Practice Test Question

First, there is no such thing as "reverse sensing". That is a misnomer which has confused the heck out of folks for many decades. The needle will deflect in the proper direction regardless of whether you're on the front or back course, and it will not reverse as you transition from front to back or back to front.

Second, this is a localizer, not a VOR radial, so there is no difference in needle deflection direction between the front and back courses because you cannot select a course to fly as you can with a VOR. Unlike a VOR signal where if you twist the OBS knob 180 degrees, the needle will reverse sides because you have reversed the selected course, the needle deflection on a localizer is unaffected by the selected course which is "hard wired" into the LOC signal, and this the same for both a CDI OBS knob and an HSI course pointer.

As a result, the needle will deflect in the same direction relative to the course pointer head regardless of whether you are east or west of the antenna (located in this case at the west end of the runway for this RWY 27 LOC) and regardless of where you set the HSI course pointer.

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This is a very important distinction, whose understanding is necessary to make sense of HSI presentation on localizers.

Now, in this HSI ("G" in the test"):

97_figure97g.gif


...the course pointer is correctly pointed 270 (the forward direction of the localizer). The needle is deflected to the south, meaning the localizer is south of you, so you are north of the localizer, which means you must be 1, 7, 10, or 11. You can see you are heading west, which eliminates 1 and 10, but is consistent with both 7 and 11. Since there is no TO/FROM on a localizer, you have no way with the information given to distinguish between the two, so Answer C (7 and 11) is the best answer.

Note also that several of the pictured HSI's (B, C, D, and E) have the course pointer turned backwards (i.e., pointing to 090). This just makes things worse, since now you have effectively turned your CDI upside down, making it appear that it's sensing backwards when in reality you have just stood yourself on your head. So, in this case (G), if you twisted the CDI course pointer to 090 (i.e., turned the CDI portion of the HSI upside down), there is no change in the needle deflection relative to the head of the pointer, making it falsely appear to be reversed, when in reality you've just turned it upside down.

If you always keep the HSI course pointer pointed in the forward direction for a localizer course (i.e., 270 on this one), you will never be confused as to which direction the localizer course is in relation to you. IOW, the questions on those four HSI's which are rotated the wrong way (I.e., course pointer on 090) have you doing something no pilot his his/her right mind would ever do, just to make it harder for you to figure out the correct answer. Really bad questions, I think.
 
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Thank you for clearing that up for me. I was under the impression that on a localizer back course you turn away from the needle to center it. Now that I think about what you said, this would be correct if I was flying towards rwy 9.

If I was flying towards 9 on a back course approach and north of the course, I'd see a left hand deflection, but would turn right (south) to correct it. Right?
 
Ron is right. Read his explanation a couple times at least, it will start making sense. If it does not, PM me your number and we can talk tonight.

Just a hint: for these kinds of questions, the shaded area on the localizer arrow is a dead giveaway.

Also, read AIM 1-1-0, it explains the differences in all the navigation aids. Very useful.
https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0101.html
 
Also, be absolutely certain you understand front course vs back course and approach vs departure. Those are two different terms and can be combined. And that's where most students run into trouble.
Once you understand the difference between these, it will be clear as day.
My phonecall offer stands.
 
gms5002 said:
I'm looking at this question in a practice test. Here's a copy I found online: http://www.exams4pilots.org/faatest.cgi?m=cv&b=inst_rat&q=5.4.5.6.9.A.1 It says the correct answer is position 7 and 11. I thought it was 7 only. In position 11, I thought there would be reverse sensing at this point and the needle would be on on the right.

What am I missing?
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I've answered this (or one similar) for somebody else, somewhere:
Answer ‘C’ (#11) is the only choice offered that fits, but aircraft #7 would fit also.

