Why is LNAV+V advisory only?

kicktireslightfires

kicktireslightfires

Pre-takeoff checklist
Joined
Jun 11, 2020
Messages
343
Display Name
Display name:
kicktireslightfires
Why is LNAV+V advisory only on the glide slope? What is it using to provide the “advisory” glide slope? GPS?
 
kicktireslightfires said:
Why is LNAV+V advisory only on the glide slope? What is it using to provide the “advisory” glide slope? GPS?
Click to expand...
Because it’s a GPS derived vertical path and doesn’t meet FAA requirements for LNAV/VNAV or LPV. With LNAV+V, you are still shooting the approach to LNAV mins.
 
§ 91.175 Takeoff and landing under IFR.
(a) Instrument approaches to civil airports. Unless otherwise authorized by the FAA, when it is necessary to use an instrumentapproach to a civil airport, each person operating an aircraft must use a standard instrument approach procedure prescribed in part 97 of this chapter for that airport.


What are the LNAV+V minimums for your approach to the airport?

The approach is an LNAV approach with a little help descending the MDA.
 
Most succinctly, it's an "advisory" glidepath because it has not been evaluated in any way by the developers of the procedure (typically the FAA). Garmin/Jeppesen put it in the database as an aid for you.

Less succinctly, there are various obstacle clearance surfaces underlying any procedure. On a vertically-guided procedure, these provide for additional safety in the event of glidepath deviations. Most pertinent here is the VGS, the Vertical Guidance Surface, which underlies final from the runway threshold to the DA point. If it is penetrated by an obstacle, a vertically guided procedure (ILS, LNAV/VNAV, LPV) is not allowed. But Garmin/Jepp may still publish an advisory glidepath, which means you will or may NOT have the normal amount of clearance on final (specifically once below the MDA).
 
But, isn’t a dive and drive which many (perhaps most) suggest, no better and perhaps worse clearance than following the vertical guidance?
 
I remember reading in one of the flying magazines where they found a couple of LNAV+V approaches that flew you into or very close to hills / trees / other obstructions if you followed the vertical guidance.
 
Salty said:
But, isn’t a dive and drive which many (perhaps most) suggest, no better and perhaps worse clearance than following the vertical guidance?
Click to expand...

Down to the MDA, it doesn't particularly matter how you get there from an obstacle clearance standpoint. You are guaranteed obstacle clearance. It's BELOW the MDA where there's a difference. For a vertically-guided procedure, there is some level of obstacle protection below the DA. For a non-vertically guided procedure, there is not necessarily ANY obstacle clearance provided down to the runway. That's like the example @Country Flier mentions. However, with an advisory glidepath, since it looks just like a "real" glidepath on the instruments, you can be deceived into just treating it as such. Most importantly, and dangerously, at night where you can't see those trees or the hillside that you possibly aren't protected from, but you assume you are.
 
With a +V approach, you have to watch carefully for additional step down fixes on the approach. The +V will provide a smooth continuous glidepath from the FAF to the MDA, but as shown in the picture above, it will fly you into a hill if you don't observe the intermediate step downs.
 
midwestpa24 said:
With a +V approach, you have to watch carefully for additional step down fixes on the approach. The +V will provide a smooth continuous glidepath from the FAF to the MDA, but as shown in the picture above, it will fly you into a hill if you don't observe the intermediate step downs.
Click to expand...
There aren't any final segment step-down fixes on that N23 Runway 07 approach. It's the high HAT of the MDA that should be a major clue to the wary.
 
aterpster said:
There aren't any final segment step-down fixes on that N23 Runway 07 approach. It's the high HAT of the MDA that should be a major clue to the wary.
Click to expand...

True, misinterpreted what it was pointing out, missed that LOTMY was before the hill. Descending below MDA can kill you regardless of navigation source, but the advisory glideslope can help district an unwary pilot. But I've seen scenarios, can't find one off-hand, where the +V will actually cause you to end up below the step-down altitude, another trap of the +V approach.
 
Country Flier said:
I remember reading in one of the flying magazines where they found a couple of LNAV+V approaches that flew you into or very close to hills / trees / other obstructions if you followed the vertical guidance.
Click to expand...

The Sidney, NY airport was the iconic one. But, to be fair, the approach (or any approach), flown as directed, does not fly into any obstacles. The problem occurs if pilots follow the advisory vertical guidance beyond the MDA. LNAV+V advisory vertical guidance does not imply obstruction clearance below the MDA. The brouhaha got the Sidney approach made N/A at night for a long while.

Personally, I like "dive and drive" for non-precision approaches. It gives me a better chance of breaking out and spotting the airport sooner than a gradual descent. It also assures that you monitor your stepdowns correctly. Dive and drive is not unsafe as long as you properly control your aircraft. For Sidney and all the other Central NY airports nestled in the hills here, the sooner you get to visual conditions, the better the chances of making it in in our perpetually gloom during parts of the year.
 
