Why higher LPV minimums?

[Brad Z]

Except that in the case of the approach that started this thread, it does, and that's the issue. What do you do if you want to fly to the lower LNAV/VNAV mins but the system automatically takes you to LPV? I think you're stuck with the Garmin WAAS units.

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.

 

[poadeleted20]

I guess I just didn't explain myself well; on a localizer approach, one doesn't have to follow the GS if working do they? Must one follow the Vert guidance on an LNAV if it's provided?

Dave

On an LNAV+V, no problem. On an LNAV/VNAV, there could be issues arising.

 

[Brad Z]

Not difficult, Necessarily, just not legal, and possibly unsafe.

Ron, I don't want to be difficult and I respect your judgement, but can you provide a citation that flying to LNAV/VNAV minima with TSO C146 equipment is illegal or unsafe?

 

[olasek]

Thus why I believe there is an error.

I don't think so.
Max Trescott in his WAAS/G1000 book specifically mentions that it is theoretically possible and it can happen albeit very infrequently and this airport is just such an example.

 

[poadeleted20]

Thus why I believe there is an error.

Didnt Wally address that a while back?

 

[poadeleted20]

Ron, I don't want to be difficult and I respect your judgement, but can you provide a citation that flying to LNAV/VNAV minima with TSO C146 equipment is illegal or unsafe?

Check the AFMS and manuals for the unit -- you aren't permitted to fly to lower minimums than the active mode. So, if the box says LPV, and the LNAV/VNAV mins are lower, you're stuck with the LPV mins.

 

[Captain]

I'm not sure how you could justify flying the LNAV/VNAV minimums with an LPV selected, especially if the LNAV/VNAV wea lower. In Essenes you'd be blowing right past the LPV DA saying, "no worries, I'm on LNAV/VNAV today". If the LPV was lower and you broke off at the higher LNAV/VNAV DH I could see it(you can always raise your personal minimums)... .but with a lower than LPV DH I'm pretty sure you'd have some splainin to do if things went south.

 

[Brad Z]

Check the AFMS and manuals for the unit -- you aren't permitted to fly to lower minimums than the active mode. So, if the box says LPV, and the LNAV/VNAV mins are lower, you're stuck with the LPV mins.

I just went back and read the manual for the 530W. I didn't see anything to that effect.

My point is this: if the approach is only published with LNAV/VNAV minima (i.e. no LPV), our Garmin 530W box can fly it perfectly safely, because a C-146 certified WAAS GPS receiver is suitable to fly to L/V minimums. In the case of TTA, the LPV is annunciated because LPV is a higher level of service.

I haven't kept up with every thread on every board regarding TERPS discussions, but there should never be a situatation where an L/V yields a lower minima than a LPV. If there was, something wasn't applied correctly when the approach was designed.

 

[Brad Z]

I'm not sure how you could justify flying the LNAV/VNAV minimums with an LPV selected, especially if the LNAV/VNAV wea lower. In Essenes you'd be blowing right past the LPV DA saying, "no worries, I'm on LNAV/VNAV today". If the LPV was lower and you broke off at the higher LNAV/VNAV DH I could see it(you can always raise your personal minimums)... .but with a lower than LPV DH I'm pretty sure you'd have some splainin to do if things went south.

We're talking the logical equivalent of a LOC approach with lower minimums than the ILS. What would you do in that situation?

 

[COFlyBoy]

We're talking the logical equivalent of a LOC approach with lower minimums than the ILS. What would you do in that situation?

So in that case you have to assume that there is an obstacle that penetrates the glidslope protected space, but the LOC stepdown occurs after said obstacle. In which case it would not be prudent to follow the glideslope all the way.

In the case of LPV vs LNAV/VNAV the virtual glideslope generated by both should be identical.

As I understood the description in the 430W manual LPV is selected based on the level of accuracy and integrity that the GPS is recieving. It will downgrade to LNAV/VNAV when accuracy no longer supports LPV. However the glideslope that the box calculates would not change.

I think the approach in question must be an error.

 

[COFlyBoy]

Here's a snip from the 430W manual:

A strict interpretation here would be that you must honor the higher LPV minimum even though the GPS unit can also provide L/VNAV guidance.

