HSI question

alfadog

alfadog

Final Approach
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alfadog
One of my weak points on my IR study is understanding the HSI. I am starting to get it but a question on a quiz just threw me. If the HSI is tuned to the localizer and the little airplane is flying away from the CDI, is there an instance when you are actually flying toward the localizer course, i.e. a reverse sensing situation? I thought the HSI eliminated reverse sensing.
 
alfadog said:
One of my weak points on my IR study is understanding the HSI. I am starting to get it but a question on a quiz just threw me. If the HSI is tuned to the localizer and the little airplane is flying away from the CDI, is there an instance when you are actually flying toward the localizer course, i.e. a reverse sensing situation? I thought the HSI eliminated reverse sensing.
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I'd like to help you out, but my brain refuses to go to work unless it's sure what you're saying:
"If the HSI is tuned to the localizer..."​
The localizer frequency is 'tuned'? Or the HSI needle is 'turned' to the inbound course? Or the HSI needle is 'turned' to the final approach course?

dtuuri
 
dtuuri said:
I'd like to help you out, but my brain refuses to go to work unless it's sure what you're saying:
"If the HSI is tuned to the localizer..."​
The localizer frequency is 'tuned'? Or the HSI needle is 'turned' to the inbound course? Or the HSI needle is 'turned' to the final approach course?
dtuuri
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I mean nav radio controlling the HSI needle is tuned to the localizer. I thought it made no difference where you turn the OBS to if you're on the localizer.
 
alfadog said:
I mean nav radio controlling the HSI needle is tuned to the localizer. I thought it made no difference where you turn the OBS to if you're on the localizer.
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Ah, yes it matters. Not on a VOR approach (funny aside, a good friend used to spin the pointer around during procedure turns for years, a habit from before flying with a PN-101 until I pointed out the needlesness of it all. The look on his face was priceless), just a localizer. You need to set it to the inbound course of the front course for proper 'sensing'. Glad Cap'n Ron is still on vacation, so he doesn't engage me about 'sensing'. :)

dtuuri
 
dtuuri said:
Ah, yes it matters. Not on a VOR approach (funny aside, a good friend used to spin the pointer around during procedure turns for years, a habit from before flying with a PN-101 until I pointed out the needlesness of it all. The look on his face was priceless), just a localizer. You need to set it to the inbound course of the front course for proper 'sensing'. Glad Cap'n Ron is still on vacation, so he doesn't engage me about 'sensing'. :)

dtuuri
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Wow, trick question then. It looks like HSI is set to the reciprocal of the inbound front course. HSI E below.

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OK, the best way to remember this IMO is for any LOC approach be it the front course or the BC, always set the OBS to the front course. If you do that the HSI will always give you normal sensing (ie fly towards the needle) regardless if you or flying the localizer or the BC.
 
That's not how I learned it. OBS doesn't matter on Localizer. And if you fly a back course, you are the needle (unless you have a switch that can switch it).
 
stratobee said:
That's not how I learned it. OBS doesn't matter on Localizer. And if you fly a back course, you are the needle (unless you have a switch that can switch it).
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I learned on Glass and fly Glass today and have never flown a mechanical HSI so maybe that's the difference, but you are right in that it's a technique thing to spin the OBS to the inbound course even for a BC--that's how I was taught to keep the picture straight in my head. As for the a BC button, I thought that was for standalone CDIs to eliminate "reverse sensing" (ie flying away from the needle) and not HSIs.
 
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tsts4 said:
I learned on Glass and fly Glass today and have never flown a mechanical HSI so maybe that's the difference, but you are right in that it's a technique thing to spin the OBS to the inbound course even for a BC--that's how I was taught to keep the picture straight in my head. As for the a BC button, I thought that was for standalone CDIs to eliminate "reverse sensing" (ie flying away from the needle) and not HSIs.
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The 480 has a BC mode. When in BC mode, it outputs reversed sensing to an external HSI so that, if you turn the OBS to the inbound course (back course) when flying a LOC BC approach, you will get "normal sensing". Otherwise, with the OBS turned to the inbound course (back course), you would get "reverse sensing".

