Correct way to enter a DME arc?

[Cessnaflyer]

So I have been looking for the correct way to enter a DME arc. A few people I've talked to say you can only enter it from the outside and others say you can enter on the side the navaid is as well. I've looked at the instrument flying handbook and it show their little airplane on the inside of the arc intercepting it so this looks like it is acceptable to enter on either side.

Is there anyway to see a good FAA write up on the correct way to fly the arc?

 

[Crockrocket94]

Ive entered from the inside before when shooting practive approaches. Also come to think of it, I've entered the DME Arc in Tuscon for 29L from the inside as well. I dont think it matters.

 

[Air Pirate]

Both ways are used. Obviously the way it is depicted on the Approach Plate is the only correct way for a specific approach.

"Generally, you intercept the arc while flying to OR from a VOR/DME or VORTAC. To join the arc, you should turn approx. 90* from your inbound OR outbound course..."
~Jeppesen Instrument/Commercial Manual pg 2-87

there ya have it!

 

[Cessnaflyer]

Ive entered from the inside before when shooting practive approaches. Also come to think of it, I've entered the DME Arc in Tuscon for 29L from the inside as well. I dont think it matters.

That's what I thought as well. I was entering the arc here for the GEG VOR 3 approach on the inside and the controller wanted to know what I was trying to do. I told him to enter the arc and he said I was entering it the wrong way. I then posed this question to other pilots and no one could give a definite answer.

 

[Air Pirate]

Here's an example of an approach where both may be used:

KHDN VOR/DME-B
-you could start it from the Hayden (CHE) VOR outbound to ZOMBU to intercept the arc southbound
-or inbound from SBURG to HABRO and intercept the arc north bound

 

[tgrayson]

I was entering the arc here for the GEG VOR 3 approach on the inside and the controller wanted to know what I was trying to do.

The only problem with doing that on this approach is that there is no published route from the inside. The depicted radials are just radials to define the IAF, they aren't feeder routes or initial segments. If you weren't being vectored or didn't have a specific clearance, why were you doing what you did?

 

[Cessnaflyer]

Well there isn't anything published for the outside of the arc either. He told me to navigate to the arc and that is what I did. I have a feeling he forgot what I was doing.

 

[tgrayson]

Well there isn't anything published for the outside of the arc either. He told me to navigate to the arc and that is what I did. I have a feeling he forgot what I was doing.

I assume you were performing VFR practice approaches? "Navigate to the arc" is a pretty vague instruction and I'd hope the controller would do better in actual IMC.

There is probably an airway that overlies the depicted radials and the "normal" way to fly the approach non-radar would be flying the airway to the IAF (from the outside). If you were already at the VOR, I think most people would have just performed the PT, rather than fly the arc. So the controller's criticism that you were "doing it wrong" probably just meant different from the way most people do it.

There is nothing inherently wrong with flying the arc from the inside; if you're doing it non-radar, you just need to make sure you have published routes. If you were on an airway, you could surely fly to the VOR and then fly the airway to the IAF (assuming this airway exists) and start the approach from there, as long as this was compatible with your clearance. You've got a 90 degree turn from either side and you've got 4 nm of primary protected airspace on each side of the arc, so obstacle protection isn't an issue.

Edit: V120 and V253 overlie these radials.

 

[MidlifeFlyer]

Well there isn't anything published for the outside of the arc either.

Yes there is.

Something to remember about (almost?) every approach. They are not simply tossed into space. Unless it's a pure radar approach, there will be a way to get from the enroute environment to an IAF. It may be a feeder from an airway or an airway itself. You should always look at an approach in that context.

What I'm confused about is exactly what happened. If you were coming in from the west, you should have been on V120 and intercepting from the outside. If you were coming from the southeast, you should have been on V253 and intercepting from the outside.

If you were coming in from the area from R214 clockwise to R142, what would have made the most sense would have been the PT not the arc, but if you were expected to do the arc (and intercepting from the inside would be fine), someone would have mentioned which IAF you were using.

If you just got a "navigate to the arc" instruction, they there's no way you could do it "wrong" (unless you were on the outside an airway, crossed the intercept, headed to the VOR and then turned around) and the controller should have expected the unexpected.

 

[Cessnaflyer]

I assume you were performing VFR practice approaches? "Navigate to the arc" is a pretty vague instruction and I'd hope the controller would do better in actual IMC.

There is probably an airway that overlies the depicted radials and the "normal" way to fly the approach non-radar would be flying the airway to the IAF (from the outside). If you were already at the VOR, I think most people would have just performed the PT, rather than fly the arc. So the controller's criticism that you were "doing it wrong" probably just meant different from the way most people do it.

