Thursday, March 29, 2012

Big Turn, Little Turns, Big Turn

DME arcs can be problematic for infrequent IFR flyers because, like holding patterns, arcs are not that common. A DME arc simply involves flying a circular course around a VOR/DME or VORTAC station (aka the station) at a specified distance, as if the aircraft were attached to the station on a string. Distance Measuring Equipment (DME) onboard the aircraft displays the distance from the station, though an certified GPS receiver can substitute for DME. It's easy to visualize a DME arc if you have a moving map and many late-model GA aircraft with fancy GPS units will fly a DME arc on autopilot while you sit back and watch. Without a moving map, joining an arc and tracking it accurately is more challenging, but it can be an enjoyable challenge. To that end, here are well-tested techniques for flying DME arcs, with or without GPS.

Anatomy of an Arc

The primary protected area around an arc is quite large (4 miles either side), but during a check ride or proficiency check pilots are expected to remain within one mile of the specified arc distance. Approaches incorporating a DME arc will have at least one minimum altitude to maintain, but altitude step-down fixes may be defined at particular radials along the arc.
DME arc Required Obstruction Clearance

Instructors and examiners often use the term right arc for an arc that keeps the station on the right wingtip of the aircraft, or left arc where the station stays on the left. Air traffic controllers usually refer to the general direction from the station where you will fly the arc, such as "arc Southeast." I've see at least one approach chart use the phrase arc clockwise. With calm winds, the station should remain directly off the wingtip. 

Remote, non-radar environments tend to have instrument approaches that use DME arcs because they help air traffic controllers who lack radar to maintain separation between aircraft. I saw a lot of approaches with DME arcs flying in the Caribbean for that very reason. Arcs also allow pilots to fly on their own navigation and get established on an approach course from en route environment. Consider the KLOL VOR/DME-A approach where you could be arriving via the Lovelock 219˚ or 054˚ radial, which happens to correspond to Victor 6.

Get Ready, Get Set ...

From a proficiency standpoint, flying a DME arc demonstrates your ability to plan and maintain orientation. Here are the basic steps.
  1. Get established on the correct radial leading to the arc
  2. Fly toward the arc
  3. Turn approximately 90 degrees onto the arc
  4. Maintain orientation as you track the arc
  5. Depart the arc on the appropriate radial
More simply, flying a DME arc consists of:
  • Big turn
  • Several small turns
  • One last, big turn

Consider the three basic navigation equipment set-ups found in most GA aircraft:
  • Two CDIs (Course Deviation Indicators)
  • One HSI (Horizontal Situation Indicator) and one CDI
  • One HSI and one RMI (Radio Magnetic Indicator, G1000's call it a bearing pointer)

If you have two separate VOR receivers, set one to navigate to the arc and the other to the fly the radial or course you'll use to exit the arc. Early editions of the FAA's Instrument Flying Handbook cautioned the reader that DME arcs should not be attempted without an RMI/bearing pointer. That wording was removed several years ago.

Tune your DME to the correct station and identify the Morse code id, remembering that these identifiers are broadcast just twice a minute. Beware of DME units that automatically use a frequency tuned on one of your VOR receivers, sometimes called remote channelling, because it's easy to inadvertently get distance information from the wrong station. When possible, consider manually tuning and identify the DME frequency. If you're using an IFR-approved GPS receiver to fly an arc, you have several options covered later.

Fly to the Arc

In the example shown, the aircraft is Southbound, tracking inbound on the CEC 343 radial toward the 11 DME arc. You could set HSI course pointer is set to exit the arc on the I-CEC localizer with the course needle set to 114 degrees and set the #2 CDI is set to fly to navigate around the arc. In this example the aircraft is headed TO the station to join the arc, but other approaches may have you headed FROM the station to join an arc. The first common mistake pilots make is they lose track of the station's position relative to their aircraft. Don't let this happen.

Some folks get all wound up about whether the radial should be at the top or bottom of the CDI. Keep it simple: Set the top of the OBS so it matches the general direction you need to fly to join the arc, in this case 163 degrees. For an HSI, set the course pointer so the arrow generally points in the direction you need to fly.

The Big Turn

Do yourself a big favor and get established accurately on the arc from the get-go. If you blow through the arc by turning early or late, you've made a lot of work for yourself and you're more likely to bust the 1 mile tolerance. It may sound simple, but you need to clearly understand which way you'll be turning initially to join the arc.

Approach charts are oriented North-up and if you're a Track-up person, the chart representation can seem upside-down: There's no shame in rotating your approach chart to a track-up orientation to help you visualize your situation.

With the chart oriented Track-up, it's obvious you want to turn right and keep the station off the left wing. If you're flustered or task-saturated and you simply guess, you have a 50-50 chance of getting it right. You certainly want better odds than 50-50 when flying in the clouds, right?

Many pilots recall a DME arc as being a series of small turns, forgetting that the initial turn onto the arc requires a big turn - roughly a 90 degree turn. You need to turn right, so look at the heading at the three o'clock position of the HSI or CDI. In this case, you'll be turning to a rough heading of 253 degrees. If you have a heading bug, bug that heading.

