ILS or RNAV request

MyassisDragon

MyassisDragon

Line Up and Wait
Joined
Dec 18, 2013
Messages
585
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Michigan
Display Name
Display name:
Mr Fred
Quick question with following scenario.
1) IFR flight plan (direct to airport, not FIX)
2) Flying a plane equipped /G (430WAAS)
3) Getting Vectors from approach
4) Active runway has both ILS and RNAV
5) LPV and ILS minimums are the same (or within 1-2ft)

I always default to requesting and flying the ILS. This way I load the approach set the CDI to VLOC and get vectored in like a standard ILS. My thinking is if I have any issues picking up the ILS I can always change the CDI to GPS and use the software generated glide slope as backup (or confirmation I didnt intercept too high).

Yes, no, maybe so?
 
In theory you could but in practical terms if the ILS failed you should go missed, fly the missed as published or instructed, delete the ILS from the flight plan, and load the GPS approach. I will sometime punch in the GPS LPV into my 530W and the ILS in my second NAV/CDI for giggles and grins but only when the approach is a give-me.

I've yet to fly into a controlled airport that has an ILS and a GPS-LPV and have ATC assign the GPS approach. They seem to always default to the ILS; most likely because the primary traffic is airliners and many do not yet have GPS LPV capability.
 
I prefer the LPV if the minimums are the same, and you can get vectors to final there too.

Why? I find the LPV to be a more stable approach.
 
Might be wrong, but I think the primary issue with your "plan" is the GPS only loads one approach at a time (ILS or GPS). If you push the CDI button I believe the GPS will simply revert back to the standard guidance to the last fix/airport; not the GPS approach.

To change from the ILS to the GPS you must first delete the ILS approach then load the GPS approach. That is not something I would be doing while descending in the soup.
 
MyassisDragon said:
I always default to requesting and flying the ILS. This way I load the approach set the CDI to VLOC and get vectored in like a standard ILS. My thinking is if I have any issues picking up the ILS I can always change the CDI to GPS and use the software generated glide slope as backup (or confirmation I didnt intercept too high).
Click to expand...
The 430W generates a GS for an ILS? Or did you mean you have the RNAV loaded in the GPS but the nav radio tuned to the localizer? I always have the ILS loaded in my 480 when I'm flying an ILS -- much better situational awareness than cross radials on the #2 CDI, though I try to have those tuned in as well as backup.

More than one CFII has told me that I should be timing from the FAF on an ILS as well, in case I lose the GS, but my own feeling is that if I lose the GS I'm going missed anyway and try to figure it out later. I don't like switching approaches in midstream.
 
azure said:
The 430W generates a GS for an ILS? Or did you mean you have the RNAV loaded in the GPS but the nav radio tuned to the localizer? I always have the ILS loaded in my 480 when I'm flying an ILS -- much better situational awareness than cross radials on the #2 CDI, though I try to have those tuned in as well as backup.

More than one CFII has told me that I should be timing from the FAF on an ILS as well, in case I lose the GS, but my own feeling is that if I lose the GS I'm going missed anyway and try to figure it out later. I don't like switching approaches in midstream.
Click to expand...

You have it right, in my opinion.
 
azure said:
The 430W generates a GS for an ILS?
Click to expand...

Good call, Maybe when I pushed the CDI button while practicing ON the ILS (just to see what would happen:yikes:) I must have been so darn close to perfect that I saw the needle zero out when vertical guidance got disabled and figured it was the difference from LPV to ILS. The only other question is did I see the GS flag pop up???? (dont think so , but I will go out and check next time, just for grins:D)

For what its worth, when I fly into my non towered home airport KBIV with light traffic the controlling ATC - KMKG will almost always ask if I want the ILS26 or RNAV26 if the winds are out of the west (must be when they see the /G on my flight plan) . I just default to ILS not because the DA is 65ft less (or any other logical reason being the greenie that I am) but more due to what I have become more accustomed to from flying planes without WAAS.

