Sign in to follow this  
dmwalker

Question about the 777 and Runway ILS Identifiers/Courses

Recommended Posts

This involves runways which have the same ILS frequency for both directions, in this case EGLL Rwy 27R/09L.

 

It's the same whether using the FSX default EGLL or the Aerosoft EGLL.

 

For 27R, the identifier is IRR and the direction is 271°. For 09L, the identifier is IAA and the direction is 091°.

 

First, if I start with the 777 on Rwy 27R and enter 110.30/271, it is correctly displayed as IRR/271° in the PFD but then immediately changes to IRR/272° and, if I taxi to the 09L end of the runway, the display changes to IAA/272° instead of IAA/091°.

 

Secondly, if I am flying the 777 to EGLL Rwy 27R, the PFD automatically shows Rwy 27R ILS as 110.30/271° but, at about DME29, it changes to IAA/271° and, at about DME26, it changes to IRR/272°.

 

Without getting into endless details, the same sort of thing thing happens with KJFK Rwy 13L/31R, which both have the ILS frequency 111.50.

 

I am wondering:

  1. Why does the 777 show 272° when ILS frequency/course is entered manually but 271° when entered automatically?

  2. Why does the 777 show IAA/272° instead of IAA/091° at the far end of Rwy 27R?

  3. Why does the 777 show identifier IAA before correcting to IRR when at DME29?

Has anybody else noticed these or is it just a problem with my installation?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

 

 


I am wondering:

Why does the 777 show 272° when ILS frequency/course is entered manually but 271° when entered automatically?

Why does the 777 show identifier IAA before correcting to IRR?

Why does the 777 show IAA/272° instead of IAA/091° at the far end of Rwy 27R?

 

The 777 gets the ILS freq/crs from the navdata, unless you enter it manually.

FSX defaults to one end or the other depending on your location in relationship to the runway. This answers both 2 and 3.

Share this post


Link to post
Share on other sites

1. FSX has a vary old magnetic deviation data base.  As such, you'll see a few degree difference often between the FSX runway alignment (i.e. old variation data) and modern charts with current deviation data.  Navdata as used on the PMDG uses current data even though FSX does not.  Somewhere in the 777 options is an option to use FSX heading instead of Navdata.

 

2. Is IRR the reciprocal ILS to IAA (i.e. 09L instead of 27L)?  I'm not sure how the FSX logic works, but in real life you're dealing with the same emitter station often and it takes a moment for the aircraft to figure out which side it's on sometimes.

 

3. Not clear I can tell you for sure what happens behind the scenes.  But effectively it's a VOR.  You can fly 271 inbound, then 271 outbound (for instance on a missed approach).  Just how a VOR doesn't turn to the reciprocal radial once you overfly it.

Share this post


Link to post
Share on other sites

Sorry for the delay. My wife just made a batch of decadent chocolate chip cookies and then I just found a recent thread about adding airports to FMC database, which explains the arpt_rwy.dat file. My file shows RW runway heading and ILS front course are both 271 for 27R and FS runway heading and ILS front course are both 273.

 

Anyway, the reason I am asking is that the 747-400 doesn't do any of these things under the same circumstances. If I enter 110.30/271 manually, it stays as 110.30/271. If I taxi it from the 27R end to the 09L end, the PFD does change from IRR/271° to IAA/091°. If I fly to EGLL Rwy 27R, the PFD doesn't show IAA at all; it shows only IRR/271° at DME26. Is the 747 right or wrong or just different? Both aircraft use the same databases.

 

When the 777 shows IAA/271°, it's approaching Waypoint LAM about 25 nm from the threshold of 27R and about 20° off the centreline. I always thought ILS transmitters were highly directional. Maybe I don't understand this specific situation where one frequency is used for both directions.

 

Eric Szczesniak: Yes, IRR is the reciprocal ILS to IAA.

