An Angle On DME Arcs

What is all this DME jazz, anyway? Make no mistake, there are some special tricks to getting the most out of your DME — VFR and IFR.

What’s a DME arc? Well, first, DME stands for Distance Measuring Equipment. It’s a navigation aid that’s used by VFR and IFR pilots alike, though it’s used a bit more by the latter group. Unlike VORs, the signal for which originates from the station, the DME equipment in your aircraft sends out paired pulses at around one gigahertz (in the UHF “band”). Those pulses “interrogate” a ground unit, which then sends two pulses back (at a slightly different frequency). Then, the box in your panel measures the time difference between the send and receive and uses it to determine the distance to the ground unit (slant range) and also your ground speed.

DME frequencies are paired with VOR frequencies, and in fact DME receivers usually display the paired VOR frequency, not the DME’s. What’s cool about DME is that you no longer need triangulation from two VORs to determine where you are on an airway. The DME gives you a fairly exact measurement and can work at distances of nearly 200 nm.

The types of navaids having DME capability are:

  • VOR/DMEs (obviously),
  • VORTACs,
  • ILS/DMEs, and,
  • TACANs (for the helmeted set).

LIMITATIONS: For obvious reasons, you don’t use DMEs when you’re right on top of the facility, unless you’re interested in seeing your altitude displayed in miles — when you’re above the ground unit, you’re still measuring distance from that unit. While the horizontal distance may be zero, the vertical distance still registers. While this does create some error in the measurement, as long as you’re more than one mile off for each 1000 feet of height above the ground unit, this error isn’t going to matter much.

DME arcs are what instrument pilots might use for transitioning from enroute flight to a final approach to a runway (for those approaches with DME arcs, that is). They’re a piece of cake if you have an RMI (more acronymns!) which provides combined heading and relative bearing information from either a VOR, an ADF or both, atop a slaved compass card. An RMI really helps put that mental picture together in a hurry. The larger arcs (bigger radials) are easiest to fly, because things change slowly. VFR pilots might use something like it to circumnavigate a cylinder of controlled airspace.

Example: In theory you could just pick a distance on from the VOR on your DME and then keep your RMI’s VOR needle pegged on the three or nine o’clock reference, for clockwise or counter-clockwise arcs, respectively by flying the appropriate constant rate turn. However, in practice it may be easier to just fly successive segments. In this case, you’d let the pointer move a few degrees behind the wingtip reference on the DME, then turn a bit towards the station until the pointer is ahead of the wingtip reference a bit and repeat the procedure when the pointer is back behind the wing again.

Speaking for myself, the only time I’ve ever been in an aircraft with an RMI was as a passenger. None of our broncos have ‘em, anyway. When I was an instrument wanna-be, I learned a neat trick for flying DME arcs that was so clever, so sweet, and so simple, I’ve just gotta pass it along to you. So here it is:

If you don’t have an RMI… First, there isn’t only one way to fly DME arcs without an RMI. You could theoretically take part of your brain and play microprocessor, furiously chasing the DME readout by quickly altering course left or right as need be, whenever the DME readout changes by 0.1 mile. (That does get a mite tiring, though, and instills about as much peace of mind as working on a bomb squad.) Perhaps, there’s a better way…

A paragraph worth figuring out: Imagine inscribing an 18-sided polygon inside a circle, how many degrees does each chord line (side of the polygon) cover? Lessee … 360 divided by 18 equals … TWENTY! OK, now the sum of all three interior angles of each of those 18 isosceles triangles (or any triangle, for that matter) is 180 and we already know that the ‘small’ angle (at the center of the circle) of each triangle is 20 degrees. By definition, an isosceles triangle contains two angles that are of the same measurement, so if the small angle is 20 degrees, the other two big angles have to equal 80 degrees, right? Stay with me now. The supplementary neighbors for those 80-degree angles (the angles on the other side of the chord line) must then be exactly 100 degrees!

Translation: Fly up to the next radial on the plan view and simply add (if clockwise) or subtract (if counterclockwise) 100 degrees to it and you’ve got the heading to fly for the next 20 degree chord line!

BOTTOM LINE: If you can follow that method, it will keep you well within the allowed tolerances for flying a DME arc. (For those math nerds out there, ‘well within the allowed tolerances’ is true because the worst deviation of a 20 degree chord line from its corresponding arc is only 1.5 percent of the radius. If you want the exact answer, just take the cosine of 10 degrees.) Plus, this method involves MUCH less headwork than chasing the DME number! Pretty cool, huh?