Thursday, August 14, 2014

RNAV (GPS) approaches - what happened to LNAV+V?

(8/18/14 update - looks like I was incorrect regarding the Garmin 430W/530W and LP+V! As of software revision 5.1, released in April 2014, these WAAS receivers now support LP+V. However, it looks like the Garmin 650/750 do not, at least as of software revision 5.0 which is the most current for them. Hopefully soon!)

Like my last article, this one comes from a question posed at a seminar at Oshkosh, where I was in the audience but was fortunately able to help answer the question.

The question pertained to RNAV (GPS) approaches with an "LP" line of minima, and was, in essence, "Why don't we get an advisory glideslope when conducting an LP approach, like we do when flying an LNAV approach?"

A little background (but brief, I don't want to get too much into types of RNAV minimums in this article).

The most basic line of RNAV (GPS) minimums is the "LNAV" line, meaning "Lateral Navigation". There is no vertical guidance, it's like a VOR approach, and can be flown without WAAS. GPS manufacturers thought that, if you had WAAS, it would be helpful to include an "advisory" glideslope to help make a nice, stabilized descent instead of the "dive and drive" that non-precision approaches typically resulted in. This was termed "LNAV+V" to show that an advisory glideslope was available and could be used for situational awareness.

However, this "advisory" glideslope was NOT evaluated by the FAA, and the pilot had to make sure to still comply with all pertinent altitude restrictions, to include leveling off at the MDA and going missed as appropriate. There was nothing depicted on the approach chart, because this capability was provided by the manufacturer, not the FAA.

Compare this to the LNAV/VNAV line of minimums. LNAV/VNAV minimums and glideslope ARE evaluated by the FAA for obstacle clearance and all the other factors, and they are flown to a DA like an ILS, meaning you don't have to level off at the DA, you just need to start your missed approach at that point.

When LNAV/VNAV minimums were published on the same chart as LNAV minimums, though, it could get a little confusing. For example, a Garmin 430W would annunciate the two, respectively, as "L/VNAV" and "LNAV+V". This is a little too easy to confuse if you don't look at it closely. Kind of reminds me of a cartoon of someone trying to fool the police with a license plate that is something like "8BB8B8" or "I1II1I1"!

Garmin 430W LNAV+V annunciation
This has actually been the source of some confusion throughout the pilot community. I've seen the questions - "My GPS says LNAV+V, does that mean I fly to the LNAV/VNAV minimums?" (no) or "If I'm flying LNAV+V, do I need to comply with the stepdown fix altitudes in final?" (yes).

Although more training could resolve this confusion, there is also a "human factors" issue - the two terms DO look alike and ARE confusing. I wish the manufacturers had used some other term to indicate the presence of an advisory glideslope, but that's the way it is.

Undoubtedly due to this confusion, when LPV and later, LP approaches started getting published the FAA originally disallowed manufacturers from providing an advisory glideslope with LP approaches. Can't say I blame them too much - I can imagine some confusion if the GPS started annunciating "LP+V" on the approach shown below. Is that a DA or MDA? Wait, there's no LPV. Do I have the right chart? What about that stepdown fix?

(Incidentally, that's the RNAV (GPS) RWY 22 at "Sporty's", I69.)

Since the FAA initially disallowed advisory glideslopes on LP approaches, manufacturers did not program them into their GPS receivers. However, in 2011, the FAA published AC 90-107 which changed that rule and allowed manufacturers to include an advisory glideslope with LP approaches (paragraph 6e(2)).

However, to enable this functionality, the manufacturers needed to develop and certify new software. I know that Garmin, for one, has not yet added it to either their new GTN 650/750 or as a software update to the GNS 430W/530W series.

This has created an unfortunate unintended consequence. If an approach has only LNAV minimums, the GPS will show an advisory glideslope as LNAV+V. But if it has LP and LNAV lines of minima, the GPS will by default annunciate "LP" and NOT provide the advisory glideslope. Since most IFR GPSes in use won't allow you to go in and select a "lower" line of minima to use, the advisory glideslope is for most practical purposes unavailable. In some ways, this means the "old" approaches are better than the "new" ones.

Take the above example at I69 - prior to the latest amendment, it had only LNAV minimums, and therefore had an LNAV+V advisory glideslope. Now, with both LP and LNAV minimums, it effectively doesn't!

Hopefully the manufacturers will be able to add this functionality at some point. Until then, just remember that there is no advisory glideslope on LP approaches. Be careful on those descents!

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