Location in relation to a VOR

[MedicRyan]

I am currently studying for my CFII written and I am having difficulty understanding something. I am getting confused about how to determine where I am in relation to a VOR (i.e. Which aircraft corresponds with the indication of HSI "B"? or Using the indication of the HSI below, determine where you are in relation to the VOR. a. northeast b. southwest, etc.) I had difficulty with this same concept when I was studying for Instrument test 6 years ago. I want to make sure I fully understand it this time. Can someone please show my an easy way (if there is one) to wrap my head around this concept?

Thanks!

Ryan

 

[socal_pilot]

The easiest way for me to figure it out is to cover up the cdi and also the to/from flag and whatever quadrant is not covered on the HSI thats the quadrant you are on the choice below.

 

[MedicRyan]

Wow that works great...thanks a lot!

 

[NJA_Capt]

Instead of learning a trick.....just learn how one works. They are actually very simple.

To/From indicator: Pretty staight forward. You are either heading toward or away from the station (VOR/WPT).
Bearing pointer is the selected radial (From) or course (To).
The compass rose is always showing the aircraft's heading.
If you are heading To the station and the CDI is off to the left, you are RIGHT of Course. If the CDI is right, you are left of course.
To get on course, you "follow" the needle (split).

http://upload.wikimedia.org/wikipedia/commons/7/72/HSI.PNG

 

[MedicRyan]

Instead of learning a trick.....just learn how one works. They are actually very simple.

To/From indicator: Pretty staight forward. You are either heading toward or away from the station (VOR/WPT).
Bearing pointer is the selected radial (From) or course (To).
The compass rose is always showing the aircraft's heading.
If you are heading To the station and the CDI is off to the left, you are RIGHT of Course. If the CDI is right, you are left of course.
To get on course, you "follow" the needle (split).

http://upload.wikimedia.org/wikipedia/commons/7/72/HSI.PNG

Well, the part that confuses me is when reverse sensing comes in to play. This is where I go south real quick. I just can't seem to wrap my head around the heading, the course select pointer, the CDI displacement and the TO/FROM indication to figure out where I am. And I do want to learn the correct way to solve this since I will soon be teaching someone else to solve it.

 

[NJA_Capt]

....since I will soon be teaching someone else to solve it.

Yea.....I can see where that would be helpful
You do have to think to keep your perspective sometimes. Try not to think of it as reverse sensing. It ALWAYS senses "forward" if you will. When you are flying the 360 "course" (To), the heading and the Big Pointer are on 360. To track..... "follow the needle." When you are flying outbound on the 180 radial, the heading and the big pointer are on 180. Here is where it is tricky(maybe). When flying a BC, you always put the big pointer on the "inbound" heading, not the course. A BC to Rwy 36 would have the big pointer on 180. Then you are still able to follow the needle deviation. All you do is engage the BC button on the Flight Director so it guides you "inbound" instead toward the big pointer.

Clear as mud?

 

[fish314]

Instead of learning a trick.....just learn how one works. They are actually very simple.

To/From indicator: Pretty staight forward. You are either heading toward or away from the station (VOR/WPT).
Bearing pointer is the selected radial (From) or course (To).
The compass rose is always showing the aircraft's heading.
If you are heading To the station and the CDI is off to the left, you are RIGHT of Course. If the CDI is right, you are left of course.
To get on course, you "follow" the needle (split).

http://upload.wikimedia.org/wikipedia/commons/7/72/HSI.PNG

The bit about the To/From indicator is not quite correct. It doesn't really matter which way you are "heading" (or in other words, it does not take into account where the nose is pointing). What it does is indicate on which side of a line perpendicular to the selected course you are on.

"Say wha?" I'm sure you are thinking.... Well here it is in plainer English:

Say for example you have 360 (straight North) selected as your desired course in your HSI. A line perpendicular to that would go through the VOR station at 090 and 270 (East and west, right?). All your To/From indicator does is tell you which side of that line you are on. If you are anywhere south of that line (if you are between the 091 and 269 radials) it will indicate "TO" or if you have one with the little arrow head on it, the arrow head will point towards the course arrow on the HSI. If you are anywhere north of that line (271 to 089 radials) it will indicate "FROM" or the arrow head will point away from the course arrow. If you happen to be right ON the perpendicular line? Well, it will be in transition from "To" to "From" or vice versa.

