DME Arc question?

[cyco]

Normally a DME arc is done on the VOR/DME, if this aid goes u/s for an extended period of time, could an arc be published using the ILS/DME.

I guess the question becomes what is the diff between a VOR/DME and the ILS/DME apart from the obvious diff locations and idents.

[OzExpat]

Tracking along a DME arc requires that you first intercept it. That point of interception is normally shown as an IAF and the fix requires an azimuth navaid as well as DME, in this instance. The Localiser radiates in one direction only, so you need either a VOR or an NDB, to provide track guidance to the IAF and, therefore, to properly identify the fix.

Then it's a good idea to have an azimuth, so that you can monitor your progress around the arc. It becomes even more important for identifying the point at which to turn off the arc. A Localiser signal can't do any of that for you.

[cyco]

What initially raised the question is the state of NSFA (Apia) where the VOR/DME has gone u/s until December.

Now the NDB is still servicable, so my question is can you track in on the NDB to 15 DME IAP then join that arc to intercept the LLZ for the ILS/DME 08

[Dan Winterland]

I would say yes, providing the NDB and VOR are located reasonably close together. Tracking inbound to the co-located VOR id obviously the best way to get onto the arc. If the NDB is close, there's no reason why you shouldn't use it.

It could also be used to maintain the arc by using the beacon in front of the wingtip if outised the arc (and vice versa) method.

[Slasher]

Ozexpat is 100% correct. If you dont have the published AZIMUTH navaid the whole DME-yark procedure goes tits up. In a case where the VOR is stuffed, its best you go with an alternative approach (usualy an overhead tear-drop from the NDB) that gets you on the LLZ. So youll be going for the Runway Whatnot NDB-ILS-LLZ procedure or similar.

[Stan Evil]

NDB/DME arcs are not unknown. The Gloucestershire NDBDME 27 procedure has the option of an 8 dme arc

[OzExpat]

That's a VERY different question to your original one cyco. The answer is that the responsible aviation authority must make a determination that the NDB is close enough to the VOR to be used in lieu. I expect that they would've done so by now, if that was the case - and there would, most likely, be a Class 1 Notam to that effect.

I assume that your reference to the VOR/DME being U/S is really only a reference to the VOR. If the DME is also U/S, all bets are off. :eek:

In either event, you're better off doing as Slasher suggests and fly a procedure from overhead the field. I assume that such an alternative method for joining the Localiser is available there.

In PNG, we have several approaches where either VOR or NDB can be used for the azimuth guidance to, on and from a DME arc.

[Tinstaafl]

is done on the VOR/DME, if this aid goes u/s


This isn't a single aid. It's two: A VOR and a DME.

Which one of these two do you mean when you fail the aid?

If the DME is the one then you can't fly the arc specified on the approach plate using another DME unless that other DME is specified as an alternative.

If the VOR is the failed aid then you could fly the arc however you may not be able to determine your position around the arc. This means that any azimuth related part of the procedure such as the IAF, minimum altitudes &/or lead radials become unuseable, having the practical effect of making the arc useless for the approach.

Unless of course an alternative azimuth aid is specified for the procedure.

Of course if BOTH fail then it's time the operator of the aid paid his electricity bill... :p

Aids that are available to be used in a procedure are determined by the procedure designer, not the pilot.

[cyco]

What originaly raised this question was the fact that the NSFA VOR/DME (both units) have gone u/s. Now the NDB in Apia is not available to bring you onto the ILS so the NZ Airways has issued an alternate approach using a GPS arc which mirrors the VOR/DME arc.

However this is obviously only available for GPS equipped aircraft, so my thinking was why cant an arc be published using the ILS/DME.

I guess it would need to be resurveyed but if so is it a viable option.

The other option would be for all non GPS operators to only use the NDB teardrop approach and live with the high minimas.

[mcdude]

Cyco - I've pasted the NOTAM below, and note it is not just a GPS route - Airways NZ have promulgated a temporary FMS or RNAV or GPS route which should allow most operators to transition to the ILS no problems. Obviously the onboard equipment must be "suitably approved" for such use.

I think to promulgate a NDB/DME(ILS) arc would as you say require a flight check and technical survey. Rgds mcdude


P0296/02 - DURING PERIOD OF 'FA' VOR/DME U/S AN ALTERNATE STAR PROCEDURE MAY BE APPROVED FOR SUITABLY APPROVED FMS/RNAV/GPS EQUIPPED ACFT 16 NOV 02:15 UNTIL 08 DEC 03:00 ESTIMATED

[OzExpat]

Looks to me like they're doing the best they can in a very bad situation. In general terms, yes, it would be possible to base the DME arc on the ILS/DME but...

1. The arc will probably not be in exactly the same place, therefore requiring a survey; and
2. The NDB may not be well situated to provide lead-in to the arc, step-down on the arc (if necessary), and lead-off the arc onto the Localiser. But, even if it IS situated where it can be used, it would have to be surveyed.

As a procedure designer, I can tell you that it takes a long time to design such an approach procedure. Then it needs to be flight checked by suitably qualified personnel, who may not be readily available for the job. I'm guessing that ACNZ would have to do that and they have lots of other work commitments under contract so this job would have to wait.

All in all, it would be a lot of work for such a temporary outage problem. In short, the job couldn't be done - and the approach couldn't be published - in such a short timeframe. The method that has been used is clearly a stop-gap and, from where I sit, it seems like a good temporary solution.

Best of luck!

[4dogs]

Folks,

Am I missing something here?

[Yes, and now I have found it!]

Brisbane ILS/DME 19 comes in two versions - one using BN and the other using IBS DMEs.

[hoss]

food for thought,

one thing to be careful of is 11-2 (IBS DME) doesnt have an arc but it does refer to a lead radial.

11-3 (BN DME) has an arc which has a lead radial and a lead bearing.

cyco,no you can't track the NDB(inplace of the VOR) to 15 IAP to arc around for the 08 ILS/DME. (unless the chart refers to a lead bearing in addition to a lead radial,which I doubt it has)

Now if only Ernst K Gann was here.......;)

Hoss