Initial heading DME arc?

[bflynn]

Reference, the VOR approach at KBUY, which contains a DME arc.

https://flightaware.com/resources/airport/BUY/IAP/VOR_DME-A/pdf

When arriving at Stase (IAF), you turn into the 27 DME arc. How do you know what heading to turn to? Foreflight breaks the arc apart for you, so I was able to read the heading, but it didn't occur to me until I was approaching it that I'm not sure how to do this on my own.

 

[MauleSkinner]

If you fly the radial to or from the VOR, a 90-degree turn to get started works pretty well.

 

[midlifeflyer]

On a DME arc, you are always flying a 90 degree tangent so, as @MauleSkinner said, if you are coming from LIB it's just a 90° turn to the left. That is going to be (almost?*) universal when the arc IAF is on a published route. When you are doing what we often do with GPS, go direct to STASE off route, a 90 degree turn will put you inside the arc. So you are still going to use 90° off the VOR radial for your initial course, a heading of 326°.

Understand that doing this is totally artificial. If you have GPS, the chances are you would never fly this approach except for training or a checkride/IPC. You might not even be able to during a GPS failure.

Great meeting you yesterday.

[* "almost?" universal - anyone seen a DME arc where it's not 90° off a published route?]

 

[midlifeflyer]

Approaching STASE off route form the southeast, your VOR will look something like this until it centers when reaching STASE. The turn is to the yellow arrow I added.



In case anyone is wondering, this is my home area. This approach has been a staple for the ground based navaid non-precision approach required for checkrides, In fact, although I thought is was retained (despite an ILS and two RNAV approaches) for a practical reason, local controllers told me it was retained specifically for training, as unrealistic as it is in a large portion of the training fleet.

 

[dmspilot]

It threw me off because it doesn't look like it's a 90 degrees turn on the chart OP provided. But it is according to the numbers. Drawing error or optical illusion?

 

[asicer]

Approaching STASE off route form the southeast, your VOR will look something like this until it centers when reaching STASE. The turn is to the yellow arrow I added.

View attachment 118166

In case anyone is wondering, this is my home area. This approach has been a staple for the ground based navaid non-precision approach required for checkrides, In fact, although I thought is was retained (despite an ILS and two RNAV approaches) for a practical reason, local controllers told me it was retained specifically for training, as unrealistic as it is in a large portion of the training fleet.

I'd imagine it'd be much easier with an HSI.

 

[midlifeflyer]

I'd imagine it'd be much easier with an HSI.

DME arcs are basically easy anyway (for some reason, like holds, we tend to teach them in a complicated way).

Everything is easier with an HSI, and definitely once you get started and turn the course needle inbound because i's pointing to the VOR. But you still need to do the first turn in a way that put you on the arc at the appropriate distance. I'm not sure that part makes much difference between a VOR CDI and an HSI/DG combo.

 

[midlifeflyer]

It threw me off because it doesn't look like it's a 90 degrees turn on the chart OP provided. But it is according to the numbers. Drawing error or optical illusion?

Dunno. Looks like 90 to me. And the chart is georeferenced so I'd guess an illusion, maybe because the arc is short?

 

[RussR]

Dunno. Looks like 90 to me. And the chart is georeferenced so I'd guess an illusion, maybe because the arc is short?

It certainly doesn't quite look like 90 degrees to me. Looks slightly more obtuse.

Although the route to STASE is not indicated on the chart itself (and is not required to be), the expected arrival at STASE would be on the radial depicted along V454 from either direction (from the Low Enroute chart). Since the radial being flown is from the same NAVAID as the DME arc, then it HAS to be a 90 degree turn (because of geometry).

I will say on the FAA chart as shown, it is either a drawing error, or "cartographic license" if you want to be polite, showing the arc as it is. I would bet that the center of the arc drawn is NOT the VOR, but something a little closer to the arc.

 

[RussR]

And there are some out there where it's not a 90 degree turn onto the arc, but this only happens when the inbound route is NOT from the same VOR as the DME arc is. One example nearby here is WDG ILS 35, the IAF PODDS has almost no turn when flown as published from PER. And since PODDS is not on any airway, that's the only was to fly it using VOR navigation.



https://skyvector.com/files/tpp/2306/pdf/00136IL35.PDF

 

[bflynn]

the chances are you would never fly this approach except for training or a checkride/IPC.

Or a checkride. Out of Burlington. With a DPE who reportedly loves that there's a DME arc to fly.

I just replotted this in Foreflight. It's 068 heading from LIB to STASE, then turn left 324. That's 104 degrees, so the initial question is still up in the air. The advice I hear on the internets is to turn 90 and adjust afterwards. I can do that, but I dislike the ambiguity.