With an HSI, remember this: For proper sensing on a localizer course you must always place the arrowhead under the inbound course of the front course regardless of the direction being flown or location of the aircraft. That way, no matter which direction you fly (toward or away from the airport) or which side of the airport you’re on (back course or front course side), you can fly toward the CDI to return to the centerline. Being a wonderful HSI, turning to any other direction after a proper setup still maintains correct left/right sensing. Sing praises to the inventor of the HSI. However this HSI (panel ‘I’, Fig 97)) is not set up correctly—it’s 180° off. Consequently, it has reverse-sensing. To center the CDI you must fly away from it. If it shows you off to the left, you’re really off to the right.
​
I happen to LIKE the term "reverse sensing". :)

dtuuri
 
gms5002 said:
Thank you for clearing that up for me. I was under the impression that on a localizer back course you turn away from the needle to center it. Now that I think about what you said, this would be correct if I was flying towards rwy 9.

If I was flying towards 9 on a back course approach and north of the course, I'd see a left hand deflection, but would turn right (south) to correct it. Right?
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No. You'd dial the front course (270) into the HSI, the needle would still deflect south, and you'd still turn right to get on course.
 
Ron, thanks for taking the time and effort to give such a thorough explanation. That's what makes this board so useful.
 
gms5002 said:
Thank you for clearing that up for me. I was under the impression that on a localizer back course you turn away from the needle to center it. Now that I think about what you said, this would be correct if I was flying towards rwy 9.

If I was flying towards 9 on a back course approach and north of the course, I'd see a left hand deflection, but would turn right (south) to correct it. Right?
Click to expand...
Only if you set the HSI course pointer backwards for that localizer, i.e., on 090 instead of 270. If you set it on 270, then when you turn around towards the east, the course pointer remains pointed west, and if you are north of the course, the needle will still deflect to the south, which is now to your right. That's why it's so important to set the course pointer of the HSI to the forward direction of the localizer regardless of whether you're flying it forwards (i.e., inbound on the front course or outbound on the back course) or backwards (i.e., inbound on the back course or outbound on the front course).
 
Ron Levy said:
Only if you set the HSI course pointer backwards for that localizer, i.e., on 090 instead of 270. If you set it on 270, then when you turn around towards the east, the course pointer remains pointed west, and if you are north of the course, the needle will still deflect to the south, which is now to your right. That's why it's so important to set the course pointer of the HSI to the forward direction of the localizer regardless of whether you're flying it forwards (i.e., inbound on the front course or outbound on the back course) or backwards (i.e., inbound on the back course or outbound on the front course).
Click to expand...

Ok, I think the HSI is what's confusing me because I don't fly with one. Is this correct:

Assuming a regular CDI if I was flying the LOC 27 backcourse approach to rwy 9 my needle would be backwards. If I had an HSI, I could spin the course pointer around to 270 so that right was right and left was left.
 
gms5002 said:
Ok, I think the HSI is what's confusing me because I don't fly with one.
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That wouldn't be surprising.

Is this correct:

Assuming a regular CDI if I was flying the LOC 27 backcourse approach to rwy 9 my needle would be backwards.
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Well, sort of. If you learned the CDI properly, you would not think it backwards, because the CDI only shows the cardinal direction of the selected course from your present position, not the left-right direction. If you set up for this localizer on a CDI, and correctly set the OBS to 270 (the forward direction of the localizer), if you're north of the localizer course, the needle will still deflect towards south on the CDI regardless of what your current heading is. With 270 selected while flying backwards (i.e., heading 090), south is to the left on the instrument, while the course is to your right, but you still need to turn to a heading somewhere south of 090 to get to the localizer course. As such, the needle is not indicating backwards.

However, since the CDI doesn't use the OBS setting when interpreting a localizer signal, if you incorrectly set the OBS to 090, the needle will still deflect to the left side of the case, where north is now improperly located. In that situation, yes, it appears to be indicating backwards. However, if you correctly set the OBS to the forward direction (just like with the HSI course pointer), the OBS ring will have south on the side where the needle is deflected.

