DME soft key

[ememic99]

"DME" is distance measuring EQUIPMENT hardware that uses signals from some VOR/DME navaids to measure distance to the VOR. It's a popular OPTION in European Diamonds since it's required equipment in some countries (Germany?), and is turned on via a separate softkey on the G1000 PFD. But (along with ADFs) it's very rare to find in North American Diamonds unless the aircraft was originally owned by a European operator and then imported.

DME is mandatory equipment for all EASA registered aircrafts that perform IFR operations and I believe it's mandatory for N-reg aircrafts that operate in Europe under IFR.

In contrast, pilots sometimes refer to "DME distance" -- which can be measured by the G1000's GPS as a substitute for hardware DME (as shown in the photo above).

DME distance is different than GPS distance. Usually this difference is very small (DME measures slant distance) but in cases of offset ground station (rare installations) can be significant.

[Rich]

I'd be interested to see an approach where DME displayed slant-distance is significantly different than GPS (lateral distance). Consider this: Normal DME equipment only displays to 0.1 nm, which is a 600 ft. uncertainty right there.

[ememic99]

I'd be interested to see an approach where DME displayed slant-distance is significantly different than GPS (lateral distance). Consider this: Normal DME equipment only displays to 0.1 nm, which is a 600 ft. uncertainty right there.

A few examples from Europe:
NL Ponta Delgada 0.897NM
PR Porto 0.177NM
ILB Lisboa 0.189NM
ILI Lisboa 0.176NM

Check the plates for the airports LPPD, LPPR and LPPT for the above DMEs and you’ll see where the problem is.

The offset is programmed into DME, usually to show 0 distance at RW threshold although DME is not physically there while GPS lateral distance reflects the position of station itself.

[Rich]

I'd be interested to see an approach where DME displayed slant-distance is significantly different than GPS (lateral distance). Consider this: Normal DME equipment only displays to 0.1 nm, which is a 600 ft. uncertainty right there.

A few examples from Europe:
NL Ponta Delgada 0.897NM
PR Porto 0.177NM
ILB Lisboa 0.189NM
ILI Lisboa 0.176NM

Check the plates for the airports LPPD, LPPR and LPPT for the above DMEs and you’ll see where the problem is.

The offset is programmed into DME, usually to show 0 distance at RW threshold although DME is not physically there while GPS lateral distance reflects the position of station itself.

I haven't had any luck getting to any of these. US and Canada are pretty much the limit of my access. Would you have a screen shot?

[ememic99]

I'd be interested to see an approach where DME displayed slant-distance is significantly different than GPS (lateral distance). Consider this: Normal DME equipment only displays to 0.1 nm, which is a 600 ft. uncertainty right there.

A few examples from Europe:
NL Ponta Delgada 0.897NM
PR Porto 0.177NM
ILB Lisboa 0.189NM
ILI Lisboa 0.176NM

Check the plates for the airports LPPD, LPPR and LPPT for the above DMEs and you’ll see where the problem is.

The offset is programmed into DME, usually to show 0 distance at RW threshold although DME is not physically there while GPS lateral distance reflects the position of station itself.

I haven't had any luck getting to any of these. US and Canada are pretty much the limit of my access. Would you have a screen shot?

Here’s Porto LPPR, ILS RW17, been there, can be really challenging in low clouds situation with strong westerly winds from the ocean (prevailing) and mountains to the east of the airport. Following GPS lateral distance to physical location of DME instead of DME distance would clearly smash you to ground before runway threshold.

[Rich]

Emir,
I’m having a hard time with this plate, as the text is blurred to where I can’t read it. If I understand you correctly, you’re saying that using a DME number displayed in the GPS would have you closer to PRT than you think and therefore descend too soon? The reality is the reverse.

Ignoring difference in measurement uncertainties in the two technologies, if we consider a value “X”, “X” GPS distance will never be closer to the fix than “X” DME. “X” GPS will always be either further from or equal to the position of “X” DME from that fix. This is because DME is always measuring the hypotenuse of a right triangle, whereas GPS is measuring the horizontal side of that same triangle. The linear amount of discrepancy is a function of the angle which the two legs meet and the distances involved. For illustration, I attach part of the plate for a VOR-A approach to KRDM, which has similar characteristics to what I think you’re describing.