 

[aterpster]

Yeah, but my understanding is you can still select individual approaches in Garmin, for example you can select LNAV rather than LPV or LNAV/VNAV over LPV. You still have this choice or are you suggesting otherwise?. I may have to try it out in my G1000 trainer tonight but have to pick the right airport.

No, you cannot.

 

[aterpster]

Ah ok. I thought they used the table in 8260.3B (page 3-21). Do they use that for non precisions only?

No. Use Table 3-4 for PA and APV.

 

[Brad Z]

Folks, take a look at AC90-107:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-107.pdf

In the first section, under definitions, it says:

n. Lateral Navigation/Vertical Navigation (LNAV/VNAV). An RNAV function that computes, displays, and provides both horizontal and approved vertical approach navigation. Both WAAS vertical guidance and baro-VNAV support approaches to LNAV/VNAV lines of minima.

Later, it says:

WAAS avionics with an appropriate airworthiness approval can enable aircraft to fly to the LPV, LP, LNAV/VNAV and LNAV lines of minima on RNAV (GPS) approaches.

And finally:

WAAS avionics display the best level of service supported by the combination of the WAAS signal-in-space, the aircraft avionics, and the selected RNAV (GPS) instrument approach.
NOTE: This AC does not preclude the use of WAAS receivers to fly to LNAV and LNAV/VNAV lines of minima.

Sounds to me like the FAA is fine with using WAAS to fly to LNAV/VNAV minima.

 

[Captain]

We're talking the logical equivalent of a LOC approach with lower minimums than the ILS. What would you do in that situation?

Obviously hypothetical:

If I shot a LOC with lower mins than the associated ILS (say DH is 850 and MDA is 300) then I would dive and drive to the LOC mins of 300. But that's the difference. Both LPV and LNAV/VNAV use vertical guidance so I'm having a hard time seeing myself with an approach loaded for a LPV and going past the DH of the approach I'm on and instead going to the DH of an approach I'm not on.


The whole things weird.

 

[Brad Z]

Obviously hypothetical:

If I shot a LOC with lower mins than the associated ILS (say DH is 850 and MDA is 300) then I would dive and drive to the LOC mins of 300. But that's the difference. Both LPV and LNAV/VNAV use vertical guidance so I'm having a hard time seeing myself with an approach loaded for a LPV and going past the DH of the approach I'm on and instead going to the DH of an approach I'm not on.


The whole things weird.

Who says you can't have a stabilized descent using the glideslope down to 300, provided you don't bust any step downs along the way?

 

[Captain]

Who says you can't have a stabilized descent using the glideslope down to 300, provided you don't bust any step downs along the way?

Nobody. But the concept is different. Step downs must be observed along the way, the final altitude is a hard deck as opposed to a DH and most importantly you are being given vertical guidance on the LPV, LNAV/VNAV, and ILS. not so with a lLOC.

Oh, I see your point. What if the LOC is lower why can't you ride the GP down to it...

Okay, you can.

 

[aterpster]

Folks, take a look at AC90-107:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-107.pdf

In the first section, under definitions, it says:

n. Lateral Navigation/Vertical Navigation (LNAV/VNAV). An RNAV function that computes, displays, and provides both horizontal and approved vertical approach navigation. Both WAAS vertical guidance and baro-VNAV support approaches to LNAV/VNAV lines of minima.

Later, it says:

WAAS avionics with an appropriate airworthiness approval can enable aircraft to fly to the LPV, LP, LNAV/VNAV and LNAV lines of minima on RNAV (GPS) approaches.

And finally:

WAAS avionics display the best level of service supported by the combination of the WAAS signal-in-space, the aircraft avionics, and the selected RNAV (GPS) instrument approach.
NOTE: This AC does not preclude the use of WAAS receivers to fly to LNAV and LNAV/VNAV lines of minima.

Sounds to me like the FAA is fine with using WAAS to fly to LNAV/VNAV minima.

Yes they are. But, the order of service in most, if not all, WAAS navigator will select LPV when it is charted.

The FAA is not addressing the ARINC and WAAS TSO specs directly.

 

[Captain]

Maybe these things need to be split out into separate procedures instead of separate minima on the same approach? Did I just say something smart?

 

[poadeleted20]

I just went back and read the manual for the 530W. I didn't see anything to that effect.

My point is this: if the approach is only published with LNAV/VNAV minima (i.e. no LPV), our Garmin 530W box can fly it perfectly safely, because a C-146 certified WAAS GPS receiver is suitable to fly to L/V minimums.

