Why higher LPV minimums?

Ron Levy said:
There is a very big difference between in lateral precision between the LPV and LNAV modes, and if the LPV procedure is removed, the unit will only go to LNAV.
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On a WAAS GPS, the accuracy is unaffected by the approach type, it is what it is at a point in time and space. The CDI scaling is essentially the same as well and is angular and +/-2 degrees FSD (Full Scale Deflection) and tapers to +/- 350 feet at the threshold. If the FAS is 7 NM or greater, the LNAV FSD is fixed at +/- .3 NM at and beyond the 7 NM point from the threshold. This is different from the C129 GPS where the FSD after the FAF is fixed at +/- .3 NM.

What changes for the different types of approaches are the integrity alert limits, with the LPV HAL being 40 meters and with both the LNAV/VNAV and LNAV HAL set to 556 meters. The integrity alert function is there to notify the pilot when the required accuracy can't be assured and the pilot will either be instructed to downgrade or abandon the approach, or if it is really bad, to switch to other navigation means.

On vertically guided approaches, the vertical integrity requirements are the toughest to meet due to the geometry of determining a vertical position. On the approach in question, the vertical integrity requirements are the same for the LPV and the LNAV/VNAV.
 
Ron Levy said:
and the only time it says anything about using LNAV procedures/mins after starting with the LPV is when the system downgrades
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I hope you are not suggesting that pilot can only fly LNAV when equipment downgrades. Clearly the pilot's free will can downgrade the approach as well.
 
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Ron Levy said:
I thought you were talking about LPV versus plain LNAV. LNAV/VNAV (L/VNAV in Garmin annunciation-speak) is another story entirely. So now you're talking about leaving the LNAV/VNAV up but killing the LPV, so you get L/VNAV when the system brings it up? I don't see an issue there.
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Don't know where you got that idea. My math was for LNAV/VNAV vs. LPV.
 
Ron Levy said:
That seems legit to me, although I'd need more research into the AFMS to be sure, and mine's in the airplane. Of course, you give up the LPV precision, but if an extra 24 feet of descent is worth that to you, you go right ahead.
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Wow, you're fast. I deleted my post because I decided I'd rather read what John Collins and Wally aterpster had to say, but since you replied... what I was getting at (and in my earlier post as well) is that it sounds totally ridiculous to insist that it's illegal to base what you fly solely on what's annunciated, when what it annunciates is partially under the pilot's control. Of course it would be stupid to turn off SBAS to fly the LNAV -- and of course there are reasons other than "an extra 24 feet of descent" to do that -- but it just seems to me to be an overly literal reading of the AFMS to say that you would *have* to do that in order to be legal to dive and drive to the LNAV MDA.

Just my "common sense" reading.
 
azure said:
Wow, you're fast. I deleted my post because I decided I'd rather read what John Collins and Wally aterpster had to say, but since you replied... what I was getting at (and in my earlier post as well) is that it sounds totally ridiculous to insist that it's illegal to base what you fly solely on what's annunciated, when what it annunciates is partially under the pilot's control. Of course it would be stupid to turn off SBAS to fly the LNAV -- and of course there are reasons other than "an extra 24 feet of descent" to do that -- but it just seems to me to be an overly literal reading of the AFMS to say that you would *have* to do that in order to be legal to dive and drive to the LNAV MDA.

Just my "common sense" reading.
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45 feet. :D
 
azure said:
Of course it would be stupid to turn off SBAS to fly the LNAV -- and of course there are reasons other than "an extra 24 feet of descent" to do that -- but it just seems to me to be an overly literal reading of the AFMS to say that you would *have* to do that in order to be legal to dive and drive to the LNAV MDA.

Just my "common sense" reading.
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Of course, I agree, especially since the overly literal reading of the AFMS was based on an accidental misquote of the AFMS.
 
Hey John do you have any examples where the LPV is higher than the LNAV/VNAV DA. I'm reading online that there are a few out there but they are miss prints and the LPV can't be higher than an LNAV/VNAV. Anyone????
 
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Velocity173 said:
Hey John do you have any examples where the LPV is higher than the LNAV/VNAV DA. I'm reading online that there are a few out there but they are miss prints and the LPV can't be higher than an LNAV/VNAV. Anyone????
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There may be a FAA policy to only publish a LPV if there is an operational advantage, but if your read my post 78 and Wally's post 79 you can see that obstacles close in to the threshold can result in the situation where the LPV DA is higher than the LNAV/VNAV DA. This is simply due to two separate sets of geometric criteria being applied. It is rare, but does happen and in that case, it is not a misprint.

An example N14 RNAV RWY 19.
 
Brad Z said:
Thus why I believe there is an error.

As far as flying the approach, there is nothing to prevent a pilot from flying an approach to LNAV minima, even when LPV is annunciated on the box. You just have to abide by LNAV minimums (ie the LNAV MDA rather than the LPV DA). Along the same logic, there is nothing to prevent a pilot with WAAS LPV to fly to LNAV/VNAV minima instead of LPV minima. WAAS is sufficient to fly to LNAV/VNAV minima.

