DME/TDME

[Airbusboy]

I am currently a student flying cross country. Although I don't entirely rely on them, I find DMEs so useful when tuned to a VOR freq, to locate myself on the map and know how far along the track I am etc.

My question is would the TDME aid me in anyway? Would it measure how far im from the airport if I am 10 miles inbound for example?

Thanks

[IO540]

Sure, you can use them. Just remember their range is generally much less; of the order of 10-20nm. Having said that, yesterday I was picking up Shoreham from ~50nm and that is common.

To be picky, the airport DME will be set up to read zero when you are at a certain place (not necessarily at the centre of the runway) and this can cause small errors (a fraction of a mile) if you are in an altogether different direction. I don't know of any cases where you would get huge errors.

Most of the time, a DME is within a fraction of a mile of the GPS distance, and that's plenty good enough for backup purposes.

[Airbusboy]

Thanks for the reply! Gonna try it out in my next nav ex :)

[A and C]

Just to put a little more flesh on the bones of what IO540 has said, the TDME is usualy paired with an approach aid like an ILS, the reading is usualy set to zero at the touch down point of the runway.

Most of the single runway airports in the UK have ILS on both ends but only one frequency so they switch off the ILS on the end that they are not using, the TDME is located at the runway mid point and is adjusted so it will read zero at the touchdown point at both ends (if you are nearer to the DME than the TDZ then you get an error flag).

There are moves afoot to stop this practice because it feeds errors into the FMS computor of airliners, most modern airliners use DME/DME as the primary navigation sorce, for this they are fitted with 5 DME units three of them permanently connected to the FMS system, this DME/DME fixing is more accurate than GPS untill they get hold of a TDME that has its own error (of half the runway length built in).

For your VFR navigation using the TDME is a very good idea for backing up DR & map reading, at the alitudes that you are flying DME range should not be a problem unless it says Narco on front of the DME box.

[IO540]

That's really interesting, A&C. I would have thought that the FMS navaid databases would have corrections built in for this; let's face it; it is pretty well known :)

Is DME more accurate than GPS? It is more robust for sure (interference wise) but I thought the resolution (and thereofre the accuracy limit, unless you can manage to interpolate) is 0.1nm.

unless it says Narco on front of the DME box.

Scarily accurate :ok:

The other problem with a DME is that the aircraft box can fail without any warning yet return plausible values. I used to fly with one such in my IMCR training, in a PA28-181. It always read something plausible between 3D and 10D - perfect for instrument approaches ;)

[A and C]

The DME will only show the pilot a reading down to 0.1nm but the FMC in an airliner sees the distance much more accuratly.

In my opinion IFR navigation will be all GPS with DME/DME back up in twenty years.................................now who has one of those Narco NS900 starnav's that the GA community did not understand (& buy) it was a system yeard ahead of its time.

(but I would interface it with a King DME!)

[Airbusboy]

Also guys, for the TDME, is it the same procedure, tune in the frequency e.g. Southend TDME(111.35) and centre the needle indicating TO ? Or can I just enter 111.35 ?

[IO540]

DME is just a box where you select the frequency and it reads the distance to the DME beacon. There is no TO/FROM flag or bearing to set etc - that is a VOR.

There is the VHF/UHF channel pairing thingy with DME but you don't need to know about that if you are a pilot because the equipment does the conversion internally.

[Airbusboy]

Ah yes! I was a bit confused. Thank you IO540

[Pace]

Airbusboy

The other point that hasnt been covered here is that DME is reading a slant angle so for instance if you are 3 miles high it will never read less than three miles even overhead.

GPS is giving you a distance between points regardless of slant angle.

Pace

[lasseb]

Most DME's has a "memory" build in in case they loose the signal.

This just means that they continuously calculate what the new range should be given the current GS towards or away from the DME.

So if the signal for some reason fails, the DME will continue to display theoretical calculated data for 5 to 10 seconds event though it has no signal.

If you continue to fly in the same general direction it will match quite nicely, but if you for some reason turn just when the signal is lost, you have 5 to 10 seconds of false indication.

[capt.sparrow]

most modern airliners use DME/DME as the primary navigation sorce

My aircraft uses GPS. Then DME DME. Then IRS.