I have a few issues with the presentation.
1) Baro Aiding is not Baro VNAV. Baro-Aiding is an aspect of the RAIM algorithm that uses pressure altitude to substitute for one satellite, so with Baro-Aiding, RAIM can be determined with four satellites in view rather than requiring 5. Most GPS use this. Baro-VNAV is a means of using MSL altitude to determine a vertical path. It can be used for enroute descent, terminal or for final approach. GPS is used for the lateral and Baro-VNAV can be used for the vertical to fly an RNAV approach to a LNAV/VNAV minimum. If Baro-VNAV is used to fly the procedure, it has temperature limits unless the Baro-VNAV is temperature compensated. SBAS can also be used to fly the vertical of an approach to an LNAV/VNAV minimum, at least in the US,.
2) Pilots may fly any of the minimums charted on the same procedure if the Annunciate Accuracy is equal to or better than the minimum requirements. So if LPV is annunciated, the pilot may fly the procedure to the LPV, LNAV/VNAV, LNAV or LNAV circling minimums. If LNAV/VNAV is annunciated, the pilot may fly to the LNAV/VNAV, LNAV or LNAV Circling minimums. If LNAV is annunciated, only the LNAV or LNAV Circling Minimums may be used. In some cases, the DH for LPV exceeds that of LNAV/VNAV and in more cases, the LNAV MDH is below the LPV or LNAV/VNAV, and many more cases, the visibility requirement is lowest for the LNAV. These are due to the fact that the LPV and LNAV/VNAV use different specifications and when there are close in obstacles in the visual segment, the LPV may end up with a higher DH than the LNAV/VNAV. Also in the case of LNAV, step down fixes can allow a lower MDA than the constant GP angle allows
3) Although it is technically possible that a WAAS GPS could fail down from LPV to LNAV/VNAV, this is not supported by the Garmin Navigators, all of which fail down to LNAV without any vertical guidance and never fail down to LNAV/VNAV. Also, if the navigator has the capability to fail down from LPV to LNAV/VNAV, it would be next to impossible for this to occur. The vertical requirements for a 200 DH LPV are a VPL of 35 meters and for 250+ DH it is 50 meters. The VPL for LNAV/VNAV is 50 meters. So there is a very narrow range of VPL which would fail for LPV but work for LNAV/VNAV and only for a DH of less than 250 feet. If the cause of the failure was for HPL, VPL would almost certainly already be out of tolerances for both LPV and LNAV/VNAV.
I can’t reconcile these two statements:
NCYankee wrote:
… if LPV is annunciated, the pilot may fly the procedure to the LPV, LNAV/VNAV, LNAV or LNAV circling minimums.
In some cases, the DH for LPV exceeds that of LNAV/VNAV…
I know that the specifications for LPV and LNAV/VNAV are different, but the specs. for LPV being what they are, how can you fly a LPV to LNAV/VNAV minima if they are lower than the LPV minima?
Airborne_Again wrote:
I know that the specifications for LPV and LNAV/VNAV are different, but the specs. for LPV being what they are, how can you fly a LPV to LNAV/VNAV minima if they are lower than the LPV minima?
A WAAS GPS is authorized to fly to an LNAV/VNAV minima. This is from AC 90-105A, Appendix A:
A.6.4 LNAV/VNAV Line of Minima Qualification.
A.6.4.1 Stand-Alone Systems. Stand-alone TSO-C146 Class 2 or 3 systems meet the aircraft qualification requirements for RNP APCH operations using the LNAV/VNAV line of minima provided that the installations meet at least the performance and functional requirements of this AC.
The vertical path using an SBAS is the same for LPV annunciation or LNAV/VNAV annunciations although in some cases, the integrity and accuracy will be better. The lateral path will be the same but the integrity requirements have to be better with an LPV annunciation than is required for LNA/VNAV. So regardless of the annunciation, the vertical and lateral path center lines are the same, they both have a CDI FSD the same on the final approach leg with the same PFAF, the only difference is the MDH which is defined by the minimum.
Here are a few procedures where the normal order of LPV lowest DH, LNAV/VNAV next, and LNAV MDH highest is not true.
NCYankee wrote:
The vertical path using an SBAS is the same for LPV annunciation or LNAV/VNAV annunciations although in some cases, the integrity and accuracy will be better. The lateral path will be the same but the integrity requirements have to be better with an LPV annunciation than is required for LNA/VNAV. So regardless of the annunciation, the vertical and lateral path center lines are the same, they both have a CDI FSD the same on the final approach leg with the same PFAF, the only difference is the MDH which is defined by the minimum.