So now you may be thinking, "well why is it a To/From indicator then?" Well, if you WERE on course (i.e. if you had the CDI centered) and you were tracking in the direction of the course arrow then you would be headed TO the station when it says "TO." Eventually you would pass over the station, and if you continued to track in the direction of the course arrow, you would have passed the station and would now be heading "FROM," the station. Since the vast majority of the time we are using the instrument we are on centerline (or close to it) and tracking in the direction we have dialed in on the selected course the terms "to" and "from" make literal sense. When we haven't yet centered up the CDI, or when we are flying in the opposite direction of our selected course, it gets a little screwy and the words "to" and "from" on the indicator don't equate as easily in our minds.

Ok, one last quick example just to demonstrate:

We have selected 235 as our desired course (that's SW). That means that a line perpendicular to that course goes through the VOR station from 315 (NW) to 135 (SE).

If we are on the 360 radial HEADING North our TO/FROM reads:




TO. Explanation: If we were to get on centerline and then track in the direction that we selected as our desired course, it would take us TO the station.

Next question: If we were on the 360 radial, but this time headed south our TO/From would read:




Still TO. Explanation. Same as before. The to/from does not know which direction the airplane is pointed (heading), it only knows what radial you are on compared to what you selected.

This link might explain a little better if my explanation still doesn't do it for you: http://stoenworks.com/Tutorials/HSI, the complete.html. The only tough part on their diagram is that it is a little hard to see, but their selected course is 300 degrees, which means their perpendicular line that divides "to country" and "from country" goes through the VOR from 030 to 210 degrees.

Clear as mud?

 

[shdw]

Well, the part that confuses me is when reverse sensing comes in to play.

The only thing doing reverse sensing is the pilots head. So long as you know it is a pilot head error, and not an instrument error, we can move on.

Next, reread what NJA posted. Reread it again.

Now, let's do an example:

1) The CDI is 2 dots to the left, pointed at heading 330, and the flag reading TO the station. Where are we in relation to our course? Where are we in relation to the station? What radial are we trying to be on?

--

To answer this, first we must know what is important. Here, below is a repost of the question with the relevant information bolded:

The CDI is 2 dots to the left, pointed at heading 330, and the flag reading TO the station. Where are we in relation to our course? Where are we in relation to the station? What radial are we trying to be on?

--

Let's look at what each bold item means, now, then answer our questions:

CDI is left - The CDI line is our course. If it is off to the left of us, then the course is off to our left. Thus, we are right of course.

Pointed at 330 - It is important to distinguish between aircraft heading and VOR setting. This is what leads to pilots brains reversing things, but we will get back to that later. The heading the VOR is pointed at is the radial you wish to fly TO, or on (FROM).

Flag reading TO - Another important distinction must be made here. In this case, we are flying TO the radial. You have likely heard that you are flying to the VOR. While this is also true, if you realize that you are also flying TO the radial it can help you better understand the instrument.

--

Ok, so what are the answers? Here we go,

Where are we in relation to our course?

If the course needle (CDI) is to our left, then our course is to our left. So, in this case, we are to the right of our course.

Where are we in relation to the station?

Hmm, maybe we want to know what radial we are on before we figure out where we are? Ok, so what radial are we trying to be on?

Recall above that I said it was important to realize that we are flying TO radial 330, if 330 is set into the VOR with the TO flag showing. So, if we are flying to it, there is a really good chance we are not on it at the same time. If we aren't on radial 330, but we are flying to radial 330, what radial are we on?

The answer is simply the reciprocal, or 330 - 180 = 150. We are on radial 150, flying to radial 330.

