GPS/WAAS approaches

flyingcheesehead · Mar 6, 2007

We just got the WAAS upgrade installed in the G430 in the 182. So, I've been digging around a bit to see what I don't know about all this stuff.

Questions so far:

1) What's the difference between a GPS approach (not an overlay, just straight-up GPS RWY x) and an RNAV(GPS) approach? I notice that the GPS approaches have minimums for S-1 etc. whereas the RNAV(GPS) approaches have LNAV MDA minimums.

2) What's up with all of these different minimums? I'm seeing the following:
Circling (duh)
LNAV MDA
LNAV/VNAV DA
LPV DA
GLS PA DA (though all of these that I've found are currently marked "NA")

3) What can I do with the G430W?

I was mostly trying to find the closest WAAS approach to test it with... JVL has some of the GLS PA DA's, but as noted they're all NA. (When will these be operational?) I knew that OSH had LPV mins (they were the first actually), but it looks like FLD does now as well.

Everskyward · Mar 7, 2007

Try this. We don't have WAAS so we're stuck with the LNAV minimums but I've been recently reading about the other stuff.

Joe B · Mar 7, 2007

Everskyward said:
Try this. We don't have WAAS so we're stuck with the LNAV minimums but I've been recently reading about the other stuff.

An article about WAAS written by someone who's never flown the approaches reads a lot like a dog trying to describe color television - they just don't get it. For the official description see AIM 1-1-20. Unnofficially,

LNAV: GPS approaches with no vertical guidance. You fly them the old-fashioned way: head for the MDA and motor along until you see the runway.

LNAV/VNAV: GPS approach with a synthetic glideslope. You fly it just like an ILS. According to the FAA this has been implemented on 1/3 of all LNAV approaches. I think they've focussed on the stand-alone GPS approaches because my experience has been more like 2/3. At any rate, if VNAV is available your GPS will give it to you automatically. Both Jepp and NACO show an extension of the approach path down to the end of the runway with either a dotted line or shaded triangle, respectively, when VNAV is available. There will only be a seperate line for LNAV/VNAV minima if those minima are different from LNAV MDA. Being able to fly a stabilized approach into Dogpatch Muni is wonderful. Even more wonderful if your autopilot couples to the glideslope.

LPV: GPS approach with synthetic glideslope and greater precision than LNAV/VNAV. Minima are typically around 250' AGL. For most purposes this can substitute for a Cat I ILS. There are 676 of them at the moment. You can download a spreadsheet of them at the above link.

GLS PA DA: High precision GPS approaches down to better than Cat I ILS minimums. Not implemented yet. Think "Years from now".

flyingcheesehead said:
What's the difference between a GPS approach (not an overlay, just straight-up GPS RWY x) and an RNAV(GPS) approach?

There is no difference. The approach plate hasn't been updated to the new terminology yet. Chances are VNAV hasn't been charted for that approach yet, either.

flyingcheesehead said:
I was mostly trying to find the closest WAAS approach to test it with... JVL has some of the GLS PA DA's, but as noted they're all NA.

All of the GPS approaches at JVL have LNAV/VNAV with lower minima than just straight LNAV. Give them a try. There might be some closer to you but since I don't know where you are... (Edit: Once out of compose mode I can see your base. At C29 you must have a note from the Administrator to use VNAV minima different from the MDA. That suggests LNAV/VNAV to me.)

Regards,
Joe

flyingcheesehead · Mar 7, 2007

Thanks Joe... That'll give me plenty to chew on until I get to fly the plane sometime in the next few days. I'm going to try to fly several different types of GPS approaches with it and see how it works.

Joe B said:
LNAV: GPS approaches with no vertical guidance. You fly them the old-fashioned way: head for the MDA and motor along until you see the runway.

The 430W/530W also has an "LNAV+V" mode which gives "advisory vertical guidance."

LNAV/VNAV: GPS approach with a synthetic glideslope. You fly it just like an ILS. According to the FAA this has been implemented on 1/3 of all LNAV approaches. I think they've focussed on the stand-alone GPS approaches because my experience has been more like 2/3.

Do any of the overlay approaches have VNAV? :dunno:

GLS PA DA: High precision GPS approaches down to better than Cat I ILS minimums. Not implemented yet. Think "Years from now".

According to the front of the approach plate books, "The GLS (NA) minima line will be removed from existing RNAV(GPS) approach charts when LPV minima is published."

