GPS and VLOC course guidance discrepancies...

[fiveoboy01]

So I have been doing a good number of non-precision approaches through my remedial training and a lot of these I'm flying right to a VOR for the IAF and then flying outbound, completing the procedure turn to get back inbound.

I have a 430W and so I do load the approach into it. I've been using the CDI screen on the GPS more and more, and of course NAV #1 is set to VLOC.

Lots of times if I'm within a few miles of the VOR, the CDI will show me to one side of the course, and the GPS will show me I'm on the other side. I figure there are a few reasons for this, VOR sensitivity, or I might have the OBS set a degree off, etc...

So legally, I need to follow the CDI with the VLOC data, correct? If someone asked me which one was more accurate, I'd say the GPS as the sensitivity doesn't change as you get closer/farther from the navaid, but my memory tells me that legally I need to be using the navaid itself for primary navigation and the GPS can only be backup in this instance.

But it confuses the hell out of me sometimes. And if it comes up on the checkride, I want to be able to explain properly to the DPE what's happening and how I deal with it correctly.

Would it maybe be better to not use the CDI screen on these non-precision approaches so I don't get conflicting course guidance?

Any input is appreciated. I might be overthinking it, which is what I like to do lol.

 

[dmspilot]

So legally, I need to follow the CDI with the VLOC data, correct?

Only during the final approach segment, in other words, from the final approach fix to the missed approach point. You may use GPS instead of VOR for the other phases of the approach. And if I am reading it correctly, if using a localizer, even if you're not on the final approach segment, you should at least have access to the raw localizer data.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf

 

[midlifeflyer]

There are any number of reasons, including the fact that the VOR radial may not be equal to the actual magnetic direction. VORs don't read compasses; their radials are set and periodically (very periodically). Or, the VOR receiver is off by the maximum regulatory amount allowed by the 30-day VOR check rules (you did the check, right?). And, (just in case) when you press the CDI source on the GPS box and change it to VLOC, it is a VOR or LOC, not a GPS-derived representation of one.

So, if you have two navs, one a GPS with the ability to select VLOC and a nav2 with just a plain old VOR, it doesn't matter which CDI you follow.

But that line on the moving map? It's not what you follow, even on a GPS approach. Moving maps are situational awareness tools, even on a fully certified IFR box. You are supposed to follow the CDI, whether GPS or VOR or LOC or....

 

[James331]

For anything other then an ILS/LOC, GPS overlay while watching your VOR or NDB to keep legal is the way to go.

 

[John Collins]

You can legally substitute the GPS guidance for the approach up to the FAF on a VOR approach. Once on the final approach segment, you must use your VOR via VLOC CDI as your guidance unless the approach has "or GPS" in the title. The GPS is generally more accurate. It can be used for situational awareness when you are on the final approach segment and can also be used to identify along track position, but the VOR CDI must be used for lateral guidance. However, I don't use VOR for approaches anymore as there is almost always a better RNAV option, so it is mostly academic for me.

 

[fiveoboy01]

There are any number of reasons, including the fact that the VOR radial may not be equal to the actual magnetic direction. VORs don't read compasses; their radials are set and periodically (very periodically). Or, the VOR receiver is off by the maximum regulatory amount allowed by the 30-day VOR check rules (you did the check, right?).

Yes, that was one other reason for the difference that at the time had escaped my brainm at the time of my OP.. the allowed discrepancy(thanks for the reminder, we talked about it earlier this week and it didn't get done).

And, (just in case) when you press the CDI source on the GPS box and change it to VLOC, it is a VOR or LOC, not a GPS-derived representation of one.

Yessir, that I knew.

So, if you have two navs, one a GPS with the ability to select VLOC and a nav2 with just a plain old VOR, it doesn't matter which CDI you follow.

Understood. I know that the source is the source whether it goes through the GPS or my nav #2, but if you switch the GPS to VLOC, you'll get that data on Nav head #1 but you will still see GPS data on the CDI screen of the box itself. If that's incorrect, someone correct me now, but I think I have it right, otherwise there's no reason for the conflicting course guidance.

But that line on the moving map? It's not what you follow, even on a GPS approach. Moving maps are situational awareness tools, even on a fully certified IFR box. You are supposed to follow the CDI, whether GPS or VOR or LOC or....

That's correct, which is why I have been using the CDI screen on the GPS(even for GPS approaches), and not the moving map. It took some getting used to, but I find it to be more helpful in conjunction with the CDI needle.