Well, now that we know what radial we are on, figuring out where we are in relationship to the VOR becomes easy. Draw a dot on a piece of paper, assume up is north, and draw a line that represents radial 150. You are somewhere on that line. In other words, we are south east of the VOR, on radial 150, flying to radial 350.


--


I told you I'd get back to reverse sensing, but only if you remember that it is a pilot head problem and not the instrument. In fact, when a pilot is experiencing reverse sensing the instruments are probably the only things in the airplane that aren't confused. But what the heck is it!?!

Take our example from above, flying to radial 330 and on radial 150. Our compass is pointed at heading 330, since we are trying to fly to that radial. Now, if we were to continue flying heading 330, but for some odd reason turn the VOR dial to 150, then we would be experiencing an instrument acting in reverse.

In this situation, if the course needle deflected left, and we turned left to chase it, it would go left faster. This is because the course is to our right now because we have the instrument upside down. Well, not literally upside down, but it is pointed to the exact opposite heading we are trying to fly.

Ok, hopefully we are on the same page with what this is. So, how do we fix it? Simple! Make sure your heading on your compass (or DG assuming you made sure these match) make sense.

For instance, if your VOR is set to 250, you might have a heading of 225, 247, 262, 273, etc then your VOR and heading make sense. Your situational awareness is in tack. Conversely, if you see 043, 057, 080, 092, etc on that compass then you know something is screwy and if you try to read the VOR like this it will be backwards. Or, if it is easier to remember, just check accidentally turned the VOR upside down.


Hope that helps and I agree with NJA. Rule of thumbs are great, but when you are trying to teach this stuff they provide little help without a solid understanding. It's time to buckle down and really figure it out so you can ace that CFI ride. Good luck.

 

[NJA_Capt]

It's time to buckle down and really figure it out so you can ace that CFI ride. Good luck.

Ouch. I'm getting a headache....and I've been using them for 15+ years.

When you get a chance to use an HSI, it all falls into place.

 

[MedicRyan]

Yea.....I can see where that would be helpful
You do have to think to keep your perspective sometimes. Try not to think of it as reverse sensing. It ALWAYS senses "forward" if you will. When you are flying the 360 "course" (To), the heading and the Big Pointer are on 360. To track..... "follow the needle." When you are flying outbound on the 180 radial, the heading and the big pointer are on 180. Here is where it is tricky(maybe). When flying a BC, you always put the big pointer on the "inbound" heading, not the course. A BC to Rwy 36 would have the big pointer on 180. Then you are still able to follow the needle deviation. All you do is engage the BC button on the Flight Director so it guides you "inbound" instead toward the big pointer.

Clear as mud?

Well, getting there...ha ha. Ok, well I have just spend the last hour trying to absorb the information that you all have so graciously presented to me. And I have come to a conclusion...RMI oriention is not something that can be taught in words alone. I am getting closer to understanding it, though, and knowing that the HSI does not care what heading you are on has helped. I do understand the basic principle of operating an HSI and if I were to go on a trip with an airplane equipped with one I know I would be just fine using it to navigate. It's funny that I have been flying for 8 years, have commercial and instrument tickets, and this is the only thing in aviation that has confused me up to this point. Well, I guess that's not too bad! I have an IPC coming up next week and what I am going to do is sit down with my instructor and go over this stuff. Thank you NJA, fish314 and shdw for taking the time to try to explain this to me, but I think the demonstration-performance method will be what I need to get it!

Ryan

 

[jrh]

Medic: Do you have access to a G1000 simulator? Even a simple PC-based sim will do.

The G1000 uses an HSI for the standard display. It also has something called "bearing pointers" in the background, which are essentially RMI needles.

A good instrument instructor could have you up to speed after 30 minutes with a G1000 sim I bet.

 

[MikeD]

Medic: Do you have access to a G1000 simulator? Even a simple PC-based sim will do.

The G1000 uses an HSI for the standard display. It also has something called "bearing pointers" in the background, which are essentially RMI needles.

A good instrument instructor could have you up to speed after 30 minutes with a G1000 sim I bet.

