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

 

[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."

 

[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: