Question e Knowledge Test - Determining Distance on En Route Chart

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Question re Knowledge Test - Determining Distance on En Route Chart

The closeup below is from Fig. 47 of the testing supplement for the instrument knowledge test - an en route chart. It's used for a question that asks you to figure out the total ETE for a flight to KPDX. So, for each leg, I need to figure out the distance and ground speed in order to calculate the leg time.

One of the leg distances I need to determine is the BTG to PDX leg (blue). I know from peeking at the answer on Sporty's that it has a distance of 10.

How would I know that for sure? Is it 10 simply because the PDX VOR is located at KPDX and because the blue circle indicates Class C airspace that normally has a radius of 10 NM? Or, is there a more definitive way of determining that it's 10 from this chart?

Thanks!

IFR_Fig47_BTG-to-KPDX.jpg
 
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Compass rose radius around VORs is 5nm. The edges of PDX and BTG touch. 5+5=10 (unless they are smaller versions of five).
 
Remember a minute is a mile - use the longitude scale...
 
Cpt_Kirk said:
Measure it with your plotter.
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Plotters don't work on the L-charts on the IR written because the charts aren't always to scale. You have to use the chart data. In this case, there is no distance published for this direct off-route leg, and the chart scale is cut off, so you can't use that (see the full figure on page 104 of the full document here). About the only way I can see to get the distance is to compare that distance to some of the published distances already on the chart, like 10 miles from BTG north to ROARK or 11 miles from ROARK north to COUGA.
 
The plotter issue always reminds me of an old Dave Bedard column in on one of the magazines which was allegedly the instructions that came with your (to warp in the sun) plotter.

For sectional charts, use the scale marked 1:500000
For WAC charts, use the scake marked 1:1000000
For charts marked Not For Navigation, use the scale marked Not For Plotting.

While the chart isn't necessarily to a 1:500000 scale, it is to some scale and if they didn't put a scale on the chart, you can use one of the other measured distances (there is a 10 mile leg on the chart between BTG and ROARK for example).
 
Thanks all! I can definitely measure. :)

I thought I was missing something because this was the seventh route I worked through and, for the first time, I couldn't determine a leg distance based on distance number(s) actually printed on the en route chart.

Appreciate it.
 
Or do what I did on my IR knowledge exam, and told the examiner when we went over the code(s) I got wrong.

"Wasn't my cross country, didn't feel like doing calculations for a flight I'm not going to fly."

He thought it was funny.
 
EdFred said:
Or do what I did on my IR knowledge exam, and told the examiner when we went over the code(s) I got wrong.

"Wasn't my cross country, didn't feel like doing calculations for a flight I'm not going to fly."

He thought it was funny.
Click to expand...

I echo the examiner.
 
I'm pretty sure I remember that question from my days of studying for the IR written. I'm also pretty sure I came to the conclusion that the best way to get that one right was just to memorize the answer, something that's true of several of the cross country questions. The problem is that the possible choices are usually close enough together that round-off error (and in this case, measurement error, since I also agree that the only reliable way to measure the distance is by first measuring a marked distance with the plotter) are enough to throw you into the neighborhood of one of the wrong answers even though you're doing the calculation correctly.

It's just another stupid guvmint test question that is not worth raising your blood pressure over. If you were doing it for real, you'd at least have a usable scale. (More likely, you'd have ForeFlight.* ;))


*Seriously, yes of course you should know how to calculate this yourself without any fancy EFB. But it's ridiculous to expect you to do it with one hand tied behind your back.
 
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Clark1961 said:
Remember a minute is a mile - use the longitude scale...
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Actually use a north/south line of latitude.
Longitude (E/W) are not accurate for distance.

One arc minute of latitude = 1nm.
One degree of latitude = 60nm.
60 arc minutes per degree
 
Clark1961 said:
Remember a minute is a mile - use the longitude scale...
Click to expand...

BillTIZ said:
Actually use a north/south line of latitude.
Longitude (E/W) are not accurate for distance.

One arc minute of latitude = 1nm.
One degree of latitude = 60nm.
60 arc minutes per degree
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Use a meridian, they're the ones that are north-south, the markings on them indicate latitude. The markings on parallels of latitude, the lines that run east-west, indicate longitude.
 
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As others have mentioned, measuring and comparing to other known distances looks like the best bet.

An alternate option is to note that BTG-mules is 18 nm and BTG-PDX is slightly more than half of that distance (you know that because the circle is centered on PDX and BTG is a bit closer to the edge than mules). Half the distance is 9nm, that makes 10nm is a reasonable guess for slightly more than half.

If you need a more accurate value and for some bizarre reason you can't measure it, you can bust out some geometry and calculate a reasonably accurate estimate.
 
Is the AFD for PDX one of the supplements available? Those usually tell you distances to nearby Navaids. If I remember right. An arrival chart will also give that info.
 