I was taught to always turn the HSI OBS to the front course inbound course and eschew that particular bell/whistle, but I actually prefer BC mode and usually use it when flying a LOC BC. I haven't flown one in a couple of years, though.
 
stratobee said:
That's not how I learned it. OBS doesn't matter on Localizer. And if you fly a back course, you are the needle (unless you have a switch that can switch it).
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The OBS on an HSI doesn't have any effect on the CDI, but it CAN make it look like you're flying toward the localizer when you're actually flying away from it.

I learned this when I was acting as safety pilot for a guy who had an HSI, and watched him struggle with this problem! (He didn't have the OBS set at all - just some random direction.)
 
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alfadog said:
Wow, trick question then. It looks like HSI is set to the reciprocal of the inbound front course. HSI E below.

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I wouldn't consider it a "trick question"...they're evaluating your understanding that the course selected doesn't have any impact on which way the CDI deflects.

Another way to think about CDI deflection with a localizer is that it represents which side of the feathered arrow the deflection represents. In the examples shown in your post, G, H, & I show the airplane on the shaded side. B, D, & E Show the airplane on the unshaded side. You could spin the course needle all the way around and the needle deflection wouldn't change, as it represents which side of the feathered arrow the airplane is on.

Back when pterodactyls were using localizers for navigation, the CDI actually had a better representation of that...a blue mark on one side for the shaded side and a white or yellow mark on the other for the unshaded side.
 
Now I'm confused.

HSI E shows a heading of 45°.

Only arrows 3 & 8 fit that.

And the fly right indication would place the plane south of the localizer.

Couldn't it be either one?

I think. It's been a long time since those particular brain cells got exercised.
 
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FastEddieB said:
Now I'm confused.

HSI E shows a heading of 45°.

Only arrows 3 & 8 fit that.

But the fly right indication would place the plane north of the localizer.

I think. It's been a long time since those particular brain cells got exercised.
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I haven't yet puzzled it all out myself but since the HSI OBS is set to the reciprocal of the inbound front course, the unit is in a reverse sensing mode for positions 3 and 8.
 
Note my edit from north to south of the localizer.

You must mentally turn the little plane to agree with the front course heading - 270° - for the needle to make sense.

That is, fly right, though that would not apply on a 045° heading.
 
FastEddieB said:
Now I'm confused.

HSI E shows a heading of 45°.

Only arrows 3 & 8 fit that.

And the fly right indication would place the plane south of the localizer.

Couldn't it be either one?

I think. It's been a long time since those particular brain cells got exercised.
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Okay, let's see if I get this. Isn't it true that the relative position between the arrow of the OBS indicator and the deflected CDI remains constant no matter how you spin the OBS needle? It's always showing you that the localizer course is to your right if you were flying the inbound front course. So if you are flying 270, the course is to your right and therefore, you're south of course. Your heading doesn't matter, and the rotation of the OBS line on the HSI doesn't matter.
 
alfadog said:
Okay, let's see if I get this. Isn't it true that the relative position between the arrow of the OBS indicator and the deflected CDI remains constant no matter how you spin the OBS needle? It's always showing you that the localizer course is to your right if you were flying the inbound front course. So if you are flying 270, the course is to your right and therefore, you're south of course. Your heading doesn't matter, and the rotation of the OBS line on the HSI doesn't matter.
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BINGO!

I think.

Lord knows, I used to teach this stuff, but it's been a long time.
 
G, H, & I are on the shaded (north, in this case) side of the course. G & I are on westerly headings, so they are representative of 7 & 11 interchangeably. H is on a southwesterly heading, so that's number 1.

HSI I should never exist in real life...all it'll do is confuse you. But the FAA isn't testing you on "what you'd do in the real world" with this question. They're testing you on your understanding of the mechanics of an HSI.
 
MauleSkinner said:
But the FAA isn't testing you on "what you'd do in the real world" with this question. They're testing you on your understanding of the mechanics of an HSI.
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Indeed. Same as with other questions folks whine about on written tests, the proper button for slaving a remote compass comes to mind.

dtuuri
 
dtuuri said:
Indeed. Same as with other questions folks whine about on written tests, the proper button for slaving a remote compass comes to mind.

dtuuri
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Didn't know I was whining. :D
 
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