The key difference when using a CDI to track a localizer backwards is that you look at the bottom of the CDI, not the top, to find the heading to fly. IOW, during normal VOR ops with a CDI, you see where the needle is deflected to pick the heading to fly to intercept the needle. For example, if you're trying to track the 270 course, and the needle is deflected to the north, you'd look north of 270 to, say 300, to find an intercept heading. You'd do the same to track a localizer forwards.

However, if you're tracking a localizer backwards using a CDI, you look at the bottom of the CDI rather than the top. In the case under discussion, where you're north of the localizer tracking east (backwards), with 270 at the top and 090 at the bottom, the needle will deflect to the south (i.e., the left side of the CDI). You would then look at the bottom of the CDI to find a heading to the south (i.e., to the left on the instrument) of 090, and see the needle is pointing near 120, which would be your heading for a 30-degree intercept. Yes, you'll have to turn right to get from 090 to 120, but that's what your heading indicator is for -- to decide which way to turn to get to the heading the CDI says to fly.

The very most important thing to remember here is that the CDI tells you the heading to which you should turn, not which way left-right you need to turn. It's your heading indicator which tells you which way to turn to get to the heading the CDI says you need to intercept the course.

If I had an HSI, I could spin the course pointer around to 270 so that right was right and left was left.
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Correct.
 
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Alright I got it now, thank you for the the thorough explanations. I'll have to look around for a back course approach so I can practice one.
 
gms5002 said:
Alright I got it now, thank you for the the thorough explanations. I'll have to look around for a back course approach so I can practice one.
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Very few left, and they're getting fewer all the time as the FAA replaces them with RNAV(GPS) approaches. Ones of which I am aware include New Bedford MA (KEWB ) and Oakland-Pontiac MI (KPTK).

However, you don't need a published BC approach to practice flying a LOC backwards -- just find an ILS or LOC and fly the front course outbound (as is required for the full procedure with course reversal on quite a few of them). You may not be able to get ATC to clear you to actually do that at the published altitudes (it interferes with the inbound flow), but you can certainly overfly it at 2500 AGL or so to get backwards practice. It's also easy to practice this in the sim, if you have access to one -- ask your instrument instructor about that.
 
Ron Levy said:
Very few left, and they're getting fewer all the time as the FAA replaces them with RNAV(GPS) approaches. Ones of which I am aware include New Bedford MA (KEWB ) and Oakland-Pontiac MI (KPTK).

However, you don't need a published BC approach to practice flying a LOC backwards -- just find an ILS or LOC and fly the front course outbound (as is required for the full procedure with course reversal on quite a few of them). You may not be able to get ATC to clear you to actually do that at the published altitudes (it interferes with the inbound flow), but you can certainly overfly it at 2500 AGL or so to get backwards practice. It's also easy to practice this in the sim, if you have access to one -- ask your instrument instructor about that.
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Seventy, spread out across the country (as of the Approach Inventory today).

Bob Gardner
 
Ron Levy said:
Very few left, and they're getting fewer all the time as the FAA replaces them with RNAV(GPS) approaches. Ones of which I am aware include New Bedford MA (KEWB ) and Oakland-Pontiac MI (KPTK).
Click to expand...
Might be worth noting that at KPTK, there is already an RNAV(GPS) approach to the same runway (27L) as the LOC BC, and it has existed for several years now. I suspect the reason they don't get rid of the LOC BC is that it has lower minimums than the RNAV does unless you have WAAS - and probably there are enough heavies still that do not have WAAS to justify keeping it around.

I wonder how many of the other remaining LOC BC approaches are going to be kept running indefinitely for similar reasons.
 
gms5002 said:
Alright I got it now, thank you for the the thorough explanations. I'll have to look around for a back course approach so I can practice one.
Click to expand...

Don't know where you're at but I flew the BC 28 approach today at CGI during an IPC.
 
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