A word of caution about LNAV+V advisories

[bradp]

I’ve been flying with a WAAS navigator for a few months and one of the perceived benefits is getting a virtual glide path even for VFR/VMC at night.  

Here’s an example that shows the limitations of virtual glide paths.  The example Airport is IGX - it’s about a stones throw from my house but because the owner (UNC) seems to be wanting to close it down most of us locals are forced south to another airport.  It does make for a nice pickup and drop off (fly up the night before a trip, load up a mile from my house, and keep the family happy by avoiding a 45 minute drive to the airport).  

As part of the effort to choke out the Airport, the university hasn’t trimmed the tees on final in many many years.  I’ve flown there in the daytime many times and know this.  There is a displaced threshold.  The visual GSI (VASI) is 4-degrees. The GPS approach to 27 is NA at night and is not a WAAS Approach (probably because of the darned trees making terps issues).  

Knowing all this I still like to load approaches at night in VMC for safety / redundancy but the lesson here is that there are limitations to this technology. I had an LNAV+V advisory glide path pop up on final.  Cool I say.  It’s a 3 degree path however. Hold on here I say the VASI is a 4 degree.   So I fly above it using the VASI like I should and all is well. Regardless those trees always make me a bit nervous.  

So so today during the day I went to see what would happen if I was tooling along VFR and didn’t essentially brief the Instrument plate nor know that the VASI is 4 degrees and not coincident with the descent angle. So I fly the approach down following the virtual glide path instead of the visual.  Two reds on the VASI all the way down and would have cleared the trees by about 20 feet if I followed it all the way down . 

Lesson’s here - all this situational awareness technology can  sometimes make things unnecessarily more complex.  Second the LNAV+V is only good till the MDA on a non-LPV GPS approach.  Third is that an approach needs to be briefed if I’m pulling it up on the GPS as if I was IFR even if just plugging it in for added SA when VFR.   Fourth before flying into any unfamiliar airport at night either know exactly what the environment is like or a better idea is to have flown into it during daylight.   

[jlunseth]

We have a program in Minnesota where you fly to 135 of the 138 airports and get a leather jacket.  I used to go out and do a bunch in a single day.  On one trip I was coming home past a small rural airport at night, in the dark, and decided to get it off my list of airports to do.  Strip was unusually narrow so it was challenging in night VFR.  Coming down on short final I happened to look out the left window, couldn’t believe what I saw, so looked right and same thing.  I was in a tree line and the tops were higher than me on both sides.  Thankfully, they had trimmed a path to the runway, but I will no longer land at unfamiliar small fields at night without a really really good reason, and I always keep the glideslope high.

[jaylw314]

2 hours ago, bradp said:

Second the LNAV+V is only good till the MDA on a non-LPV GPS approach.

Do WAAS navigators (I have a 530W) always use a 3 deg slopoe on LNAV+V, or or they adjusted for each approach so you don't bust any minimums on a stepdown approach?  Or does it vary by navigator?

[bradp]

Here we go: this is exactly the trip up that LNAV+V can give you:

 

http://www.planeandpilotmag.com/article/tripped-up-by-stepdowns/#.Wjf2gqJOmaN

[jaylw314]

Thanks, that's what I thought.  In retrospect, the 530W's do not have altitude information on approach waypoints, so there's no way they could protect you from busting altitudes.  I was thinking specifically of approaches with a stepdown fix inside the FAF.

[kortopates]

4 hours ago, jaylw314 said:

Thanks, that's what I thought.  In retrospect, the 530W's do not have altitude information on approach waypoints, so there's no way they could protect you from busting altitudes.  I was thinking specifically of approaches with a stepdown fix inside the FAF.

Not really true as long as you are referring to step down fixed inside of the FAF to MDA. Those step down fixes will be right at the computed descent angle as plotted on the chart and you'll be able to follow the +V glide slope angle to MDA. But you should not be following the glide slope through step down fixes outside of the FAF because those could have mandatory altitudes or maximum altitudes that are below the glide slope. You're not going to have a problem following +V crossing intermediate step down fixes past FAF before MDA. 

The biggest mistake is following any non-precision approach glide slope during the visual portion of the approach beyond MDA or DH without the required visual cues of 91.175 - this includes LNAV/VNAV, not just +V. Specifically anytime there is not a Visual descent point (VDP) plotted on the profile view, you should be alert for obstacles that penetrate the 20:1 slope between the TCH and the MDA or after descending below MDA. Only if a VDP is plotted are you relatively safe of descending without finding an obstacle penetrating the slope in the visual segment. But this intended to help you fly a stabilized approach to landing - not bust minimums.

The key point is to level off at MDA and not descend before we have the cues required by 91.175.

It wasn't that long ago, when Garmin provided +V on every non-precision approach with a computed descent angle and then the FAA started zero'ing the computed descent angles out when the FAA discovered pilots where not paying attention and trying to follow them to the runway despite  a note that read "VGSI and descent angles not coincident". That note is to warn us that the Visual glide slope/VASI etc angle is not the same as the approach descent angle and therefore the need to level till we pick up the VASI before descending to the threshold because of obstacles. So for quite some time we lost +V on many approaches  until Garmin made some updates. The FAA seems to be relaxing their constraints again and is allowing computed glide slopes on approaches that have the coincident issue but lets not forget that these end at MDA.

Edited December 18, 2017 by kortopates

[kortopates]

5 hours ago, jaylw314 said:

Do WAAS navigators (I have a 530W) always use a 3 deg slopoe on LNAV+V, or or they adjusted for each approach so you don't bust any minimums on a stepdown approach?  Or does it vary by navigator?

No it varies for every approach and is depicted in the profile view. Unlike the government charts though there may be a calculated descent angle even though its not depicted on the gov chart. But it is always depicted on Jepp charts if the TERPs guys provided one. Consequently, you never know for sure with government charts if you'll get +V on a non-procession approach if one isn't shown, you might fly it and find it does. But with Jepp you always know by seeing if one is depicted or not.

 

Edited December 18, 2017 by kortopates