It has to be like that (with a fake DME offset)
? The approach chart from the other side just shows the distance from that side, you don’t have to fake anything, do you?
Also, how does that work when the ILS and DME frequency are paired?
Peter, are you talking about the ILS DME being used for the outbound part of a procedural ILS, and/or the missed approach?
The outbound procedure is going to be on the same side as the ILS course anyway so I can’t see the difference being significant for something like “remain inside 10 DME on the procedure turn”.
And in the missed approach case, any difference would err towards staying nearer the airfield, so erring on the safe side.
That is unless there are ILS DMEs which are located off the airfield entirely to give several miles difference, and I very much doubt that. Normally they are offset by runway length or less, to give zero at the threshold?
Do you have a real world example where this is actually an issue?
Flyer59, take a look at say the Gatwick ILS plates to see where the DME is located, and the notes that say it is ranged to read zero at the threshold, not at its true location.
I think they do something with the DME delay setting to zero range set it?
Do you have a real world example where this is actually an issue?
Probably not for an ILS without doing a lot of digging but for sure there are many approaches where there is a “distant” DME whose displayed readings are used for stepdown fixes.
At a more basic level, sure, in most cases you won’t get killed if the “DME waypoint” you put into your GPS is half a mile off, but the point is that the GPS waypoint is simply not correct. And I am not even talking about the slant range error which is usually miniscule anyway.
Balliol wrote:
I think they do something with the DME delay setting to zero range set it?
Yes!
At a more basic level, sure, in most cases you won’t get killed if the “DME waypoint” you put into your GPS is half a mile off, but the point is that the GPS waypoint is simply not correct.
If I wouldn’t have the DME, I’d simply have the approach chart on the MFD. If you open the view with both the lateral track and the profile you just check the altitudes when you overfly the checkpoints … 4.0 DME … Alt xy … If you have the right altitude at the FAF, stay exactly on the GS and double check with the active chart … I’d say the precision is exactly the same. I always do that anyway … and then check if the DME says the same.
(No, Achim, no need to tell me how much the fine can be :-) After all i invested the 10 K …) I am just sayin’ that with the modern possibilities to check the altitude the result is the same. (Yes, I know it’s illegal :-))
Flyer59 has it.
If you fly a procedure with a GPS, you can fly the overlay in the GPS, you won’t need the underlying nav-aids (VOR, NDB, DME, whatever) at all. Just keep the HSI needle centered and twist the knob in time at turning points. Depending on your attitude to prudence and law, you will monitor the underlying nav-aids while doing that, or even pretend you are using them as primary. Except – I wouldn’t follow it on the moving map, it would be loaded in the flight plan and I would follow it as I would follow the en-route portion of the flight plan.
So the only time the DME is relevant at all is on final approach after passing the final approach fix. You don’t need it for the FAF itself because it is in the database. The only thing you cannot reliably do is do an altitude / distance check based on the DME while on the final approach.
In theory, there is the runway threshold point – RWxx, part of the procedure, and the distance from that point can be used instead of a DME that reads zero at the threshold. No need to fiddle with identifiers etc, it will be right on the NAV page of the GPS, or on the EFIS if you have one.
The snag is – not all DME are zero-ranged, no all approaches have a RWxx waypoint coded, so you need to do some homework.
Personally – I would buy the DME. It’s not cheap, but it takes the hassle out of it. But I also subscribe to Jeppesen because I like charts that I can fly right out of he folder (or right on the iPad, these days) instead of squinting for small print and calculating minima.
The ADF, however – just buy a dead unit, cut off the display, and stick it somewhere to your panel with double-backed tape when the inspector comes. It will be just as useful as a unit that works, but a lot cheaper.
Peter, now you are talking about “step down fixes” which means:
A) You are talking about the inbound course again, so I maintain my previous argument that as long as the DME location is coded into IFR databases as the intersection of the zero range circle and the inbound course, there will be no issue.
B) You are talking about non-precision approaches only. And from this we can exclude VOR and NDB approaches which would surely never have a DME offset (since also used for enroute nav). Which means we are only talking about this issue being relevant for localiser-only approaches anyway?
I agree with Flyer59 that there are safe alternatives to using DME (legal or otherwise). What I am trying to understand is, are there any technical reasons why GPS-for-DME substitution can’t be done properly/officially/safely in Europe, without redesigning some approaches. I can’t see any issues, other than perhaps on a missed approach that uses the DME.
ortac wrote:
What I am trying to understand is, are there any technical reasons why GPS-for-DME substitution can’t be done properly/officially/safely in Europe, without redesigning some approaches. I can’t see any issues, other than perhaps on a missed approach that uses the DME.
Precision approaches require check altitudes. Therefore you have to combine two technologies, typically ILS and marker or ILS and DME. You could design approaches with other methods such as ILS and RNAV but that does not exist.
I doubt EASA will give up on the check altitude.
I completely understand the need for the check altitudes.
My question is, why can’t GPS serve this purpose? Why can’t the DME location in the GPS database be encoded to provide check altitude information accurately? If the DME location is coded as the intersection of the “zero DME circle” and the localiser inbound course, it will provide everything that is required, no new approach design required.
Several people on here seem to be saying that “using GPS in lieu of DME is dangerous as GPS will give an incorrect reading in the case of an offset DME”, based on the (possibly incorrect) assumption that DMEs are always encoded in the GPS database at their physical location.