RussR said:
Down to the MDA, it doesn't particularly matter how you get there from an obstacle clearance standpoint. You are guaranteed obstacle clearance. It's BELOW the MDA where there's a difference. For a vertically-guided procedure, there is some level of obstacle protection below the DA. For a non-vertically guided procedure, there is not necessarily ANY obstacle clearance provided down to the runway. That's like the example @Country Flier mentions. However, with an advisory glidepath, since it looks just like a "real" glidepath on the instruments, you can be deceived into just treating it as such. Most importantly, and dangerously, at night where you can't see those trees or the hillside that you possibly aren't protected from, but you assume you are.
Click to expand...
Well if you’re below MDA and can’t see the obstacles there are other more fundamental problems.
 
chemgeek said:
The Sidney, NY airport was the iconic one. But, to be fair, the approach (or any approach), flown as directed, does not fly into any obstacles. The problem occurs if pilots follow the advisory vertical guidance beyond the MDA. LNAV+V advisory vertical guidance does not imply obstruction clearance below the MDA. The brouhaha got the Sidney approach made N/A at night for a long while.

Personally, I like "dive and drive" for non-precision approaches. It gives me a better chance of breaking out and spotting the airport sooner than a gradual descent. It also assures that you monitor your stepdowns correctly. Dive and drive is not unsafe as long as you properly control your aircraft. For Sidney and all the other Central NY airports nestled in the hills here, the sooner you get to visual conditions, the better the chances of making it in in our perpetually gloom during parts of the year.
Click to expand...
I prefer the dive and drive as well...for the same reasons. I also find it easier, as descending to an altitude only requires the precision of leveling at the altitude, with no real "precision" on the descent (if that makes sense).
 
Salty said:
But, isn’t a dive and drive which many (perhaps most) suggest, no better and perhaps worse clearance than following the vertical guidance?
Click to expand...
It shouldn't be. Those stepdowns before the MDA are specifically there for obstacle clearance.

I stopped diving and driving before the CDFA became a thing. The "dive" part anyway, where I was taught to descend at 1000 FPM. I hate cockpit math but even I can get a quick take on dividing feet to lose by 3 to see if I need more or less than 3 degrees to a VDP.
 
An LNAV or LP procedure does not consider obstacles below the MDA. Basically the OCS (Obstacle Clearance Surface) is set to be 250 feet above the highest obstacle in the final approach segment area that is evaluated. Once the highest obstacle is found, the MDA is set to its height plus 250 feet, rounded up to the next 20 foot increment, so if the highest obstacle is 363 feet in elevation, the MDA will be set to be at 613 feet which is rounded up to 620 feet, There is no evaluation by the procedure designer of any obstacles along the VDA below the MDA. When the procedure is flight tested, if the obstacles along the VDA require the aircraft to adjust the flight path to avoid obstacles, this is reported back to the design team. If adjustments can't be made (example increasing the TCH or increasing the VDA), then the procedure still gets published, a VDA still gets determined, and a note is added to the profile view "Visual Segment - Obstacles" and in the case of the FAA charts, the VDA and TCH are not charted. Jeppesen charts include the VDA and TCH along with the profile note. The GPS +V is defined by the VDA and TCH, regardless if it is charted. For the latest version of software on the Garmin systems, they provide the advisory +V guidance for either LP or LNAV minimums. What is different about LPV, it is simply that a primary sloped surface from the FAF to the runway is evaluated for obstacles and if obstacles penetrate the primary sloped surface, the LPV has to be adjusted to make it comply without any penetrations or the LPV can't be issued. LNAV no obstacle evaluation below the MDA, LPV is evaluated all the way to the runway, even below the DA. LPV also requires a more extensive survey than an LNAV because of the differences. This is an over simplification, but is the basic reason why +V is not authorized below the MDA.
 
John Collins said:
There is no evaluation by the procedure designer of any obstacles along the VDA below the MDA.

...

This is an over simplification
Click to expand...

Mostly correct, but it's a bit too much of an oversimplification in my opinion.

There is absolutely evaluation by the procedure designer of obstacles below the MDA - these are the 20:1 and 34:1 visual segment obstacles that I know you are aware of.

Now, they have ZERO effect on the MDA, that is true. But they are definitely evaluated and affect the visibility minimums and whether the procedure is available at night.
 
There's been a lot of discussion in several forums here of how vertical guidance provided by GPS navigators for LNAV+V approaches should not be used below MDA.

So the perhaps obvious question is...why doesn't the GPS navigator limit vertical guidance to MDA? After the advisory GP hits MDA, it could either throw a flag on vertical guidance, or switch to constant altitude guidance at MDA.

Is anyone aware of the thinking/logic behind current behavior by Garmin and othe mfrs?

Bonus Question: I learned that the official analysis of an IAP is legally documented by the FAA in a "letter of transmittal". The FAA website only seems to give access to such letters issued in the last 24 months. Is there a way to access letters older than that?
 
Turbo-Arrow-Driver said:
There's been a lot of discussion in several forums here of how vertical guidance provided by GPS navigators for LNAV+V approaches should not be used below MDA.

So the perhaps obvious question is...why doesn't the GPS navigator limit vertical guidance to MDA? After the advisory GP hits MDA, it could either throw a flag on vertical guidance, or switch to constant altitude guidance at MDA.

