Longbow64

Can someone please explain me about DME arc, and localizer back course approaches?

Cubdriver

Your best bet is to read up on them in your Jeppeson Instrument/ Commercial textbook or Instrument Flying Handbook (FAA).

DME arcs are drawn at certain distances around VORs and occasionally other navaids, to serve as a flexible IAF to an approach. They once were only VOR-based but now GPS has them. DME arcs are often used with back course approaches. BC approaches are simply the opposite side of a typical localizer. BC approaches makes two approaches from one localizer, saving the taxpayer some money. Just make sure you engage the "BC" button the autopilot, if you are using one that does not know which way the approach is being flown front or back. Older Bendix Kings and S-Tecs are what I have in mind, but I think they all need to be set for back course localizer even on the G1000/ GFC700 autopilots.

The nice thing about DME arcs is that they can be intercepted at any angle. This saves time and makes the approach more efficient because you have an almost unlimited number of initial approach fixes. They can be a bit challenging to track with only one VOR receiver and DME, but it gets easier with two VORs plus DME, and it becomes a no brainer with gps.

block30

To qoute John Eckalbar, author of IFR A Structured Approach,

"...we are not to turn onto a DME arc at a randomly encountered location on the arc-arcs are to be entered only at the depicted IAF's on the arc." "...if we come up the (XXX radial) towards (XXX VOR), we can't take a quick left on the 10 DME arc and take advantage of its "NoPT" feature."

That was a question I had when I was training for my instrument, "can I just turn onto the arc and be good to go?" Unfortunately not. I'm trying to find AIM reference, but so far Eckalbar's words have been quickest to find.

I say this only to clarify how arcs are supposed to be entered.

Best to you, block30

Cubdriver

An airline pilot may wish to comment on this because they fly only IFR and would know more about real world practices; but I think if you are cleared to intercept a particular DME arc by a controller then you may do so, and you do not have to fly to one of the two llustrated IAFs on the arc if you have the clearance.

TheGreatChecko

ATC will clear you to a fix.

ATC cannot just let someone fly where they want, they need to ensure terrain clearance and separation. A DME arc is just another type of transition to an approach and like all transitions, they have to be started at an IAF. ATC will clear you to the IAF and if you are in a non-radar environment will clear you for the approach from that IAF. Also, if the arc IAF isn't convenient, there is probably an easier transition closer to your position anyways.

In the training environment, we create arcs just so we can practice, so essentially we intercept them wherever we want. The only difference between that and "real life" is that point at which you intercept the arc has a name and you would fly a radial to that point and then intercept the arc exactly like we practiced when getting out instrument rating.

Cubdriver

There ya go.

frozenboxhauler

Easy, lead the turn on to the arc by 10 percent of your ground speed.( Eg. GS 250 kts, start the turn 2.5 miles prior to the arc.) If you have an RMI and DME it's a piece of cake. Put the RMI needle at 90 degrees off you nose (zero wind) and just bracket the DME.
On the localizer back course, you "become" the localizer needle. If it's off to the right of center, turn left to join the loc.
hope that helps.
fbh

the King

Actually, you can be vectored onto the DME arc in a radar environment. We used to do it all the time while training. Now they've removed the arc. If you are non-radar, you're going to the fix first like TheGreatChecko said.

From personal experience, the "vector" was "intercept the __DME arc, expect the approach."

Clue32

I concure. The Tacan 07 to Wiesbaden AAF begins with a DME Arc. Frankfurt approach always sends us to the fix that begins the approach.

The only thing I need the RMI for is confirming the fix and locating the lead radial to turn and intercept the final course. Otherwise, I fly the DME. Numbers get bigger, turn in towards the Tacan. Numbers get smaller, hold the heading and wait to reintercept the arc. The whole turn ten tune ten thing works fairly well too, but it really isn't necessary.

SomedayRJ

Unless otherwise vectored to a position from which the approach may be made without flying the arc, the way I understand it, you must start the approach *AT* a published IAF—and for DME Arcs, that includes a specified "where" (radial) as well as a specified "how far" (DME).

(Consider being out in the mountains, IFR, but non-radar, and you get "Airline 1234, cleared for an approach to the Blank airport—call us at such-and-such when you're on the ground." You're going to stay on an airway or other terminal route and fly a full procedure--not just veer off and go intercept an arc at other than a published fix.)

A RMI makes a DME arc as easy as shooting fish in a barrel; even without, they're not overly difficult.