...if, and only if, the L/VNAV light is lit.

In the case of TTA, the LPV is annunciated because LPV is a higher level of service.

OK, I'm good with that. I thought you were saying that in reference to this approach where the LNAV/VNAV DH is lower than the LPV DH, you could fly to the lower LNAV/VNAV DH with LPV indicated on the unit. That you cannot do.

I haven't kept up with every thread on every board regarding TERPS discussions, but there should never be a situatation where an L/V yields a lower minima than a LPV. If there was, something wasn't applied correctly when the approach was designed.

That's Wally's department, and he's covered that.

 

[poadeleted20]

Here's a snip from the 430W manual:

A strict interpretation here would be that you must honor the higher LPV minimum even though the GPS unit can also provide L/VNAV guidance.

It all depends which light is lit -- LPV or L/VNAV. You fly no lower than the mins associated with the mode indicated in that box in the lower left corner of the display. In this case, where the LNAV/VNAV mins are lower than LPV, if the LPV light is lit, you start the missed no later than reaching the LPV mins, and you do not continue down to the LNAV/VNAV mins. The problem is that there's no way to manually override the automatic selection of LPV when the LPV criteria are met, so there's no way to push the unit back to L/VNAV in order to take advantage of the lower LNAV/VNAV DH.

Phew --- everyone got that?

And I'll leave to Wally the Terpster the issue of why the LNAV/VNAV DH is lower than the LPV DH.

 

[LearDriver]

Obviously hypothetical:

If I shot a LOC with lower mins than the associated ILS (say DH is 850 and MDA is 300) then I would dive and drive to the LOC mins of 300. But that's the difference. Both LPV and LNAV/VNAV use vertical guidance so I'm having a hard time seeing myself with an approach loaded for a LPV and going past the DH of the approach I'm on and instead going to the DH of an approach I'm not on.


The whole things weird.

Wouldn't both of those approaches be utilizing the same glide path.

Dive and drive is becoming the less preferred method of flying non precision approaches since most aircraft are having more sophisticated avionics installed. I just finished recurrent and they stressed that VNAV was to be used if at all possible. Minimums for your approach type must always be respected. My operator is approved for LNAV/VNAV and my sim partners operator is only LNAV approved. Yet we both flew the approaches using VNAV. We each used they type of minimum (DH or DA) procedure for missed approach as necessary for dealing with our approach type approvals.

 

[Captain]

My understanding when it comes to missed for these approaches is:

ILS, LPV, LNAV/VNAV: it's a DH. Descent below is allowed as long as the decision is made AT the DA.

LNAV: its an MDA. Descent below is NOT allowed unless visual criteria is met.

 

[LearDriver]

My understanding when it comes to missed for these approaches is:

ILS, LPV, LNAV/VNAV: it's a DH. Descent below is allowed as long as the decision is made AT the DA.

LNAV: its an MDA. Descent below is NOT allowed unless visual criteria is met.

Thats it.

 

[poadeleted20]

Wouldn't both of those approaches be utilizing the same glide path.

It's not the path, it's how the vertical steering is generated. LPV and LNAV/VNAV use different methods to generate that steering, and there are different issues and requirements for setting the DA/DH.

Dive and drive is becoming the less preferred method of flying non precision approaches since most aircraft are having more sophisticated avionics installed.

Agreed.

I just finished recurrent and they stressed that VNAV was to be used if at all possible.

That's a nice goal, but sometimes the regular LNAV MDA/HAT is lower than the LNAV/VNAV DA/DH, and that may make the difference in getting in or not.

Minimums for your approach type must always be respected.

Exactly. If the LPV light is lit, you must use the LPV DA as your missed approach point, not the (in this particular case) lower LNAV/VNAV DA.

My operator is approved for LNAV/VNAV and my sim partners operator is only LNAV approved. Yet we both flew the approaches using VNAV. We each used they type of minimum (DH or DA) procedure for missed approach as necessary for dealing with our approach type approvals.

If your partner's operator is only LNAV approved, your partner should not be flying to the LNAV/VNAV DA/DH, but instead should be stopping the descent no lower than the LNAV MDA/HAT, even if using the VNAV steering to get there.

