DME - any point in having one?

achimha wrote:

How would you do that on a GNS4x0W?

AUX, Last Page, SBAS Selection

Good luck with Avidyne. At least you’ll only be lied to and cheated, not killed.

I agree to everything Timothy says. Just so that you should not think he is alone with his views. ADF is maintainence-heavy, weight-heavy, installation-heavy, and obsolete. DME is dead simple to manage*, simple to interpret, and likely to be useful for many years, and apart from simply being a requirement for most non-GPS procedures, it is also very good for cross-checking what you think the GPS/FMS is telling you.

*(Garmin managed to completely screw up the inherent simplicity of the DME in the G1000 by hiding it in two separate incredibly non-intuitive menu systems. Garmin really deserves severe punishment for that).

huv wrote:

Garmin managed to completely screw up the inherent simplicity of the DME in the G1000 by hiding it in two separate incredibly non-intuitive menu systems. Garmin really deserves severe punishment for that

While huv is kind enough to agree with me, I’ll agree with him. It’s appallingly unintuitive. I don’t fly G1000 that often (there might be several weeks between times) and when I do, I really struggle to remember this bit of the interface.

carlmeek wrote:

Am I correct in saying that essentially that accident can be summarised as someone not following a published approach? I.E. It was published as a DME approach and the pilot ignored it.

I believe that is oversimplifying. I also think that Timothy is oversimplifying and there is no way we can be sure that the pilot would not have done the same mistake even with a DME.

First of all it seems like he may not even have selected the NAV as source for the CDI but was using the GPS – which precluded the GS from displaying.

The GPS gives excellent situational awareness, and you you have other ways of knowing your distance from the threshold than using the distance to the DME. If you have loaded (and somehow activated) the procedure it will sequence through the waypoints of the approach, including the FAF, and eventually PN09 which would be the threshhold of RWY09 (which probably would be very close to I-DDE given the DME is according to the plate supposed to show 0 DME at the threshold). Still he commenced his descent at 6.7 NM from DDE although he should have seen on the GPS that he was still 2.7 NM outside the FAF.

The pilot used distance from DND instead of I-DDE. That shows that DME substitution may not have been very present in the pilot’s mind.

In my opinion this is more a case of a poorly planned and executed approach than a DME substitution issue.

I find that explanation confusing.

There was no RNAV approach at the time, so PN09 wouldn’t have existed.

If he had had a working DME he would not have been using DME substitution, he would have had a real distance from the threshold, which he would have multiplied by 300 for the 3° slope and would have descended quite safely.

There are two things that resulted in the crash. He used DME substitution and he made a mistake on where the waypoint which he was substituting was. As @huv says, the DME UI is pretty simple, hundreds of manual pages simpler than the GNS530W, and it would almost certainly have alerted him that he was desperately low.

I stand by my statement that a working DME would have saved the day.

Aviathor wrote:

I believe that is oversimplifying.

Maybe. For me, that was just one example for an accident out of an instrument approach which was caused by loss of situational awareness, in this case most probably “wrong assumption regarding one’s location” and not helped by using the navaids provided in the intended way. There have been plenty more of these accidents, many of them by airliners with experienced crews, e.g. Alitalia 404 who for their ILS approach relied on one defective GS indicator alone instead of using all other instruments available to them, dual DME included.

Many ILS approaches that I fly regularly have a caption “DME required” on the approach chart. Theoretically one could substitute that with a GPS fix (at least in private operation) but this has its pitfalls. The location of the DME may not be immediately obvious or not coded in the GPS database which leads to the kind of error that (most probably) lead to the accident in Timothy’s link.

Personally I would not fly IFR without a DME, not even when flying privately. It is one more safety net and a good backup for GPS (which is not 100% reliable, mostly because of onboard equipment failures of which I have had two already) if one wants to get home in the evening in poor weather.