I appreciate the clarification on where I can use GPS data. I knew that you could use it on some segments of a non-GPS approach, but wasn't sure exactly where you had to switch.. the 430W switches it automatically if I don't get it first - I've watched it happen a few times. I need to dig into the manual and see where exactly it does this. I assume when you're within a certain distance from the FAF and/or close to being on the final approach course.

 

[James331]

You can legally substitute the GPS guidance for the approach up to the FAF on a VOR approach. Once on the final approach segment, you must use your VOR via VLOC CDI as your guidance unless the approach has "or GPS" in the title. The GPS is generally more accurate. It can be used for situational awareness when you are on the final approach segment and can also be used to identify along track position, but the VOR CDI must be used for lateral guidance. However, I don't use VOR for approaches anymore as there is almost always a better RNAV option, so it is mostly academic for me.

Book yeah, but real world most everyone flys the overlay and monitors the NDB/VOR, as long as the NDB/VOR doesn't go crazy folks typically fly the overlay (as in nav head 1 set to GPS and nav two head set to VOR/NDB) all the way in, most times its a lot more accurate and with GPSS much smoother too.

Agree 100%, as times goes on most places with VOR or NDBs are ether going to be replaced with GPS approaches or have GPS approaches in addition.

 

[fiveoboy01]

Yeah I won't be flying many of those as long as there is a GPS available, but it's good to know what's going on with them in case I have to. Airplane is GPSS equipped as well, so there's that.. but I'm hand flying at this time. Some days I do a couple hours of flying with foggles on and I walk away feeling like I know so little.

 

[MAKG1]

Yes, you're right. Between FAF and MAP, you must follow the VOR (or NDB or LOC or DME). "VOR twist" can cause them to differ, by several degrees. Make sure you did set the OBS correctly -- down to the degree -- and follow it. The approach (and its minimum) was designed for that. And sometimes, having everyone make the same error is better than having some people improve on it.

 

[MAKG1]

Yeah I won't be flying many of those as long as there is a GPS available, but it's good to know what's going on with them in case I have to. Airplane is GPSS equipped as well, so there's that.. but I'm hand flying at this time. Some days I do a couple hours of flying with foggles on and I walk away feeling like I know so little.

Just a bit of real world experience -- it really sucks to lose the GPS in the clouds on a GPS approach. BTDT. If you're on a VOR approach or especially ILS approach with a GPS overlay, you can usually salvage it minimally using backup receivers. Having said that, VOR approaches with GPSS (especially VNAV) are the highest workload approaches of them all. ILS, not so much.

 

[John Collins]

Yes, you're right. Between FAF and MAP, you must follow the VOR (or NDB or LOC or DME). "VOR twist" can cause them to differ, by several degrees. Make sure you did set the OBS correctly -- down to the degree -- and follow it. The approach (and its minimum) was designed for that. And sometimes, having everyone make the same error is better than having some people improve on it.

Just an FYI, there is another source of error, that is the difference between the radial dialed on the OBS verses the value read by the avionics. So you set the OBS to precisely 270, but the avionics such as the Nav board interprets it as 268 degrees. The calibration has introduced a 2 degree error.

 

[MAKG1]

Just an FYI, there is another source of error, that is the difference between the radial dialed on the OBS verses the value read by the avionics. So you set the OBS to precisely 270, but the avionics such as the Nav board interprets it as 268 degrees. The calibration has introduced a 2 degree error.

Absolutely. But a 430W displays the OBS setting on the self test. That can (and should) be noted upon engine start.

 

[midlifeflyer]

Y
Understood. I know that the source is the source whether it goes through the GPS or my nav #2, but if you switch the GPS to VLOC, you'll get that data on Nav head #1 but you will still see GPS data on the CDI screen of the box itself. If that's incorrect, someone correct me now, but I think I have it right, otherwise there's no reason for the conflicting course guidance.

The magenta line on the nav screen is GPS-driven. The CDI, whether on the box or remote, is driven by whatever you select as the nav source.

I appreciate the clarification on where I can use GPS data. I knew that you could use it on some segments of a non-GPS approach, but wasn't sure exactly where you had to switch.. the 430W switches it automatically if I don't get it first - I've watched it happen a few times. I need to dig into the manual and see where exactly it does this. I assume when you're within a certain distance from the FAF and/or close to being on the final approach course.

I'll add my voice to the the ones who already mentioned this. You may use GPS data for everything except lateral guidance on the final approach course. What I think most do (in the absence of a GPS that accomplishes the switching itself) is use GPS data until in a position to intercept the extension of the final approach course. That would be on the final intercept if vectored or on that last turn to intercept when doing a PT or HILPT. There is some variation but a lot of folks use that since the location ends up being the same whether vectored or doing the full approach. What you choose is yours to make so long as you are using VLOC data before crossing the FAF or intercepting the glideslope on an ILS.