That said, using VNAV or +V vertical steering on an LNAV approach to an MDA is going to cramp your style a bit, as you'll have only one shot to spot the runway on breakout before you're too close in to make a normal descent to the runway. In that case, dive-and-drive gives you a better chance of spotting the runway in time to make a normal descent to it.

 

[LearDriver]

If your partner's operator is only LNAV approved, your partner should not be flying to the LNAV/VNAV DA/DH, but instead should be stopping the descent no lower than the LNAV MDA/HAT, even if using the VNAV steering to get there.

That said, using VNAV or +V vertical steering on an LNAV approach to an MDA is going to cramp your style a bit, as you'll have only one shot to spot the runway on breakout before you're too close in to make a normal descent to the runway. In that case, dive-and-drive gives you a better chance of spotting the runway in time to make a normal descent to it.

He was doing the level off then executing the missed. I was not.

Personally i like the dive and drive method if you are only LNAV approved and the approach is right at minimums. But thats not how it is now being taught. You know the motto. Cooperate and graduate.

 

[Captain]

He was doing the level off then executing the missed. I was not.

Personally i like the dive and drive method if you are only LNAV approved and the approach is right at minimums. But thats not how it is now being taught. You know the motto. Cooperate and graduate.

I agree. If its really at minimums then you're not going to get in using VNAV. The round out to level off at MDA is going to make you high so even if you do break out now you're not in a position to land.

I'd say that if you're doing a straight VNAV then dive and drive is preffered. I also agree with 'cooperate and graduate' though, so there's the rub.

: (

 

[gismo]

It's not the path, it's how the vertical steering is generated. LPV and LNAV/VNAV use different methods to generate that steering, and there are different issues and requirements for setting the DA/DH.

Ron, I'm pretty certain that TSO C146 (WAAS) boxes use the exact same mechanisms (software) to generate LPV and LNAV/VNAV vertical guidance. The VAL (vertical accuracy requirement) is the same 50M for both types (this is reduced to 35M if the DA is less than 250 AGL) and I can see no technical reason why there would be any issues flying to LNAV mins when the box is happy with LPV operation. AFaIK the only reason a WAAS navigator would revert from LPV to LNAV/VNAV is when the tighter HAL (horizontal accuracy requirement) for the LPV (40M) was exceeded but not by so much that the HAL for LNAV/VNAV (556M) was also unavailable. And FWIW, the AFMS and PG (included by reference in the AFMS) for my GNS480 do not contain any language dictating otherwise. I suspect that the verbiage in the 530W PG you quoted was written by someone who naively assumed that LPV mins would never be higher than the LNAV/VNAV mins on the same approach.

WRT the title question, upon further review I found that it is indeed possible albeit uncommon for the LPV mins to be slightly higher than the LNAV/VNAV mins on the same approach. The geometry of the OCS (Obstruction Clearance Surface, which lies below the approach path) is slightly different for LPV than for LNAV/VNAV approaches. There's a diagram of this issue on page 64 in Ekalbar's "Instrument Flying Update" book.

 

[Captain]

Ron, I'm pretty certain that TSO C146 (WAAS) boxes use the exact same mechanisms (software) to generate LPV and LNAV/VNAV vertical guidance. The VAL (vertical accuracy requirement) is the same 50M for both types (this is reduced to 35M if the DA is less than 250 AGL) and I can see no technical reason why there would be any issues flying to LNAV mins when the box is happy with LPV operation. AFaIK the only reason a WAAS navigator would revert from LPV to LNAV/VNAV is when the tighter HAL (horizontal accuracy requirement) for the LPV (40M) was exceeded but not by so much that the HAL for LNAV/VNAV (556M) was also unavailable. And FWIW, the AFMS and PG (included by reference in the AFMS) for my GNS480 do not contain any language dictating otherwise. I suspect that the verbiage in the 530W PG you quoted was written by someone who naively assumed that LPV mins would never be higher than the LNAV/VNAV mins on the same approach.

And WRT the title question, everything I've read about the TERPS requirements for LPV and LNAV/VNAV approaches (8260 and Ekalbar's book) suggests that there is no valid reason why the LPV mins should exceed the LNAV/VNAV mins so I strongly suspect that the TTA GPS-21 has them reversed. I'd love to hear from aTerpSter or anyone else in that line of work, under what conditions could this be correct. If that's the case the discussion about using the lower LNAV/VNAV mins when LPV HAL and VAL are met is moot.

