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DME Arcs in a Cessna

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DME Arcs in a Cessna

Old 14th Aug 2019, 09:03
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DME Arcs in a Cessna

Hi all, as we had got our DME box recently fixed, I thought I would try to fly a DME Arc around a local VOR, just like the ATPL types do in their A320s.

So I headed straight for the VOR and when the distance was near 10nm, I turned 90 degrees, and tried to keep the distance constant.
Well after quarter of an hour of 'thrupenny bit' type flying, I saw that I was only about one third of the way around. A quick mental calculation estimated that the full circle would be 62.8nm and take about 40 minutes..! So maybe I should have used a 4 nm dme instead.

I wonder how the ATPL types practice these arcs, as they are used for several Standard Arrival Routes into airfields. These 10nm DME Arcs should be easier in an A320, as they go a bit faster, and have a lot more inertia, so that one angle of bank might be the solution. But how do you practise them in a Cessna..?
.
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Old 14th Aug 2019, 11:06
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If you're practicing in a Cessna, you probably have lots of time. I used to fly them all the time in the Cheyenne, but it flies considerably faster than a [single] Cessna, so the time was about right. If you're just practicing in a slower plane, and not "on airways with a clearance", I'd fly smaller arcs. Don't get so distracted you forget to watch for traffic.
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Old 14th Aug 2019, 12:29
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Haven't seen a long DME arc in Europe for donkey's years, they probably do exist but are petty unusual. The few that one does see are such short segments that the technique below hardly has time to work.

On a long arc technique is to fly 'thrupenny bit' shape, you can't do it in a continuous arc.

Pick a heading that will take you a little into the arc, (perhaps .2 to .4 miles on a 10 mile arc) fly out through the circumference by the same amount, turn to cut inside the arc and back out again. Repeat as necessary.

Cumbersome procedure.
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Old 14th Aug 2019, 14:19
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Once established in the arc at the appropriate distance just keep it with the bank that will give you a closure speed of 00 😉
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Old 14th Aug 2019, 15:14
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Easier with an NDB co-located, as you can just keep the NDB on the beam.

There are still a few around - some of the Gloucester approaches have short DME arcs as part of direct joining procedures.
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Old 15th Aug 2019, 17:25
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You mean like like this one to Martin State in Maryland - a DME arc all the way to the runway...

https://aeronav.faa.gov/d-tpp/1909/05222VDTZ15.PDF

Watsonville, CA has a neat one, where the arc is "backwards" off a distant VOR:

https://aeronav.faa.gov/d-tpp/1909/00805VA.PDF

Sadly all the really good DME arcs are disappearing though, used to be a very impressive 90+ degree one to Arcata, CA, but it seems to have gone away now.
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Old 15th Aug 2019, 18:14
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For most arcs, say 7 to 15 DME, fly 10 degree increments, the mantra is "twist 10, turn 10." After making your initial turn onto the arc, fly a straight line segment that is actually tangential to the arc. As your OBS needle swings from full scale to center (or, from plus 5 to minus 5), you turn another 10 degrees heading, and twist the OBS selection 10 degrees. You have to think about which way you're going, and which way to turn the OBS dial. Kind of hard to explain without pictures, hope this makes sense.
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Old 15th Aug 2019, 19:41
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Originally Posted by meleagertoo View Post
Haven't seen a long DME arc in Europe for donkey's years, they probably do exist but are petty unusual. The few that one does see are such short segments that the technique below hardly has time to work.
An instructor once asked me to plan a radio nav route for IMCR training. Just to make a change from VORs and NDBs I put in something like 30 miles' worth of a DME arc. Instructor seemed to think this was a moderately silly plan for that particular route, but he let me fly it and gave me some tips as we were doing so.
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Old 15th Aug 2019, 20:33
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Originally Posted by meleagertoo View Post
Haven't seen a long DME arc in Europe for donkey's years, they probably do exist but are petty unusual. The few that one does see are such short segments that the technique below hardly has time to work.

On a long arc technique is to fly 'thrupenny bit' shape, you can't do it in a continuous arc.

Pick a heading that will take you a little into the arc, (perhaps .2 to .4 miles on a 10 mile arc) fly out through the circumference by the same amount, turn to cut inside the arc and back out again. Repeat as necessary.

Cumbersome procedure.
Zakinthos ASTUS 2F is quite interesting with very long Arcs as you fly the 313 radial inbound towards KFN VOR to join the 25DME arc then intercept ZAK 266 radial inbound to then join the ZAK 12 DME arc to intercept the final approach.

How do you fly it in a boeing, well it's easy as the FMC tells the autpilot what to do whilst you try to go down and slow down after been left in a mess by Andravida approach. All very entertaining and much more fun than been vectored to the ILS back in the UK.

https://www.google.com/url?sa=t&sour...GnshKfCfCEpL9G
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Old 15th Aug 2019, 21:26
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I was taught how to fly an arc by a pilot whose professional career started in the 1930s. His method was the thrupenny bit , breaking the arc into ten degrees segments and changing the heading accordingly. His DC-2/3 wasn't much faster than your Cessna.
The Airbus I fly now can determine its position to a few metres and it varies the bank to stay on the arc. It is easier to leave the autopilot engaged whilst it does this
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Old 15th Aug 2019, 21:44
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Benbecula and Stornoway also have DME arcs if one wants to venture to the far north.
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Old 15th Aug 2019, 21:45
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Originally Posted by tubby linton View Post
I was taught how to fly an arc by a pilot whose professional career started in the 1930s. His method was the thrupenny bit , breaking the arc into ten degrees segments and changing the heading accordingly. His DC-2/3 wasn't much faster than your Cessna.
The Airbus I fly now can determine its position to a few metres and it varies the bank to stay on the arc. It is easier to leave the autopilot engaged whilst it does this
i remember the first one in the UK was at Biggin and caused much confusion. It was around the mid eighties. How many on here remember QDM approaches? I flew my last QDM approach in 1989, the controller and I were chatting about impending retirement over coffee today.

SND
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