 

[sarangan]

Or a checkride. Out of Burlington. With a DPE who reportedly loves that there's a DME arc to fly.

I just replotted this in Foreflight. It's 068 heading from LIB to STASE, then turn left 324. That's 104 degrees, so the initial question is still up in the air. The advice I hear on the internets is to turn 90 and adjust afterwards. I can do that, but I dislike the ambiguity.

Regardless of where you are coming from, when you get to STASE and the CDI is centered, you only have two options. Turn to 326 or 146. The CDI shows you both numbers. There is no need to calculate this in your head, or flight plan it on Foreflight. Since you already know you need to go northeast for a counterclockwise arc, 326 is the correct heading. Whether you turn left or right is irrelevant as long as you get to 326. But I agree with you that the ambiguity between 326 and 146 could be discomforting. There are ways to narrow this down further, but it is really not necessary given that the two headings are 180 deg apart.



I am not sure how you came up with 068 from LIB to STASE. The track should be R-056 from LIB, and then -90 deg to get to 326. Is the 068 and -104 with wind correction? Sure, you need to be aware of the winds, but calculating the WCA at every stage of the arc is really not necessary. You have to adjust for winds as you fly.

 

[midlifeflyer]

Or a checkride. Out of Burlington. With a DPE who reportedly loves that there's a DME arc to fly.

Yep, that's why I mentioned a checkride. There really aren't too many choices for a realistic non-precision approach around us, given the equipment you are flying. I know some of the DPEs like using either the ILS or LOC at BUY or TTA with a pretend "Glideslope Unreliable" NOTAM, but the only realistic ones near us would the at RDU and GSO.

I just replotted this in Foreflight. It's 068 heading from LIB to STASE, then turn left 324. That's 104 degrees, so the initial question is still up in the air. The advice I hear on the internets is to turn 90 and adjust afterwards. I can do that, but I dislike the ambiguity.

I am not sure how you came up with 068 from LIB to STASE.

@sarangan I think can account for it.

Brian, remember, this is a VOR approach, not a GPS approach. VORs are calibrated periodically to match magnetic heading but not often and they can be wayyyy off*. This approach involves the 056° Radial, not a GPS or other determination of the actual magnetic bearing from waypoint LIB to waypoint STASE. As Sarangan said, there are only two choices, one of which joins the arc and one of which turns away from it. If the VOR OBS or HSI course needle is on 056, there's nothing to calculate because it's just a right angle to the centered needle.

The discrepancy between VOR and what GPS (or Foreflight) will show is important to remember. I know of someone who failed an instrument checkride at your school on this approach because of it. I don't know the applicant but their CFII asked me what happened - they didn't realize it either!

Basically, there is no ambiguity to account for. Don't overcomplicate this simple approach. All that's involved with this one is the OBS set on 056, the needle centering, turning 90° to the centered needle and adjusting to maintain the 27 NM distance until ready to turn inbound on the FAC.

Edit: Looks like LIB was last set in 1963 based on a 3°W magnetic variation. There's 4° right there. And don't forget that a VOR can be up to 6° off.

 

[EdFred]

Y'all make this way too complicated. Just put it at something in the neighborhood of 300 and the DME will tell you pretty soon whether you guessed right or not. Wind could have you +/- 20 anyway

 

[midlifeflyer]

Y'all make this way too complicated. Just put it at something in the neighborhood of 300 and the DME will tell you pretty soon whether you guessed right or not. Wind could have you +/- 20 anyway

I sort of agree, but I wonder whether 30° toward the inside of the arc might put someone out of tolerance rather quickly on a checkride. Personally, I'm bad with numbers, so I always found the picture easier.

 

[John Collins]

Edit: Looks like LIB was last set in 1963 based on a 3°W magnetic variation. There's 4° right there. And don't forget that a VOR can be up to 6° off.

This VOR's radials are 6 degrees off of current magnetic variation. ForeFlight shows the 056 degree radial to be at 062 current magnetic variation with no wind, so 068 would have to include wind. I think 6 degrees is a supposed maximum difference between the VOR declination and the current magnetic variation, but I don't think it is actually gets fixed as it is just too much work. So many of the VOR's in service today were established back in the 1960's and have never been realigned.

 

[midlifeflyer]

This VOR's radials are 6 degrees off of current magnetic variation. ForeFlight shows the 056 degree radial to be at 062 current magnetic variation with no wind, so 068 would have to include wind. I think 6 degrees is a supposed maximum difference between the VOR declination and the current magnetic variation, but I don't think it is actually gets fixed as it is just too much work. So many of the VOR's in service today were established back in the 1960's and have never been realigned.

I'm thinking this one will be realigned at some point since it's being retained for MON. but who knows?