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Localizer Autotuning


dlrk
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There’s a whole complex dance that the FMSs have to execute, but in normal ops the conditions are:

  • There is a suitable (read: LOC or ILS) approach procedure linked in the flight plan that the plane is in, or that is coming up down track in your FPL;
  • The plane is within 31nm of the approach airport
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Should it do anything else than just tune the frequency alone? I'm not sure if I'm doing something wrong (or if I'm just too impatient) but every time I've tried to fly an ILS approach on LNAV so far, the ILS did not seem to activate by itself although approach mode was armed. The frequency gets auto-tuned but I appear to need to manually switch the Nav Source. The course also always remains pre-selected at 0°, even after the ILS has already been auto-tuned.

What I would have expected to happen from similar auto-tune functionality on other aircraft systems if I remember correctly:

  • localizer course for the selected approach gets automatically entered into the localizer pre-selection window
  • ILS identification is being displayed while still on LNAV (maybe I just did not find it on the displays?)
  • localizer & glideslope indicators become visible when in range
    • that sort of appears to happen if I change from LNAV to heading mode (blue bars?)
  • if approach mode is armed, AP should switch automatically from LNAV to LOC & GS

Maybe the Challenger's systems just work differently and don't do that? But I think it's more likely that I'm still doing something wrong or maybe I'm just still a bit lost on the displays. Would be good to have a confirmation if any of the points listed above are actually being performed by the Challenger and when/how that happens. :) 

Edited by EnQ
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The course will never appear in preset nav, that’s one of the oddities of nav-to-nav transfer (the name for FMS to ILS auto-transition), and why I recommend setting it manually if radar vectors are expected. I found this weird too, and not how I would have designed it, but it’s just how the Challenger is.

When nav-to-nav transfer is able to work, you’ll see blue indications of localiser and glideslope deviation, both as blue diamonds on the deviation scales in the attitude/svs part of the PFD, as well as the CDI at the bottom with blue dashed “ghost” needles.

You should have FMS1 as the nav source on the left side, and FMS2 on the right side. Make sure baro settings are sync’d too.

Press the approach mode on the autopilot and you should see white LOC1 and GS indications on the FMA strip at the top of the PFD. Even then, it will hang onto FMS nav for as long as it can, until nearly established on the final approach track, then it will swap to LOC navigation. If you’re not on the glideslope, you may need to adjust VS or pitch to capture the glide properly.

Normal pilots rules apply : if it’s not doing what you expect, don’t just sit there, make it do it! Change nav source manually and fly the ILS. This is why, personally, I always set the course on preset nav as part of approach preparation.

Hope that makes sense. 

Edited by Graeme_77
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I was having the same issue. 
From the Bombardier docs, looks like 30 nautical miles and less than 105 degree intercept (section 04-10-50 page 201 of volume 2 of the flight crew operations and section 04-10-30 page 181).

You need CDU1 and CDU2 to have the same flight plan active (I just copy CDU1 from CDU2 and for good measure put a copy in CDU3 as well).  It took me a while to realize the copy of approach (localizer) data between the flight computers wasn't automatic on this aircraft (there apparently isn't a primary FMS that can be selected), and importantly there is no message that localizer/glideslopw capture is disabled such as "LOC UNABLE" via a message on the CDU when the avionics don't have at least two independent systems to track from.  This had me plow the internets and landed me in Smartcockpit's documentation on the Challenger 605 which also uses the Collins Pro 21 avionics.  The 605 is similar to the 650 except for engines and range looks like but it looks pretty much the same in terms of operations.

Edited by Muchimi
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  • 9 months later...
On 1/16/2022 at 2:48 AM, Graeme_77 said:

The course will never appear in preset nav, that’s one of the oddities of nav-to-nav transfer (the name for FMS to ILS auto-transition), and why I recommend setting it manually if radar vectors are expected. I found this weird too, and not how I would have designed it, but it’s just how the Challenger is.

When nav-to-nav transfer is able to work, you’ll see blue indications of localiser and glideslope deviation, both as blue diamonds on the deviation scales in the attitude/svs part of the PFD, as well as the CDI at the bottom with blue dashed “ghost” needles.

You should have FMS1 as the nav source on the left side, and FMS2 on the right side. Make sure baro settings are sync’d too.

Press the approach mode on the autopilot and you should see white LOC1 and GS indications on the FMA strip at the top of the PFD. Even then, it will hang onto FMS nav for as long as it can, until nearly established on the final approach track, then it will swap to LOC navigation. If you’re not on the glideslope, you may need to adjust VS or pitch to capture the glide properly.

Normal pilots rules apply : if it’s not doing what you expect, don’t just sit there, make it do it! Change nav source manually and fly the ILS. This is why, personally, I always set the course on preset nav as part of approach preparation.

Hope that makes sense. 

One of THE most important posts on this forum! 

Thanks Graeme

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  • 2 weeks later...

Possibly the 650 might have changed, this is from the 605 FMS Operator's Guide - FMS v4.0:

Quote

 

Selecting APPR mode on the FCS arms the system for NAV-to-NAV capture. Use discretion in choosing the proper time to arm for the approach since radar vectors or published approach routes can prematurely get close to or cross the localizer signal. Select the APPR mode only after making sure the FMS has completed the NAV-to-NAV set up, the aircraft is in a position to capture the localizer, and cleared for the approach. The set up is complete when the localizer is tuned, LOC is the preselect navigation source, and the localizer course is set.

The NAV-to-NAV transfer works only when the same-side FMS is selected as the active navigation source and it is using the same-side navigation radio.