So why'd they put that line there in the first place? To tell us which approaches will have LPV minima in the future? :dunno:

All of the GPS approaches at JVL have LNAV/VNAV with lower minima than just straight LNAV.

Yes, but I'm noticing that in a LOT of places, the LNAV/VNAV minima are actually higher than the straight LNAV minima. What's up with that? For instance, all of the approaches at the home drome (KMSN) are that way. Example: The RNAV(GPS) RWY 21 approach. LNAV MDA is 1380/1/2 but LNAV/VNAV DA is 1380/1 1/4. :dunno:

The other somewhat silly thing at the home drome is that 5 of the 6 runway ends have IAP's and all of those have RNAV(GPS) approaches, but then only 3 of the 5 have LNAV/VNAV minima and all three of those already have ILS approaches. Why wouldn't they try to get better approaches to the runway ends that don't already have an ILS?

(Edit: Once out of compose mode I can see your base. At C29 you must have a note from the Administrator to use VNAV minima different from the MDA. That suggests LNAV/VNAV to me.)

Only LNAV listed at C29, but I'm based at MSN anyway.

Dave Siciliano · Mar 7, 2007

Kent:

I don't have my charts handy but I believe Rockford has a GPS to 19 with VNAV also if you get down that-a-way!

Joe was sure right about terminology; it's changed and can be confusing. Once I get the W upgrade on our plane, I'll probably try to take some kind of course on these new approaches; although, they seem pretty straight forward.

We've already had one point of discussion about them on another board; there is no way to verify where the GS is intercepted via a different nav aid (or to get other confirmation of location). That is, on an ILS, for instance, usually there is an altitude at which GS is intercepted and one can verify that. There are also either DME or markers on the way in to confirm where you are. VNAV doesn't have those.

Brave New World <g>

Best,

Dave

flyingcheesehead · Mar 7, 2007

Dave Siciliano said:
I don't have my charts handy but I believe Rockford has a GPS to 19 with VNAV also if you get down that-a-way!

Yeah, RFD has lots of LNAV/VNAV but no LPV's.

Once I get the W upgrade on our plane

I thought you had a 480?

We've already had one point of discussion about them on another board; there is no way to verify where the GS is intercepted via a different nav aid (or to get other confirmation of location). That is, on an ILS, for instance, usually there is an altitude at which GS is intercepted and one can verify that. There are also either DME or markers on the way in to confirm where you are. VNAV doesn't have those.

Hmmm. Yeah. Hadn't thought of that...

Kinda scary!

tonycondon · Mar 7, 2007

well you pretty much blindly trust it is correct laterally, why not vertically?

bbchien · Mar 7, 2007

Ugh! There is a bug in the new 430W software. The E6B doesn't work. Others have confirmed this.

sigh.

tonycondon · Mar 7, 2007

pull out the ole whiz wheel doc

bbchien · Mar 7, 2007

tonycondon said:
pull out the ole whiz wheel doc

Yup, and grease pencil. It's almost (but not quite) as quick....

"now where did that da_ged grease pencil slip off to....?!!!"

"Oh, here, on the floor...sigh."

Off to OHare to get Susan home....

gprellwitz · Mar 7, 2007

Dave Siciliano said:
Kent:

I don't have my charts handy but I believe Rockford has a GPS to 19 with VNAV also if you get down that-a-way!

Okay, Rockford has a number of GPS approaches. The following have VNAV:
RNAV (GPS) RWY 1 (DA 1080) (ILS 929, NDB 1220)
RNAV (GPS) RWY 7 (DA 1080) (ILS 942, ILS CAT II 892, ILS CAT III 0)
RNAV (GPS) Z RWY 19 (DA 1060, LOC BC 1140)
RNAV (GPS) Z RWY 25 (DA 1140, no alternative)

The following DON'T have VNAV:
RNAV (GPS) Y RWY 19 (LNAV 1080)
RNAV (GPS) Y RWY 25 (LNAV 1160)

For those with VNAV, I indicated any other approaches and their minimums. Let me know if you're authorized to fly ILS CAT II or III!

These are from the 3/15 plates.

