GTN750 not picking up GS on RNAV/LPV approach (resolved)

[TimothyNathan 11]

Stefan, 

18 hours ago, StLu1000 said:

How is RXP GTN calculating and get the glide slope information since there is no glide path transmitter at the airport?

I think that you have a deep misunderstanding of how RNAV (GNSS) works!

Firstly, may I offer you a copy of the PBN Manual, which I co-author, which takes you through, in what I hope are bite-sized chunks, what RNAV, RNP and GNSS (GPS) are all about.  It is quite an easy read, I hope, of quite a challenging subject.

As a direct response to your question, lateral and vertical navigation on the GTN works entirely on GNSS, which is space (satellite) based and has no ground element.

Thus the question of transmitters at the airport does not arise.  On a small pedantic point, but one which causes confusion, only ILS (which does have transmitters at the airport) has a glideslope.  If the vertical navigation is satellite derived, it is called a glidepath.

The GTN calculates the approach path, laterally and vertically, on a Visual Approach purely based on geometry.  It derives a line which is on the runway QDM, descends at 3° and crosses the threshold at 50' above the threshold elevation.  If terrain or obstacles make that path unsafe, the glidepath is removed and it only provides lateral guidance.

This calculation is not based on any published surveyed approach data.  It comes purely from an algorithm in the GTN software.

My suggestion is that you should improve your understanding of how RNP APCH works by skimming my book, and that might help you work out what is going wrong with your expectations and usage of the GTN and of RNP approaches.

[StLu1000 2]

Hi Timothy

Very nice work with the manual. Thank you. 

I will read the manual to better understand how it really works and from that it will be more easy to trace strange behaiviour from RXP GTN 750 that could be normal?

I have tested RNP approach at Insbruck airport Austria it is follow calculated glide path but on ESTA Angelholm, Sweden it does not. Your manual will probally answer that question.

I heard today that ESTA is only approved as 2D approach. 

Its a learning curve to work thru to fully understand the system. Many issues in this forum is more a user understanding problem perhaps and I is one of them.

Thanks again for your attention in this matter

BR

Stefan

 

 

[scottb613 1,188]

Hi Folks,

I can't download the manual at the moment - but here is a brief summary on the RNAV approach types:

- LPV: Localizer performance with vertical guidance. Offers localizer accuracy and a WAAS-based glideslope, like an ILS. Also like an ILS, lateral sensitivity increases as the aircraft approaches the runway. Minimums are published as a decision altitude (DA).

- LNAV/VNAV: Lateral/vertical navigation. Unlike an LPV procedure, lateral sensitivity does not increase as the aircraft nears the runway. Glideslope information is based on WAAS or barometric altitude. Minimums are published as a DA.

- LNAV: Lateral navigation. Similar to an LNAV/VNAV procedure except no glideslope is presented. Sensitivity does not increase as one nears the runway. Minimums are a minimum descent altitude (MDA), as with a VOR or localizer-only procedure.

- LP: Localizer performance. No vertical guidance provided. Sensitivity increases as the aircraft nears the runway, as with a traditional localizer. Minimums are an MDA.

- Stand-Alone GPS: The old-style GPS approach is basically an LNAV procedure but restricted to using only GPS. Offers an MDA.

Regards,

Scott

[TimothyNathan 11]

@scottb613

I am sorry to have to correct some of that.

LPV - spot on

LNAV/VNAV - not quite right.  If you are using a TSO-146() receiver, such as the GTN or GNSx30W, both the lateral and vertical elements are angular.  If you have a TSO-129() receiver, such as the GNS430 (without the W) then you could fly it as an LNAV approach, with linear FSD, but only if you have Baro-VNAV.  This is a GTN forum, so all LNAV/VNAV we discuss will be angular.

LNAV - again, close, but no banana.  If you fly an LNAV approach on a GTN, or any other TSO-146() receiver, it will present angular lateral guidance.  Only on TSO-129() boxes is it linear.

LP - Yes.  The main difference between LP and LNAV is that the spec is different.  You cannot fly an LP approach on a TSO-129() receiver, so it will always be angular.

From a technical point of view, LP and LPV are differentiated from LNAV because they have a FAS Data Block which defines the Final Approach Segment.

