VOR CDI Interpretation

[FlyHighM]

Hey guys, I've recently been preparing for my PPL written exam, I've been studying VOR interpretations, from what I understand, when the CDI needle is left it means I'm right of radial and need to correct left, if the CDI needle is right it means I'm left off and I need to correct to the right. Do these indicators change with TO/FROM flags?
My instructor mentioned that they reverse depending on TO/FROM positions, but that's not what I'm understanding from the material that I've been studying, any insight on how the CDI works with TO/FROM flags in relation to position from the radial would be much appreciated.

cheers

[Metaltubepilot]

Stop thinking about it as left or right. If you fly the heading indicated at the top or bottom of the CDI ring, you will fly parallel to the radial. If the CDI is on the left side of the indicator any heading on the left side of the CDI ring will take you towards the radial and vice versa. If the Flag says TO then the heading at the top of the CDI ring will take you towards the VOR.

Once you know how to identify what headings will take you towards the radial and either towards or away from the VOR then you can decide to do a 90, 45, or 30 degree intercept as desired or instructed. Only then can you determine where the tuned radial is in relation to yourself as you figure out the shortest direction to turn from your present heading to your desired heading. Don't let your instructor make this more complicated than necessary.

[Canadaflyer46]

I’m amazed they’re still teaching/examining that stuff. What the hell is a To/From flag?! Got an Astro compass handy?

[Fly0nTheWall]

Have you used a VOR in a plane/sim before? Or do you have access to one? Or even a decent VOR/CDI emulator? The reason I ask, is it makes a lot more sense when you can see it and play around with it. The key is to not think of it as headings, but as your position relative to the radial. I won't go into the details of that though, as I'm sure you've gone through that in your ground school. But to answer your question...

In real life, The CDI is set by the pilot in the cockpit, so you have to know how to set it up properly for what you are trying to to do. Otherwise yes, it is possible (and pretty easy) to set it up wrong and go in the opposite direction. Basically, the only time you'll get the reverse sensing is if you set it up incorrectly. Let's say that you are currently on the 020 radial and you're trying to set up to fly a 360 radial outbound (FROM) the station. If you incorrectly put 360 at the bottom of the CDI, the instrument will think that you are trying to fly TO the station on the 360 radial (and will show a TO flag). Thus the CDI will show a deflection to the right. So if you then pick up a heading of, say, 050 outbound to intercept the 360 radial outbound, you'll be flying in the opposite direction and will never intercept the 360 radial. So when flying, a common sense check always helps when setting up the CDI and should help prevent reverse sensing (if I know I'm on the 020 radial and trying to fly outbound on the 360 radial, I should have a FROM flag, and the CDI should be to the left). Hope this helps.

[dialdriver]

I’m amazed they’re still teaching/examining that stuff. What the hell is a To/From flag?! Got an Astro compass handy?

Hey magenta boy, how old are you, 12?

[TahuluLimia]

Hey there,

You've got the right idea about the needle itself - left needle = right of course, right needle = left of course. The TO/FROM flag is the key to understanding where you are relative to the station.

Think of TO/FROM like direction signs. TO = You need to fly TOWARDS the radial (so the needle deflection tells you which way to turn to get there). FROM = You're flying AWAY from the radial (so again, the needle tells you which way to turn to get back on course).

Your instructor is right, it can seem counterintuitive at first. But trust the system - the needle and flag work together to show you the way. If you're unsure, double check your heading with your chart to confirm which direction you actually need to fly.

Good luck with the PPL written!

[Fly0nTheWall]

TO = You need to fly TOWARDS the radial (so the needle deflection tells you which way to turn to get there). FROM = You're flying AWAY from the radial (so again, the needle tells you which way to turn to get back on course).

Not quite. TO = TOWARDS the station, and FROM = AWAY from the station. You're always trying to fly towards a radial, regardless of whether you have a TO or FROM. Because the goal is always to intercept a radial.

And it's important to note that the pilot sets the CDI to show either a TO or FROM flag. So if you incorrectly set a TO flag to fly a radial AWAY from the station, the CDI will show a deflection opposite to your desired radial. This is what is referred to as reverse sensing.

Lastly, the needle will NEVER tell you the direction to turn on course. You have to figure out which HEADING you need to fly to intercept your desired radial. Then figure out which direction (a left or right turn) will be the shortest turn to intercept your desired heading. Could be a left turn, a right turn, or could even be no turn at all.

[Me262]

easy way to remember is you always go towards the deviation.

If the deviation is shown left, you turn left to get on the selected radial. Same with glide slope on an ILS. You go towards the deviation, so if glide slope indicator is below you, you lower nose and reduce power.

[Fly0nTheWall]

easy way to remember is you always go towards the deviation.

If the deviation is shown left, you turn left to get on the selected radial. Same with glide slope on an ILS.

Ugh noo , it's a good thing nobody actually does VOR approaches anymore. So let me break down why I keep saying this is wrong.

Suppose you want to intercept the VOR radial 360 outbound. So you have 360 at the top and you are showing a FROM flag, with the CDI deflected full scale left. Life is looking good. Now suppose your heading is currently 180. If you think to yourself, "needle to the left, turn left" and turn left to pick up a heading of, say 090. Now you will be flying in the complete opposite direction to your desired radial.

What you should've done in that scenario is look at the CDI and think, "gee, I guess I have to pick up a heading west of 360". Suppose you decide on 330. So that would require a turn to the RIGHT to intercept the 360 radial outbound.

[Me262]

easy way to remember is you always go towards the deviation.

If the deviation is shown left, you turn left to get on the selected radial. Same with glide slope on an ILS.

Ugh noo , it's a good thing nobody actually does VOR approaches anymore. So let me break down why I keep saying this is wrong.

Suppose you want to intercept the VOR radial 360 outbound. So you have 360 at the top and you are showing a FROM flag, with the CDI deflected full scale left. Life is looking good. Now suppose your heading is currently 180. If you think to yourself, "needle to the left, turn left" and turn left to pick up a heading of, say 090. Now you will be flying in the complete opposite direction to your desired radial.

What you should've done in that scenario is look at the CDI and think, "gee, I guess I have to pick up a heading west of 360". Suppose you decide on 330. So that would require a turn to the RIGHT to intercept the 360 radial outbound.

Radial outbound you always put the tail on the radial.

So 360 radial outbound, your CDI would point 180 and your tail is 360. Exact same setup if you had to intercept 180 inbound

[Fly0nTheWall]

Radial outbound you always put the tail on the radial.

That is incorrect, and the exact opposite. Radial outbound, the radial goes at the top of the CDI. Radial inbound, the desired radial goes at the bottom of the CDI (tail on the radial, as you put it).