By the way, WHY does CHICAGO/ROCKFORD come after CHICAGO O'HARE INTL but before CHICAGO/DU PAGE? Okay, I KNOW why, but it really aggravates me that the order goes:

Code:

[FONT=Courier New]
[/FONT][FONT=Courier New][SIZE=1] CHICAGO                       CHICAGO MIDWAY INTL
CHICAGO                       CHICAGO O'HARE INTL
CHICAGO/ROCKFORD              CHICAGO/ROCKFORD INTL
CHICAGO (WEST CHICAGO)        CHICAGO/DU PAGE
CHICAGO/AURORA                CHICAGO/AURORA MUNI
CHICAGO/LAKE IN THE HILLS     CHICAGO/LAKE IN THE HILLS/LAKE IN THE HILLS
CHICAGO/PROSPECT HEIGHTS/WHEELING  CHICAGO EXECUTIVE
CHICAGO/ROMEOVILLE            CHICAGO/LEWIS UNIVERSITY
CHICAGO/WAUKEGAN              CHICAGO/WAUKEGAN REGIONAL[/SIZE][/FONT]

I mean, this takes pages 121 - 222, literally more than 100 pages. Trying to find an airport is problematic. How in the world did Rockford wind up there? You need to use the index to find an airport, not something I want to be doing when faced with a diversion!

gismo · Mar 7, 2007

flyingcheesehead said:
Questions so far:

1) What's the difference between a GPS approach (not an overlay, just straight-up GPS RWY x) and an RNAV(GPS) approach? I notice that the GPS approaches have minimums for S-1 etc. whereas the RNAV(GPS) approaches have LNAV MDA minimums.

GPS is the old name for the LNAV (only) version of a RNAV(GPS) approach, the only difference there is the name. As you noticed, an RNAV(GPS) approach chart may offer options for LNAV/VNAV and/or LPV.

2) What's up with all of these different minimums? I'm seeing the following:
Circling (duh)
LNAV MDA
LNAV/VNAV DA
LPV DA
GLS PA DA (though all of these that I've found are currently marked "NA")

As I said, LNAV is the new name for the old GPS standalone approaches. LNAV/VNAV and LPV are similar except that LPV requires more accuracy from the GPS and therefore often has lower mins. Both use angular guidance which like a VOR or LOC get progressively more sensitive as you get closer to the runway (navaid for VOR). I see you noticed that some LNAV/VNAV approach mins are actually higher than LNAV. I'm not completely clear on the reason for that but I know it has to do with the rules that approach makers must adhere to, and I think it may be related to the point where the missed approach begins and the altitude one might be at that point. I just purchased a book called "Instrument Flying Update" by ABS BPPP instructor John Eckalbar:

(http://www.skyroadprojects.com/images/ifucover.jpg)

According to the website it's about the impact of GPS and other recent changes to the IFR system and it may hold the answers to some of your questions.

3) What can I do with the G430W?

I was mostly trying to find the closest WAAS approach to test it with... JVL has some of the GLS PA DA's, but as noted they're all NA. (When will these be operational?) I knew that OSH had LPV mins (they were the first actually), but it looks like FLD does now as well.

AFaIK, GLS PA requires local differential correction. I suspect that this is still a ways into the future. Then again, I know that the WAAS system is being upgraded and perhaps that's what is needed to implement GLS PA. Supposedly those approaches will have typical ILS mins (1/2 mi vis & 200 ft DA). Some LPVs go down to 200 HAT, but AFaIK none have visibility mins down to 1/2 mile or less.

BTW I found what seems to be a problem with several GPS approaches that use the "basic T" format. Specifically when arriving from beyond the "top" of the 'T', the procedure states that no procedure turn is required but the GNS-480 tries to fly one anyway. I've gotten different stories on this from Jeppesen and Garmin, and it could be a problem in the DB, a bug in the GNS-480, or some FAA mandated stupidity.

Welcome to the world of WAAS.

Joe B · Mar 8, 2007

flyingcheesehead said:
Do any of the overlay approaches have VNAV?

According to the front of the approach plate books, "The GLS (NA) minima line will be removed from existing RNAV(GPS) approach charts when LPV minima is published."

So why'd they put that line there in the first place? To tell us which approaches will have LPV minima in the future?

Dunno and dunno, sorry.

flyingcheesehead said:
Yes, but I'm noticing that in a LOT of places, the LNAV/VNAV minima are actually higher than the straight LNAV minima. What's up with that? For instance, all of the approaches at the home drome (KMSN) are that way. Example: The RNAV(GPS) RWY 21 approach. LNAV MDA is 1380/1/2 but LNAV/VNAV DA is 1380/1 1/4.

Note that the difference between the two is flight visibility, not altitude. If you're LNAV/VNAV on glideslope and at DA you're 1.4 miles from the runway end. So they're giving you a visibility minimum that'll just barely let you see the beginning of the approach lights.