Hence my thought provoking post..;)
 
Reading the replys and looking back at my my first post, maybe it should have read more like this:

MyassisDragon said:
Quick question with following scenario.
1) IFR flight plan (direct to airport, not FIX)
2) Flying a plane equipped /G (430WAAS)
3) Getting Vectors from approach
4) Active runway has both ILS and RNAV
5) LPV and ILS minimums are the same (or within 1-2ft)
Click to expand...

When given the option by ATC what do you choose and why? :)

(my wife says I talk to much, so sometimes people get confused and it just lowers my perceived IQ):lol:
 
TMetzinger said:
I prefer the LPV if the minimums are the same, and you can get vectors to final there too.

Why? I find the LPV to be a more stable approach.
Click to expand...
Ditto. Too many things can interfere with or warp a LOC or GS signal, causing you to wander back and forth trying to keep the needles centered. GPS is invariably rock-steady all the way down the approach. I have enough trouble keeping the needles centered without having to chase a moving target. So, when I have the choice, I'll take a GPS-LPV over an ILS, and a GPS-LNAV over a VOR or LOC any day of the week and twice on Sundays.
 
MyassisDragon said:
Reading the replys and looking back at my my first post, maybe it should have read more like this:



When given the option by ATC what do you choose and why? :)

(my wife says I talk to much, so sometimes people get confused and it just lowers my perceived IQ):lol:
Click to expand...

RNAV, as Time mentioned.
 
MyassisDragon said:
My thinking is if I have any issues picking up the ILS I can always change the CDI to GPS and use the software generated glide slope as backup (or confirmation I didnt intercept too high).
Click to expand...
As noted above, the only thing that pushing the CDI to GPS will do at that point is take away your GS needle and generate a warning MSG on the GPS. Further, even quickly reloaded the RNAV(GPS) approach so you can use that instead of the ILS has a number of difficulties, starting with a lot of button pushing to select and activate the approach, and going to the probability that the programmed missed approach for the RNAV(GPS) procedure will be different than that for the ILS for which you were cleared, which could create serious problems if you miss -- controllers usually get really upset when you do something other than what they cleared you to do.
 
Ron Levy said:
Ditto. Too many things can interfere with or warp a LOC or GS signal, causing you to wander back and forth trying to keep the needles centered. GPS is invariably rock-steady all the way down the approach. I have enough trouble keeping the needles centered without having to chase a moving target. So, when I have the choice, I'll take a GPS-LPV over an ILS, and a GPS-LNAV over a VOR or LOC any day of the week and twice on Sundays.
Click to expand...

Wouldn't the LPV approach (or any GPS approach really) also have more redundancy due to the RAIM/WAAS in the GPS constantly double checking the system? The ILS doesn't have this and thus you're trusting that it's calibrated correctly and reading correctly.
 
tehmightypirate said:
Wouldn't the LPV approach (or any GPS approach really) also have more redundancy due to the RAIM/WAAS in the GPS constantly double checking the system? The ILS doesn't have this and thus you're trusting that it's calibrated correctly and reading correctly.
Click to expand...
I suppose that's true to an extent. But the ILS equipment on the ground is supposed to have internal self-test features which alarm and shut down the system if a failure is detected, at which point the nav flags appear and you go missed the same as if you got a RAIM/INTEG warning with GPS. In that regard, you have similar protection as with WAAS/RAIM, although unlike your GPS internal checks, it's only checking the ground equipment, not the onboard gear.
 
LPV

More accurate
Less button pushing
Don't have to listen to Morse code
Easier missed approach guidance
 
TMetzinger said:
I prefer the LPV if the minimums are the same, and you can get vectors to final there too.

Why? I find the LPV to be a more stable approach.
Click to expand...

Because the LPV has the same sensitivity the entire way down, right? Whereas the localizer gets more sensitive the closer you get to the station.

