kmw510

No glidescope capture when flying in level change

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I just started flying with vatsim. I was flying the star to SFO in LNAV and VNAV when atc instructed me to descend to an altitude and turn to  a different heading. I used level change to make the descent and used heading select to fly the new heading. I am now out of LNAV and VNAV. Atc continued to give me instructions to intercept the ILS approach. I engaged VORLOC and it activated. Upon this I engaged APP. The pfd showing the white g/s, however, the glidescope was never captured. Having never had this happen before I tried several flights trying to enter the ILS approach using heading select and level change. I was never able to get glidescope capture and a couple times couldn't get VORLOC. I always get VORLOC and G/S capture when approaching using LNAV and VNAV and when flying manually. Is there an issue using MCP autopilot to transition to VORLOC and APP?

 

Kevin Wilson

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Hello Kevin, welcome to VATSIM!

Take a look at the KSFO Runway chart (www.airnav.com) and you'll find the altitude you need to be at, for the position your aircraft is at, in order to capture the Glideslope.  Remember that the glideslope is a finite vertical beam (localizer is a finite laternal beam) that you have to be inside in order to capture it.  You should always capture the glideslope with your aircraft underneath and flying into the beam rather than descending into the beam (glideslope).

The bottom of the runway chart will help you visualize the glideslope as well as provide you with the altitudes you need to be at.

I hope this is helpful to you.

 

Best wishes.

 

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51 minutes ago, Chock said:

Did you engage both autopilots?


I don't believe this is necessary in order to lock onto the glideslope.

 

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Nope, it isn't, I am just trying to determine what the problem might be. :cool:

 

 

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The reason I'm asking that, is because Autopilot A uses the Captain's (i.e. left) Course selector and the number one (Nav 1) radio's frequency, but Autopilot B uses the Co-Pilot's Course selector and the Nav 2 radio's frequency, so if you initiate a dual channel approach and you don't have both course selectors the same and both nav radios the same, the autopilot will freak out and not perform an automatic approach, but you might think it still has the localiser if the aircraft keeps on flying straight.

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On March 26, 2017 at 1:44 PM, DaveCT2003 said:

Hello Kevin, welcome to VATSIM!

Take a look at the KSFO Runway chart (www.airnav.com) and you'll find the altitude you need to be at, for the position your aircraft is at, in order to capture the Glideslope.  Remember that the glideslope is a finite vertical beam (localizer is a finite laternal beam) that you have to be inside in order to capture it.  You should always capture the glideslope with your aircraft underneath and flying into the beam rather than descending into the beam (glideslope).

The bottom of the runway chart will help you visualize the glideslope as well as provide you with the altitudes you need to be at.

I hope this is helpful to you.

 

Best wishes.

 

Hello DaveCT2003,

The ATC directions did have me descend to an altitude low enough to capture the glideslope in that way. For some reason when I am in level change I can't capture the glidesope. All my approaches using VNAV work fine. 

 

Kevin Wilson

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23 hours ago, Chock said:

Did you engage both autopilots?

Hello Chock,

No I use only the pilot side autopilot and have performed hundreds of vnav approaches successfully doing this. Thanks for the help.

 

Kevin Wilson

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Kevin, you said "The pfd showing the white g/s, however, the glidescope was never captured," which leaves me with no idea of what the GS was indicating.  Were you level and below GS, level and above GS or level and at GS?

With either LOC or GS, capture can take place from hand flying or any mode of automated flight (with or without a coupled approach, the GS mode will be activated).  I have been at an acquired altitude reached via FLCHG and captured GS many times, this is probably not your problem so the question is what did go wrong?  And for that we need a better understanding of the circumstances and configuration.

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6 hours ago, downscc said:

Kevin, you said "The pfd showing the white g/s, however, the glidescope was never captured," which leaves me with no idea of what the GS was indicating.  Were you level and below GS, level and above GS or level and at GS?

With either LOC or GS, capture can take place from hand flying or any mode of automated flight (with or without a coupled approach, the GS mode will be activated).  I have been at an acquired altitude reached via FLCHG and captured GS many times, this is probably not your problem so the question is what did go wrong?  And for that we need a better understanding of the circumstances and configuration.

Hello Dan,

I was flying level below the GS expecting that as soon as I intercepted the GS then it would be captured thereby having the autopilot follow the glideslope. For some reason upon intercepting the GS nothing happened and I remained in the GS waiting to capture mode. It has always worked when approaching in VNAV or flying manually. Just doesn't work when flying in lvl change mode. Is that normal?

 

Kevin Wilson

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Did the FMA pitch show G/S in small white indicating that glideslope was armed for capture?  You are not mentioning details like this that are important.  FCOM 4.20.17 does not lists any specific pitch modes that must be active to capture the glideslope (except that there is an option to inhibit GS capture before LOC capture).  Your level change mode changed to altitude acquired mode when the glide slope came alive and began to descend to center... it should be captured about 1 1/2 dots high.  Maybe you didn't have glideslope armed?

 

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25 minutes ago, downscc said:

Did the FMA pitch show G/S in small white indicating that glideslope was armed for capture?  You are not mentioning details like this that are important.  FCOM 4.20.17 does not lists any specific pitch modes that must be active to capture the glideslope (except that there is an option to inhibit GS capture before LOC capture).  Your level change mode changed to altitude acquired mode when the glide slope came alive and began to descend to center... it should be captured about 1 1/2 dots high.  Maybe you didn't have glideslope armed?

 

Hi Dan,

I am confused now. In my original post I state that upon Vorloc capture I engage APP and see the white G/S in the pfd. Does that not answer your question or is there something else. Based on your statements there should not be any reason that a VNAV approach should be any different from a lvl change approach. Something must be haywire in my sim software.

 

Kevin Wilson

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Yes Kevin the GS armed is clear to me at least. At this point could you replicate the situation and post a screenshot of your Pfd and mfd indications ? VORLOC was engaged in green right ? 

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1 hour ago, kmw510 said:

In my original post I state that upon Vorloc capture I engage APP and see the white G/S in the pfd.

I interpreted that to mean you saw the white glide slope deviation indicator on the PFD, which is why I asked about the FMA indication.  Let's go down this path, did you see both a white small letters G/S on the FMA and the white glide slope deviation indicator?

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Two myths I'd like to dispel: it is not required to capture the GS from below and it is not required to capture the LOC before arming APP. Throw those out of the equation. 

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