Please help me with this. HSI depiction - IR test question

J

JasonM

Pattern Altitude
Joined
Mar 24, 2012
Messages
1,837
Location
West Virginia
Display Name
Display name:
JM
I feel like an idiot for having to ask for help on this. I am studying for my IFR written and there are a handful of these types of questions. I am having a hard time understanding this for some reason.

Is there any pointers someone can give me to make this type of question easy to understand?

I dont understand why some of these mean I am high or low, left or right. I seem to get them wrong when I treat it like a VOR signal. example, the course is shown as 9 deg off to the left/below. I would think I am high and need to head south to fix this, but as you can see there is only one airplane on this heading, so my assumption to fix this situation is wrong. :mad2:

I would assume the answer to be the only one on the heading of 315 as shown, which is prob. correct due to the process of elimination, but of other questions like this there will be multiple aircraft on the same heading and i cant figure out what is the correct one.:confused:

Please help..

16h30k.png


1szl1x.png
 
Jason...you're not wrong that there can be more than 1 airplane as the correct answer. On some of the questions, the answers will have 2 airplanes listed as the answer. It appears you're using software to study. If so, set the software to display only the correct answer, then read the explanation. Keep paper & pencil handy and sketch the radials and compass and anything else the explanation describes. Don't even bother looking at the list of possible answers, it only confuses you while studying.
 
I actually did just that and it never registered with me. I have been studying for this test for about 3 months and this is honestly my biggest problem. I am scoring in the 90% for the practice tests, but for some reason, I cannot grasp this. I feel like I am missing something I should be seeing the instant I see the pictures.
 
murphey said:
Jason...you're not wrong that there can be more than 1 airplane as the correct answer. On some of the questions, the answers will have 2 airplanes listed as the answer. It appears you're using software to study. If so, set the software to display only the correct answer, then read the explanation. Keep paper & pencil handy and sketch the radials and compass and anything else the explanation describes. Don't even bother looking at the list of possible answers, it only confuses you while studying.
Click to expand...


That's an HSI, not a DG.

HSI says 315. The plane is heading 315.

Oh, and they have the HSI dialed in "backwards", so that's why it adds to the confusion.
 
The reason the HSI seems to be giving you the wrong information is that the OBS is set up incorrectly. The approach to 27 is the front course but the OBS is set to 9. Turn it to 27 and the CDI will show the localizer course to the north.
 
Ron Levy showed me a REALLY nice iPad app to help learn the concept behind this question (where are you based on the HSI or CDI depiction) and practice other CDI related skills.

NAV Trainer, $4.99 in the App Store.

During our training, Ron used it many times to demonstrate a concept associated with using VORs for various IFR Flying tasks. It was a great tool to help visualize and practice these tasks.

Included in the program is a quiz similar to the IFR Written question the OP references. You are shown CDI's (or HSI), the referenced VOR, and must pick which aircraft is the correct one based on it's position relative to the VOR. It was a great tool to learn how to determine what side of the course line you are on, and if the VOR is ahead or behind you.

Totally worth the download.
 
The reason you get it wrong is because you're treating it like a VOR signal. LOC signals are not the same. Unlike VOR signals, which radiate omnidirectionally from the station, a localizer signal is essentially a single radial radiating from a point an infinite distance beyond the departure end of the runway. As such, there is no TO/FROM, and the OBS knob on a CDI (or the course pointer on a CDI) has no effect on needle deflection. As a result, if you do as some of the test pictures show and twist the course pointer to the opposite direction from the localizer course (in this case, 090 instead of 270), the indications become very confusing.

In the case of HSI "D", in order to make sense of things, you must mentally turn the course indicator around from the 090 selected in the picture to the 270 of the actual localizer course. Since course selection has no effect on needle deflection, the needle will remain in the same position with respect to the course pointer arrow. Thus, after you rotate the course indicator around, the needle will be deflected towards North (not South as originally depicted). That means the localizer course is north of you, so you are south of the course. That limits the possible answers to 2, 3, 5, 8, and 13. The heading portion of the HSI shows 315, or northwest. Of the five aircraft south of the localizer course, only #2 is headed northwest, so that's the correct answer.

Now, try the same process to analyze HSI "E".

And this all goes together with my discussion regarding the term "reverse sensing" in another thread.
 
Nav Trainer is also available for Android at the Google Play Store. Same price. Thanks, I needed to spend a quick $5 for a new toy.

Scott
 
azure said:
The reason the HSI seems to be giving you the wrong information is that the OBS is set up incorrectly. The approach to 27 is the front course but the OBS is set to 9. Turn it to 27 and the CDI will show the localizer course to the north.
Click to expand...

How can you tell from that picture which way it should be pointed? Its pointed at 090 in the picture. How did you know that is wrong?
 
AggieMike88 said:
Ron Levy showed me a REALLY nice iPad app to help learn the concept behind this question (where are you based on the HSI or CDI depiction) and practice other CDI related skills.