You can see from the chart that the LNAV VDP is coincident with the point in space where you hit DA under LNAV/VNAV. This should give you some idea how the LNAV approach is going to work out in real life. But the MAP for the LNAV approach is the runway end. There's no reason you can't fly to the MAP under 1/2 mile visibility and helicopter down to the runway from there, though quite a few planes would find it challenging.

Not shown on the NACO chart, but Jepp shows it clearly, is that when you get to DA via LNAV/VNAV, you can simply level off at LNAV MDA and complete the LNAV approach to the MAP. That defeats the whole concept of being stabilized on either kind of approach. It's legal, but riskier than flying either approach stabilized well before you get to the airport.

More interesting are the approaches to 18/36. There the LNAV/VNAV DA is higher than the LNAV MDA. I've no idea what little quirk in the TERPS is driving that.

flyingcheesehead said:
The other somewhat silly thing at the home drome is that 5 of the 6 runway ends have IAP's and all of those have RNAV(GPS) approaches, but then only 3 of the 5 have LNAV/VNAV minima and all three of those already have ILS approaches. Why wouldn't they try to get better approaches to the runway ends that don't already have an ILS?

Both NACO and Jepp show a computed descent angle for the 14/32 approaches. When you get up in the air and fly them you might find that you get vertical guidance from your GPS that corresponds to those descent angles. That's your LNAV+V mode. If so, you can fly it just like LNAV/VNAV, treating MDA as DA.

The real answer to your question, though, is that it takes a lot of work to design a new approach. I'm guessing that the additional work for LNAV/VNAV is fairly minimal where the ILS designer has already done all the heavy lifting for obstacle clearance on the glideslope (can you say "cut and paste"?) But it's a different deal to incorporate a glideslope that's never been surveyed. Besides, why should they? The approach designers know that when the weather really goes in the dumper you're going to be flying the ILS anyway.

Regards,
Joe

flyingcheesehead · Mar 8, 2007

Joe B said:
Note that the difference between the two is flight visibility, not altitude. If you're LNAV/VNAV on glideslope and at DA you're 1.4 miles from the runway end. So they're giving you a visibility minimum that'll just barely let you see the beginning of the approach lights.

Aha, that makes sense.

More interesting are the approaches to 18/36. There the LNAV/VNAV DA is higher than the LNAV MDA. I've no idea what little quirk in the TERPS is driving that.

My guess is that since it's a DA, not an MDA, they're allowing for somewhat of a dip (down to where the LNAV MDA is) while still being able to climb out on the miss.

Both NACO and Jepp show a computed descent angle for the 14/32 approaches. When you get up in the air and fly them you might find that you get vertical guidance from your GPS that corresponds to those descent angles. That's your LNAV+V mode. If so, you can fly it just like LNAV/VNAV, treating MDA as DA.

Aha. I will have to try that. I'm going to try several different approaches, and see which ones give vertical guidance and in what mode.

flyingcheesehead · Mar 8, 2007

gprellwitz said:
By the way, WHY does CHICAGO/ROCKFORD come after CHICAGO O'HARE INTL but before CHICAGO/DU PAGE? Okay, I KNOW why, but it really aggravates me

What gets me is... Why is it marked "CHICAGO/ROCKFORD" at all? Rockford ain't anywhere near Chicago!

Why does everyone want to be Chicago anyway? Palwaukee is "Chicago Executive," Milwaukee tells everyone to remain clear of class charlie... And now Rockford, through some sort of magical warp in the space-time continuum, is somehow magically transformed into a place where you're in Chicago on short final but you're 75 miles away in Rockford when you get to the terminal. Wow, I never knew I could taxi at 750 knots!

flyingcheesehead · Mar 8, 2007

OK, I'm gonna go up with Pete and maybe Joe sometime in the next few days to try the new toy box out.

I think the plan will go like this:

1) Madison direct Watertown NDB, full-procedure NDB or GPS 23 approach. We'll see what the new box does on an overlay approach (shouldn't be any vertical guidance on this one), and we'll let the autopilot fly the whole thing, including the procedure turn (new capability).

2) Direct SABLL IAF for the RNAV(GPS) 36 FLD. This is an LPV approach. I'll let the autopilot fly the T and cut it off on the intermediate segment, hand-flying final.

3) Miss to EDENN intersection just south of FLD and fly the transition to the GPS 20 at UNU. Old GPS approach with a calculated descent angle. We'll see if we get LNAV+V on this. Published miss to RANDO with the autopilot flying the hold.