Share this post


Link to post
Share on other sites

One frequency for both directions is very common.  As I said, the direction sensed depends on FSX and FSX depends on your position relative to the runway.  Also, ILS LOC is not highly directional; only the overlap between left beam and right beam, which creates the center of the localizer, is " directional."  The B747 is very different from the B77X, for many reasons including the FMS programming which is essentially a legacy product.

Share this post


Link to post
Share on other sites

 

 


Maybe I don't understand this specific situation where one frequency is used for both directions.

In real life only one runway direction would be transmitting an ILS at any one time so there would be no confusion. The aircraft would sense a backcourse localiser from the opposite direction. FSX doesn't switch the two ILS transmitters in the same way. Both ends are live and the the sim determines which one is being received.

Share this post


Link to post
Share on other sites

Sorry for the delay. My wife just made a batch of decadent chocolate chip cookies

 

 

I was going to inquire further about those cookies, but then I changed my mind. 

Share this post


Link to post
Share on other sites

Because FSX uses a very old magnetic variation database, many ILS localizer courses differ from the published localizer courses found in current charts and nav databases. In some cases, the FSX localizer course can be up to 3 degrees off from the current R/W alignment. The localizer course for any given airport and runway in the FMS will reflect the current value. The 777 has the option to use the actual FSX localizer course stored in the airport AFCAD, which is why the course may change by a degree or two when the aircraft gets close to intercept.

 

The older MD-11 does not have that feature.

Share this post


Link to post
Share on other sites

 

 


I was going to inquire further about those cookies, but then I changed my mind.

I had only one but I think I overdosed.

 

The references to the magnetic variation database reminded me that I hadn't updated it since my last reinstall. There is now a 2015 update which took care of the 272° question.

 

I didn't know about backcourse localisers but now I understand a bit better what is happening.

 

Thanks to all.

Share this post


Link to post
Share on other sites

Because FSX uses a very old magnetic variation database, many ILS localizer courses differ from the published localizer courses found in current charts and nav databases. In some cases, the FSX localizer course can be up to 3 degrees off from the current R/W alignment. The localizer course for any given airport and runway in the FMS will reflect the current value. The 777 has the option to use the actual FSX localizer course stored in the airport AFCAD, which is why the course may change by a degree or two when the aircraft gets close to intercept.

 

The older MD-11 does not have that feature.

Mmm...interesting!

If you ask me, we can delve deeper here.....because it is fun and complicated :-)

 

I know what you mean....see way below, but the 777 (or any FSX aircraft) does not have an OPTION to use the actual AFCAD loc course.......it can ONLY use the actual AFCAD loc.(=FSX scenery stored).

There is nothing else that CAN be used other than the scenery stored course and frequency.

You could have a nav database saying 108.5/360 but if the scenery AFCAD does not incorporate this facility then nothing will be indicated in the aircraft.

 

correct me if I wrong though.

 

The PMDG navdata base (navigraph or aerosoft) only holds the frequency and inbound course info.

 

The frequency is used so that the PMDG777 autotunes this frequency.

after that, the 777 will receive what FSX is "broadcasting" on that frequency.

If you have a recent addon for the airport you are doing the ILS approach at, then most likely the PMDG777 database and the scenery product have the correct and the same ILS frequency/course stored.

If you are using default FSX airports then, if the real world ILS frequency has been changed since eeeh 2006?....,then you will have that frequency stored in your PMDG777 database info.

So it will tune todays frequency.

But since FSX is not "broadcasting" anything on that frequency......the PMDG777 will receive nothing (or something wrong).

 

The inbound course is not actually used for loc deviation.

Just like GS deviation does not need you to set a 3 degrees glidepath somewhere, the loc deviation does not require you set the inbound course.

The antennas on the ground send out left/right and high/low radiation lobes that the aircraft receives.

The instrument just translates that into a pointer that tell you that you are left/right and high/low relative to something.

So it is your position that determaines the loc and gs pointer deflection.

 

Based on that deflection you or the AFDS (autopilot flight director system) must correct back to the inbound course.