Is anyone aware of the thinking/logic behind current behavior by Garmin and othe mfrs?

Bonus Question: I learned that the official analysis of an IAP is legally documented by the FAA in a "letter of transmittal". The FAA website only seems to give access to such letters issued in the last 24 months. Is there a way to access letters older than that?
Click to expand...
Get them from me. Or, make a FOIA and you might get them in 18 months. I have them back to 2007.
 
Turbo-Arrow-Driver said:
Bonus Question: I learned that the official analysis of an IAP is legally documented by the FAA in a "letter of transmittal". The FAA website only seems to give access to such letters issued in the last 24 months. Is there a way to access letters older than that?
Click to expand...

The Transmittal Letters contain the form 8260-3 or -5 for approaches, and 8260-15a/b for departures. This is the "source document" we occasionally reference here, and is distinct from the 8260-9 which contains the obstacle data and such "behind the scenes" stuff.

When a new procedure is pending publication, all of the proposed forms can be found on the FAA IFP Gateway, search for an airport and select the Coordination tab. That allows for public comment, and they all stay up there for "a while" (I don't know how long).

Once the procedures are published, the -3/5 and -15 forms are placed in the "IFP Documents" tab permanently. This is "fairly" recent though, meaning last 5 or 6 years maybe (not sure), so anything older than that won't have documents in there. But more recent stuff will.
 
Turbo-Arrow-Driver said:
There's been a lot of discussion in several forums here of how vertical guidance provided by GPS navigators for LNAV+V approaches should not be used below MDA.

So the perhaps obvious question is...why doesn't the GPS navigator limit vertical guidance to MDA? After the advisory GP hits MDA, it could either throw a flag on vertical guidance, or switch to constant altitude guidance at MDA.

Is anyone aware of the thinking/logic behind current behavior by Garmin and othe mfrs?

Bonus Question: I learned that the official analysis of an IAP is legally documented by the FAA in a "letter of transmittal". The FAA website only seems to give access to such letters issued in the last 24 months. Is there a way to access letters older than that?
Click to expand...

How would Garmin know the MDA that applies to the approach? The MDA may be affected by category, remote altimeter, NOTAM, circling ... The path is defined from the FAF or step down approach to the TCH. The pilot is required to use their altimeter to determine the MDA or DA, not all installations have that information available and it can't be determined from GPS altitude. It is simple, use the advisory GP down to the MDA and compy with all step down minimums using your altimeter.
 
John Collins said:
How would Garmin know the MDA that applies to the approach? The MDA may be affected by category, remote altimeter, NOTAM, circling ... The path is defined from the FAF or step down approach to the TCH. The pilot is required to use their altimeter to determine the MDA or DA, not all installations have that information available and it can't be determined from GPS altitude. It is simple, use the advisory GP down to the MDA and compy with all step down minimums using your altimeter.
Click to expand...

Excellent point. All of the info required to determine MDA would not be available to the firmware. You could conceivably add a system setting for approach category to answer some of the questions, but then what about the altimeter setting? Is it going to ask you where you got that? At some point it would become more trouble than it's worth. The only option would be for the pilot to manually enter the MDA...and I'm not sure how much sense that makes.
 
Even if the MDA was a value that was entered by the pilot, it is an MSL altitude and not a GPS altitude. So comparing a GPS altitude with a baro altitude is not straight forward as you also need to know the altimeter setting, the pressure altitude with some precision and the temperature. So even if all those items are input to the GPS or more likely a PFD, at best it is a calculated value whose only value would be to suppress the +V below the MDA, when alternatively, the pilot could simply follow the rules and use visual cues below the MDA. Even with an LPV, pilots are required to determine the DA using their altimeter and only continue the approach if the appropriate visual cues are available.
 
John Collins said:
Even if the MDA was a value that was entered by the pilot, it is an MSL altitude and not a GPS altitude. So comparing a GPS altitude with a baro altitude is not straight forward as you also need to know the altimeter setting, the pressure altitude with some precision and the temperature. So even if all those items are input to the GPS or more likely a PFD, at best it is a calculated value whose only value would be to suppress the +V below the MDA, when alternatively, the pilot could simply follow the rules and use visual cues below the MDA. Even with an LPV, pilots are required to determine the DA using their altimeter and only continue the approach if the appropriate visual cues are available.
Click to expand...

I'm not agreeing with the idea of providing guidance for leveling off at MDA, but most modern PFDs (think G1000) already have a "Minimums" bug that can be set, and it is based on barometric altimetry. So the setting exists to be able to implement the idea.
 
Yes on a glass PFD, but not as a part of the GPS function to disable the advisory GP. With my G500TXi, I can set the minimums, but the GTN 750 has no clue and provides the advisory GP to the threshold.
 
My point was just that all the infrastructure exists to do it on some glass panel setups. It would just be a matter of software to implement it.
 
My point is that it does not exist on a GPS, which was the suggestion, and all you need to implement your idea is another $40,000 or more to an aircraft without a glass panel. It is impractical otherwise.
 
You must log in or register to reply here.
Back
Top