DME soft key

[Rich]

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.

DME involved or not, missed approach points for many nonprecision approaches are too close to land, as MDAs can be well above the surface. You'll note that very often the GPS/DME discrepancy is far below the 0.1 nm resolution and often is of the order of the dimensions of the airplane itself. What's real fun is breaking out where the runway is well off to the side (and maybe well below to boot), due to the nature of the approach itself. Here in the States the LDA-A for KVUO and GPS-A to KLDJ have such interesting challenges.

[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.

DME involved or not, missed approach points for many nonprecision approaches are too close to land, as MDAs can be well above the surface. You'll note that very often the GPS/DME discrepancy is far below the 0.1 nm resolution and often is of the order of the dimensions of the airplane itself. What's real fun is breaking out where the runway is well off to the side (and maybe well below to boot), due to the nature of the approach itself. Here in the States the LDA-A for KVUO and GPS-A to KLDJ have such interesting challenges.

I believe there are many approaches like that around the world, here's one from my neighbourhood. It's IGS approach (like ILS but not aligned with runway direction) and I've flown it several times but never in conditions close to published minima. My lowest break was around 1600 ft (some 1400 AGL).

[chili4way]

Bold Method published a nice write up on DME today.
https://www.boldmethod.com/learn-to-fly ... s-and-vfr/

[ememic99]

A quote from text:

If you want GPS DME to match traditional DME on an ILS approach, you could type in "Direct IXXX" to get distances that match your approach chart.

This simply isn't correct and in Europe it's not legal.

[chili4way]

I agree.

I think the example was intended to explain that when the FAF is 5 miles from the LOC antenna and your plane is 2 miles further out from the FAF (presumably just passing a step-down fix defined as DME 7), the GPS distance would be displayed to the FAF (2 miles) instead of what a real DME would display (7 miles).

I think their example was intended to be theoretical (not operational) and explain why the GPS distance wouldn't match what the approach plate shows as the DME distance.

At least they had the sense to include the following immediately after their unwise (and illegal) example: "We don't recommend you do it, however, because you might end up placing a direct-to line on your map, leading to confusion about which course you're following to the runway."

Bad explanation & example!

[CFIDave]

I often set GPS distances in my G1000 Flight Plan page to CUM to display approach waypoint cumulative distances to the runway, rather than show individual leg distances. I think it improves situational awareness and makes it easier on the radio to tell the tower or CTAF how far out you are (and is closer to what a real DME might give you).

I'm just frustrated that the G1000 won't remember this as a preference -- requiring me to re-set the FPL page to CUM every time I load an approach.

[ememic99]

I always set GPS distances in my G1000 Flight Plan page to CUM to display approach waypoint cumulative distances to the runway, rather than show individual leg distances. I think it improves situational awareness and makes it easier on the radio to tell the tower or CTAF how far out you are (and is closer to what a real DME might give you).

I'm just frustrated that the G1000 won't remember this as a preference -- requiring me to re-set the FPL page to CUM every time I load an approach.

I use the same setting but my G1000 remembers the last chosen option.

[Rich]

I often set GPS distances in my G1000 Flight Plan page to CUM to display approach waypoint cumulative distances to the runway, rather than show individual leg distances.

With the 530/430 combination I can put 3 fields on the 530 and the remaining 2 options on the 430. So I put the track and distances on the 530 with the times on the 430.

There's also the NRST chapter where you can simultaneously display running distance/bearings/etc. to a bunch of VORs (and other stuff) that may or may not be on your flight plan.

[Boatguy]

I think the example was intended to explain that when the FAF is 5 miles from the LOC antenna and your plane is 2 miles further out from the FAF (presumably just passing a step-down fix defined as DME 7), the GPS distance would be displayed to the FAF (2 miles) instead of what a real DME would display (7 miles).

Bad explanation & example!

I don't see the conflict here as the PFD will display both distances.

The nav status bar at the top of the PFD will show the distance to the next waypoint in the approach; 2nm to the FAF in your example.

One of the Bearing displays (1 or 2) below the CDI/HSI will show the GPS distance to the LOC itself, which in your example is 7nm.

[blsewardjr]

With my 530WAAS/430NON-WAAS, I have Direct to Destination in my 430 and display the distance to destination as a way of being able to know how far out I am for announcing on the approach.