All that said, some operators may have different operating limitations that prevent them from downgrading from a vertically guided approach to a stepped non-precision approach. For part 91, it's not an issue.
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I'm guessing there's an obstacle that violates the 50:1 approach surface slope used to determine the LPV minumum, but not the 34:1 approach surface slope used to determin the LNAV/VNAV minumum. It's not just about the minima, it's also about what's in between the FAF and the threshold, and the requirements vary based on the type of approach.

In addition, the FAA is commissioning GPS approaches like crazy. The LPV approaches need obstruction surveys that are conducted to different standards. So it may also be that the obstruction survey is OK for LNAV/VNAV, but not for LPV, so the default values change.
 
Ken Ibold said:
I'm guessing there's an obstacle that violates the 50:1 approach surface slope used to determine the LPV minumum, but not the 34:1 approach surface slope used to determin the LNAV/VNAV minumum. It's not just about the minima, it's also about what's in between the FAF and the threshold, and the requirements vary based on the type of approach.

In addition, the FAA is commissioning GPS approaches like crazy. The LPV approaches need obstruction surveys that are conducted to different standards. So it may also be that the obstruction survey is OK for LNAV/VNAV, but not for LPV, so the default values change.
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Both require a vertical survey under airport standards and both must pass the GQS.

Beyond that the approach and missed approach criteria for the two are quite different.
 
John Collins said:
There may be a FAA policy to only publish a LPV if there is an operational advantage, but if your read my post 78 and Wally's post 79 you can see that obstacles close in to the threshold can result in the situation where the LPV DA is higher than the LNAV/VNAV DA. This is simply due to two separate sets of geometric criteria being applied. It is rare, but does happen and in that case, it is not a misprint.

An example N14 RNAV RWY 19.
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Hey thanks John. I did some basic TERPs in the Army with Emergency Copter GPS. I read a little in the 8260.54 and it's way more complicated then how we made a Copter. Good stuff though.
 
Velocity173 said:
Hey thanks John. I did some basic TERPs in the Army with Emergency Copter GPS. I read a little in the 8260.54 and it's way more complicated then how we made a Copter. Good stuff though.
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In context, before WAAS became operational all GPS IAPs were LNAV only (Excepting a few Baro VNAV IAPs). The FAA criteria were in Order 8260.38(A), which were funny paper simple compared to 8260.54A.

Since WAAS came about both LNAV/VNAV and LPV criteria have changed several times. The only part that hasn't changed are the LPV final segment criteria.
 
Velocity173 said:
Awesome. Another one. For some reason the original link didn't work but this one does. Thanks. :)
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Since the link expires at the end of the 56 day TPP cycle, it is better if the original poster posts the name and the approach or downloads the pdf as an attachment. Unfortunately, this happens all the time and post links go stale. Someone reading these posts a few months from now will not be able to use the new post of the link and not be able to figure out where the Op is talking about. Sometimes, one can read thru the thread and come across someone commenting on the airport or a waypoint on the approach, and a good sleuth can figure it out, but it does waste a lot of time.
 
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aterpster said:
Garmin WAAS navigators will downgrade from LPV to LNAV/VNAV. Having said that, the Garmin engineer I work with said the chances of seeing it happen are very unlikely.
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Strangely, I've had this happen to me 3 times (out of about 250 approaches in my log book). All three were at the same airport on the same approach.

Never seen it anywhere else.
 
Martymccasland said:
Strangely, I've had this happen to me 3 times (out of about 250 approaches in my log book). All three were at the same airport on the same approach.

Never seen it anywhere else.
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Terminology:

Fail Down: approach initially annunciates LPV and one minute prior to the FAF, the integrity does not meet the requirements for the approach, so a message is displayed and the approach changes to LNAV without any vertical guidance.

Approach Annunciation: When an approach with a LPV and LNAV/VNAV minimums depicted on the same approach is flown and the integrity meets the LNAV/VNAV criteria but not the LPV criteria, the approach is initially annunciated as LNAV/VNAV. Although permitted by the TSO, very unlikely to occur.
 
John Collins said:
Terminology:

Fail Down: approach initially annunciates LPV and one minute prior to the FAF, the integrity does not meet the requirements for the approach, so a message is displayed and the approach changes to LNAV without any vertical guidance.

Approach Annunciation: When an approach with a LPV and LNAV/VNAV minimums depicted on the same approach is flown and the integrity meets the LNAV/VNAV criteria but not the LPV criteria, the approach is initially annunciated as LNAV/VNAV. Although permitted by the TSO, very unlikely to occur.
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It sure would be nice if the TSO required a WAAS navigator to provide the new minimum altitudes (MDA and step downs) if and when the GPS reverts to lateral only guidance. Wouldn't be difficult to accomplish either.
 
gismo said:
It sure would be nice if the TSO required a WAAS navigator to provide the new minimum altitudes (MDA and step downs) if and when the GPS reverts to lateral only guidance. Wouldn't be difficult to accomplish either.
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It doesn't provide the minimums or step down altitudes for LPV DA, and for most GPS units they don't provide any altitudes. It would have to be psychic to figure out your intentions (what authorized minimums do you choose to fly) and what category airplane or speed, and if you plan on circling, ...
 