I'd still choose the ILS. I've never been a fan of GPS other than for enroute navigation.
 
Cpt_Kirk said:
Because the LPV has the same sensitivity the entire way down, right? Whereas the localizer gets more sensitive the closer you get to the station.
Click to expand...
No -- for the reason I stated (wandering guidance on ILS).
I'd still choose the ILS. I've never been a fan of GPS other than for enroute navigation.
Click to expand...
Your choice. But if you get trained on it properly, I'll bet your opinion changes.
 
Cpt_Kirk said:
Because the LPV has the same sensitivity the entire way down, right? Whereas the localizer gets more sensitive the closer you get to the station.

I'd still choose the ILS. I've never been a fan of GPS other than for enroute navigation.
Click to expand...

No, the LPV uses +/- 2 degrees angular CDI. The WAAS GPS is smoother, less button pushing, fewer steps to remember. I haven't flown an ILS in the past almost 7 years other than for practice, but I have flown a slew of LPV approaches.
 
I vote the LPv approach. Not as much needle chasing if it gets bumpy.
 
Extra steps for an ILS using a GPS for navigation enroute and terminal include:

1) After approach selection, activate the ILS frequency as the current Nav frequency.
2) Identify the Morse code for the localizer, GNS530 will decode this for you but the GNS430 and GTN units won't.
3) Select CDI to VLOC or verify it auto-switches as intended (auto switch doesn't work with close turn on to final or distant turn on to final).
4) Some GPSS installations will be disabled when VLOC is chosen.
5) Course width is angular but varies with runway length, the longer the runway the smaller the localizer angle.
6) ILS hold lines need to be respected by those on the ground, aircraft on the approach in front of you can mess up the signal you receive.
7) ILS can have bends from ground reflections off of buildings and obstacles and notes such as do not use GS below a certain altitude or autopilot not approved below certain altitude are possible notes on the approach chart.
8) Going to missed approach requires a CDI switch back to GPS to obtain missed approach procedure navigation guidance.

Edit: If there are step downs inside the FAF for the localizer version or it has a MAP at a DME distance location, these are not included in the database and the pilot needs to calculate ATD for these step downs or MAP.
 
Last edited:
Ron Levy said:
the programmed missed approach for the RNAV(GPS) procedure will be different than that for the ILS for which you were cleared.
Click to expand...

Good point!
Also the more I think about it the MAP sequence on RNAV sometimes seems more more straight forward.
My local Rnav26 is climb runway heading to xxx alt and hold over fix, whereas the same ILS MAP is something like climb runway HDG to xxxx alt, turn to HDG xxx, intercept vor radial xxx then run to a intersection of VOR radial xxx for a hold.
 
Cpt_Kirk said:
Because the LPV has the same sensitivity the entire way down, right? Whereas the localizer gets more sensitive the closer you get to the station.

I'd still choose the ILS. I've never been a fan of GPS other than for enroute navigation.
Click to expand...

No. LPV sensitivity scales as well. But the signal doesn't wander if a cow/deer/fuel truck/other airplane wanders in front of the LOC or GS antenna.
 
Great article on how the LNAV, LNAV+V, and LPV approach limits all look: https://allaboutairplanes.wordpress.com/tag/lnavv/

As stated before the LPV approach limits are angular. Note that the LNAV approach will also adjust the limits of full scale deflection between enroute, terminal, and approach phases, with adjustments taking place over a short period (1 min for enroute to terminal, 2nm for terminal to approach). Thus, while linear between the final approach fix and the missed approach point even an LNAV approach will technically "taper" between the initial approach fix and and final approach fix. This is actually somewhat important to note as if you're off-course but turned to re-intercept the magenta line during the switch to 0.3 nm approach mode it could give you the impression that you're actually moving farther away from the course as the scale just got cut down by 60%.
 
tehmightypirate said:
Great article on how the LNAV, LNAV+V, and LPV approach limits all look: https://allaboutairplanes.wordpress.com/tag/lnavv/

