Question about RNAV descent profile

[Jaybird180]

The subject for discussion requires the reader to view the approach plate for VKX RNAV(GPS)6, which can be found here.

In the profile view, the initial altitude at WOBUB is 1700. Leg from WOBUB - CRROL is 6.2nm with an altitude loss of only 200feet. The runway is 4.2nm from that point with a LNAV MDA of 562, which means the descent rate must be increased to arrive over the threshold in a position to make a normal landing.

How come the profile view descent profile isn't drawn like a non-precision approach (chop and drop)?

The note says the VGSI angle is 5.00° @19ft where as the approach angle is 3.04° @118.

What am I missing or misunderstanding?

 

[flyingron]

How come the profile view descent profile isn't drawn like a non-precision approach (chop and drop)?

The FAA doesn't believe in "chop and drop". They believe (as it may) in stabilized approach. The government always shows descents as continuous straight lines wither you're talking about an approach with vertical guideance (like ILS, LPV, LNAV/VNAV) or one without. Of course it's silly in the case where it's not a constant angle like this. Just a reason why the FAA charts suck. It's Jepp that shows the dive and drive profiles for approaches without vertical guideance. When my ipad charges up enough to come to life, I'll post the Jepp version of this approach.

The note says the VGSI angle is 5.00° @19ft where as the approach angle is 3.04° @118.

What am I missing or misunderstanding?

The VGSI is for the panel VASI that serves that runway. It's set at a steeper angle which is really more appropriate for that runway especially since downwind takes you over higher terrain. The 3.04 glideslope is the constant descent from the FAF to the impact on the runway.

 

[DGlaeser]

Not missing anything. This is why you brief an approach!
Always check nonprecision descents to estimate the descent rate required.

 

[flyingron]

Here's the Jepp depiction

 

[Jaybird180]

Not missing anything. This is why you brief an approach!
Always check nonprecision descents to estimate the descent rate required.

Are only WAAS approaches precision approaches?

 

[Everskyward]

Here's the Jepp depiction

Which is one of the reasons I like Jepp better. Also probably because that is what I have used all my life. The only time I see NACO charts seems to be on this board.

 

[Jaybird180]

FlyingRon, thanks for posting the Jepp chart, that helps understanding a bit. Are those descent rates on the chart where it lists angles (corresponding to groundspeeds)?

 

[Jaybird180]

The FAA doesn't believe in "chop and drop". They believe (as it may) in stabilized approach. The government always shows descents as continuous straight lines wither you're talking about an approach with vertical guideance (like ILS, LPV, LNAV/VNAV) or one without. Of course it's silly in the case where it's not a constant angle like this. Just a reason why the FAA charts suck. It's Jepp that shows the dive and drive profiles for approaches without vertical guideance. When my ipad charges up enough to come to life, I'll post the Jepp version of this approach.


The VGSI is for the panel VASI that serves that runway. It's set at a steeper angle which is really more appropriate for that runway especially since downwind takes you over higher terrain. The 3.04 glideslope is the constant descent from the FAF to the impact on the runway.

What app are you using?

 

[flyingron]

Are only WAAS approaches precision approaches?

Eh? Technically, no RNAV approach is a precision approach (due to more goofy FAA ICAO boondoggling, a precision approach is no longer defined as one with vertical guidance).

Only LNAV/VNAV and LPV have vertical guidance (i.e., the WAAS enabled approaches). Some navigation units may compute an advisory glideslope in other cases, but that's beyond anything the FAA deals with.

 

[flyingron]

What app are you using?

Jepp FD.

 

[flyingron]

FlyingRon, thanks for posting the Jepp chart, that helps understanding a bit. Are those descent rates on the chart where it lists angles (corresponding to groundspeeds)?

Yeah, the little table where you'd normally see time to the MAP on approaches where it's determined off some fix, is indeed the fpm for the listed groundspeeds to make the 3.04 degree glideslope.

Note that the FAA publishes a general climb/descent table for various groundspeeds in the front of the plate book, but as more people get away from the paper charts, they tend to forget how to find/compute it.

 

[coloradobluesky]

Feet per minute.

