GPS Approach question

Z

zmiller

New Member
So I've got a GPS approach question that may be pretty obvious...or maybe just confusing. If you have an approach-certified, moving-map GPS that has an external CDI (i.e. you can change the CDI on one of the VOR heads over to the GPS), can you legally fly an approach with the map page open on the GPS if you've got the external CDI linked to the GPS?

I've looked through the Garmin 430 pilot's manual and flight manual supplement, the AIM, and what I thought were the pertinent AC's, but I couldn't find a definitive answer. Anyone got a quick answer before I call Garmin tomorrow?

-Zach
 
Zach, I'm pretty sure you can. I always kept the 430 in map page. I think the thing is, garmin doesn't want people shooting the approach with the map displayed because someone might decide to use the map as a cdi instead of the actual cdi.

I broke my ankle, and I'm out 3-4 months. Sucks, huh?
 
The only way to legally fly the GPS approach would be to link it to your CDI. Following the purple line is not how to fly GPS approaches.
 
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The only way to legally fly the GPS approach would be to link it to your CDI. Following the purple line is not how to fly GPS approaches.

[/ QUOTE ]I think he was asking whether he can ffly using the CDI that co-uses the VOR head instead of the CDI on the GPS, not asking about flying the purple line.

I agree with viper. You can use either CDI.

.
 
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The only way to legally fly the GPS approach would be to link it to your CDI. Following the purple line is not how to fly GPS approaches.

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I'm very aware that following the purple line is not acceptable. MidlifeFlyer is correct, I'm asking if you could have the map page open for situational awareness but fly the approach off the external CDI. The word from Garmin technical support is that it's completely OK to have the map page open on a GPS overlay or stand-alone approach if you use the external CDI.


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I broke my ankle, and I'm out 3-4 months. Sucks, huh?


[/ QUOTE ]

It certainly does suck--sorry to hear that. I hope it heals faster than they're saying!
 
You need to always be getting your course information from the External CDI that you have slaved to the GPS (either CDI #1 or #2), so it doesn't matter if you have the GPS on the map page or Nav 1 page (for the garmin).

In one of the GPS manuals (I believe the KLN), they even recommend that you keep the GPS on the map page during the approach for situational awareness.
 
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You need to always be getting your course information from the External CDI that you have slaved to the GPS (either CDI #1 or #2), so it doesn't matter if you have the GPS on the map page or Nav 1 page (for the garmin).

[/ QUOTE ]I wasn't aware that a GPS installation even had to have a slaved external CDI? Can you point us to something that says that?
 
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You need to always be getting your course information from the External CDI that you have slaved to the GPS (either CDI #1 or #2), so it doesn't matter if you have the GPS on the map page or Nav 1 page (for the garmin).

[/ QUOTE ]I wasn't aware that a GPS installation even had to have a slaved external CDI? Can you point us to something that says that?

[/ QUOTE ]

I'll try to find that for you. I know I've read it somewhere, but for a GPS to be IFR certified, it has to be coupled with a CDI.
 
AC 20-138A Deals with this issue. Although it doesn't state that an external CDI is required, it does state on Page 23:


(5) Cross-Track Deviation Display. Minimum discernible movement, accuracy of the centered display, resolution of the electrical output, linearity of the display and/or electrical output, and display latency should be as specified in section 2 of RTCA/DO-229C for applicable navigation modes. Deviation display designs with display ranges and resolutions outside the bounds of the required values should be substantiated by demonstration, by reference to appropriate empirical data, or by similarity to previous certified range/resolution displays.

So, technically speaking, the aircraft doesn't need an external CDI, but I haven't seen a GPS unit that has resolution or display refresh rate required by DO-229C.

So, we have external CDI's!
 
Ok back to the question, is there anywhere that states you should not be on the map page or anywhere that states you must be on a certian page?
The reason im asking is because I have always used the map page for the approach, I just like it better because it gives a nice plain view (little bit more SA). However, I have just recently been bit*#ed at by some of the check airmen at the school. They seem to think that students shouldnt be using the map page. Their reasoning is that students will fly the line not the CDI, which makes sense. However, thats just something that you have to teach your students.
Any thoughts?
 
There's nothing that states that you must use a particular page on the GPS. However, a pilot should use the page that's going to give the most information that will assist him/her with the approach.

If you're concentrating all of your attention on the map, you can get pretty screwed up. Keep a normal scan going, and the map can be just one little addition to the scan.

