert to the polar form? While you can do it correctly that way, it's error prone and complex.

That's a lot of keypunches to do what an E-6B does in three much simpler steps (usually with the steps written right on the face of the device).

Hey, I didn't say it was practical...
Of course this guy also held the state altitude record in gliders, and a number of people followed that statement up with "high or low?"

 

[denverpilot]

Well, there are situations where that's an issue -- like if you're in the middle of a large strike package and you have a TOT (time over target) which must be made +/- 60 seconds or you may have the previous plane's bombs go off in your face or your bombs go off in the next crews' faces. Other than that...

There's some nifty flight planning software for multiple aircraft converging at different airspeeds for that... But I am required to say that I've never seen it.