John Collins said:
Terminology:

Fail Down: approach initially annunciates LPV and one minute prior to the FAF, the integrity does not meet the requirements for the approach, so a message is displayed and the approach changes to LNAV without any vertical guidance.

Approach Annunciation: When an approach with a LPV and LNAV/VNAV minimums depicted on the same approach is flown and the integrity meets the LNAV/VNAV criteria but not the LPV criteria, the approach is initially annunciated as LNAV/VNAV. Although permitted by the TSO, very unlikely to occur.
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John, how do the provisions of AC 90-107's Foreword, under Contingency Procedures, play into this?

"(b) After the aircraft passes the FAF an alert may result in a fail-down to LNAV-only operations.
• Pilots can continue to the LNAV minimums if the aircraft is above the MDA or the next step-down fix altitude for the LNAV approach.
• Pilots must initiate a missed approach if below a required altitude on the approach and cannot transition visually to land."

Bob Gardner
 
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bobmrg said:
John, how do the provisions of AC 90-107's Foreword, under Contingency Procedures, play into this?

"(b) After the aircraft passes the FAF an alert may result in a fail-down to LNAV-only operations.
• Pilots can continue to the LNAV minimums if the aircraft is above the MDA or the next step-down fix altitude for the LNAV approach.
• Pilots must initiate a missed approach if below a required altitude on the approach and cannot transition visually to land."

Bob Gardner
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Bob,


I was merely pointing out that there is no fail down from LPV to LNAV/VNAV, just LPV to LNAV, or LNAV/VNAV to LNAV. What was being discussed as a "fail down", IOW annunciating one thing (LPV) and changing the annunciation to another later on the approach to LNAV or another way of stating it is that the annunciation changes on the approach.

It is a different situation if the original annunciation is lower than the highest annunciation possible, IOW the annunciation is LNAV/VNAV but the approach has a LPV minimum available. This isn't a fail down, because the annunciation was never LPV and started out as LNAV/VNAV. This is theoretically possible, but I have never seen it occur. It might be more common on the West Coast where the LPV availability isn't as high as it is in the rest of the country. But even this should be a rare situation as the HPL for LPV is 40 meters and the highest VPL is 50 meters. The HPL for LNAV/VNAV is 556 meters but the VPL is the same 50 meters. So the HPL would have to exceed 40 meters while the VPL had to not be greater than 50 meters. This is an extremely difficult situation to occur because if the HPL is 40 meters, the likely VPL will be at least 60 meters due to the poorer geometry for a vertical coordinate.
 
John Collins said:
Bob,


I was merely pointing out that there is no fail down from LPV to LNAV/VNAV, just LPV to LNAV, or LNAV/VNAV to LNAV. What was being discussed as a "fail down", IOW annunciating one thing (LPV) and changing the annunciation to another later on the approach to LNAV or another way of stating it is that the annunciation changes on the approach.

It is a different situation if the original annunciation is lower than the highest annunciation possible, IOW the annunciation is LNAV/VNAV but the approach has a LPV minimum available. This isn't a fail down, because the annunciation was never LPV and started out as LNAV/VNAV. This is theoretically possible, but I have never seen it occur. It might be more common on the West Coast where the LPV availability isn't as high as it is in the rest of the country. But even this should be a rare situation as the HPL for LPV is 40 meters and the highest VPL is 50 meters. The HPL for LNAV/VNAV is 556 meters but the VPL is the same 50 meters. So the HPL would have to exceed 40 meters while the VPL had to not be greater than 50 meters. This is an extremely difficult situation to occur because if the HPL is 40 meters, the likely VPL will be at least 60 meters due to the poorer geometry for a vertical coordinate.
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According to my Garmin engineer friend the difference between LPV and WAAS (as opposed to Baro) LNAV/VNAV is sufficiently minimal that it is next to impossible to see grade of service on an LPV IAP downgrade to LNAV/VNAV. Further, if it downgrades from LPV to LNAV, you are probably no longer in the WAAS mode at all.

Nonetheless the computational issues are quite different for the LPV and WAAS VNAV vertical paths. LPV has a complex path record in the database; WAAS VNAV does not. So far as the avionics are concerned (as opposed to the TERPs) there is no difference between the WAAS VNAV path and the advisory vertical path (LNAV+V).

The availability on the west coast is no longer an issue according to the gurus. (It always works on my G-1000 trainer in any case. :wink2:)

The path record for every LPV vertical path is contained on the 8260-3 series forms that comprise the regulatory record for the approach. That makes certain transmittal of the proper path record to Jeppesen, LIDO, and any other database vendor (if there are any, at least for IFR).
 
Martymccasland said:
Strangely, I've had this happen to me 3 times (out of about 250 approaches in my log book). All three were at the same airport on the same approach.

Never seen it anywhere else.
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On the west coast perhaps a couple or more years ago?
 
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