Once I'm on the final approach segment, I fly the approach no differently than I fly a VOR approach. I keep the needle centered, and monitor my distance from the next fix. IMO, the map is irrelevant. As far as SA goes, if you get disoriented on a GPS approach, you're going to fly the published missed approach, right? So, why do you need to know where the Boondock VOR is with relation to you?
smirk.gif
 
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Ok back to the question, is there anywhere that states you should not be on the map page or anywhere that states you must be on a certian page?
The reason im asking is because I have always used the map page for the approach, I just like it better because it gives a nice plain view (little bit more SA). However, I have just recently been bit*#ed at by some of the check airmen at the school. They seem to think that students shouldnt be using the map page. Their reasoning is that students will fly the line not the CDI, which makes sense. However, thats just something that you have to teach your students.
Any thoughts?

[/ QUOTE ] So, their argument is that some students are morons, so all other students should be treated like they are morons too.

The only valid argument to not use the map page is to make sure your student can keep their situational awareness if they do not have a moving map. This is not really a big deal to most, but if you have a student who is relying on the map too much, turn it off.

Some check airmen like to turn off the map page on partial panel approaches as well, just to make sure the student can keep their SA with out the map.
 
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They seem to think that students shouldn't be using the map page. Their reasoning is that students will fly the line not the CDI, which makes sense. However, that's just something that you have to teach your students.
Any thoughts?

[/ QUOTE ]It's a variation on the "don't allow students to use GPS for primary training" theme. I can understand the sentiment form a training perspective. There =will= be a tendency to fly the map rather than the CDI. But I don't think the answer is to avoid using it. You might =start= by teaching without it, incorporate it a bit later and, needless to say, "fail" it periodically as a cross-check.
 
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So, their argument is that some students are morons, so all other students should be treated like they are morons too.

The only valid argument to not use the map page is to make sure your student can keep their situational awareness if they do not have a moving map. This is not really a big deal to most, but if you have a student who is relying on the map too much, turn it off.

Some check airmen like to turn off the map page on partial panel approaches as well, just to make sure the student can keep their SA with out the map.

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yeahthat.gif


As an instructor, I don't know why an other instructor wouldn't want you to use all means necessary to keep situational awareness. If you can fly an NDB/VOR/ILS approach without a map then you can fly a GPS without one too, but obviously if you are flying a GPS approach then you have a moving map in your cockpit! If your GPS screen fails then you obviously would not be able to continue the approach anyway since there would be no way of telling if the approach is armed, if you have RAIM and no way to identify the fixes.
 
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If you can fly an NDB/VOR/ILS approach without a map then you can fly a GPS without one too, but obviously if you are flying a GPS approach then you have a moving map in your cockpit!

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Not necessarily, there are GPSs out there that do not have moving maps and yet are approach certified.

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If your GPS screen fails then you obviously would not be able to continue the approach anyway since there would be no way of telling if the approach is armed, if you have RAIM and no way to identify the fixes.

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Not necessarily, there are many installations that use an external MFD that displays everything you need. As long as you got the approach loaded and armed before the GPS screen died, your home free.

Even in installations that don't have an MFD to display the loss of RAIM, you don't always need the GPS screen. When we're flying a GPS approach, we can have the fixes displayed on the MFD, the distance ticks down on our HSI, and if the FMS loses RAIM, then the HSI (which was displaying course information in white to signify GPS data) reverts to green needles showing that it's using VOR data. We never need to look at the FMS screen.

The problem with GPSs is that there is no standardization. Every installation is different. In some installations, you have to have the GPS on a specific page to get all the required information. In other installations, you could have a movie playing on the GPS screen for all the good it's doing you. There is no general rule that you can use for what needs to be shown on the GPS screen.

Do you need to see distance to the next fix? Maybe maybe not. Some GPSs show each step down as a seperate fix in the database. Distance is trivial in this case, since the GPS will indicate when a fix is reached, at which point you step down to the next one. Other GPSs don't show each stepdown as a seperate fix, and require you to monitor distance from another fix to determine when you've reached a stepdown.

Can you have the GPS in map mode on an approach? Sure you can. There is no rule saying you can't. To have such a rule would be dumb, on some GPS models, the only way you can get the required data is to be in map mode.

As for the accuracy of following a magenta line on a moving map as compared to an HSI, try zooming all the way in on the map of a 430/530. It's more precise than an HSI is, but the refresh rate isn't very good.
 
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So, we have external CDI's!

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Oh, well. Guess our new G1000 172 isn't IFR certified since it doesn't have an external CDI.
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So, we have external CDI's!

[/ QUOTE ]

Oh, well. Guess our new G1000 172 isn't IFR certified since it doesn't have an external CDI.
smile.gif


[/ QUOTE ]

Oh, but it is! If you read the specific verbiage of AC 20-138A, and airplane doesn't neccesarily have to have an external CDI, assuming that the display meets the requirements listed. Clearly, the LCD panels in the G1000 aircraft will meet the requirements!

They have a bit more resolution than the itty-bitty screen on the GNS-430 or 530. Heck, they're approved to be the primary flight display! Not all CDU's can do that, though.
 
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