I know (+), but it doesn’t answer my question. Apparently, compared to LNAV/VNAV, the LPV procedure design calls for greater obstacle clearance, or for considering obstacles in a larger area, or both. There must be a reason for that and whatever the reason why can you simply ignore that just because there is also a LNAV/VNAV procedure?
(+) Actually, I believe that the vertical path for LPV is straight, while due to the curvature of the Earth for LNAV/VNAV it is slightly curved, but the difference can’t be operationally significant.
Airborne_Again wrote:
Actually, I believe that the vertical path for LPV is straight, while due to the curvature of the Earth for LNAV/VNAV it is slightly curved, but the difference can’t be operationally significant.
That is correct if flown using baro-vnav and the basic TERPS design takes that into consideration. However, when flown with a WAAS navigator providing for the vertical, they are identical paths, a straight line in space, The Baro-VNAV has greater error allowances but needs to be limited in temperature to keep the vertical path within acceptable tolerances. The WAAS vertical does not have a temperature limitation.Airborne_Again wrote:
I know (+), but it doesn’t answer my question. Apparently, compared to LNAV/VNAV, the LPV procedure design calls for greater obstacle clearance, or for considering obstacles in a larger area, or both.
The lateral obstacle protection area for an LPV is smaller than and fully contained within that for an LNAV/VNAV. The vertical obstacle protection path for an LPV is a single slopping surface whereas the LNAV/VNAV uses a sloping surface down to a point and then transitions to a level surface. The method for determining the effect on a DH for a penetration in the visual segment is different, resulting in different DH. The vertical path generated by the WAAS GPS navigator is identical, regardless of annunciation of LPV or LNAV/VNAV or LNAV+V, the differences being the required vertical integrity in some cases, that is tor LPV DH < 250 ft, the VPL has a lower value. So here is a thought experiment, assume an approach has only LNAV/VNAV and LNAV minimums. There should be no argument that the WAAS GPS is certified to fly the LNAV/VNAV line of Minima when the annunciated integrity is L/VNAV. So then assume that an LPV is added to the procedure at a later date and the same position of the FAF, GS angle, and TCH are used. The LNAV/VNAV is unchanged, but the new LPV because of its different rules for determining a DH for low close in obstacles in the visual segment ends up with a higher DH. So the day before the addition of the LPV, it was legal to fly the LNAV/VNAV with the L/VNAV annunciation, but the day that the LPV is published, even though the vertical integrity requirement is unchanged and the later integrity is way over kill for the LNAV/VNAV and the path provided by the navigator is identical before and after, why would an annunciation of LPV not permit the use of the LNAV/VNAV line of minima?
NCYankee wrote:
why would an annunciation of LPV not permit the use of the LNAV/VNAV line of minima?
because
the new LPV because of its different rules for determining a DH for low close in obstacles in the visual segment ends up with a higher DH.
We’re not discussing safety or common sense. We’re discussing procedure design and ultimately regulations.
As you imply, it makes no sense that if you have
the same position of the FAF, GS angle, and TCH
the minima should be different. But they are.
It is simply the byproduct of two sets of TERPS rules and in some rare cases, one set of rules produces a higher DH. The TERPS rules were designed for different types of vertical navigation systems, and after the fact, WAAS vertical guidance was approved for use on the LNAV/VNAV version, Both are safe. There isn’t a single way of determining a safe DH. Any time two separate geometric rules are applied, there are instances where the differences show up. The easier to understand is the LNAV verses one of the vertically guided procedures TERPS. The LNAV allows for step downs, and has a fixed ROC of 250 feet above the controlling obstacle, the LPV has an ROC along along an inclined plane. The vast majority of the time the LPV has a lower DH than the MDH, but some cases it is the other way around.
NCYankee wrote:
It is simply the byproduct of two sets of TERPS rules
PANS-OPS is the relevant document for approaches in Europe… And my impression is that it is frequently the case that LPV minima are higher than LNAV/VNAV minima.
I am not familiar with PANS-OPS approach design rules or whether other countries permit WAAS GPS to fly LNAV/VNAV line of minima. In the US, with TERPS, there isn’t a restriction, so my comments only apply to US. In the US it happens that the LPV has a higher DH than a LNAV/VNAV, but it is relatively rare.
