Flying a DME based arc.

[pax anglia]

Having spent an idle few minutes looking at some approach plates recently, I am prompted to ask how difficult it is to fly a DME based arc. I imagine there is some rule of thumb to follow, presumably bank angle being the first factor to take into account.
It is a rather long time ago that I worked for Nats and its previous incarnations, and I have to admit that I lost touch with aviation for many years (yes, shame on me, I know). In the meantime these procedures appeared. I expect they were implemented on the basis of the requirement for aircraft to carry the appropriate kit in the enroute phase of flight and could therefore be used as an approach aid, together with the widespread introduction of DME equipment on airfields whether linked to the ILS or not.
Having looked at just a few procedures, I haven't come across any that require descent at the same time as flying an arc which I imagine would increase the workload to an unacceptable degree.
Perhaps I'm missing the obvious and they have really been devised to let the FMC do the hard work !
Any thoughts on the subject however brief, would be gratefully received.

Thank you, Gentlemen

Edited for Sp. error

n

[OzExpat]

DME arcs were implemented in Pans Ops at least as far back as 1986. I guess its a matter of how rapidly the various authorities implemented them, if indeed they even needed to implement them at all. I don't know about letting the FMC fly them, I do it manually without any major difficulty.

It's a matter of having an idea of the aircraft's radius of turn for the turn onto and off the arc. There is usually no guidance for the former but I know that my aircraft's turn radius, in standard initial approach configuration is about 1.5 NM. For the turn off the arc, the lead radial/lead bearing normally provides 2NM lead for the turn. You can pro-rate that to find out what lead brg/radial you need for a turn of lesser radius.

To maintain track around the arc, I use heading changes every 10 degrees of azimuth navaid shift. Thus, I'm really just flying a series of short straight legs around the arc. All I then have to watch is that I remain within the requisite DME tracking tolerance.

It's not unusual to descend on an arc either. Most aircraft fly the arc as part of their profile descent, as long as ATC doesn't get in the way. It's not difficult to initiate a descent while tracking a DME arc.

Hope this helps.

[Intruder]

The US Navy taught a 10 degree method of flying arcs:

Intercept the arc. If you don't know the number of miles required to do so (your radius of turn), LEARN IT!

Put the VOR needle 10 degrees ahead of your beam Fly a constant heading essentially perpendicular to the VOR until the needle is 10 deg behind your beam (e.g., if intercepting from the 180 radial, flying counterclockwise, initial heading is 080 until crossing the 160 radial).

Adjust heading 20 degrees, so the VOR needle is again 10 degrees behind your beam.

Repeat as necessary until reaching the lead-in radial (if not published, CALCULATE IT for your airplane).

[pax anglia]

OzExpat and Intruder.........
Many thanks for your responses, Gentlemen. I've taken a print and armed with this information will wrap a wet towel around my head and read your replies in conjunction with my book of Approach Plates. On paper, it would appear a somewhat complex procedure, but perhaps it is easier to actually fly than it seems to someone of my limited experience. (VFR only but proud of it !)
Thanks again, chaps !

[Young Paul]

It's not too hard to fly, as long as your spatial awareness is good. You need to be able to absorb the information that your radio kit is telling you and respond to it - and also be aware of the "end point" of the arc, and what you need to do when you get there. Just a new trick.

Alternatively, you can hope the procedure is programmed into the FMGC and let LNAV fly it - or program yourself a series of points that approximate to the arc .......

[411A]

DME arcs have been in widespread use in the USA since the 1970's...not that difficult, provided your situational awareness is good...and it doesn't require glass or an FMS either.

[reynoldsno1]

They are used widely in New Zealand as well for initial approaches. They can help conserve airspace, though arcs should not be designed at less than 7DME.

[Earthmover]

We also use the manual method - but if suitable, back it up with a fix-page circle from the FMC to confirm the situation looks Kosher.

pax anglia, enjoy your wet towel - and remember that 'Ford Prefect' in the 'Hitch-hiker's Guide to the Galaxy' put anti-depressants in one corner of his towel - for just such a moment as this!!:D

[mustafagander]

A DME arc is easy-peasy.

Allow 1% G/S to turn onto or off the arc.

When initially inbound to the station, overturn onto the arc (i.e. 95 or 100 degrees) and V.V. Remember the scale effect - the further away from the station, the less "turn" during the arc.

Use 1 in 60 for situational awareness, especially when to configure and turn onto final. e.g a 20 DME arc through 30 degrees is 30*20/60 = 10 miles.

Remember your tolerance (+/- 2nm in Oz) and use it to make the straight bits longer. It's not illegal to track a 10 DME arc at 11 DME if the turn onto final is tight.

Keep the VOR needle (it MUST be co-located!!) on the wing tip and watch the drift - it is NEVER constant (think about what you're doing, flying a circle).

It's much easier to fly it than to think how to explain it!! :)

[big pistons forever]

I find the easiest way is to just fly the DME. In a light twin start the intial 90 deg turn about .7 NM from the desired distance and watch the DME. As it starts to increase above the desired distance just turn in a little untill it starts to decrease, steady up wait for it to increase and repeat as necessary. When you get to the lead radial turn to the procedure turn inbound heading and then intercept the inbound track. Personnaly I found all the book procedures just complicated my life.

[TomPierce]

Big pistons has it right in my book. That's always how I flew them. Once you cracked it, it was straightforward. You just keep checking for distance changes - which are very easy to correct.

Better than all that mental arithmatic agility, just do it!

In fact one tip is to plan a trip specifically to do a 360 round the beacon. It concentrates the mind.

[411A]

Many GPS approaches in the USA overlay VOR/DME procedures (or are stand alone GPS approaches) using arcs as a transition to the final.
Simply select the procedure, attach same to the active flight plan and fly the CDI...couldn't be easier.