RNAV approach question And my buddy does a good

AggieMike88

AggieMike88

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The original "I don't know it all" of aviation.
this evening, I had the pleasure of being safety pilot for a buddy who is working in his IFR rating.

Part of the plan tonight included shooting the RNAV 35 at Bowie Municipal, Bowie, TX (http://155.178.201.160/d-tpp/1703/05567R35.PDF) starting at JIRSO

The minima shows LNAV minimums, which to me would be a "dive and drive" style of stepping down. But as we got near the IAF, our magic CNX80/GNS480 shows us the "LNAV+V" annunciation and provided us the artificial glideslope on the CDI once we were inside FENUP. And he does a good job keeping both the lateral and vertical needles bullseyed to the MDA to which he powered up and flew the missed.

Question is about the proper practical wisdom on an approach that shows only LNAV mins but we're provided the "+V" guidance from the GPS. Is "dive and drive" the proper way to execute? Or take advantage of the artificial glideslope and come on in with a steady descent?

(By the way, I don't think he busted any rules or was unsafe. The situation just creates the opportunity to ask a "what would you do?" of our more experienced membership d expand my personal knowledge.
 
What Jordane said. Any time I can get a stabilized approach, I will. There really isn't much stable about dive and drive.
 
There is a picture somewhere in the IFR study material where the FAA advocates the dive. Basically if there's a chance of dropping below the clouds on the dive and acquiring the airport, there's a greater chance for a successful landing. That being said, I prefer to follow the advisory glideslope down. Just make sure to be aware of your position and MDA.


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In almost 40 years of flying I have only missed one approach for real. I will take a stabilized glide path any day over dive and drive if I have the option.
 
Definitely use the GS, as it's far safer IMO. Just remember that, unlike DH/DA, regardless of the +V, MDA assumes you won't bust it, so when you get close to it, count down the feet above and be ready to pull the trigger and do the missed, if needed.
 
AggieMike88 said:
And he does a good job keeping both the lateral and vertical needles bullseyed to the MDA to which he powered up and flew the missed.
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Just keep in mind that an LNAV approach, even with advisory glide path, is still an MDA...unlike a DA, you aren't allowed to penetrate that altitude while initiating the missed approach.

Your friend obviously kept the needles bullseyed to JUST ABOVE MDA. ;)

Oops...RotorDude beat me to it, by a fair margin. :rolleyes:
 
drjcustis said:
There is a picture somewhere in the IFR study material where the FAA advocates the dive. Basically if there's a chance of dropping below the clouds on the dive and acquiring the airport, there's a greater chance for a successful landing. That being said, I prefer to follow the advisory glideslope down. Just make sure to be aware of your position and MDA.


Sent from my iPad using Tapatalk
Click to expand...
Good point. I think it's sometimes yes and sometimes no.

There have always been problems with the lack of stability in dive and drive. But it was a standard recommendation on non-precision approaches because there was often no good way to calculate a stable glidepath that would get you to a VDP - a point that was both at the MDA and in a position to make a normal approach to landing. You needed to get down to MDA early if you had any chance if landing when the weather was low. I think the goal of the advisory glideslope is do both jobs. Stable descent that gets you to the MDA before the MAP and in position to land.

That said, there are downsides, the primary one being, following the advisory glideslope could conceivably put you in a position where you bust either the MDA (as already mentioned) or step down minimums, so it's not quite the same as flying an LPV or ILS.
 
drjcustis said:
There is a picture somewhere in the IFR study material where the FAA advocates the dive.
Click to expand...
THis comment produced an unexpected good thing.

I went into Chapter 4 of the IPH looking for that picture. Scanning that chapter brought to my attention 1) that there are many topics there that I should have family with that I really don't (many not covered very deep during training) and 2) how much information leakage I have experienced since my checkride on the topics that were well covered.

So looks like I'll have some good reading material to cover over next few weeks during lunch and evenings. :thumbsup: :thumbsup:
 
Every few years, the FAA flips its stance on almost any issue, including "stabilized descent to MDA at VDP" vs "dive-n-drive". I went to a safety seminar recently where they preached not to use the latter.
And FWIW, when I descended to the MDA on my last approach, I reached MDA before the VDP and got a warning from the tower "low altitude alert, check altitude". I told them "I'm at MDA" and got a "roger" but their computer tells them that I descended "too fast" because I got to MDA before the VDP. (I climbed to the tower afterwards to ask for the reason for the low altitude alert)

So do whatever is safe and whatever keeps you safe. Just don't bust the MDA, as others pointed out. The false security of the artificial ADVISORY glideslope can be deceiving. But you're not flying an ILS with guaranteed obstacle clearance on the GS.

