Question on ILS

[john82088]

For some reason, my localizer is off with this plane. It does not line up with the runways. Some have been way, way off. I have not had any problem with any of my other planes. I also have an updated subscription with navigraph. 

Appreciate any advice anyone can offer. 

[frumpy]

How about a screenshot?

[XPlanePort]

Are both of your NAV radios (left and right) tuned to the ILS frequency? Also, make sure your left and right course selectors are both set to the ILS heading.

Edited November 23, 2016 by XPlanePort

[john82088]

Yes, I have both left and right tuned to the ILS frequency. However, I did not make sure the right course selector was set to the ILS heading. I have not done that before on any other plane. I will try that. Maybe I missed that when I watched the ILS videos from this forum.

[Litjan]

This is normal and not an IXEG problem. Happens with all other aircraft as well. Reason is that runways are not placed correctly in the X-Plane world, sometimes.

You need to set the CRS selectors to the correct inbound course, otherwise the plane does not know which track to align to initially - this is the same on other aircraft as well, they just set the course automatically from their database (modern aircraft). The ILS signal contains no course information, only "you are right, you are left".

But even if you set the course slightly wrong, the plane will still track the localizer exactly (it will just "think" there is a crosswind).

Cheers, Jan

 

Edited November 23, 2016 by Litjan

[john82088]

That did it!  Thank you very much for your help.

[Rodeo]

On 11/23/2016 at 7:01 PM, Litjan said:

This is normal and not an IXEG problem. Happens with all other aircraft as well. Reason is that runways are not placed correctly in the X-Plane world, sometimes.

You need to set the CRS selectors to the correct inbound course, otherwise the plane does not know which track to align to initially - this is the same on other aircraft as well, they just set the course automatically from their database (modern aircraft). The ILS signal contains no course information, only "you are right, you are left".

But even if you set the course slightly wrong, the plane will still track the localizer exactly (it will just "think" there is a crosswind).

Cheers, Jan

Can you elaborate on that? The difference between an ILS localizer and a VOR beacon are not 100% clear to me, and it seems you know something about it

Edited November 25, 2016 by Rodeo

[Litjan]

Sure.

The VOR is sending out a dual-phase signal, the on-board receiver can detect which part of the "phase" he is on - and hence determine the radial it is on. This in turn allows the pilot to know which way he would have to fly to get TO or FROM the station. So if the detected phase-shift is 180, you must be somewhere "due south" of the station, so flying 360 would take you right to it, while 180 straight from it.

https://en.wikipedia.org/wiki/VHF_omnidirectional_range

The ILS localizer antenna is creating two overlapping beams, one contains a "fly to right!!" code, one a "fly to left!!" code (simplified). If you are straight in the middle, both infos cancel each other out, and you know you are centered on the LOC.

https://en.wikipedia.org/wiki/Instrument_landing_system_localizer

However, you (or the plane) have NO idea which way to fly to stay there! Imagine the runway is 36-18. You are ON the LOC. Unless you know that the LOC is running 360 degrees, you will have a hard time staying on it.

If you know that its APPROXIMATELY north, you can react to the drift in time - so if you fly 355, you will notice it drift to the right, and can correct. But if you start out flying 270 degrees, you will be in the "fly right!!" zone so deep, that you have no idea how far right you need to go, to get it centered again (280? 290? 330?)

You can observe this in the 737. Pick KSEA 34R. Dial 325 into CRS1 and intercept the LOC. The plane will INITIALLY turn to 325, then goes "whoa, this loc is running to the right like crazy, better bank to keep it centered!". Once it tracks it, you will find that its keeping exactly the correct course (to null the drift).

If you have a crosswind (but set the CRS correctly), the effect is the same. Plane turns to inbound course, but notices the "drift" away from the LOC and corrects.

Hope this explains it, Jan

 

[Rodeo]

It appears to make sense, thanks!

[captain_alligator]

On 23/11/2016 at 6:01 PM, Litjan said:

Reason is that runways are not placed correctly in the X-Plane world, sometimes

There is a facility in X-Plane to fix problems with navaids, but like most things in X-Plane 10 the user interface is not exactly user friendly.

Location --> Local Map

Check the "Edit" checkbox in the top right of the screen

Select the offending ILS (remember the localiser is located at the far end of the runway as you approach to land) and use the controls in the bottom left of the screen to nudge the position and/or the direction of the localiser.

It helps if you respawn the aircraft on the runway concerned, as X-Plane will then place the aircraft exactly on the centreline of the runway. Then you can zoom in and nudge the controls to bring the localiser centre line into alignment with the aircraft silhouette.

IIRC, you don't need to hit any button to confirm - the changes are live immediately in the simulator as soon as you exit the local map screen. However, X-Plane will ask whether or not you want to save the updated data when you exit the sim - so you can feel free to experiment with this, and you can always abandon your edits if you make a complete hash of it.

Also please be aware a few airports have offset localisers - please check the approach chart before editing! LOWI is a good example, the initial approach from the east tracks the localiser on 255 degrees, but the runway itself is at 258 degrees. Whilst it is easy to fix the alignment of a normal localiser using the actual runway as your reference, an offset localiser is more difficult to fix (and more difficult to see any problem in the first place).

Here is a video of someone editing the LOWI localiser (although he actually calls it the glideslope, which doesn't exactly instil confidence). I am fairy sure he is doing something badly wrong here, which is why I put the video at the end of my post. The charts I have for LOWI say you should track this localiser inbound at 255 degrees, but here it seems the localiser is set to 256.89 degrees in X-Plane. However, rather than adjusting the localiser heading to 255 (which may not be correct but would at least have had some logic to it), he instead moves the position of the localiser south until the centre line crosses the centre of the threshold of RWY26. Sorted! 

Anyway, with that disclaimer out of the way, here's the video... at least it does demonstrate how easy it is to edit navaids...