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Bavafa

Well Known Member
Can any one here help me out to understand these approach a bit better?

My understanding of LNAV is that only lateral guidance is provided by the GPS but I have flown a number of LNAV approach that I also get the vertical guidance. Does any one know how this works?

Also, is there any other difference between LPV and LNAV+V beside the facts that minimums are lower for LPV?

Thanks in advance
 
LNAV: A non-precision GPS approach with only lateral guidance. The approach will have altitude step-downs with an MDA.

LNAV + V: This non-precision approach has an extra feature in that the glideslope needle comes alive. This gives you a stabilized approach, but does not give you obstacle clearance. The glideslope portion is added by Jeppesen and keeps you at or above any published step-downs.

LNAV/VNAV—or L/VNAV: This approach gives you lateral and vertical guidance down the final approach course. This is not a precision approach like an ILS, but you are assured of clearance above terrain and obstacles. Fly it like a precision approach, and trust the increased decision heights to compensate for the lower accuracy of a precision approach.

LPV: This is the most accurate WAAS approach, giving you lateral and vertical guidance to decision altitudes as low as 200 feet agl. On an LPV approach, accuracy is 16 meters laterally, and four meters vertically. The terrain and obstacle clearance zone is that of a Cat 1 ILS approach.
 
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TCONROY said:
LNAV: A non-precision GPS approach with only lateral guidance. The approach will have altitude step-downs with an MDA.
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The part that has confused me is that most LNAV approach I have made, my glideslop needle has become active and provided vertical guidance. I am wondering where is this coming from, how it works and what causes it to come alive or NOT.

My home airport KPVF has a LNAV approach and I have always got the vertical guidance in this approach.

BTW, a very good and complete explanation of all type of GPS approach
 
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You might check your GNS-430W during that approach again. It will probably say LNAV+V which means you use LNAV minimums shown on the chart but the GNS-430W is giving you an advisory glideslope only.
 
TCONROY said:
You might check your GNS-430W during that approach again. It will probably say LNAV+V which means you use LNAV minimums shown on the chart but the GNS-430W is giving you an advisory glideslope only.
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Good point and I will test it on my next approach. Much appreciate the very good, logical and useful info.
 
Sometimes, the approach plates don't show ..

...that vertical guidance is available either. This happened recently at KOKZ. My buddy runs the FBO there and told me that they had VNAV now, so the next time I flew over, I loaded the approach in the 430W and...son-of-a-gun---glideslope bar! The added benefit was ahead of printing the approach.

Best,
 
Sometimes, the approach plates don't show ..

pierre smith said:
...that vertical guidance is available either. This happened recently at KOKZ. My buddy runs the FBO there and told me that they had VNAV now, so the next time I flew over, I loaded the approach in the 430W and...son-of-a-gun---glideslope bar! The added benefit was ahead of printing the approach.
Best,
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I don't know what the criteria are for showing VNAV on the approach charts, but I've seen quite a few, including the airport where I instruct, that have had LNAV+V for a while (many chart cycles) but it is not shown on the chart.
You don't have to wait to see a glide slope - the approach name in the GPS will tell you.
 
Yeah but...

...if I'm planning on a cross-country and the weather is forecast to remain 500 overcast and the LNAV DA is 800', I'm not going.

If the approach has a published VNAV or LPV and the minimums are below that 500', I'd feel much better going on the trip. I can only safely go by what the approach charts have published, not guessing that they might have vertical guidance when I get there.

Best,
 
The +V guidance is not legal and cannot be used towards lower minimums.. so it's irrelevant if it's there or not.. Sure helps if it's there, but legally speaking it doesn't matter. You'll never see +V on approach plates.
 
Bavafa said:
Can any one here help me out to understand these approach a bit better?

My understanding of LNAV is that only lateral guidance is provided by the GPS but I have flown a number of LNAV approach that I also get the vertical guidance. Does any one know how this works?

Also, is there any other difference between LPV and LNAV+V beside the facts that minimums are lower for LPV?

Thanks in advance
Click to expand...


The vertical guidance data is coded in the ARINC 424 database that you get from your database supplier. (probably Jeppsen) Either the FAF altitude and the threshold crossing height are in the database and your Garmin uses those altitudes to create the Advisory Vertical Guidance or the angle for the FAF to threshold crossing height is in the ARINC 424 database coding and your Garmin uses that for the Advisory Vertical Guidance.

The LPV is very different, in that an addition set of precision data is included in your navigation database for the WAAS LPV guidance. This additional data is called a FAS data block and it consists of 20 elements which provide the precision guidance needed to provide guidance for CAT I minima.


The AIM Chapters 1 and 5 and Advisory Circular 90-107 provides very good information for LPV, LNAV/ VNAV and LNAV operations.

http://www.faa.gov/regulations_poli...cfm/go/document.information/documentID/903350
 
It is important to note that, despite how it may appear, "LNAV+V" is not an approach type and will not show up on any approach chart. LNAV+V displayed on the system in a list of approaches means that advisory vertical guidance is available in the database and will be provided for that LNAV approach, if activated. LNAV+V annunciated on the GPS or HSI display as an active approach simply means that advisory vertical guidance is being or will be displayed for the active LNAV approach. This is why you have to use LNAV minimums when LNAV+V is annunciated. It was done this way to distinguish the "+V" advisory vertical guidance from the standard vertical guidance provided with LNAV/VNAV and LPV approaches.

I think it confuses the matter to call them "LNAV+V approaches," but I have seen and heard them referred to that way many times. They are just LNAV approaches.
 
Another question.

I shot an approach into Americus, Ga. a while back (Rwy 24, I think), and had GS but it disappeared close to DA. Is this normal for a 430W?

Best,
 
pierre smith said:
I shot an approach into Americus, Ga. a while back (Rwy 24, I think), and had GS but it disappeared close to DA. Is this normal for a 430W?

Best,
Click to expand...

In the case of RNAV (GPS) RWY 23 at KAJC, you would have seen "L/VNAV" annunciated unless the approach was downgraded for lack of fidelity detected by the system--GS indication with LPV or L/VNAV minimum approach means that it was not downgraded because a downgrade would mean no GS at all. Vertical guidance is provided and legal to follow until DA--at which point you are not supposed to need it any longer. ;)

LNAV+V vertical guidance is not legal to follow to MDA, but the standard "dive and drive" is an incredibly sucky alternative by comparison, IMO.
 
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