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!

Wednesday, August 6, 2014

0/0 takeoffs for Part 91?

Was at EAA AirVenture (Oshkosh) this past week, and at one of the instrument seminars one of the attendees asked a question I thought would be interesting to others as well. It was involved, but the part I'm going to discuss boiled down to the following (paraphrased):

I am based at an airport with some high terrain nearby that drives up the MDA for the approaches to about 900 AGL. However, I know that I can depart the airport with 0/0 ceiling and visibility. Why is that?

The presenter wasn't able to answer the question (I think due mostly to not understanding what it was), but I was able to help.

First, some background.

As discussed in other blog posts of mine, the MDA (or other minimums) are often determined by nearby terrain. If you have high terrain within a few miles of the airport, on final, you will often have high MDAs for the approach.

This makes sense, and of course 14 CFR 91.175 tells us that if you need to make an instrument approach, you cannot go below the DA or MDA unless certain parts of the runway environment are in sight, as every instrument student learns (I hope).

However, for departure, instrument students are taught that you can legally take off in 0/0 conditions (zero visibility and ceiling at zero feet) and what's more, that's true!

91.175(f) states the standard minimums required for takeoff, such as "aircraft having two engines or less - 1 statute mile visibility", and also requires that aircraft comply with an obstacle clearance procedure.  HOWEVER, this subparagraph only applies to "persons operating an aircraft under part 121, 125, 129, or 135". In other words, NOT part 91 operators, which applies to many of us (including the person asking the question).

Also, from the Instrument Procedures Handbook, page 1-8:

Aircraft operating under 14 CFR Part 91 are not required to comply with established takeoff minimums. Legally, a zero/zero departure may be made, but it is never advisable.

So as a part 91 operator, I have to comply with high minimum altitudes on an approach due to high terrain, but yet I can depart the same airport with zero visibility? Why is that?

The simplest answer is that the takeoff minimums for part 91 are not determined by obstacles or even evaluated in any way. 0/0 is the rule only because it's not prohibited by 91.175. You can make up your own departure minimums if you will.

An examiner once told me to consider three things when thinking about doing something in an airplane - Is it legal? It is safe? Is it smart?

Taking off in 0/0 weather may be legal. It might even be debatably safe-ish, depending on aircraft performance and terrain. It is smart? I don't think so. Not many "outs" in the event of any kind of problem.

What should you do as a Part 91 operator? In my opinion, always comply with at least the established takeoff minimums for the runway.

Looking at a specific example, South County Airport in San Martin, California (E16) has the following departure minimums:

And then has an admittedly lengthy definition of the departure procedure to follow. But the standard takeoff minimums (the ones in 91.175 that do not necessarily apply to Part 91) can only be used if you can maintain a climb gradient of 324 feet per nautical mile (not per minute) to a certain altitude, depending on which runway you depart. Or if you can't meet that, it allows a departure in visual conditions (called a Visual Climb Over Airport, VCOA) with at least a ceiling of 1700 and a visibility of 2.5 miles. This is to get you high enough that once you enter the clouds you can keep climbing and have a good safe cushion over the nearby terrain. If you can't meet that, maybe you should wait a while until the weather improves.

But VCOAs are a good topic for another blog. Fly safe!