 

[John Collins]

Absolutely. But a 430W displays the OBS setting on the self test. That can (and should) be noted upon engine start.

Good point, but rarely done. Also a single reading does not provide the error as you move around the compass rose. Unfortunately, most pilots don't stop long enough on the self test page to read it, much less verify all the feedback it provides.

 

[fiveoboy01]

The magenta line on the nav screen is GPS-driven. The CDI, whether on the box or remote, is driven by whatever you select as the nav source.

Not trying to be argumentative, but why then would the remote show me to one side(say two dots) of course and the CDI on the GPS screen says I'm on the opposite side? I've twiddled with the OBS to make sure it's set right, with the same result.

I do use the self test page on boot up. If there's something wrong with the unit there is an early indication right in front of me. Better found there than in the air.

 

[MAKG1]

Not trying to be argumentative, but why then would the remote show me to one side(say two dots) of course and the CDI on the GPS screen says I'm on the opposite side? I've twiddled with the OBS to make sure it's set right, with the same result.

Because the default NAV page is always GPS, and you have the remote CDI set to VLOC. They are reading different things. Use the remote.

If you mess with the OBS knob, you can always make it agree (for a VOR), but that's not recommended.

 

[midlifeflyer]

Not trying to be argumentative, but why then would the remote show me to one side(say two dots) of course and the CDI on the GPS screen says I'm on the opposite side? I've twiddled with the OBS to make sure it's set right, with the same result.

I do use the self test page on boot up. If there's something wrong with the unit there is an early indication right in front of me. Better found there than in the air.

Sorry. My bad. Brain fart. @MAKG1 is absolutely correct. The VLOC head drives the external OBS, not the one depicted on the GPS. That's why the Garmin manual tells you to use the external CDI for non-GPS approaches and even cautions against it. For example:

NOTE: The external CDI (or HSI) must be coupled to the VLOC receiver
for approaches which are not approved for GPS.
See the ILS example in the Procedures section​

 

[fiveoboy01]

Thank you. I thought that was the case, but I try not to argue with people who have vastly more knowledge than me

I appreciate the input. Learning to use the equipment has been almost as much work as the instrument flying itself.

 

[midlifeflyer]

Thank you. I thought that was the case, but I try not to argue with people who have vastly more knowledge than me

I appreciate the input. Learning to use the equipment has been almost as much work as the instrument flying itself.

Nice of you to say but I'm just SGOTI. As the saying goes, "trust but verify."

 

[John Collins]

For a VOR navigation with VLOC selected, the CDI course is selected by the OBS setting. For GPS navigation, the GPS course for leg mode of operation is defined by the great circle route between the from and to waypoints being navigated. If the GPS navigation is via direct-to, the from waypoint is the latitude-longitude where the aircraft was at the time of the direct-to navigation was activated. Only if the GPS is in OBS mode will the OBS affect the selected course for GPS navigation and even then the magnetic course can be different from a VOR radial with the same OBS setting because VOR's once sited or last aligned use the magnetic variation of the time it was accomplished, whereas the GPS uses the current variation. The difference can be up to 6 degrees. If the GPS is in OBS mode to a VOR, then variation will be used that is the same as of the time that the last alignment of the VOR took place.

 

[azure]

Sorry. My bad. Brain fart. @MAKG1 is absolutely correct. The VLOC head drives the external OBS, not the one depicted on the GPS. That's why the Garmin manual tells you to use the external CDI for non-GPS approaches and even cautions against it.

If true, that's different from the 480 then. On the 480, the internal pseudo-HSI on the NAV page is driven by whatever source you have selected with the CDI toggle. If the source is GPS, the GPS drives the CDI. If it's VOR/LOC, the internal nav receiver drives the CDI.

I think I prefer the 480's functionality.

 

[EppyGA]

Good point, but rarely done. Also a single reading does not provide the error as you move around the compass rose. Unfortunately, most pilots don't stop long enough on the self test page to read it, much less verify all the feedback it provides.

It's on my check list prior to an IFR flight. Also the half left, no flag on the CDI; half up, no flag on the CDI; To/From flag should be TO; desired track should show 149.5 (except it can't do decimals) and then I crank on the CDI to make sure the selected OBS course tracks.

 

[TheGolfPilot]

You have to tune the VOR to stay legal but you can use the GPS or even a handheld to supplement it to fly more accurately. If the VOR isn't tracking, going missed would probably be the smart thing to do but 99.9% or maybe more (when tuned correctly) of the time it will stay center or close to center as you follow your GPS.