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#21
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LPV vs LNAV/VNAV?
Stan Prevost wrote:
"Sam Spade" wrote in message ... Sam Spade wrote: If I read it correctly (assuming WAAS is available and passes muster) you will always get LNAV+V on an LNAV-only IAP, thus you will have vertical guidance to MDA. Can an RNAV approach have stepdown fixes on the final approach segment? Yes. When VNAV first came in they could only be an an LNAV-only IAP. Then, quite a few years ago that was changed to permit them for LNAV minimums on an LNAV/VNAV IAP. |
#22
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LPV vs LNAV/VNAV?
On 6 Jan 2007 21:33:52 -0800, "Andrew Sarangan"
wrote: Peter R. wrote: Andrew Sarangan wrote: Something I never understood is why LNAV approaches don't automatically show a glideslope so that the airplane arrives at the MDA at the VDP. Perhaps because it wasn't part of the certification at the time? Perhaps my comment was not clearly stated. When you fly an LNAV approach (or any nonprecision approach for that matter) instead of the traditional dive and drive you can mentally calculate the vertical speed required (VSR) to arrive at the VDP at a constant glide angle . That mental calculation could be easily performed by the GPS and displayed as a glideslope. But I have not seen any GPS do that. Certification is irrelevant. We are not talking about a lower minimum or anything new that we not already allowed to do. The CNX80/GNS480 with current SW will do that for many (not all) LNAV approaches. It is called advisory vertical guidance and available when published. I believe the presence of a VDP is one factor that usually triggers this availability. Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA) |
#23
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LPV vs LNAV/VNAV?
Peter R. wrote:
Andrew Sarangan wrote: That mental calculation could be easily performed by the GPS and displayed as a glideslope. But I have not seen any GPS do that. Certification is irrelevant. Certification is most certainly relevant to your query, as that is most likely what kept that feature out of the TSO C129a certified IFR GPS's. A handheld Garmin 196 (I think that is the model a pilot-friend had with him a couple of years ago) that we took up on a practice IFR flight did just that. It displayed a glideslope for a non-precision approach. If the cheaper handhelds can do it, then why don't their IFR-certified TSO C129a big brothers do it? But handhelds aren't certified, so there's no guarantee of correctness. The basic requirement they don't meet is the Integrity requirement, e.g. the guarantee a) that the error can be bounded and b) that sufficient warning can be provided when Integrity can not be met. 129 boxes aren't certified for Vertical Guidance, so I suspect that, even for an NPA (LNAV) approach the same would hold true. Because it wasn't part of the certification and therefore, regardless of their ability to provide this feature, are restricted from doing so due to the certification. The lack of certification is based on Standalone (Unaugmented) GPS not being certified for Vertical guidance. This traces to the fact that the dominant error (after SA was turned off) is the Ionospheric component and the recevier's model (Klobuchar) is not certified to provide sufficient Integrity for the Vertical component. With SBAS (e.g. WAAS in the US), the Integrity requirement has been proven to be met with sufficient Availability over the Service Volume, to approve approaches with Vertical Guidance,. Note that when even when the 145/6 boxes were deployed up in Alaska (Capstone project), WAAS had yet to be commissioned, and thus the published approaches were LNAV only. Also note, there had been talk of building newer 129 boxes, but with the 145/6 boxes now out, the manufs. apparently can't cost justify upgrading a box that still wouldn't perform as well as the 145/6 -- Peter Regards, Jon |
#24
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LPV vs LNAV/VNAV?
Andrew Sarangan writes:
I am sure you are correct, but it doesn't seem to make a whole lot of sense too keep a useful feature out of certification unless there is something dangerous about it. I don't see anything unsafe about providing a glideslope to a nonprecision approach. The cost of certification is probably an important factor. -- Transpose mxsmanic and gmail to reach me by e-mail. |
#25
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LPV vs LNAV/VNAV?
"Sam Spade" wrote in message ... Stan Prevost wrote: Can an RNAV approach have stepdown fixes on the final approach segment? Yes. When VNAV first came in they could only be an an LNAV-only IAP. Then, quite a few years ago that was changed to permit them for LNAV minimums on an LNAV/VNAV IAP. An "advisory" guide slope could only be applied to a final approach segment without stepdown fixes, or one in which the glide slope clears the obstacle(s) by the required margin. I doubt that all LNAV-only approaches have been TERPSed for that, especially those without VDPs. |
#26
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LPV vs LNAV/VNAV?
