Distance Measuring Equipment

[El Cid Av8or]

When flying an IFR approach (such as a VOR/DME approach), does the DME equipment have to be an actual DME receiver. Could it not legally be a GPS or Loran receiver as well?

 

[Singlecoil]

I know GPS suffices, but I doubt LORAN does. I would never trust LORAN to let down, which is what you would be trusting it to do on a VOR/DME approach. GPS seems to work or it doesn't, LORAN will lie to you.
I seem to recall that even a VFR GPS is legal for DME, but I can't back that up with anything.

 

[midlifeflyer]

I seem to recall that even a VFR GPS is legal for DME, but I can't back that up with anything.

Because it's not. In order to substitute for a primarycomponent for IFR navigation, the GPS must be IFR-certified.

What you may have heard is that there is nothing wrong with using a VFR GPS, even a handheld, as a situational awareness backup.

 

[100LL... Again!]

Check the FAR/AIM in the navaid section of the AIM.

Unless, like most pro pilots, your FAR/AIM is a 1996 model, and you got by with making your students keep theirs up-to-date (my method).

 

[100LL... Again!]

Which approach are you referring to?

Are you talking about direct overlay (in other words on the same plate)?

Or a stand-alone GPS that is for the same runway?

Curious on this one.

 

[172driver]

How do you set the GPS up to shoot a VOR/DME approach (not an overlay)? Is it good enough to step down when the GPS says you are over the fix or must you set it up to measure distance directly from the VOR?

In other words, is it good enough to simply load the approach, which will give you distances to each of the fixes or do you have to plug the VOR identifier into the flight plan?

Another one...can you fly airways using GPS navigation or do you have to use VOR's?

 

[Wiggums]

If you want to use GPS for a DME/VOR approach just tell the GPS to go direct to the VOR you want DME from. I use that distance to determine when to begin a stepdown, and have never had a problem. On the Garmin 430s you can load non-GPS approaches (like an ILS), but if you do that it will give you the distance to the next waypoint instead of the total distance to the VOR. Obviously, you can't use the GPS to navigate on non-GPS approaches.

If your GPS is IFR enroute certified you can fly airways using the unit. On the Garmin 430s the airways aren't pre-programmed, but you just add the VORs to your flightplan, and any fixes if the airways have bends. Then just couple the autopilot, sit back and relax.

 

[Singlecoil]

It's a VOR/DME or GPS approach (same plate) with multiple stepdown fixes after the FAF. If doing the VOR/DME, stepdown at 2.7 miles, if the GPS, stepdown at 2.0 miles. If you stepdown at 2.7 on the GPS you will be in my bedroom, or in a football stadium...

I saw your earlier post on this and I found it quite surprising. Does this happen in more than one airplane? Are you sure the GPS is not showing distance off the airport, or distance of the missed approach point? If, as was pointed out by Wiggums, you loaded the approach, then it might be showing you DME from the MAP. That .6 miles is just a huge error and way bigger than anything I've ever seen in 9 years working with GPS. I'm certainly not trying to attack your competence and hope that is not how this is received, I would just like to get to the bottom of this one.

 

[172driver]

If you want to use GPS for a DME/VOR approach just tell the GPS to go direct to the VOR you want DME from.

I understand that it can be done this way and that is how I've always done it. However, if a VOR/DME approach is in the database, can I not use the fixes themselves? Or, do I absoutely have to load the navaid and use the 'DME' from it? It is a little more work and the AIM and AC 90-94 seem to support using the fixes alone.

Anyone know any good reference books out there for GPS, especially the 430? The AIM just seems way too foggy on a lot of this stuff. For example, it lists all the substitutions and limitations for GPS but never once mentions navigating airways/VOR's enroute. Obviously, if you can get cleared direct or VOR to VOR, this is OK but what about the airways? Makes sense to just plug them in the FPL, but is it legal and why is it not addressed?

 

[midlifeflyer]

An IFR GPS is legal to use in place of standard DME. However, it can be tricky sometimes when you have a GPS approach overlaying a VOR-DME approach (or any other approach). The DME stated for the VOR-DME approach could be way off compared to the GPS. We have this at our home airport. VOR on the field, and 0.6 miles difference between the stepdown fixes for the approaches.