They are not reversed. There are many instances where the LPV is higher than LNAV/VNAV or LNAV/VNAV is higher than LNAV.

I found KMOB GPS 14 with just a few minutes search.

 

[gismo]

They are not reversed. There are many instances where the LPV is higher than LNAV/VNAV or LNAV/VNAV is higher than LNAV.

I found KMOB GPS 14 with just a few minutes search.

I now agree and will edit the post you quoted.

 

[poadeleted20]

Ron, I'm pretty certain that TSO C146 (WAAS) boxes use the exact same mechanisms (software) to generate LPV and LNAV/VNAV vertical guidance. The VAL (vertical accuracy requirement) is the same 50M for both types (this is reduced to 35M if the DA is less than 250 AGL) and I can see no technical reason why there would be any issues flying to LNAV mins when the box is happy with LPV operation.

I don't know enough about the mechanisms to discuss that, but I do know that flying to lower minimums than those for the mode shown on the box isn't kosher.

 

[Captain]

I don't know enough about the mechanisms to discuss that, but I do know that flying to lower minimums than those for the mode shown on the box isn't kosher.

Amish don't fly WAAS approaches....or do they? [cue evil music]

 

[poadeleted20]

Amish don't fly WAAS approaches....or do they? [cue evil music]

Amish don't even drive cars, no less fly airplanes. Well, maybe the ones in Ohio do...

 

[aterpster]

Ron, I'm pretty certain that TSO C146 (WAAS) boxes use the exact same mechanisms (software) to generate LPV and LNAV/VNAV vertical guidance. The VAL (vertical accuracy requirement) is the same 50M for both types (this is reduced to 35M if the DA is less than 250 AGL) and I can see no technical reason why there would be any issues flying to LNAV mins when the box is happy with LPV operation. AFaIK the only reason a WAAS navigator would revert from LPV to LNAV/VNAV is when the tighter HAL (horizontal accuracy requirement) for the LPV (40M) was exceeded but not by so much that the HAL for LNAV/VNAV (556M) was also unavailable. And FWIW, the AFMS and PG (included by reference in the AFMS) for my GNS480 do not contain any language dictating otherwise. I suspect that the verbiage in the 530W PG you quoted was written by someone who naively assumed that LPV mins would never be higher than the LNAV/VNAV mins on the same approach.

WRT the title question, upon further review I found that it is indeed possible albeit uncommon for the LPV mins to be slightly higher than the LNAV/VNAV mins on the same approach. The geometry of the OCS (Obstruction Clearance Surface, which lies below the approach path) is slightly different for LPV than for LNAV/VNAV approaches. There's a diagram of this issue on page 64 in Ekalbar's "Instrument Flying Update" book.

But, the paths are different. The LPV approach has a final approach segment path record in the database. LNAV/VNAV does not have that level of sophistication.

But, indeed both paths are WAAS vertical paths.

 

[Dave Siciliano]

Great discussion; really excellent info folks!
Of course the LPV is DA not DH. As a matter of fact, at recurrent this year, we looked up DH in the AIM and Instrument Approach Handbook and references to that were for Cat approaches with a RADAR altimiter--height above touchdown. DA is a decision altitude using the baro altimeter. Small stuff, but it was in there.

I see how the LNAV/VNAV is different than a localizer approach with when getting glide slope because the former has a DA and not an MDA; so, one would follow the vertical guidance down. On the localizer, one could go right down to MDA which is different. Of course that would apply to LNAV alone or LNAV plus V.

Nice to see everyone with their thinking hat on and digging out facts. Makes this a great board for discussions like this.

Best,

Dave

 

[John Collins]

It's the geometry. When there are obstacles close in to the runway that penetrate the OCS (Obstacle Clearance Surface) that provides a surface below the glideslope that provides a ROC (Required Obstacle Clearance) that decreases as you go from the glidepath intercept point down to the DA. The general concept is that if an obstacle penetrates the surface, the DA must be moved back and up higher so that when you break out at the DA you can see the offending obstacle and avoid it. The way this is accomplished is by projecting the obstacle height back to a point where it intersects the OCS, and then the GS directly above this point becomes the DA.