To do it in your head. The feet is the height above the runway of the Final Approach Fix. Say it is 2000'. The minutes is the "distance" from the runway in nautical miles. This is true at 60 nm per hour, which is a nautical mile a minute. Say it is 5 nautical miles. Take 2000 and divide by 5 and you get 400 feet per minute. That is at 60. At 90 it is half again as much which is 600 feet per minute. at 120 is twice as much, 800 feet per minute. ETC...

If you have a tailwind, you need to come down at more fpm.

 

[Jaybird180]

Eh? Technically, no RNAV approach is a precision approach (due to more goofy FAA ICAO boondoggling, a precision approach is no longer defined as one with vertical guidance).

Only LNAV/VNAV and LPV have vertical guidance (i.e., the WAAS enabled approaches). Some navigation units may compute an advisory glideslope in other cases, but that's beyond anything the FAA deals with.

I guess I'll have to look in the IFH for that definition?

 

[tsts4]

The upshot is ICAO for whatever reason declined to define WAAS GPS approaches (eg LPV) as "precision" even though in many instances they can provide the same minimums as a traditional ILS.

 

[tsts4]

Of course the RNAV GPS approach in question is a traditional non- precision ( the so called dive and drive approach--a misnomer IMO) as it only has LNAV MDA and circling mins.

 

[olasek]

which means the descent rate must be increased to arrive over the threshold in a position to make a normal landing.

You are right but it isn't actually that bad, if you calculate where this 3.04 slope intersects the runway - it is about 490' beyond the threshold. Or, like they promise, you will be crossing runway threshold at 38' AGL.

 

[aterpster]

Only LNAV/VNAV and LPV have vertical guidance (i.e., the WAAS enabled approaches). Some navigation units may compute an advisory glideslope in other cases, but that's beyond anything the FAA deals with.

The FAA indeed has a hand in the advisory WAAS path for LNAV+V or LP+V. The source document for this approach has the 3.04 degree final segment Vertical Decent Angle (VDA) and the 38' TCH. If those values are omitted on the source document then Jeppesen will not code a VDA and Garmin, et al, will not enable LNAV+V.

 

[olasek]

How come the profile view descent profile isn't drawn like a non-precision approach (chop and drop)?

That's the way NACO does it, for better or for worse.

By the way I tried this approach in the G1000 trainer, I got LNAV+V, nice smooth descent with the advisory glidepath, there is a displaced threshold here and if I continued on this slope all the way to the ground it would put me in a position for easy landing. Nothing really extraordinary here.

 

[Brad Z]

That's the way NACO does it, for better or for worse.

By the way I tried this approach in the G1000 trainer, I got LNAV+V, nice smooth descent with the advisory glidepath, there is a displaced threshold here and if I continued on this slope all the way to the ground it would put me in a position for easy landing. Nothing really extraordinary here.

Except that if you actually flew the VNAV to the runway at Potomac you'd fly mighty low over houses and trees. There's a reason the VASI and VNAV are not the same angle. On this approach, you really shouldn't be flying the VNAV below the MDA, you should be visual and using the VASI for guidance.

 

[olasek]

you really shouldn't be flying the VNAV below the MDA,

I never said I would do it in real life.

 

[chemgeek]

I never said I would do it in real life.

Which is a good thing. Following the vnav profile below mda will buy a peck of trouble at selected airports. Don't try it for real at Sidney NY. Some commercial pilots were annoyed when following advisory VNAV into Sidney takes you through a hill if you go below MDA. The complaints shut down night approaches for 12 months until the FAA relented.

Personally I think the advisory vertical guidance is more of a hindrance to safety than a benefit. Dive & drive for me. Keeps you out of the weeds.

 

[olasek]

What I meant to say I would never use advisory glideslope as an excuse to dip below MDA in IMC. That would be both stupid and illegal. But if I am below MDA and (hopefully) can see and advisory glideslope doesn't conflict with terrain and leads me to the right spot on the runway - I would follow it, manually of course. But your case of approach to rwy 7 at N23 is an excellent counterexample why sometimes terrain conflict might arise and following such glideslope below MDA could be deadly, I appreciate this example.