Stan Prevost wrote:
"Sam Spade" wrote in message ... Stan Prevost wrote: Can an RNAV approach have stepdown fixes on the final approach segment? Yes. When VNAV first came in they could only be an an LNAV-only IAP. Then, quite a few years ago that was changed to permit them for LNAV minimums on an LNAV/VNAV IAP. An "advisory" guide slope could only be applied to a final approach segment without stepdown fixes, or one in which the glide slope clears the obstacle(s) by the required margin. I doubt that all LNAV-only approaches have been TERPSed for that, especially those without VDPs. They aren't TERPsed, rather Jeppesen applies an angle that assures clearance of the stepdown fixes. Example: http://tinyurl.com/y68phr |
#27
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LPV vs LNAV/VNAV?
"Sam Spade" wrote in message ... An "advisory" guide slope could only be applied to a final approach segment without stepdown fixes, or one in which the glide slope clears the obstacle(s) by the required margin. I doubt that all LNAV-only approaches have been TERPSed for that, especially those without VDPs. They aren't TERPsed, rather Jeppesen applies an angle that assures clearance of the stepdown fixes. Example: http://tinyurl.com/y68phr A very nice example it is, when comparing to the NACO chart http://tinyurl.com/yfygkv . NACO computes the descent from the FAWP to the stepdown fix. Jeppesen computes it from the stepdown fix to the TCH at the runway. I wonder how the GPS units present the glide slope: 3.67 deg from the FAWP or 3.61 deg from 0.1 nm after the FAWP. There is only about 30 feet difference at the stepdown fix between starting the two descent angles at the FAWP. |
#28
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LPV vs LNAV/VNAV?
Stan Prevost wrote:
"Sam Spade" wrote in message ... An "advisory" guide slope could only be applied to a final approach segment without stepdown fixes, or one in which the glide slope clears the obstacle(s) by the required margin. I doubt that all LNAV-only approaches have been TERPSed for that, especially those without VDPs. They aren't TERPsed, rather Jeppesen applies an angle that assures clearance of the stepdown fixes. Example: http://tinyurl.com/y68phr A very nice example it is, when comparing to the NACO chart http://tinyurl.com/yfygkv . NACO computes the descent from the FAWP to the stepdown fix. Jeppesen computes it from the stepdown fix to the TCH at the runway. I wonder how the GPS units present the glide slope: 3.67 deg from the FAWP or 3.61 deg from 0.1 nm after the FAWP. There is only about 30 feet difference at the stepdown fix between starting the two descent angles at the FAWP. NACO is not presenting a VNAV path, rather the descent angle from the FAF to TDZ elevation. |
#29
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LPV vs LNAV/VNAV?
"Sam Spade" wrote in message ... Stan Prevost wrote: "Sam Spade" wrote in message ... Example: http://tinyurl.com/y68phr A very nice example it is, when comparing to the NACO chart http://tinyurl.com/yfygkv . NACO computes the descent from the FAWP to the stepdown fix. Jeppesen computes it from the stepdown fix to the TCH at the runway. I wonder how the GPS units present the glide slope: 3.67 deg from the FAWP or 3.61 deg from 0.1 nm after the FAWP. There is only about 30 feet difference at the stepdown fix between starting the two descent angles at the FAWP. NACO is not presenting a VNAV path, rather the descent angle from the FAF to TDZ elevation. Maybe so, but the angle seems to compute out closer to the descent angle from FAWP to stepdown, limited by the precision in the numbers on the chart. The question remains, though, where does the glideslope begin in the GPS unit: from the FAWP or from the point 0.1 nm after that waypoint? |
#30
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LPV vs LNAV/VNAV?
Stan Prevost wrote:
Maybe so, but the angle seems to compute out closer to the descent angle from FAWP to stepdown, limited by the precision in the numbers on the chart. The question remains, though, where does the glideslope begin in the GPS unit: from the FAWP or from the point 0.1 nm after that waypoint? \ Beats me. Have you tried to find out in the Garmin Handbook and couldn't find it? Having said that, the advisory vertical path is provided to Garmin, and other avionics vendors, by Jeppesen. So, assuming no errors on the part of Jeppesen, it doesn't matter whether you are using their chart or NACO's. If my butt were on the line I would be keeping an eye on that stepdown fix. I guess I would have more confidence with L/VNAV. I would, however, only have complete confidence in LPV. |
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