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.