A 90 degree change in heading at 3 degrees-per-second takes time so you must to lead the arc to account for the distance covered during the big turn. A rule of thumb is to use 0.5% of your groundspeed as the lead distance. Assuming your ground speed is 120 knots, start turning onto the arc about 0.6 miles before the arc or when the DME reads 11.6 miles. Depending on winds aloft, your groundspeed may differ significantly from your indicated airspeed, so select the groundspeed readout on your DME and pay attention to it. When your DME reads 11.6, make the big turn.

After the big turn, the course pointer points to station

Turn Ten, Twist Ten

Regardless of which way you turn to get on the arc, stay on the arc by making a series of small, strategic changes in heading while adjusting your OBS or HSI to track your progress. Let's say you are using an HSI to fly the arc, twist the OBS so that you just barely have a full-scale needle deflection in the direction you are flying. Not sure which way to twist the course pointer? It's really simple, same side safe:
  1. Determine your current heading
  2. Locate that heading on the OBS or HSI
  3. Twist the OBS or HSI course pointer so the needle deflects on the same side of the CDI as where you found your current heading
Current heading on same side as course deviation bar = Same side safe

Using an RMI/bearing pointer is much simpler: Just adjust your heading to keep the arrow head of the pointer generally aimed at the station.

While you fly the arc, consult your DME to determine your distance and speed relative to the VOR/DME station. By setting the DME unit to ground speed/time-to-station mode, you can strive to see a ground speed (relative to the station) that is below 20 knots or so. That's taxiing speed and you won't get too far off the arc at those speeds.

If you have a heading bug, use it to track your chosen heading and adjust the bug each time you change heading to remain on the arc.

Let's say after joining the arc the DME indicates 11.2 miles. You're outside the arc and need to turn at least 20 degrees toward the station. Don't be afraid to make an aggressive heading change if you are outside the arc. On the other hand, if you joined the arc and the DME indicated 10.5 miles you are slightly inside the arc and that's not a bad place to be. Maintaining the current heading which should eventually take you back onto arc or turn away from the station by just 10 degrees. Wait 5 or 10 seconds and you should see the DME distance increase.

If the DME distance is within 0.1 or 0.2 miles, fly your present heading and wait for the CDI needle to center. Once it does, twist the OBS another 10 degrees, and consider turning the aircraft 10 degrees toward the station. Whether or not you change heading depends on the winds aloft and what the distance readout says.

Stop the Arc, I want to get off ...

To exit the arc on the desired radial or course, you need to make another big turn. Many instrument approaches with DME arcs depict a lead radial to alert you that your exit is approaching. Just like the turn to join the arc, exiting the arc require approximately 90 degrees of heading change.

This bring up the fact that there are four basic DME arc variations:
  • Fly TO the station, join the arc, exit the arc flying TO the station.
  • Fly TO the station, join the arc, exit the arc flying FROM the station.
  • Fly away FROM the station, join the arc, exit the arc flying FROM the station.
  • Fly away FROM the station, join the arc, exit the arc flying TO the station.

Additional Challenges

There are many places (the SF Bay Area being one) where there are few published DME arcs. That means a pilot on a check ride or proficiency check may be asked to demonstrate a made-up DME arc with no charted representation. When flying a made-up arc, your examiner or instructor should have specified either a left arc or a right arc, so figure out which way you'll need to turn to keep the station on the left or right side of the aircraft.

A particularly challenging DME arc can be found in the MTN VOR/DME RWY 15 where you arc right to the runway threshold, complying with various step-down fixes as you go. You'd obviously want to fly this arc very accurately. And if you need to fly the missed approach, you get to fly another arc to the missed approach holding waypoint. I've only flown this approach on a simulator, but would welcome the opportunity to try it in a real aircraft.

The Easy Way

If you're using an IFR-approved GPS receiver to fly an arc, here are your options:
  • Load the instrument approach defines the arc, specifying the appropriate transition
  • Make the VOR/DME station the current waypoint (proceed direct-to)
  • G1000 and G530 can display distance to any VOR station that has been tuned
The obvious choice with most late-model GPS receivers is to load the approach with the appropriate arc transisition and let the GPS guide you.

Practice Makes Perfect

The best way to maintain proficiency with DME arcs is to practice in a simulator or in a real aircraft. Like a good crosswind landing, there's a satisfying feeling to flying a good DME arc. Try it, you'll like it.


Iain D. Williams said...

A well written and informative artcle. Thanks for taking the time to put this together

David Cheung said...


In your CEC example, I believe the initial heading (and OBS setting) to join the 11 DME arc is 163, not 343, which is actually the radial, since you're heading south?

Next time we do some sim approaches, we should do one. My trip to Oregon a few years ago took me across the CEC approach. This article reminds me of that day.

John Ewing said...


Thanks for catching my error ...

JohnOCFII said...

Nice article. In this day of moving map GPS displays, it is good to switch the display and try to fly these via the needles.