4) When we're done with the hold, we'll head down V9 towards MSN and fly the RNAV(GPS) 21 with the HPILPT. Again, we'll see if the autopilot and GPS can handle the hold correctly and continue sequencing, and we'll use the LNAV/VNAV minimums provided we get the L/VNAV indication from the 430.

5) Miss to CINOT IAF for the RNAV(GPS) 32, do the HPILPT from the opposite direction and see how the boxes handle the entry. This approach also only has LNAV mins but it does have a calculated descent angle so hopefully LNAV+V will kick in.

So... That gives us one overlay approach, one old "GPS" approach, and three RNAV(GPS) approaches, one each LNAV, LNAV/VNAV, and LPV. We should get to see all of the modes of the 430 (LNAV, LNAV+V, L/VNAV, LPV). We'll also see how well the S-TEC GPSS works with the 430W's new capabilities, as it'll get to fly a procedure turn, a missed approach hold, and two HPILPT's with different entries.

Should be a good workout, too! :goofy:

gprellwitz · Mar 8, 2007

flyingcheesehead said:
What gets me is... Why is it marked "CHICAGO/ROCKFORD" at all? Rockford ain't anywhere near Chicago!

My guess is that it is an official reliever airport for the Chicago area (read: O'Hare/Midway) so it has "CHICAGO" in the title.

flyingcheesehead · Mar 9, 2007

flyingcheesehead said:
We just got the WAAS upgrade installed in the G430 in the 182.

And, deinstalled. The fan on the new unit was inop, so it got pulled out and a regular 430 put back in until another 430W comes in.

ghogue · Mar 9, 2007

flyingcheesehead said:
According to the front of the approach plate books, "The GLS (NA) minima line will be removed from existing RNAV(GPS) approach charts when LPV minima is published."

So why'd they put that line there in the first place? To tell us which approaches will have LPV minima in the future?

Kent,
From AIM 5-4-5 j. "GLS was a placeholder for WAAS and LAAS when they became available and is marked N/A. LNAV/VNAV is a new type of instrument approach called APV, with lateral and vertical navigation. The vertical portion can be flown by approach certified Baro-VNAV and by WAAS electronic VNAV as well. A new line will be added to these charts titled LPV. This will replace the GLS N/A line."

I think the chart format was designed for a specific number of lines of minima.The GLS N/A, as it says, is just to fill in the space until LPV came along. In other words, they just wanted to design one format now for future use as well.

Lance F · Mar 12, 2007

flyingcheesehead said:
And, deinstalled. The fan on the new unit was inop, so it got pulled out and a regular 430 put back in until another 430W comes in.

Interesting. I know (well, my avionics shop told me) that the old antenna will not work at all with a 430W. Apparently though the new antenna will work with a legacy box.

kgruber · Mar 12, 2007

Lance F said:
Interesting. I know (well, my avionics shop told me) that the old antenna will not work at all with a 430W. Apparently though the new antenna will work with a legacy box.

430W works just fine with the old antenna. Don't ask how I know!

flyingcheesehead · Mar 13, 2007

kgruber said:
430W works just fine with the old antenna. Don't ask how I know!

Yeah, but is it certified for approaches in that config? I think not.

flyingcheesehead · Mar 13, 2007

Lance F said:
Interesting. I know (well, my avionics shop told me) that the old antenna will not work at all with a 430W. Apparently though the new antenna will work with a legacy box.

Good thing, too, or we'd be GPS-less right now.

tonycondon · Mar 13, 2007

flyingcheesehead said:
Good thing, too, or we'd be GPS-less right now.

jeez how would you ever get anywhere?

flyingcheesehead · Mar 13, 2007

tonycondon said:
jeez how would you ever get anywhere?

Well, it's also the #1 Nav/Com radio... And while I get places fine without it, I sure wouldn't count on certain others.

GPS/WAAS approaches

Taxi to Parking

Experimenter

Pre-takeoff checklist

Taxi to Parking

Final Approach

Taxi to Parking

Gastons CRO (Chief Dinner Reservation Officer)

Touchdown! Greaser!

Gastons CRO (Chief Dinner Reservation Officer)

Touchdown! Greaser!

Touchdown! Greaser!

Touchdown! Greaser!

Pre-takeoff checklist

Taxi to Parking

Taxi to Parking

Taxi to Parking

Touchdown! Greaser!

Taxi to Parking

Pre-takeoff checklist

En-Route

Final Approach

Taxi to Parking

Taxi to Parking

Gastons CRO (Chief Dinner Reservation Officer)

Taxi to Parking