Airmarty2
Joined: 29 Nov 2012
Posts: 9
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 Posted: Wed Aug 21, 2013 5:53 pm Post subject: Fwd: Advisory Glideslope on LP approaches ISSUE=138781 PROJ |
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Hey Bill, its been a few years since working in support of the FAA's WAAS program office so my info might be dated.... That said, there is a reason why some RNAV / Gps approaches didn't quaily for LPV minima. Usual its close in obstacles...basically the same as an ILS, and why people are disappointed that it didn't solve their localizor only approach limitation. Second could be course alignment , enough for staight in but still a close in turn, and or other reasons. The FAA first priority was to build LPVs whereever possible.
LP was the alternative, with usually better minima than a LNAV (height-wise) because the obstacle protection zone is smaller. But LPs also took a lot of extra programing for Garmin who didn't see the business case while there weren't many LPs out there, and thousands of LNAV's out there. There is also a programming issue because the approach is really now a data file, and so either it has vertical (LPV) or not (LP). Its a data block in the file that's either on or off. In other words an LP + VNAv would be an LPV and the FAA would have given you an LPV if there wasn't a reason they couldn't.
Ref minima...
Visibility minima is partly based on the distance needed to see the runway from the MAP. By design, the LNAV MAP is the runway end, while the LPV, and LnAV/VNAV decision height is usually at the beginning of the approach lights, based on approach angle/height and other Terps magic.
So while the LNAV visability is nearly always lower, the aircraft usually is not in a position to land if its at MAP when seeing the runway. (Similar to other non -precision approaches.) Unlike a Vertically guided approach which will have an airplane (on course/glideslope) in position to land if seeing the runway at decision height, further, but not as high from the runway.
If not confused yet, I can keep typing....
Marty Heller, RV-7 120 hours, KSUT
From: bjudge(at)gmail.com
Date: Wed, 21 Aug 2013 13:35:26 -0700
Subject: Fwd: Advisory Glideslope on LP approaches ISSUE=138781 PROJ=1
To: SoCAL-RVlist(at)yahoogroups.com; rv-list(at)matronics.com
So I noticed a while back that "advisory glide slope" disappeared from GPS 17 at KSEE, my home field.
I thought something broke... but after flying recently with an instructor during an IPC I found out that the new LP approaches(that sometimes have lower minima) do not support advisory glideslope, at least the Garmin 430 LP implementation doesn't support advisory glideslope.
Background: For the guys that upgraded to WAAS, it was a pleasant surprise that all the non precision approaches suddenly had glideslope added, this allowed you to easily set up a stabilized approach much like an ILS on any GPS approach as long as you kept in mind that the baro altimeter and horizontal position was the actual guidance for where you belong vertically. You could still "chop and drop" but the 430 would connect the dots and give you "advisory vertical guidance" allowing for the hallowed stabilized approach.
LP came along and the FAA decided that this advisory glideslope business just has to go away. So in 2009 when Garmin implemented the LP approaches in the 430 they deleted the glideslope.
Then in 2011 the FAA recognized their mistake and published AC 90-107 and now allows advisory glideslope.
So I thought I'd see what Garmin's position was on this. The answer is below. I think basically they are saying 430 owners shouldn't hold their breath
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Marty
RV-7 (Flying)
Southport, NC (KSUT) |
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