You have missed two other approaches from your description:

LNAV+V - This is an LNAV approach, but Garmin have provided an unofficial, unapproved glidepath, which is a straight line from the FAF to the MAP.  You still have to use check altitudes and the LNAV MDA, but you are given guidance to help you stay on the correct slope.

Visual Approach - calculates the approach path, laterally and vertically, on a Visual Approach purely based on geometry.  It derives a line which is on the runway QDM, descends at 3° and crosses the threshold at 50' above the threshold elevation.  If terrain or obstacles make that path unsafe, the glidepath is removed and it only provides lateral guidance. This calculation is not based on any published surveyed approach data.  It comes purely from an algorithm in the GTN software.  It is not approved for use in IMC.

Stand-Alone GPS - I think a misunderstanding.  The other approaches above differ on the Final Segment (ie the differences are only really relevant between the Final Approach Fix through the Missed Approach Point to the end of the first segment of the missed approach.)  The Stand-Alone GPS approach is an old-fashioned way of describing the earlier parts of the procedure, ie from the Initial Approach fix(es) through the Intermediate Fix to the Final Approach Fix and all RNP/RNAV approaches have an RNAV 1 portion before the IF.  So the expression Stand-Alone GPS is no longer in use.

A couple of final points. 

Approaches are not LPV or LNAV/VNAV or LNAV.  One RNAV (RNP) approach can be all three.  It depends on the surveys that have been performed, the equipment available on the flightdeck and the integrity of the Signal-in-Space at the time and place.

The naming of approaches.  Over the years they have been called GPS, GNSS, RNAV (GNSS) and RNP.  The ICAO standard is that they must all be called RNP by 2022, but the USA has filed a difference and in the States they will continue to be called RNAV, with RNP reserved for Special Authorization Required approaches.  Most of the receiver manufacturers (such as Garmin) are American, so they intend to retain the RNAV nomenclature even when the rest of the world goes to RNP.  (Sorry, Jean-Luc, I don't know where Canada stands.)  This is going to cause infinite confusion.  A friend of mine nearly had his jet impounded by the Egyptian authorities over this ridiculous mess.

I am sorry that you have had difficulty downloading the Manual, but when you do, all this will be clear.

[TimothyNathan 11]

It occurs to me that there is another kind of approach available in the latest software (v6.50).   

You can set any user waypoint to be an airport.  You can then fly DCT to that waypoint and use VNAV to create a glidepath of any angle you choose.

In this case, both lateral and vertical guidance will be linear, so it will be pretty inaccurate, but, if you make your minima high enough it should work.  It is worth setting the CDI scale to 0.30 nm to improve the lateral accuracy.  Guidance will then be the same as an LNAV/Baro-VNAV approach on a TSO-129() receiver. 

This has the advantage that you can set up an approach to your own strip, or any PinS for helicopter pilots, but is inaccurate and doesn't have the integrity safeguards of a published approach.

[RXP 2,338]

Hi,

This is the most extensive information I've been able to read in a single document about this topic, and I particularly like the number of illustrations which really help relating the concepts to something tangible.

Thank you Timothy for sharing the PBN manual and for the quality of your comments!

 

[TimothyNathan 11]

You are too kind.  As you can probably tell, it was, and remains, a huge amount of effort, and it's good to know that it's useful, understandable and comprehensive.

You can just imagine the long debates about what should be included and what left out.

[StLu1000 2]

Hi As we brainstorming GTN iI have a question that Timothy mya have a good answer on.

Im askin If you know is it is a normal behavior that the HSI GS warning flag dosen not show when a RNP approaches that is not approved for precision approach with un-activated Visual approach on the GTN? The GS needle is visible and it stands at the top during the approach regardsless of any altitude.

However when activating Visual on GTN (as guidance GP only) the GS needle folow the GP and the AP follow it as well.

What I mean is that if the GP is not present even as guidiance only,  why is the warning flag for GS needle not showing?

Do you know if this is normal or not?

 

I hope I was right in my explanation about what is GP and GS in this matter?

 

BR

Stefan

 

 

[TimothyNathan 11]

I think the question is whether you would expect the GP flag to show on an LNAV/LP/2D approach?