That is why you need the inbound course!

To figure out the correction angle required....(example:ok, I am right of something....how do I get back on this something.....well I am right of 360 inbound so how about 350 track.)

I just tried it....funny as heck.....I entered inbound course 200 for a rwy 29 ILS.

Being right of track the loc deviation correctly showed my position relative to the rwy track.

But then the AFDS wanted to correct to the left.......to the left of 200 that is, lol.....so I was all over the place but not because the loc deviation does not know where I am but because the AFDS cant calculate an appropriate intercept angle.

 

The PMDG777 also needs the inbound course to set the heading bug automatically after loc intercept.

 

The difference between real world and FSX is ofcourse.......wel ofcourse......I am specualting here now........that in the real world you do NOT know your position but your instruments receive something that you can convert to a position.

Where FSX knows exactly where you are flying around, no frequency waves are being broadcasted so nothing is received, but your position is probably converted into loc deviation data that is then used by the PMDG loc deviation indicator.

 

In the PMDG777 options there is an option to choose if you want the 777 to set the FSX (AFCAD) stored inbound course or the navigation database inbound course.

I guess that it is almost allways better to choose "set to FSX heading".

Because it is after all the scenery inbound course that you need to follow. Be it FSX default scenery or a recent Addon airport.

It is a really nice feature too because it saves you having to go to FSX/Menu/airport xxxx to find out the FSX inbound course :-)

 

I do not know if Addon airports have an updated variation table or not.

The runway direction will still be relative to FSX true north......dont know what can be changed with the AFCAD loc.

Share this post


Link to post
Share on other sites

Yes, the option in the 777 FMS to autoselect either the AIRAC or FSX AFCAD ILS inbound course is what I was referring to.

 

It's absolutely correct that in a real aircraft, the centering of the localizer needle is solely dependent on the aircraft's physical position in space relative to the transmitted beam, and the course selector has no effect on the needle at all. In fact, when performing avionics functional checks on the NAV radios in our fleet aircraft, one of the tests is to generate a simulated ILS signal with a centered LOC, and then rotate the course selector through a full 360 degrees. If the needle on the indicator moves off center at any point, that is a "fail".

 

However, as I think you have since discovered, FSX does not seem to work this way - most likely because the whole system is a digital emulation of how a real ILS works. If the course selector does not match the ILS localizer course stored in the AFCAD, then the aircraft's inbound path during the approach may be offset to one side or the other of the runway centerline.

 

Of course, there are many real-world airports where the localizers on certain runways really ARE offset from the runway centerline. The ILS 22R at JFK comes to mind. Also, there are quite a few default FSX airports where the AFCAD lat/lon of the localizer and/or glideslope antennas are just plain wrong. I believe default KPDX has this issue on at least one runway.

Share this post


Link to post
Share on other sites

 

 


If the course selector does not match the ILS localizer course stored in the AFCAD, then the aircraft's inbound path during the approach may be offset to one side or the other of the runway centerline.

 

I have not seen that behavior, interesting.  Of course the B777 sets the course automatically and even sets the heading to match; however, I've flow the PMDG 737 NG/NGX with course errors and she still tracks the localizer dead on.  I'd like to try this again, can you reproduce and provide the initial conditions? That'd be great.

Share this post


Link to post
Share on other sites

I have not seen that behavior, interesting. Of course the B777 sets the course automatically and even sets the heading to match; however, I've flow the PMDG 737 NG/NGX with course errors and she still tracks the localizer dead on. I'd like to try this again, can you reproduce and provide the initial conditions? That'd be great.

I've never noticed the problem in the NGX, or 777, but I have in older models that may use more standard FSX SDK functionality for nav instruments and functions - including the PMDG MD-11, Level-D 767 and several default FSX aircraft. One place where there is a disparity between AFCAD and R/W localizer courses in KLAS 25L, where I have seen the offset on approach in older aircraft when using the R/W course

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this