There is a second obstacle surface that is evaluated between the DA and the threshold at the TCH elevation, but it is narrower than the OCS and picks up where the OCS ends. There are different geometrical methods used to determine the OCS for LNAV/VNAV and LPV approaches. In the vast majority of cases, the LPV will have the lowest DA and the LNAV/VNAV the second lowest. But there is a small area in which the OCS for the LPV and the LNAV/VNAV will result in a penetration for the LPV OCS but not affect the OCS for the LNAV/VNAV. This may force the LPV DA higher than the LNAV/VNAV DA. It is rare, but can happen. More common is the situation where the LNAV/VNAV results in a higher DA than the LNAV MDA. Often this is also caused by an obstacle within a few miles of the runway that penetrates the LNAV/VNAV OCS but with the LNAV, a step down can be used, once the obstacle is past, and the MDA can drop down lower than the LNAV/VNAV.

The point is there are examples of all these kinds of anomalies.

 

[John Collins]

Ron,

My understanding is that the TSO C146AR WAAS GPS may fly any minimum line approved on the approach chart. There is a hierarchy in the annunciations on the WAAS GPS (LPV, L/VNAV, LNAV). You cannot fly any minimum line that is higher than the one annunciated, but you can fly anyone below the one annunciated. The main difference in the annunciation is the integrity values that are required to be met. The vertical and lateral guidance is the same. So with LPV annunciated, you can fly to the LPV DA minimums, the LNAV/VNAV DA minimums, or ignore the vertical guidance all together and fly to the LNAV MDA. This approach is one of the unusual cases where flying the LNAV/VNAV has an advantage with the WAAS GPS.

There are no downgrades from LPV to LNAV/VNAV. All downgrades are to LNAV without ever any vertical guidance. Unless an approach has LNAV/VNAV as the highest service available on the chart (better look for one soon as all of them are slated for adding a LPV), you will never see L/VNAV annunciated.

The WAAS GPS calculates a straight line in space for the vertical guidance. A Baro VNAV system follows the curvature of the earth as it bases the vertical on the altimeter verses a GPS or FMS position. The path that the WAAS GPS will be higher than the one a Baro VNAV follows by about 20 feet at the FAF and converging to the same value at the TCH altitude. So if you fly a WAAS generated path on a LNAV/VNAV approach, you will be slightly higher than the nominal path for the Baro VNAV system at standard temperature. But this is of little consequence.

 

[John Collins]

I attached some graphics to show how the different OCS evaluations are made for LPV and LNAV/VNAV.

First the LPV because it is simpler: It starts at 200 feet from the runway and slopes upwards at a value equal to 104/GS angle, so for a 3 degree slope it is 104/3 = 34 to 1.



The LNAV OCS actually consists of two parts, a level section that is 89 feet high and the ROC is 161 feet above it (notice they total to 250 ft). The second portion is the sloped portion and it is located about 4000 +/- feet from the threshold. If there is an obstacle in the first section, the minimum DA value is 161 feet above that obstacle.



If an obstacle penetrates the sloped portion, the adjustment to the DA is similar to the one for the LPV, but because of the 4000 feet offset instead of 200 feet, it can result in a different DA. The obstacle is always left in the visual segment so you can see it and avoid it.



If you examine these graphics, hopefully you can see that an obstacle can fall close to the threshold and effect the DA differently for the LPV and not affect the DA for the LNAV/VNAV to the same degree or at all. All it takes is careful obstacle placement.

 

[aterpster]

From the horse's mouth:

LPV and LNAV/VNAV share the same controlling obstacle (KTTAT0079, 318 MSL TREE, 353555.86N/0790531.46W).

LPV: Obstacle penetrates the "W" surface, causing an automatic 250 HATh.
At 250 HATh, the obstacle is located in Missed Approach Section 1AW.
Consequently, the HATh must be raised to 300 to clear Section 1AW.

LNAV/VNAV: Obstacle is located in the level surface ROC area, prior to the point where the OCS slope starts. Level surface ROC is 161 ft. 161 + 318 = DA 479 / HATh 255.

 

[poadeleted20]

Of course the LPV is DA not DH.

Actually, it's both -- the DA you see on your altimeter, and the DH you are above touchdown when you see DA on your altimeter. So, at Salisbury MD (KSBY) on the RNAV(GPS) 32, when operating in LPV mode, your DA is 253 MSL and your DH is 200 feet above the touchdown zone elevation, or TDZE, of 53 feet MSL.