NAV Trainer, $4.99 in the App Store.

During our training, Ron used it many times to demonstrate a concept associated with using VORs for various IFR Flying tasks. It was a great tool to help visualize and practice these tasks.

Included in the program is a quiz similar to the IFR Written question the OP references. You are shown CDI's (or HSI), the referenced VOR, and must pick which aircraft is the correct one based on it's position relative to the VOR. It was a great tool to learn how to determine what side of the course line you are on, and if the VOR is ahead or behind you.

Totally worth the download.
Click to expand...

Thanks. I'll check into that.
 
Ron Levy said:
The reason you get it wrong is because you're treating it like a VOR signal. LOC signals are not the same. Unlike VOR signals, which radiate omnidirectionally from the station, a localizer signal is essentially a single radial radiating from a point an infinite distance beyond the departure end of the runway. As such, there is no TO/FROM, and the OBS knob on a CDI (or the course pointer on a CDI) has no effect on needle deflection. As a result, if you do as some of the test pictures show and twist the course pointer to the opposite direction from the localizer course (in this case, 090 instead of 270), the indications become very confusing.

In the case of HSI "D", in order to make sense of things, you must mentally turn the course indicator around from the 090 selected in the picture to the 270 of the actual localizer course. Since course selection has no effect on needle deflection, the needle will remain in the same position with respect to the course pointer arrow. Thus, after you rotate the course indicator around, the needle will be deflected towards North (not South as originally depicted). That means the localizer course is north of you, so you are south of the course. That limits the possible answers to 2, 3, 5, 8, and 13. The heading portion of the HSI shows 315, or northwest. Of the five aircraft south of the localizer course, only #2 is headed northwest, so that's the correct answer.

Now, try the same process to analyze HSI "E".

And this all goes together with my discussion regarding the term "reverse sensing" in another thread.
Click to expand...


Thanks Ron. I still am having trouble understanding. How did you know that needed to be turned from 090 to 270?
 
JasonM said:
Thanks Ron. I still am having trouble understanding. How did you know that needed to be turned from 090 to 270?
Click to expand...
Because you can tell from Figure 96 that the LOC is aligned to 270. Two ways you know:

  • Blue side is to the north
  • Outer and middle markers are to the east.
...and as I discussed above, you always want the course pointer aligned to the LOC course forward direction regardless of whether you're flying it forwards or backwards.
 
JasonM said:
How can you tell from that picture which way it should be pointed? Its pointed at 090 in the picture. How did you know that is wrong?
Click to expand...
The front course always has the right side of the arrow shaded. I forget the reference for that, but it must be in the AIM somewhere.

With an HSI, you always turn the OBS so it points in the direction of the front course. (Well, unless your panel has a back course mode that reverses the CDI indication, but that's unusual in GA aircraft.)
 
JasonM said:
Thanks Ron. I still am having trouble understanding. How did you know that needed to be turned from 090 to 270?
Click to expand...

It doesn't. It's just to help with situational awareness.
 
Ron Levy said:
...and since this is a question about situational awareness (like where you are), it sure does help.
Click to expand...

Well, I went at it the easier route. HSI shows 315, there's only one plane heading 315. No need to worry about the CDI deflection.
 
EdFred said:
Well, I went at it the easier route. HSI shows 315, there's only one plane heading 315. No need to worry about the CDI deflection.
Click to expand...
Well the OP did that too, but was still wondering why the HSI seemed to be showing the LOC course to the south.
 
EdFred said:
Well, I went at it the easier route. HSI shows 315, there's only one plane heading 315. No need to worry about the CDI deflection.
Click to expand...
If all you want to do is train for the test, that works for that particular question, but it won't help for many of the others in that group.
 
Ron Levy said:
If all you want to do is train for the test, that works for that particular question, but it won't help for many of the others in that group.
Click to expand...

For now he should be doing what is best for the test. Process of elimination gave the answer he needs.
 
Ron Levy said:
Because you can tell from Figure 96 that the LOC is aligned to 270. Two ways you know:

  • Blue side is to the north
  • Outer and middle markers are to the east.
...and as I discussed above, you always want the course pointer aligned to the LOC course forward direction regardless of whether you're flying it forwards or backwards.
Click to expand...

Thank you. That is very helpful. :D

azure said:
The front course always has the right side of the arrow shaded. I forget the reference for that, but it must be in the AIM somewhere.

With an HSI, you always turn the OBS so it points in the direction of the front course. (Well, unless your panel has a back course mode that reverses the CDI indication, but that's unusual in GA aircraft.)
Click to expand...

Also, very helpful. Between Ron and You explaining things, i am starting to have a better idea of how this is working.


TommyG said:
For now he should be doing what is best for the test. Process of elimination gave the answer he needs.
Click to expand...

I would like to understand this and some of the questions I can not simply eliminate.
 