As stated before the LPV approach limits are angular. Note that the LNAV approach will also adjust the limits of full scale deflection between enroute, terminal, and approach phases, with adjustments taking place over a short period (1 min for enroute to terminal, 2nm for terminal to approach). Thus, while linear between the final approach fix and the missed approach point even an LNAV approach will technically "taper" between the initial approach fix and and final approach fix. This is actually somewhat important to note as if you're off-course but turned to re-intercept the magenta line during the switch to 0.3 nm approach mode it could give you the impression that you're actually moving farther away from the course as the scale just got cut down by 60%.
Click to expand...

To be explicit. LNAV approaches are .3NM wide from FAF to MAP. LPV are angular and get more sensitive from FAF to MAP, just like a localizer.
 
Actually it depends on the type of GPS. A non WAAS GPS has a fixed CDI FSD (Full Scale Deflection) of +/- 1 NM and the final approach segment steps down to +/- 0.3 NM, a linear step. However, a WAAS GPS uses an angular FSD of +/- 2 degrees or +/-0.3 NM, whichever is less. Since the angular is less anytime the final approach segment is under 7 NM, for most approaches LNAV, LNAV/VNAV, or LPV when flown with a WAAS GPS, they are all angular.

Many pilots believe that a WAAS GPS uses the same 0.3 NM fixed FSD on the final approach segment for an LNAV approach, but this is incorrect, it is angular, the same as an LPV.
 
John Collins said:
Actually it depends on the type of GPS. A non WAAS GPS has a fixed CDI FSD (Full Scale Deflection) of +/- 1 NM and the final approach segment steps down to +/- 0.3 NM, a linear step. However, a WAAS GPS uses an angular FSD of +/- 2 degrees or +/-0.3 NM, whichever is less. Since the angular is less anytime the final approach segment is under 7 NM, for most approaches LNAV, LNAV/VNAV, or LPV when flown with a WAAS GPS, they are all angular.

Many pilots believe that a WAAS GPS uses the same 0.3 NM fixed FSD on the final approach segment for an LNAV approach, but this is incorrect, it is angular, the same as an LPV.
Click to expand...

Really? This is news to me, can you confirm this?
 
tehmightypirate said:
Really? This is news to me, can you confirm this?
Click to expand...
I'm surprised too -- assuming he's talking about a straight LNAV approach without LPV or LP minimums. I don't see that distinction in the AIM (though I've only checked my 2011 print copy).

To answer the question, I'm pretty much like everyone else -- I'll take the LPV any day over an ILS with comparable minimums. Personal anecdote: my very first solo approach in actual IMC was an LPV that got me in, when the approach that ATC first offered to me (a LOC BC) would not have. There are lots of fields where the ILS is only in one direction, and the LPV going the other way can come in VERY handy.
 
azure said:
I'm surprised too -- assuming he's talking about a straight LNAV approach without LPV or LP minimums. I don't see that distinction in the AIM (though I've only checked my 2011 print copy).

To answer the question, I'm pretty much like everyone else -- I'll take the LPV any day over an ILS with comparable minimums. Personal anecdote: my very first solo approach in actual IMC was an LPV that got me in, when the approach that ATC first offered to me (a LOC BC) would not have. There are lots of fields where the ILS is only in one direction, and the LPV going the other way can come in VERY handy.
Click to expand...

That's a good point, ceilings permitting I'd probably take an LNAV approach into the wind rather than a cross-wind ILS or circle to land.
 
tehmightypirate said:
Really? This is news to me, can you confirm this?
Click to expand...

Sure. RTCA DO229C or D provides the detail specification for a WAAS GPS. I have attached the specification diagram for the LNAV FSD from RTCA DO229D. You can also see them described in Appendix C of the GNS430W/530W Pilot Guide, also attached.

Next time you fly a LNAV procedure with a WAAS GNS or GTN or G1000, go to the CDI page and look at the CDI system value. It will be about 0.3 at the FAF and continue to decrease as you approach the runway threshold.