There was actually a crash where a pilot flew into a hill because his GS told him to. Well, he descended below MDA in IMC. (somebody feel free to look up the NTSB report, I am too lazy)
 
AggieMike88 said:
THis comment produced an unexpected good thing.

I went into Chapter 4 of the IPH looking for that picture. Scanning that chapter brought to my attention 1) that there are many topics there that I should have family with that I really don't (many not covered very deep during training) and 2) how much information leakage I have experienced since my checkride on the topics that were well covered.

So looks like I'll have some good reading material to cover over next few weeks during lunch and evenings. :thumbsup: :thumbsup:
Click to expand...
Yeah. What's the old saying? The more you learn the more you realize you don't know.
 
Following the +V works well when the visibility and ceilings are high and I would use it under those conditions. It is not so good when they are low. A typical NPA with a MDH of 600 feet will put the aircraft over 2 SM from the runway, and more if you add 50 feet to avoid busting the MDA, yet the minimums are typically 1 mile. An instant in time at a DA(DH) works well with approach lights to make up the for low visibility. It does not work well if the runway is not well lit, marked, and visibility is low and it is much harder to locate a runway and sort it out from the WalMart parking lot. It is not a one size fits all situation. When I fly an NPA when the weather is close to minimums, what I want is the maximum amount of time to locate the runway. I want to get down to the MDA, stabilized in level flight and start searching for that runway and once I find it, I want to convince myself it is actually a runway. Using the +V simply does not give the time needed under these circumstances. So it depends.
 
AggieMike88 said:
this evening, I had the pleasure of being safety pilot for a buddy who is working in his IFR rating.

Part of the plan tonight included shooting the RNAV 35 at Bowie Municipal, Bowie, TX (http://155.178.201.160/d-tpp/1703/05567R35.PDF) starting at JIRSO

The minima shows LNAV minimums, which to me would be a "dive and drive" style of stepping down. But as we got near the IAF, our magic CNX80/GNS480 shows us the "LNAV+V" annunciation and provided us the artificial glideslope on the CDI once we were inside FENUP. And he does a good job keeping both the lateral and vertical needles bullseyed to the MDA to which he powered up and flew the missed.

Question is about the proper practical wisdom on an approach that shows only LNAV mins but we're provided the "+V" guidance from the GPS. Is "dive and drive" the proper way to execute? Or take advantage of the artificial glideslope and come on in with a steady descent?

(By the way, I don't think he busted any rules or was unsafe. The situation just creates the opportunity to ask a "what would you do?" of our more experienced membership d expand my personal knowledge.
Click to expand...

I always determine a VDP for those D&D approaches based on what the glide slope would be to arrive at that VDP point. That gives me a rate of descent "glide slope" so that I arrive at the VDP in a descent as if on a glide slope never getting below the MDA in the process of course. If I dont see the runway at my personal VDP then my D&D time is reduced to very little because I will be close to the MAP.

I use descent rate tables to determine my own VDP is one is not shown.

tex
 
In the situation where you have the "advisory" glideslope, it's fine to follow that. The WAAS equipment provides this as a great perk. The glideslope basically carries you through the step-downs. However, unlike an LPV (or ILS), the actual descent angle on the LNAV+V may not necessarily be constant.
 
SbestCFII said:
In the situation where you have the "advisory" glideslope, it's fine to follow that. The WAAS equipment provides this as a great perk. The glideslope basically carries you through the step-downs. However, unlike an LPV (or ILS), the actual descent angle on the LNAV+V may not necessarily be constant.
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If, by that, you mean the LNAV+V vertical path (aka:VDA) can change on a given approach in the final approach segment, that is not correct.
 
Dave S. said:
If I dont see the runway at my personal VDP then my D&D time is reduced to very little because I will be close to the MAP.
Click to expand...

You can easily be over 2 NM from the runway at the VDP, for example if the MDH is 600 AGL. Most of these approaches would have a visibility minimum of 1 SM or about 0.85 NM and the MAP is usually at the threshold. If the visibility is lower than 3 SM I am doing a DND, and I want to be down, level, and stabilized at the MDA before reaching the VDA so I can maximize the time to locate the runway. Time is my friend as it takes time to sort out a runway without approach lights.
 
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