Nellis TACAN and ILS

[randomTOTEN]

Just now, Frederf said:

The magic is that the system knows both the localizer position and the localizer position trend over time. Knowing both position and position trend it's possible (and indeed how the real system works) to develop bank director commands without knowing course.

That is a more modern way for the system to work, sure. But I know many of us have experience with systems that do actually rely on course info, so the current function is not a surprise.

[Desert Fox]

8 minutes ago, randomTOTEN said:

Course should absolutely have an effect on the steering bars. The system isn't magic. It doesn't know what direction the localizer is pointing, thus could not calculate an intercept command without using the course info.

 

Hm, always thought ILS works via directed "fields" that intersect and by that one can tell if left/right/center and up/down. HSI course setting  should  have zero effect on that. Got to dig into that i guess.

 

 

[jaylw314]

36 minutes ago, randomTOTEN said:

That is a more modern way for the system to work, sure. But I know many of us have experience with systems that do actually rely on course info, so the current function is not a surprise.

At least according the flight manual, there's no reference to the ADI bank steering needle and the HSI CDItaking input from the course selector in ILS mode, just the localizer position.  The source I have is not terribly definitive, though.

 

Interestingly, it mentions the ADI bars can be used in FM ADF mode.  The banks steering needle shows direction and the pitch steering needle shows signal strength as it rises.  I'm guessing this is not modelled, anybody know?

 

[Desert Fox]

.

Edited April 8 by Desert Fox
Either something is entirely wrong with ADF or i messed up completely

[Frederf]

Desert Fox, localizer position is indeed only based on signal ratio. CDI shows position the same on the centerline and tracking center and on the centerline but rapidly moving laterally. If someone takes a photograph of a CDI centered and shows it to you, you can't tell if that airplane is stabilized on the center of the beam or if that airplane was just instantaneously centered but flying completely at right angles. But bank steering has to give different commands not just based on position but also position trend. Flying quickly across the center it doesn't give the same bank command as stable on the center. Just knowing position isn't sufficient.

 

I used to be confused about the ILS bank steering too. "How can it figure out the steering commands with just loc signal and not desired course?!" But someone instructed me that with the use of trend information it could (and does) work without knowing course. It makes sense that the system could have been designed to use course knob setting; that would have provided enough info to run everything but it wasn't.

[Zeagle]

Fred...I disagree. You always need to set the course on the HSI. However, you may be technically correct on the CDI. I don't work on them. I just used them in the air.

Edited April 9 by Zeagle

[Caldera]

Hey all,

 

Still missing the part about the "bank steer bar".  This refers to the LSB? 

 

Obviously this is not cross track error as I think this through a bit more.  That deals with the projected course line versus a fixed position, but still kind of sort of. 

 

Let me ask this then.  If the LSB only responds to bank, not direction, then why do I see the LSB line up in the very center of the ADI as I cross the center line of the localizer at a 90 degree angle with wings level?

 

Another question I have.  In the mission builder on the Nellis map there is a diagram representing the localizer at the end of runway 21L.  On the end of this diagram that is away from the runway it has the ILS frequency and another number that I had guessed was the heading of the localizer.  That number at Nellis is 22.  What or why is that?

 

Caldera

[Frederf]

The official name of the yellow vertical element on the ADI is bank steering bar. When bar is centered then current aircraft bank is equal to computed/commanded aircraft bank at least in the ILS context. The commanded bank is a computer calculation based on inputs of localizer position, trend rate, pre-v-post capture state, and has a attenuation factor (so it doesn't get too sensitive in close). DCS's implementation is simplistic in that it is a bank as a simpler function. In the real airplane you would see a command to counter bank before you reached the middle of the localizer beam. It would say "woah partner, you're flying across the beam too quickly for how far you are from the center beam, better bank the other way." The behavior of the DCS and real bar behavior is different. DCS behavior is workable but not exactly correct.

 

In the editor I see white text at N 36°23.3' W 114°51.9' as 109.10 MHz and 220°. That second number is the approach course of the localizer. Be aware that this 220° is the DCS ruler azimuth of the localizer which will not coincide with the magnetic course instruments in the A-10.

[Caldera]

Frederf,

 

OK  So that means that since number on the diagram is 220 (my bad) would indicate that the localizer center line and the runway are actually at 214?

 

This from Chucks Guide page 507.  Just what he calls them?

 

 

 

Caldera

Edited April 9 by Caldera

[Frederf]

Nevada typically has a variation of around 15 each degrees if I remember so more like 205. I would set up a sample flight right on that centerline to be sure before I trusted it in IMC though. It'll vary depending on year and even in diff parts of the map I think. Yeah, a little loose terminology is all.

[Ramsay]

10 hours ago, Caldera said:

So that means that since number on the diagram is 220 (my bad) would indicate that the localizer center line and the runway are actually at 214?

 

MagVar for the Nevada map is approx. 12° E, so the center line for Nellis RWY 21L is on approx. 208° Magnetic.

 

You can use the A-10C's CDU Position Page to read the MV for any particular date/location.

 

Edited April 9 by Ramsay
typo

[corvus5624]

13 hours ago, Zeagle said:

Fred...I disagree. You always need to set the course on the HSI. However, you may be technically correct on the CDI. I don't work on them. I just used them in the air.

 

Do you mean you have to set course in DCS (which is true), or do you mean it also works likes this in real-life? It is technically possible to provide steering direction towards the runway based on ILS alone (without setting the course). That does not mean that the A-10C does not require you to set the course, I don't know how it works in real-life.

[jaylw314]

12 hours ago, Caldera said:

Frederf,

 

OK  So that means that since number on the diagram is 220 (my bad) would indicate that the localizer center line and the runway are actually at 214?

 

This from Chucks Guide page 507.  Just what he calls them?

 

 

 

Caldera

 

 

Chuck's guide just calls them that, you could call it anything you want I suppose

 

The key is that the traditional "glideslope indicator" and "localizer deviation indicator" which show your position error are the arrow on the left and the CDI on the HSI. 

 

The yellow bars on the ADI are fancier electronic dojiggery stuff that is supposed to be a little more advanced and easier to follow than the traditional pointers.  They show some combination of your position error and your rate error.  So if you're left of centerline, but correcting to the right, the needle will start right of center.  There's a point at which the steering bar will be left of center.   That would cue you in that it's time to bank left to line up with the runway heading again.

 

Maybe an easier way to describe them is "what would the autopilot do?"  In fact, a lot of flight directors are just indicators for what the approach autopilot is doing or would do.

 

IRL, there are ILS approaches that allow you to fly to lower minimums if you have a flight director, so they are seen as being functionally superior to the traditional GS and CDI indicators

[Caldera]

Hey all,

 

Thanks for your input.

 

Caldera

[Caldera]

Ramsay,

 

How do you get to that page?

 

All I can find is:

 

 

This is CDU --> NAV --> OPTIONS

 

Caldera,

Edited April 13 by Caldera

[Desert Fox]

8 minutes ago, Caldera said:

Ramsay,

 

How do you get to that page?

 

All I can find is:

 

 

This is CDU --> NAV --> OPTIONS

 

Caldera,

 

 

If you set the "PAGE" knob to "POSITION", you should have it.

 

 

[Tippis]

…or you go via the UFC: FUNC + HACK (POS), although that's more for if you're repeating the CDU on one of the MFDs.

Edited April 13 by Tippis

[Caldera]

Thanks,

 

Taking a closer look...

 

Caldera