Not intercepting LOC when on VNAV path

[Boeing or not going 578]

For the first time in the NGX I let the aircraft fly the VNAV path to the final approach, and was lined up with the runway about 15 miles out. I selected VORLOC but it never captured the LOC. When the glideslope was nearing passing me I selected APP but it still did not capture the LOC or GS. Is there a trick to getting it to capture the LOC when on the VNAV path that I missed? Thanks - David Lee

[Budbud 227]

Hi,

 

First question, did you set the frequency and bearing of the ILS?

Second, did you try to change your heading to get a convergent route to the loc beam? If you fly LNAV/VNAV to an ILS, it may happen that you fly parallel to the loc but never capture it.

Final, you usually cannot capture the GS if you are established on the LOC, except if you have enabled the option in the CDU.

[Boeing or not going 578]

Hi,

 

First question, did you set the frequency and bearing of the ILS?

Second, did you try to change your heading to get a convergent route to the loc beam? If you fly LNAV/VNAV to an ILS, it may happen that you fly parallel to the loc but never capture it.

Final, you usually cannot capture the GS if you are established on the LOC, except if you have enabled the option in the CDU.

 

I can change to heading then intercept it like I've done before, but thought on the LNAV path of the approach fixes it would intercept the LOC.

 

I just realised I put VNAV in the topic but I meant LNAV. Early morning. - David Lee

[vonmar 123]

"I selected VORLOC but it never captured the LOC"

 

Were you using one of your PMDG saved flights?

 

Which airport / ILS?

[downscc 1,327]

 

 

I can change to heading then intercept it like I've done before, but thought on the LNAV path of the approach fixes it would intercept the LOC.

 

Usually but there is no rule that it needs to happen this way.  Be a pilot and fly the airplane, do not leave it up to the automation.  Next time this happens to you, you'll instantly recognized what is happening and use MCP HDG to intercept the LOC.

[ALF1 15]

Being on LNAV, did VORLOC arm ? Frequency set ? Proper Course set ?

 

For safety reasons, GS cannot be intercepted and active unless on the LOC captured first, but this I believe depends on the airline and you can select in the CDU in the NGX.

[Boeing or not going 578]

My settings were right, correct frequency and course. I was on the LNAV path into the last 2 approach fixes. I then armed VORLOC on the runway heading 15 miles out. At no point did the aircraft attempt to intercept the LOC. I have never gone from LNAV to LOC before I always used Heading to intercept LOC and was not sure if the airplane would attempt to capture the LOC from LNAV. I should arm the VORLOC before the aircraft on the LNAV course is intercepting runway heading. - David Lee

[pcubine 143]

I should arm the VORLOC before the aircraft on the LNAV course is intercepting runway heading.

David

I don't believe it makes any difference. When I see the diamond representing the localizer start to move toward the center of the PFD, I press the APPR button if I am below the glideslope. If I am above the glideslope, I press VOR/LOC and then the APPR button when I am almost on top of the glideslope.

 

I usually split the intercepts of the localizer 50-50 between heading intercept and LNAV. In LNAV, I have never failed to capture the localizer and I will repeat never.

 

There are situations where the airplane might fly parallel to the localizer and never intercept it. The only time I have seen this was in the FSLabs A320 ILS 25L KLAX. I have never seen this occur in any PMDG airplane be it the NGX, T7, 744, or MD-11.

[joolsd 10]

Is this just a standard ILS approach. One where you have a valid ILS frequency in  nav1 and/or 2? That is more important than the LNAV or VNAV path.

Unless you mean the R-NAV approach?

[Boeing or not going 578]

Is this just a standard ILS approach. One where you have a valid ILS frequency in  nav1 and/or 2? That is more important than the LNAV or VNAV path.

Unless you mean the R-NAV approach?

 

ils approach but arrived on the LNAV path to the last approach fixes before the runway. Frequencies entered correctly - David Lee

[Budbud 227]

Hi,

 

If I understand well, when you use HDG SEL to intercept the ILS, it works. Only when you let the LNAV follows the track, it doesn't intercept the LOC and switch to APP.