 

On the attached screenshot (1.6.1), the FMS had completed the Nav-to-nav setup (EDDB ILS 25L), but not effected the nav-to-nav transfer yet as I had not armed APPR. The "ghost" needle was (correctly) pointing to 245º, but I was expecting the PRESET NAV box CRS to also show '245', rather than '000'. 

Fabulous simulator!!

Screenshot NtoNCRS.png

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We have tested this NAV to NAV on our CL650. What counts is that CM1 is in FMS 1, but it does not matter what CM2 is in. But in the Hotstart CM 2 needs to be in FMS 2 to have the NAV-to-NAV transfer. In our opinion this is not correctly modelled. 

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Collin's Nav-to-Nav transfer has always been finnicky, whether it was the PL4, PL21, or PL21 Enhanced.  There are many things that can trip it up.  One is executing an FMS change on legs page right at the 30 NM distance from ARP.  If that happens, the LOC may tune but the CDI will auto set to the inbound course.  Found this out the hard on the LDA Rwy 25 into KEGE one day after ATC cleared direct to  QNDRY just as we were passing the 30 NM from ARP point.  The LOC tuned on both sides, and it auto set the inbound course on the right side (I was PM that leg) but on captain's side, it didn't set the inbound course correctly.   When APPR was pressed passing AIGLE, which you must do on this approach because of the stepdown fixes in the intermediate segment are above the LDA GS, we found the CDI turned about 90 degrees from the inbound LDA course.  

Hold the DME at the wrong time, like on the ILS 6 at TEB is another trap for Nav-to-Nav transfer.  Hold the DME after it has done the Nav-to-Nav transfer tuning inside the 30 NM from the ARP.

Rich Boll

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On 11/30/2022 at 6:08 PM, FYG001 said:

We have tested this NAV to NAV on our CL650. What counts is that CM1 is in FMS 1, but it does not matter what CM2 is in. But in the Hotstart CM 2 needs to be in FMS 2 to have the NAV-to-NAV transfer. In our opinion this is not correctly modelled. 

Can you clarify which nav sources are permissible on the non-steering side? Is there any checking for the active nav source at all? Or is it entirely irrelevant? We're essentially trying to reverse engineer's Collins' coding decisions here. It seemed to me that on-side FMS would be the safest/most sensible option. The other options are: any FMS source, or literally any source. Which options have you tested?

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On 11/30/2022 at 12:08 PM, FYG001 said:

We have tested this NAV to NAV on our CL650. What counts is that CM1 is in FMS 1, but it does not matter what CM2 is in. But in the Hotstart CM 2 needs to be in FMS 2 to have the NAV-to-NAV transfer. In our opinion this is not correctly modelled. 

What actually happened on co-pilot’s side when it wasn’t in FMS2 nav source? Did the ghost needles still show up? Nav2 auto-tuned? Did the switch to LOC2 happen? Etc.

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The "nav-to-nav transfer" function is designed specifically to tune each side's LOC and set each side's course, and then automatically change each pilot's nav source from FMS to ILS/LOC once APPR is armed and the aircraft approaches the LOC. Saves a lot of manual work and mode switching.

It will do so for any side whose nav source is FMS. If a pilot's nav source is not FMS, there won't be nav-to-nav automatic setup or transfer on that side. Another essential condition is that an ILS or LOC approach be loaded in the FMS. 

Quote

From the avionics manufacturer (specifically for FMS 3000,  5000, 6000 and 6100:

When a localizer-based approach (including ILS) is entered in the active flight plan, the FMS sets up for automatic Nav-to-Nav transfer once the aircraft enters the terminal area of the arrival airport. The terminal area is entered when the first STAR waypoint is sequenced or the aircraft is within 30 or 31 NM (depending on the FMS version) of the arrival airport reference point. When setting up for Nav-to-Nav transfer, the FMS attempts to tune the Nav receivers to the localizer frequency. Once tuned successfully, the Nav receivers change to MANual tuning mode.

Quote

Again from the manufacturer: (emphasis mine)

The crew must select APPR after the FMS completes the Nav-to-Nav set-up in order for the automatic Nav-to-Nav transition from FMS to LOC navigation source to occur. The FMS performs the Nav-to-Nav set-up when the aircraft is in the terminal area for the localizer-based approach that is selected in the FMS flight plan.

A ghost needle is only ever displayed (in ROSE mode) after FMS has completed its setup per the above.

Please disregard the following if, as I think, you are already aware, but I thought I'd mention it. Enroute - typically using the FMS as the nav source - the FGCs are in dual mode, meaning that the flight director is using the onside FMS (or the cros-side FMS if the XFR button has been pressed and the right FD is "flying"). You see LNV1 <- ALTS (or respectively LNV2 -> ALTS) on both PFDs. 

But for any ILS or LOC approach, the system wants to switch to split mode, where the left FD will follow LOC1, and the right FD will follow LOC2. Among other things, this provides very quick fallback capability if something fails on one side. 

Nav-to-nav transfer will also do its thing if the right side FD is "flying" and the requirements are met. 

Again, if one pilot's nav source was already switched to VHF, nothing would happen on that pilot's side, no LOC tuning and no course setting - this would be dangerous because it might undo something a pilot has manually set up.

On the 605, some electrical failures can cause the FMSes to go out of sync, and this comes with only subtle indications (a white MSG on the scratch pad). In a level D sim, nav-to-nav transfer occurs even if this has happened.

Edited by Lightcivvie
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