What amuses me is that pilots have been flying these for years and never noticed that it was angular all that time.

View attachment LNAV path.pdf

View attachment gns530w pilot guide FSD.pdf
 
John Collins said:
Sure. RTCA DO229C or D provides the detail specification for a WAAS GPS. I have attached the specification diagram for the LNAV FSD from RTCA DO229D. You can also see them described in Appendix C of the GNS430W/530W Pilot Guide, also attached.

Next time you fly a LNAV procedure with a WAAS GNS or GTN or G1000, go to the CDI page and look at the CDI system value. It will be about 0.3 at the FAF and continue to decrease as you approach the runway threshold.

What amuses me is that pilots have been flying these for years and never noticed that it was angular all that time.

View attachment 33038

View attachment 33039
Click to expand...

Definitely interesting and thanks for the PDFs. I'm surprised I'm just now learning this as all the diagrams I've seen prior to this lead me to believe that it was not angular. Do you know if any non-WAAS GPS systems do this or is it WAAS only?

For myself, I've only personally seen two LPV approaches from the right seat and the planes I've flown I either never did an LPV approach or did not have WAAS (the G1000 I use is the non-WAAS version). As for other pilots I imagine 90% of the time you're centered most of the time and wouldn't see much change in the scale.
 
John Collins said:
Actually it depends on the type of GPS. A non WAAS GPS has a fixed CDI FSD (Full Scale Deflection) of +/- 1 NM and the final approach segment steps down to +/- 0.3 NM, a linear step. However, a WAAS GPS uses an angular FSD of +/- 2 degrees or +/-0.3 NM, whichever is less. Since the angular is less anytime the final approach segment is under 7 NM, for most approaches LNAV, LNAV/VNAV, or LPV when flown with a WAAS GPS, they are all angular.

Many pilots believe that a WAAS GPS uses the same 0.3 NM fixed FSD on the final approach segment for an LNAV approach, but this is incorrect, it is angular, the same as an LPV.
Click to expand...

Doesn't the LPV have a minimum width beyond which it doesn't get more sensitive? Granted, this is normally beyond the intersection of the GS and DA IIRC, but still beneficial in low vis.
 
John Collins said:
Actually it depends on the type of GPS. A non WAAS GPS has a fixed CDI FSD (Full Scale Deflection) of +/- 1 NM and the final approach segment steps down to +/- 0.3 NM, a linear step. However, a WAAS GPS uses an angular FSD of +/- 2 degrees or +/-0.3 NM, whichever is less. Since the angular is less anytime the final approach segment is under 7 NM, for most approaches LNAV, LNAV/VNAV, or LPV when flown with a WAAS GPS, they are all angular.

Many pilots believe that a WAAS GPS uses the same 0.3 NM fixed FSD on the final approach segment for an LNAV approach, but this is incorrect, it is angular, the same as an LPV.
Click to expand...

I did not know that, thanks!
 
John Collins said:
Actually it depends on the type of GPS. A non WAAS GPS has a fixed CDI FSD (Full Scale Deflection) of +/- 1 NM and the final approach segment steps down to +/- 0.3 NM, a linear step. However, a WAAS GPS uses an angular FSD of +/- 2 degrees or +/-0.3 NM, whichever is less. Since the angular is less anytime the final approach segment is under 7 NM, for most approaches LNAV, LNAV/VNAV, or LPV when flown with a WAAS GPS, they are all angular.

Many pilots believe that a WAAS GPS uses the same 0.3 NM fixed FSD on the final approach segment for an LNAV approach, but this is incorrect, it is angular, the same as an LPV.
Click to expand...

That is how Garmin chose to implement WAAS LNAV. Nonetheless the alerting and integrity in the final approach segment are limited to 0.3 nm, unlike LPV (or LP).
 
aterpster said:
That is how Garmin chose to implement WAAS LNAV. Nonetheless the alerting and integrity in the final approach segment are limited to 0.3 nm, unlike LPV (or LP).
Click to expand...