That's interesting. I'm surprised to hear that the difference could be that great unless the pilot was tuning in the GPS to the =airport= rather than the VOR. (That's not an uncommon mistake).

You made me curious so I ran a quick (simple Pythagorean) calculation in Excel. Unfortunately, not knowing the slant vs actual distance, I had to do it "on its side", calculating a distance based o knowing the DME distance and altitude.

I used the BJC VOR/DME approach in the Denver area as an example. The IAF/FAF is BIZEN, which is 7 DME from BJC VOR. It is at 1375 AGL. Plugging in the numbers, I get a calculated slant distance of 1375.018, a difference of .018 of a foot.

Closer in, using the 1.9 DME, 305 AGL MAP, I get an even smaller difference, .0059 of a foot.

In both cases, the difference is less than a thousandth of a percent.

Taking a typical 8 NM from FAF to MAP, even a 1% difference (a 1000 times greater error than I found) would be a difference of less than 1/10th of a mile. If you're getting a .6 NM difference, that would be a 7.5% error, or 75,000 times as great as the backward calculation I made.

I probably am missing something here. Any =real= mathematicians out there.



PS: I edited this post because I found an error. I forgot to convert the lateral distance to feet, so the numbers were way off. But even with the cange, I still get only error in tiny fractions of a percent.

 

[Singlecoil]

If you want to use GPS for a DME/VOR approach just tell the GPS to go direct to the VOR you want DME from.

I understand that it can be done this way and that is how I've always done it. However, if a VOR/DME approach is in the database, can I not use the fixes themselves? Or, do I absoutely have to load the navaid and use the 'DME' from it? It is a little more work and the AIM and AC 90-94 seem to support using the fixes alone.

Anyone know any good reference books out there for GPS, especially the 430? The AIM just seems way too foggy on a lot of this stuff. For example, it lists all the substitutions and limitations for GPS but never once mentions navigating airways/VOR's enroute. Obviously, if you can get cleared direct or VOR to VOR, this is OK but what about the airways? Makes sense to just plug them in the FPL, but is it legal and why is it not addressed?

Yes you can use the fixes themselves on a VOR/DME approach, but you run the risk of getting yourself confused as the mileage will read differently from what is on the approach plate. Many approaches are being re-certified with named fixes for all DME points to lessen the confusion. The name will appear on the chart and on the GPS so you know what the mileage is reading from or to.

You can certainly use GPS for airways. Many FMS/GPS systems allow to load the actual airways. You are navigating the airway solely by GPS. ATC doesn't care how you do it, as long you meet the Required Navigation Performance, or RNP. For enroute nav, you need an RNP or 2.0, or 2 nautical miles. The GPS somewhere should be able to tell you what the Actual Navigation Perfomance, or ANP, is right now, based on existing satellite coverage. So it would be more correct to say you need an ANP of 2.0 to navigate enroute.
The magnetic course for an airway printed on the chart may not match what you see in the GPS when you load the airway either out of the database, or by going VOR to VOR. Why is that? It is because the magnetic variation has changed since the last time the airway was surveyed and flight checked. Trust your GPS. I think the VOR's are only recalibrated every 5-10 years or so. In the northwest, the airways out of Seattle were 3-4 degrees different from what the GPS was showing. Then the feds came along and recalibrated the whole area and voila, they suddenly matched the GPS courses.
If you had set your GPS up to fly the published course from or to the VOR, you would have been diverging on radar. Always set up a GPS route with fixes or navaid names out of the database, and not with courses you type in off the chart.

 

[cocknbull]

When shooting a VOR/DME approach you must use the DME specified on the approach chart regardless if you are using DME equipment or GPS. The Fix will have a "D" with the distance in the "D" with the DME identifier over that. You may not put individual step down fixes in the GPS you must put the identifier for the DME used. You will not hit a house or anything else as long as you are on the right path and altitude. Infact their is more error from DME equipment to other DME equipment then using an IFR certified GPS. Cycling through different waypoints or fixes when flying a VOR/DME approach is illegal because the approach chart does not tell you to identify them by GPS waypionts but by DME from a specific source.

