I would never fly IFR without a DME. Besides being required for many procedures, it is useful for cross checking the RNAV/GPS position. It is also dead simple to operate (exception: in the G1000 Garmin has managed to make the incredibly simple operation of DME and ADF quite complicated.) Last but not least DME is going to stay in the system for a long time, unlike the NDBs and VORs that are being decomissioned at a steady rate and in reality are nearing extinction. In contrast new DMEs are actually popping up in some terminal areas, to feature ground-based P-RNAV, I believe.
By the way, NDBs used for missed app can be navigated using RNAV in Denmark – it was on Notam for almost 6 months while the locator was down for repair 2 years ago here at EKRK.
ortac wrote:
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?
DMEs can’t really because of slant range and offsets but one could define GPS points — the runway thresholds as coded in every RNAV approach as check altitude reference point. This would be an ILS with “RNAV required” which is something that could exist but does not exist. I don’t see much value in it because an all GPS precision approach using LPV is possible and this will be the future for most airports.
ortac wrote:
based on the (possibly incorrect) assumption that DMEs are always encoded in the GPS database at their physical location.
Do you have any reference that DMEs in the GPS databases might be encoded at a different than their physical position @ortac? I would say that is impossible because the offset zero ranges them in a circle around their physical position. This cannot be mimicked by a GPS database.
I guess most of what was said is true, … But: wherever an LPV or LNAV/VNAV is available I’d rather use that anyway, with DME on board, and without …
- higher precision than ILS
- no mode change from GPS to VLOC
- less error prone
- no setting of final course on PFD
- no side lobes etc etc
In a Perspective Cirrus with VNAV it’s basically select procedure and ENTER. In mine i have to dial in the initial approach altitudes …
achimha wrote:
DMEs can’t really because of slant range and offsets but one could define GPS points
Using horizontal distance instead of slant range introduces an error of about 0.13% on a 3° glide slope. DME has a tolerance of 0.25NM + 1.25%. So overall the GPS would still be more accurate.
Would you fit an ADF in a new panel?
The long anticipated approval of IMC flight in UK homebuilts got me thinking about what would go in a brand new panel in an IFR capable SEP homebuilt.
It’s easy to decide on a modern experimental EFIS like the Garmin G3X, as you get so much capability for relatively little money. The next easy decision is to install a GPS navigator such as a GTN650 to get en route nav, GPS approaches and ILS (and radio).
But then it gets tricky, because most UK IAPs still rely on ADF for the MAP and ILS approaches often require DME too. In fact both of the IFR airfields closest to me fall into this category. Another consideration is the (EASA) IR or IR(R) renewal, as that would be best done in the same aircraft, so it would have to be legal for at least two different approaches. On the other hand, ADF receivers and DME transceivers are expensive (and, in IMHO) legacy products with a limited useful life. A new ADF and DME would roughly double the cost of the avionics in such an aircraft.
There is also the question of a backup in case the GPS navigator croaks. In my current (steam gauge) stead, there are lots of back up options. If the ILS fails there is the option of an NDB DME approach. If the ADF fails, I could substitute with GPS/OBS. There are many other permutations and, if all else fails, there’s vectors and an SRA.
So, what would you do? Install GPS only, on the basis that EASA will allow WAAS only approaches soon? Install ADF and/or DME? Something else more cunning?
I would install DME but not ADF.
If the ADF is only necessary for the Missed Approach there is nothing stopping you requesting ATC for different missed approach instructions which do not require an ADF. Agree with Jason, DME is not going away and should be fitted. In addition to ILS/DME pairing, you may be given hold instructions on a Radial/DME fix.
The ADF is a noble instrument and quite fun to teach, but outside of the bush and remote airports not really used in anger.
Why the UK has not embraced GPS overlay and allowed aircraft to have IFR approved overlay approaches on their GPS database is beyond me. There is no safety case for current policy, especially with WAAS.
I agree 100% that installing an ADF in a nice homebuilt is just mad, but there as as yet no legal ADF-GPS substitution concession in Europe, for approaches.
And there are loads of NDB approaches around Europe.
So, working on the basis that none of the hundreds of SR22s without an ADF has ever been busted (well, no verified reports ever, only rumours which nobody could verify, including one I checked out via the UK CAA) that leaves just the insurance angle. But there is also a lack of reports of insurance refusing to pay out – though that could be because almost nobody crashes while flying a published IAP.
I would fit the DME because most ILS approaches need it. It’s also generally useful. Yeah, they are not cheap, but for a homebuilt you can buy a working KN63 (remote reading) DME on US Ebay
The problem with DIY GPS approaches is that the receiver will not switch to TERM or APPR sensitivity if the approach is not on the database. Whether a 5nm sensitivity is better or worse than the magical mystery tour of an NDB I let others decide, although a DIY GPS into mountainous region may not be ideal.
You can set an IFR GPS to 1nm or 0.3nm full-scale manually.
From what I know of others, most people fly NDB and VOR approaches using the OBS mode of the GPS, using the GPS for lateral nav to fly the charted track, and using the DME as charted.
For example, to fly this one using a GPS
I would
IOW, fly the IAP exactly as charted.
I don’t think there is a waypoint here (EGKA) which corresponds to DME=0 which takes us back to the old debate… a real DME is the way to go.