As I said before it may happen that the LNAV brings you on a parallel path to the LOC and therefore never intercept the beam. This is especially true if the approach path angle to the LOC beam is very small.

 

I experienced it a couple of times (but not everytime though). If the capture of the LOC works in HDG SEL but not in LNAV, it cannot be a problem of configuration.

 

To check what really happens, keep an eye on the FMA. 

[Boeing or not going 578]

Hi,

 

If I understand well, when you use HDG SEL to intercept the ILS, it works. Only when you let the LNAV follows the track, it doesn't intercept the LOC and switch to APP.

As I said before it may happen that the LNAV brings you on a parallel path to the LOC and therefore never intercept the beam. This is especially true if the approach path angle to the LOC beam is very small.

 

I experienced it a couple of times (but not everytime though). If the capture of the LOC works in HDG SEL but not in LNAV, it cannot be a problem of configuration.

 

To check what really happens, keep an eye on the FMA. 

armed LOC right before the final LNAV turn and it picked it up. But once on final LNAV path it did not pick up the LOC. Is that by design? - David Lee

[Budbud 227]

Yes, most probably parallel track.

Think of it: To intercept the localizer you must be on a route that will make cross the beam (but obviously you will turn to establish on it). If you arm the VOR/LOC before you have turned, then with LNAV you will proceed towards the beam and the LOC mode will kick in when you're close enough to the center line with a convergent angle.
Now if you are on final course with LNAV and then you arm the VOR/LOC, chances are that your route is not convergent enough to the beam for the LOC mode to activate. It may work if in LNAV you are perfectly aligned with the localizer beam but if you are slightly on a side, then forget about capturing the loc.

I cannot tell you by memory, but there must be a minimum angle for the localizer to get captured on a convergent route and maybe a maximum distant to the centerline on a parallel route. I don't remember if they are mentioned in the FCOM.

 

In any case, there are two ways to get rid of that:

Either you arm the VOR/LOC before you have turned towards the runway,

or if you have already and you see that the LOC mode doesn't engage, just engage the HDG SEL mode and change your heading frankly towards the beam, usually as soon as the aircraft starts turning, the LOC mode will engage.

[Boeing or not going 578]

Yes, most probably parallel track.

Think of it: To intercept the localizer you must be on a route that will make cross the beam (but obviously you will turn to establish on it). If you arm the VOR/LOC before you have turned, then with LNAV you will proceed towards the beam and the LOC mode will kick in when you're close enough to the center line with a convergent angle.

Now if you are on final course with LNAV and then you arm the VOR/LOC, chances are that your route is not convergent enough to the beam for the LOC mode to activate. It may work if in LNAV you are perfectly aligned with the localizer beam but if you are slightly on a side, then forget about capturing the loc.

I cannot tell you by memory, but there must be a minimum angle for the localizer to get captured on a convergent route and maybe a maximum distant to the centerline on a parallel route. I don't remember if they are mentioned in the FCOM.

 

In any case, there are two ways to get rid of that:

Either you arm the VOR/LOC before you have turned towards the runway,

or if you have already and you see that the LOC mode doesn't engage, just engage the HDG SEL mode and change your heading frankly towards the beam, usually as soon as the aircraft starts turning, the LOC mode will engage.

I thought the aircraft might notice the LOC parallel very close to the LNAV path and pick it up, thanks for the explanation. I am getting back into the NGX and having fun flying Virgin Australia around Orbx Australia. I'll change to heading and arm the LOC and intercept it rather than expect it to find it. Thank you everyone for helping. - David Lee

[ALF1 15]

I think Budbud is right on track. Sometimes while on LNAV I press APP but FAC and G/P arm. This does not pertain to your case since you are specificaly arming VORLOC. As Budbud says, Y would arm VORLOC well before intercepting.