 

[midlifeflyer]

When shooting a VOR/DME approach you must use the DME specified on the approach chart regardless if you are using DME equipment or GPS.

Yes. I looked at the approach at http://edj.net/cgi-bin/echoplate.pl?EastCentral/CMI_vd_gr22.GIF.

Frankly, I don't see a GPS-defined MAP. All I see is a MAP co-located with the VOR.

I confess to not being very familiar with GPS, but isn't this approach a simple overlay? Aren't you supposed to be using the GPS in place of VOR and DME? Isn't the MAP the VOR no matter which set of avionics you use? Shouldn't the GPS be set for the VOR instead of the threshhold? Assuming you're flying this GPS-only, if you crank in the airport threshold into the GPS, aren't you not only creating the potential for a huge error, but not flying the approach correctly to begin with?

 

[Singlecoil]

Isn't the MAP the VOR no matter which set of avionics you use? Shouldn't the GPS be set for the VOR instead of the threshhold? Assuming you're flying this GPS-only, if you crank in the airport threshold into the GPS, aren't you not only creating the potential for a huge error, but not flying the approach correctly to begin with?

Right you are. Thanks for posting the link to the chart so we can have a look at this one. Using a slant range of 2.7 DME at 1400 MSL (651 AGL) and running the math yields an across the ground distance of 2.6979 nautical miles. Ergo, something is fishy here. Slant range DME vs. GPS DME does not make any sort of significant difference on this approach. To get the aforementioned .6 mile error, something definitely is set up wrong, or perhaps the GPS approach in the unit is programmed wrong by the company. This happens all the time. Somebody at the company makes a keystroke error and this is what you end up with when you load the new database.

 

[cocknbull]

If you are going to use the GPS for anything but DME from CMI then you need to ask for clearance for the GPS RWY 22. Then you would load the approach into the GPS and use the defined fixes by the GPS. If it is loaded right it will cycle through all of the fixes or waypoints that define the approach.

 

[cocknbull]

Someone stated the MAP was a RWY waypoint. This Chart has no runway MAP identified on it. The only MAP as depicted on this chart is the vortac. Make sure you have the right approach loaded into the GPS. If there was a RWY MAP for the GPS approach you would see a star or waypoint symbol on the approach chart with a [RW22] next to it.

 

[Rick1128]

Just finished writing a MEL. According to our FSDO, for 135 the answer is NO!!!!!! The transmitting unit has to be ground based. And the reg states DME, not DME or GPS. I have not found any Letters of Intrepretation to counter this position.

 

[Singlecoil]

Just finished writing a MEL. According to our FSDO, for 135 the answer is NO!!!!!! The transmitting unit has to be ground based. And the reg states DME, not DME or GPS. I have not found any Letters of Intrepretation to counter this position.

Your POI needs to start doing his job. The FAA approved substitution four and a half years ago! I hate worthless hacks like that who are too lazy to look something up when safety is at stake. When I flew 135, the company spent the money to put an IFR GPS in a 402, but we couldn't use it because when asked to give us a checkride on it, our POI said, "I don't even know what I'm supposed to be checking." Never could legally use the thing.
How about this link:
http://www.aopa.org/whatsnew/air_traffic/gps_in_lieu.html

 

[172driver]

Sure enough, when you pull up that approach on the GNS430, the waypoints are STADI (FA), RW22R (MA), CMI, LODGE(MH). Weird. The MAP should definitely be CMI, according to the plate, and RW22R should not be in the database for this approach, IMO.

This is a good example of my question too though. Can you descend when the GPS says you are at STADI or must you read DME from CMI and descend at 6 DME? I agree with Singlecoil...STADI suffices. Would the same logic apply if this was a VOR/DME approach only...not GPS approved.

 

[Rick1128]

Singlecoil,

Interesting. But please note on "the new policy, section the note: POI must approve. Good luck on getting them to sign off on anything except their paycheck.