Suppose I fly the DME arc using my GNS530. I’m 4900 ft above and ten nm (~60,000 ft.) out from the VOR. But that’s using GPS distance, not slant distance. The angle is about 4.7 deg. Running the calculations, 60,000 ft DME would actually be about 59,800 ft. horizontally. But I’m not using DME and maintaining 60,000 ft. horizontally. Therefore a 200 ft. horizontal difference. Of course both displays only resolve to .1 nm so there is a several-hundred ft. “slop” for either.

Now I turn inbound and descend to 5900 ft. The actual discrepancy would start to climb and peak out around .5-.6 nm at VOR crossover. This is unimportant since crossover happens at flag flip. Now I can descend to 3660. This is roughly 600 ft above the VOR in my illustration (it’s actually less, this VOR is on a hilltop a couple of hundred ft. above field elevation). Was we get to the missed approach point the critical thing is that the angle gets very shallow, about ,94 degrees and the discrepancy drops to about .02%, with my GPS having me arrive at the 6.1 DME missed approach point about 5 ft. further than would 6.1 DME.

You're example might be clearer if I could read the text of the waypoints prior to VOR passage. In any case, the amount of discrepancy is important, driven by the angles at which they occur - a function of distance and altitude. For example, at 2000 ft above and 2 nm you'd be about 170 ft. early. At 90 knots that's about 1.3 seconds or so. Established at 600 FPM descent,that'd be maybe 15 ft.

[ememic99]

Here’s the plate again a bit zoomed - I hope it’s readable.

[Rich]

Here’s the plate again a bit zoomed - I hope it’s readable.

Readable it is. Understanding is a bit murkier, as it's laid out differently than I'm used to. Would it be the DME/ALT list for PR that's of concern? Some DME are of PRT and some are of PR (the DME for the localizer). It looks like the transmitter for the localizer DME SIGNAL is at the runway threshold. Do I have this right? Also I can't seem to figure out the MDA for the localizer approach>

[Rich]

Making some guesses about the LPPR approach (we're talking mostly about the LOC approach and there's an interesting transition between using PRT to PR DME), it looks to me like:

1. You'll be about 100 ft. farther out at the IAF using GPS vs. DME. Coming inbound at that 2.7 degree slope, you'll continue to be slightly farther out than DME, but it will gradually be less and less (about 60 ft. at 6.8 PRT DME)
2. I assume you have a Marker Beacon receiver. So no big issue at FAF anyway, but about 40 ft. early otherwise.
3. I'm guessing that the MDA is something like 800 AGL. Which means that you'd get the Missed Approach Point about 50 ft. early. This is either a good or bad thing but it's something like .4 sec of flight.
4. If, however, the MDA is really high (1749 AGL?). Then you'd wind up going missed about 250-ish ft. early. Still only a couple of seconds of flight time, though.
5. The Missed approach fix, at 6000 ft and 15 DME would be about 200 ft farther out using GPS as DME.

If you've got glideslope the MAP is the VOR itself, so station passage is the trigger. I don't see DME a factor there. Another thing: here on US charts, any waypoint with a name (and even many that don't) are charted in the nav data by lat/long and displayed in various ways, so you've got that additional information.

[ememic99]

You're right - DME station is pass runway threshold rather than in front of it, so you're kind of on the safe side (I mixed PRT and PR DMEs). It's similar in the other cases I listed.

However, if you are forced to land at LOC only minima (440 ft) and you aim PR (which is physically located practically at the end of runway) using GPS lateral distance, I seriously doubt you would be able to land. So, you're right - you won't crash before threshold but you'll miss the runway.

And even if you have GS and you follow it, can you imagine confusion in IMC when distances doesn't correspond to what's on the plate? I landed there at OVC080 and I know how challenging it can be.