If so, then the answer is no.   In fact, I have never seen the flag come up, provided the VDI/GP is performing as expected.

On an EHSI, the GP indicator doesn’t show when it is not part of the specification of the approach.

To be really boring, we also need to distinguish between non-precision and 2D approaches.  Look at p102 of the manual.  ICAO have really complicated matters by distinguishing between Operations, Procedures and Runways, but in this context it is enough to say that non-precision approaches are those with a DH of 250’ or more, whereas a 2D approach is one without vertical guidance.  This confuses everyone, including regulators.

Edited October 1, 2018 by TimothyNathan

[RXP 2,338]

4 hours ago, StLu1000 said:

The GS needle is visible and it stands at the top during the approach regardsless of any altitude.

For this particular item, it might be a case of installation-only option. See what's coming up here:

V-Flag Declutter:

Normal:
Whenever vertical deviation is invalid the vertical deviation bar parks in the centered position and the vertical flag is shown.

Declutter:
Whenever vertical deviation is invalid, the vertical deviation bar is parked in the maximum UP position and the vertical flag is removed from view, except in the following cases: (i) the CDI is in VOR/LOC mode and an ILS frequency is tuned, or (ii) the CDI is in GPS mode and a GPS approach with vertical guidance is active. In these cases, whenever the vertical deviation is invalid, the vertical deviation bar parks in the centered position and the vertical flag is shown.

Edited October 1, 2018 by RXP

[StLu1000 2]

Hi

I must also say once again thanks to Timothy for a very complete explanation about how it works and I understand that this is not so easy. Im my case "the coin is not falling down" yet but Im study the manual carefully bit by bit. There is so many parameters for the system to be right and to be considered. In real life, it must be a risk for misstake compared to old fasion ILS (go or no go).

Lack of red flag for a not reliable indication is very strange even if we know that the flag will not show in the use case explained.

Im continue study...

BR and thanks for this attention.

Stefan

 

Edited October 1, 2018 by StLu1000

[TimothyNathan 11]

I have done some tests on the sim.  They are on a G600, as the GTN won't drive my (any?) mechanical HSI's VDI

The VNAV function is really designed for the world of the EHSI.

Firstly, the issue of "flagging" the GP/VDI doesn't arise, as the VDI disappears if there is no valid VNAV/GP.

If you have an active VNAV profile, the VDI appears and the indicator is a chevron (or is it a V for VNAV?)

If you are on an LNAV approach, the VDI disappears when there or no further VNAV altitudes.  There is no VDI/GP on final approach.

On an LPV, L/VNAV or LNAV+V, the chevron is replaced by a GP diamond when the Final Approach Track is intercepted.  On a procedural approach, this happens at the Intermediate Fix, on vectors it happens at any range.   If VNAV intercepts the GP from below, then a chevron shows and the GP diamond is in gray and gradually descends to the middle, at which point the GP diamond becomes active.  It's very neat.

I have not reproduced the error condition of Integrity being lost for LPV, but still being adequate for LNAV, but I am 99.99% certain that the GP will disappear as the LNAV annunciation comes up, and a warning will come up informing the pilot of the reduced integrity and capability.

So, I don't think that the question of a flag need ever arise.

[tylenolwithwings 1]

Any update on this? I'm having the issue with the Carenado 177 cardinal where it ARM light stays active and never turns off. It will follow the path correctly but won't pick up the GS. I know with the PC-12 you just have to Disable GTN 1 and re-enable it to get it to function properly. For the Cardinal I've found that if you "reload current aircraft" and then disable GTN 1 and re-enable it, it will work properly. Such a pain. Seems like there should be an easier fix here. 

[RXP 2,338]

@tylenolwithwings This most likely is something only related to some sort of 'race' condition during initialization of both the aircraft and the GTN. Can you give a try to the latest GTN v2.5.7 released a couple days ago?

[tylenolwithwings 1]

I did but I figured this one out. Similar to the Carenado PC-12 but with one extra step. You have make sue the GTN is disabled before you power on the plane. Once you flip the battery on you have to turn on the GNS530 (off by default for this plane). Once this is on you can then turn on the GTN using the plugin menu and it then proceeds to work as normal.