At this point, who else is there and I seriously doubt others when they get to market will implement it differently. The DO offers it as an option, but to me it is clear which they prefer.
 
gismo said:
Doesn't the LPV have a minimum width beyond which it doesn't get more sensitive? Granted, this is normally beyond the intersection of the GS and DA IIRC, but still beneficial in low vis.
Click to expand...

The lateral FSD width is angular all the way to the threshold at which point it may optionally remain fixed at +/- 350 feet. An ILS is calibrated so that the localizer FSD is +/- 350 feet at the threshold. The result is that the length of the runway affects the localizer FSD angle since the antenna on a long runway is further from the threshold. There is no antenna on an LPV to contend with and a reference focal point is always the same distance from the threshold, regardless of runway length.

I believe you are referring to the GS which has this behavior. FSD for the GS is +/- 25% of the GS angle, so a 3 degree GS would have a vertical CDI FSD of +/- 0.75 degrees. Nominal for an ILS GS is +/- 0.7 degrees. There is an option that permits the GS to be a fixed value anytime the FSD is equal to or greater than +/- 150 meters to be set to +/- 150 meters. Also it may be fixed at +/- 15 meters anytime the FSD is equal to or less than +/- 15 meters. For LNAV/VNAV and LNAV+V the lower limit where the GS FSD may remain fixed is +/- 45 meters. It is the manufacturer's option to use an angle for the entire GS, but if they chose to do so, then the equipment is not compatible with meeting the Baro-VNAV requirements and is not permitted to support LNAV/VNAV procedures. Garmin supports LNAV/VNAV procedures being flown with their WAAS products.
 
John Collins said:
At this point, who else is there and I seriously doubt others when they get to market will implement it differently. The DO offers it as an option, but to me it is clear which they prefer.
Click to expand...

The high-end biz jets are beginning to get WAAS. There is some flexibility in the ARINC specs. Usually, their avionics/FMS is from Collins or Honeywell. I am not sure, but I think Collins goes with the taper down to + or - 3/10s at the runway for LNAV and LNAV/VNAV. It makes it more like a very good VOR located at the runway threshold.
 
azure said:
More than one CFII has told me that I should be timing from the FAF on an ILS as well, in case I lose the GS, but my own feeling is that if I lose the GS I'm going missed anyway and try to figure it out later. I don't like switching approaches in midstream.
Click to expand...

My CFII says this too, and I had a good discussion with him about it. One can imagine a scenario where you are on an ILS approach, low on fuel or engine starts running rough, ILS craps out, and at that point you REALLY wish you'd started that timer. But in 99.9% of cases if you lose the ILS the best thing to do is to go missed, slow down, think, rebrief the approach, and do it again from the beginning as a LOC. And maybe even depending on the weather, mental state, passenger comfort, etc, you choose to divert to an airport with a working ILS.
 
spinfire said:
My CFII says this too, and I had a good discussion with him about it. One can imagine a scenario where you are on an ILS approach, low on fuel or engine starts running rough, ILS craps out, and at that point you REALLY wish you'd started that timer. But in 99.9% of cases if you lose the ILS the best thing to do is to go missed, slow down, think, rebrief the approach, and do it again from the beginning as a LOC. And maybe even depending on the weather, mental state, passenger comfort, etc, you choose to divert to an airport with a working ILS.
Click to expand...
The way I look at it, I shouldn't be up there at all if things are so bad that I REALLY need to get in. Every approach that ends in a landing is just an aborted missed. If multiple things start failing, then it's really not my day.

Yes, I'd either start the approach again after going missed, or ask for a different, more reliable approach (like an RNAV) if one was available.

Any downside to starting a timer at FAF?
Click to expand...
Not that I can think of. It's just another task to add into the mix at an already busy point in the approach.
 
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