JasonM said:
I would like to understand this and some of the questions I can not simply eliminate.
Click to expand...

Here's how I've explained it elsewhere for a similar question, FYI, with no apologies to Cap'n Ron :):
"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. In this question, the ‘shaded side’ of the chart symbol is to the right, for ‘right’ (correct) sensing as per our memory gouge, only when the arrowhead is at 270° and the aircraft is heading west. 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, etc."​

dtuuri
 
Last edited:
azure said:
The front course always has the right side of the arrow shaded. I forget the reference for that, but it must be in the AIM somewhere.

With an HSI, you always turn the OBS so it points in the direction of the front course. (Well, unless your panel has a back course mode that reverses the CDI indication, but that's unusual in GA aircraft.)
Click to expand...

Not an official "reference" but a good reminder is "Blew right in front of me" (Blue on the right when inbound on the LOC front course.
 
The way I pitched this question to my students is that if you look at the first figure, you see the Outer and Inner Markers depicted. This turns the entire question in to a LOC question and not a VOR question, then you can only have one answer.
 
Another trick to remember is when all is normal, you fly the plane TOWARDS needle. When in reverse sensing, you switch to YOU ARE THE NEEDLE. On some HSI's you can flick a switch that reverses it, but I haven't seen many that do that.
 
stratobee said:
Another trick to remember is when all is normal, you fly the plane TOWARDS needle.
Click to expand...
If you mean you fly in the cardinal direction in which the needle is pointing, I'd say yes. Otherwise, I'd say you're just asking to end up flying around in a little circle without intercepting anything.

When in reverse sensing, you switch to YOU ARE THE NEEDLE.
Click to expand...
Only if you want to confuse yourself, and you can still end up flying around in that little circle.

On some HSI's you can flick a switch that reverses it, but I haven't seen many that do that.
Click to expand...
I've seen that only on a CDI, and then only on the old Cessna ARC ones (although I think it may have been available on some Narcos back in that era). I have never seen a switch that reverses needle function on any HSI.
 
Ron

You do this for a living and I don't, so with the risk of skating on thin ice:

Explain how you could ever fly a circle intercepting a LOC? Or flying a circle flying a reverse sensing LOC back course? Key word being intercept here.

Maybe it was a CDI I saw.
 
Last edited:
stratobee said:
Explain how you could ever fly a circle intercepting a LOC? Or flying a circle flying a reverse sensing LOC back course? Key word being intercept here.
Click to expand...
Let's say you are some distance north of course on a Rwy 27 LOC with the needle pegged to the left side of the case. The needle is left, so you start turning left. You are so far north of the course that your turn radius never takes you close enough that the needle comes off the peg. You could sit there in that turn all day long going round and round, and unless a northerly wind blows you within the needle scale, you'll never see the needle move. And none of that changes if you are on the "back course" side of the localizer antenna, either.

The only way to make good intercepts is to use the needle to tell you what heading you need to fly (not which way to turn), then use your heading indicator to tell you which way to turn to get to that heading, stop on that heading until the needle starts to move in, and finally let the needle guide you through the headings to roll out on course. That's true whether you are using a CDI/HI or an HSI, although it's a lot easier to visualize what you need to do with an HSI. And it works just fine when flying "backwards" on the LOC (e.g., eastbound on that Rwy 27 LOC), too, as long as you set the OBS to the forwards direction and look down rather than up from the needle position to find the heading to fly for interception.
 
Sure, but this is part of intercept. If your course is not converging then that could happen on an old style VOR/LOC. But if they haven't been taught that by a CFI, I would say that's a pilot that needs some more training.

As long as you're less than 180 degree off radial, even on an old VOR/LOC you will at some point converge (as long as the signal remains). You might be going the wrong way, but you will cross it.

I did some IR training in my old Commander 520. It had the most rudimentary instrumentation - a single old VOR/GS. No DME. No current GPS to back it up. So all the cross radials on the IFR chart, I had to switch frequency, read it ("nah, not quite there yet") and then flip back to the radial I was tracking. Then flip back again, and again etc. It got to be such a pain in the a** for my poor instructor he refused to do it after a while. Told me to either stick another VOR in there or get a new plane.

I got the new plane, but still haven't finished the rating.
 
Last edited:
I just wanted to thank all you guys. I have been nailing these questions now. :) Looking to take the written soon.
 
Ron Levy said:
I've seen that only on a CDI, and then only on the old Cessna ARC ones (although I think it may have been available on some Narcos back in that era). I have never seen a switch that reverses needle function on any HSI.
Click to expand...
The 480 has a back course function, so if it's driving an HSI you effectively have the same thing. It's useful if you use auto-slew to set the course pointer, since it always sets it based on the leg. (But I found out the hard way that you still need to be alert when using auto-slew -- maybe especially when using it -- since the semantics of what the current leg is are fraught with subtle gotchas.)
 
You must log in or register to reply here.
Back
Top