Ron Levy said:
Plotters don't work on the L-charts on the IR written because the charts aren't always to scale. You have to use the chart data. In this case, there is no distance published for this direct off-route leg, and the chart scale is cut off, so you can't use that (see the full figure on page 104 of the full document here). About the only way I can see to get the distance is to compare that distance to some of the published distances already on the chart, like 10 miles from BTG north to ROARK or 11 miles from ROARK north to COUGA.
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I don't have one in front of me, and please correct me if I'm wrong, but doesn't each panel of a low chart have a distance measuring scale at the bottom of the page? Just mark off the distance on a piece of scrap paper, and measure it on the distance scale at the bottom.
 
desertav8r said:
Is the AFD for PDX one of the supplements available? Those usually tell you distances to nearby Navaids. If I remember right. An arrival chart will also give that info.
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Just double checked. The referenced Figs. 44, 45, 46, and 47 includes a departure plate for Yakima (YKM), but no AFD or arrival procedures for PDX.

I was finding some leg distances on those related figures for some of the other problems, separate from the en route charts, but not in this case.
 
roncachamp said:
They don't touch here.
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No they don't. On the other hand, the Class C shaded airspace shelf is a good approximation:
"Although the configuration of each Class C airspace area is individually tailored, the airspace usually consists of a 5 NM radius core surface area that extends from the surface up to 4,000 feet above the airport elevation, and a 10 NM radius shelf area that extends no lower than 1,200 feet up to 4,000 feet above the airport elevation." http://www.faa.gov/air_traffic/publications/atpubs/AIM/aim0302.html#aim0302.html.4​

dtuuri
 
Sluggo63 said:
I don't have one in front of me, and please correct me if I'm wrong, but doesn't each panel of a low chart have a distance measuring scale at the bottom of the page? Just mark off the distance on a piece of scrap paper, and measure it on the distance scale at the bottom.
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Thanks. From a quick scan, it doesn't look like any of the enroute chart segments in the IR supplement include a measuring scale, but now that I know I won't always find an explicit distance, there are certainly plenty of other distances to use as a reference.
 
Sluggo63 said:
I don't have one in front of me, and please correct me if I'm wrong, but doesn't each panel of a low chart have a distance measuring scale at the bottom of the page? Just mark off the distance on a piece of scrap paper, and measure it on the distance scale at the bottom.
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That works when you have the full chart, but not when you only have an excerpted section like that in Figure 47 of that IR test supplement.
 
The chart excerpt shows the geographic coordinates of both BTG and PDX, which gives you definitive locations on the surface of the earth. The distance can be calculated.
 
somorris said:
The chart excerpt shows the geographic coordinates of both BTG and PDX, which gives you definitive locations on the surface of the earth. The distance can be calculated.
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Thanks - Mathematically, by gleaning lat/long values from the lat/long lines and minute intervals?

How do you do it? Is that practical for the instrument exam?
 
register@teamandras.com said:
Thanks - Mathematically, by gleaning lat/long values from the lat/long lines and minute intervals?

How do you do it? Is that practical for the instrument exam?
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Geometry. You can memorize that every degree of latitude is 60nm. But measuring along the latitude it varies with latitude. So unless airports are directly N/S from each other, no it's not practical.
 
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Memorize the answer and move on. You will never have this problem in real life as the IFR charts all have a scale at the bottom that you can measure distances with (if you're the last guy to figure out ForeFlight or SkyVector)
 
18NM [BTG->MULES] / 2 = ~10NM [BTG->PDX].

BTG -> ROARK is 10NM, scale distance is the same BTG-PDX and more than the 7NM OSWG-CANBY.

Law of sines using PDX-> GLARA and PDX->HARZL.
 
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register@teamandras.com said:
Thanks - Mathematically, by gleaning lat/long values from the lat/long lines and minute intervals?

How do you do it? Is that practical for the instrument exam?
Click to expand...

For the instrument exam? No, not really practical. What I do is just set my pointer and pinky fingers on a set of latitude lines (better known as Parallels) along an edge to longitude line (better known as Meridians) and use the measured divide in minutes as miles of distance, and then use my fingers as the measuring tool which can be calibrated in distance, time, or fuel required. While this is all good real life skill, on your exam you will have little use for it.
 
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Henning said:
For the instrument exam? No, not really practical. What I do is just set my pointer and pinky fingers on a set of latitude lines (better known as Parallels) along an edge to longitude line (better known as Meridians) and use the measured divide in minutes as miles of distance, and then use my fingers as the measuring tool which can be calibrated in distance, time, or fuel required. While this is all good real life skill, on your exam you will have little use for it.
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Plus, using your fingers isn't NEARLY precise enough to choose between answers on the instrument exam, which are often so close together that even roundoff error can put you in the ballpark of one of the "wrong" answers. Agree with Hook_dupin; for the exam, memorize the answer and move on. Just make sure you really do know how to do the calculation.
 
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