There is nothing inherently wrong with flying the arc from the inside; if you're doing it non-radar, you just need to make sure you have published routes. If you were on an airway, you could surely fly to the VOR and then fly the airway to the IAF (assuming this airway exists) and start the approach from there, as long as this was compatible with your clearance. You've got a 90 degree turn from either side and you've got 4 nm of primary protected airspace on each side of the arc, so obstacle protection isn't an issue.

Edit: V120 and V253 overlie these radials.

Ok now I can explain myself better with the chart in front of me and not posting from my phone

We were being vectored off of the missed approach off of COE and he brought us way around to the south so we requested to do the arc for practice. He then said fly heading 220 and advise when established on the arc. The problem was with the wind and a heading of 220 would have made us continue on the inside of the arc and we would have never hit it. So what we did was when we got to R142 we were 7 miles away so we made a quick turn to the left to head outbound with the radial and turn back to the right again to pick up the 10 DME arc. When we were headed outbound is when said we were doing it wrong.

This was all IMC and I know nothing is tall in the area but I'd still like to stick as close as possible to the charted approach.

Yes there is.

Something to remember about (almost?) every approach. They are not simply tossed into space. Unless it's a pure radar approach, there will be a way to get from the enroute environment to an IAF. It may be a feeder from an airway or an airway itself. You should always look at an approach in that context.

What I'm confused about is exactly what happened. If you were coming in from the west, you should have been on V120 and intercepting from the outside. If you were coming from the southeast, you should have been on V253 and intercepting from the outside.

If you were coming in from the area from R214 clockwise to R142, what would have made the most sense would have been the PT not the arc, but if you were expected to do the arc (and intercepting from the inside would be fine), someone would have mentioned which IAF you were using.

If you just got a "navigate to the arc" instruction, they there's no way you could do it "wrong" (unless you were on the outside an airway, crossed the intercept, headed to the VOR and then turned around) and the controller should have expected the unexpected.

I forget the north side is off of V120 I am used to always coming in V2 so I think the northern portion is not on the airway and I was thinking that V253 pointed further to the east. But yes you are right they should all be able to transition from the enroute to the approach.

 

[tgrayson]

The problem was with the wind and a heading of 220 would have made us continue on the inside of the arc and we would have never hit it.

What he probably wanted you to do is intercept the arc outside the published segment and arc around until you got to the IAF. This isn't unusual. You had to fly through the arc in order to intercept the R142 to fly it outbound.

 

[LV-ARG]

what about asking for clarification from ATC?

 

[Cessnaflyer]

what about asking for clarification from ATC?

Wouldn't have had time it was a busy day with IMC and runway construction. Nearly every airline had to be vectored to different approach segments because of their varying equipment.

 

[Cessnaflyer]

What he probably wanted you to do is intercept the arc outside the published segment and arc around until you got to the IAF. This isn't unusual. You had to fly through the arc in order to intercept the R142 to fly it outbound.

That's what I don't understand though, why would you tell a pilot to intercept the arc when you give him a vector that would make him hit the approach segment 7 miles before the FAF in the wrong direction?

This little picture I drew here is what it looked like in the GPS. The red arrow is our vector to intercept the south arc. The blue arrow is what I did to get on the arc.

 

[tgrayson]

The red arrow is our vector to intercept the south arc. The blue arrow is what I did to get on the arc.

Your red line passes right through the arc from the outside, pretty close to where the red line starts. This is what I meant by intercepting the arc outside the published segment.

 

[Cessnaflyer]

Ok I see so you mean. Enter on a more SE radial that was at 10 DME when we crossed it.

Yeah I guess it should have been done that way. :crazy:

 

[tgrayson]

Yeah I guess it should have been done that way.

It was a new concept to me longer after I became a -II. After a similar confusing episode with ATC, I came back and called up Memphis Center on the telephone and asked "What were you expecting me to do?"

I now include that as part of my instrument training.

 

[LV-ARG]

Good stuff, I didnt know that either.

 

[Barty]

Ok I see so you mean. Enter on a more SE radial that was at 10 DME when we crossed it.

Yeah I guess it should have been done that way. :crazy:

Yes, that is perfectly acceptable. What changes in this instance is the altitude that can be assigned/flown. Until you reach the published IAFs, you have to be given an altitude to maintain until you are on the published portion of the arc. If a controller needed to arc someone around from V2 to join this approach to separate from a departure headed NE on the same airway, it would be a convenient way of providing lateral separation.

Edit: Non-radar separation for a DME arc is 5 miles up to and including 35 miles from the navaid, 10 miles on each side of the arc beyond that. This is to provide some more wiggle room in the event a pilot isn't flying the exact distance from the navaid.