Not just a G1000. The standard HSI in a military aircraft uses bearing pointers on the outer card as standard. I'm surprised you need to define what they are. Do people not know what those or an RMI are?

 

[Douglas]

Not just a G1000. The standard HSI in a military aircraft uses bearing pointers on the outer card as standard. I'm surprised you need to define what they are. Do people not know what those or an RMI are?

Very much so.
After a couple of flights in a G1000, it finally dawned on me what they were and that they were acting like an RMI.
I and most only see RMIs in the written test prep book.

"An RMI! I found one!"
It's like finding the easter bunny.

 

[MikeD]

Very much so.
After a couple of flights in a G1000, it finally dawned on me what they were and that they were acting like an RMI.
I and most only see RMIs in the written test prep book.

"An RMI! I found one!"
It's like finding the easter bunny.

Ever used an ADF? Same thing, just without the fixed card. I mean, even though someone hasn't seen one, seeing it for the first time shouldn't be like flying the Space Shuttle for the first time. Even though the SA it gives you is great.

The first 172 I ever began flying had an RMI as the heading instrument, vice a DG (Directional Gyro, for all the glass babies out there).

 

[Douglas]

Ever used an ADF? Same thing, just without the fixed card. I mean, even though someone hasn't seen one, seeing it for the first time shouldn't be like flying the Space Shuttle for the first time. Even though the SA it gives you is great.

The first 172 I ever began flying had an RMI as the heading instrument, vice a DG (Directional Gyro, for all the glass babies out there).

hehe
Sure it is the same as an ADF. It's not hard to use but it is rare enough to do a double take.
You're old.

 

[MikeD]

hehe
Sure it is the same as an ADF. It's not hard to use but it is rare enough to do a double take.
You're old.

Those Dern youngin's!!!

 

[The Gardener]

MikeD, I haven't seen a RMI in a piston aircraft in the 10 years I've been flying them. They are great tools though, it's a shame we don't see them more.

Regarding the OP's question.

Try to describe your position relative to the course selected without using the words LEFT or RIGHT.
Clue: the needle will be swung to one side or the other of center. Note the cardinal direction(s) on the OBS on the side the CDI is swung to and wallaaa, that is the direction to the course. Then just interpret the to/from indicator and fish314 described and determine an appropriate intercept angle.

If it makes you feel any better I didn't see the light of this technique until I was well into my CFI (at AF no less).

 

[MedicRyan]

Medic: Do you have access to a G1000 simulator? Even a simple PC-based sim will do.

The G1000 uses an HSI for the standard display. It also has something called "bearing pointers" in the background, which are essentially RMI needles.

A good instrument instructor could have you up to speed after 30 minutes with a G1000 sim I bet.

I don't have a G1000 sim, but I do use FSX quite a bit for procedural stuff. It comes with a baron that has an HSI. I think this will be good for me. I just need to see the darn thing in action to get it.

 

[MedicRyan]

Ever used an ADF? Same thing, just without the fixed card. I mean, even though someone hasn't seen one, seeing it for the first time shouldn't be like flying the Space Shuttle for the first time. Even though the SA it gives you is great.

The first 172 I ever began flying had an RMI as the heading instrument, vice a DG (Directional Gyro, for all the glass babies out there).

I have, and the nice thing about using an ADF for oriention is you can use a basic formula (MH + RB = MB) to get your orientation. If an HSI had a formula...I would be in good shape!

 

[fish314]

I have, and the nice thing about using an ADF for oriention is you can use a basic formula (MH + RB = MB) to get your orientation. If an HSI had a formula...I would be in good shape!

I don't have a G1000 sim, but I do use FSX quite a bit for procedural stuff. It comes with a baron that has an HSI. I think this will be good for me. I just need to see the darn thing in action to get it.

Hey MedicRyan, try taking a look at this page. It appears to be a basic instrument sim over the internet. Anywhooo.... it has an HSI and you can practice with all the settings, plus you can see a depiction on the left that shows the location of the airplane in relation to the VOR.