As to benefits of advisory glideslope - I will stick with what industry pundits say - overall it is a very good thing, it saves lives, stabilized approach is better (in probably 99.9% of cases) than step-downs.

 

[Brad Z]

Problem is you can be in VMC at night, see the runway, and still pick up a few branches and leaves following the VNAV all the way down to the runway. Not all airports are maintained to ILS standards. There's a reason why you won't see LNAV+V mentioned in the AIM of any FAA material; it's a feature that is based on obstacl clearance assumptions that have not been surveys. Otherwise there would have been a LPV procedure.

 

[aterpster]

Problem is you can be in VMC at night, see the runway, and still pick up a few branches and leaves following the VNAV all the way down to the runway. Not all airports are maintained to ILS standards. There's a reason why you won't see LNAV+V mentioned in the AIM of any FAA material; it's a feature that is based on obstacl clearance assumptions that have not been surveys. Otherwise there would have been a LPV procedure.

LNAV+V is a Garmin term for displaying the VDA when it is on source. So, the AIM indirectly speaks to LNAV+V:

(c)​

​

​

​

​

Nonprecision Approach (NPA). An instrument​

approach based on a navigation system​

which provides course deviation information, but no​

glidepath deviation information. For example, VOR,​

NDB and LNAV. As noted in subparagraph i, Vertical​

Descent Angle (VDA) on Nonprecision Approaches,​

some approach procedures may provide a Vertical​

Descent Angle as an aid in flying a stabilized​

approach, without requiring its use in order to fly the​

procedure. This does not make the approach an APV​

procedure, since it must still be flown to an MDA and​

has not been evaluated with a glidepath.

 

[luvflyin]

The subject for discussion requires the reader to view the approach plate for VKX RNAV(GPS)6, which can be found here.

..............LNAV MDA of 562..............

??????????????????????????????????

 

[luvflyin]

The FAA doesn't believe in "chop and drop". They believe (as it may) in stabilized approach. The government always shows descents as continuous straight lines wither you're talking about an approach with vertical guideance (like ILS, LPV, LNAV/VNAV) or one without. Of course it's silly in the case where it's not a constant angle like this. Just a reason why the FAA charts suck. It's Jepp that shows the dive and drive profiles for approaches without vertical guideance. When my ipad charges up enough to come to life, I'll post the Jepp version of this approach.


The VGSI is for the panel VASI that serves that runway. It's set at a steeper angle which is really more appropriate for that runway especially since downwind takes you over higher terrain. The 3.04 glideslope is the constant descent from the FAF to the impact on the runway.

There is no difference to those charts. With either chart you can chop and drop, or you can set up a stabilized descent to CRROL and then another one after CRROL. How they choose to draw the bold black lines changes nothing. I would choose to cross WOBUB at about 3200 and descend stabilized the whole way unless ATC forced me lower and have a stabilized angle of descent the whole way. Judging by the airports proximity to some busy airports in busy airspace I'd guess more often than not they'd force you down somewhere close to 1700 at WOBUB. If they didn't force me all the way down 1500 at CRROL I'd just stay at 1700, or higher if they gave me something like "cross WOBUB at or above 2000, cleared for approach, until beginning my descent however far outside of CRROL was appropriate to ensure I didn't go below 1500 before CRROL.

 

[aterpster]

The subject for discussion requires the reader to view the approach plate for VKX RNAV(GPS)6, which can be found here.

In the profile view, the initial altitude at WOBUB is 1700. Leg from WOBUB - CRROL is 6.2nm with an altitude loss of only 200feet. The runway is 4.2nm from that point with a LNAV MDA of 562, which means the descent rate must be increased to arrive over the threshold in a position to make a normal landing.

562 is the Height Above Touchdown (HAT). The MDA is 680.

 

[Jaybird180]

Thanks

 

[olasek]

Otherwise there would have been a LPV procedure.

I think there are also other requirements for an LPV to exist, like certain runway length, lighting, etc, so even though obstacles present no problem you still don't get LPV.

There is no difference to those charts.

I happen to agree with this, just another way to draw something, the key information depicted is the same.