DME Arcs Lesson by wifiCFI
What is a DME Arc?
DME Arcs are most commonly found on Instrument Approaches.
A DME Arc is a way to fly a constant distance arc around a navigation system.
Pilots may be required to climb, descend, or maintain altitude while flying a DME Arc.
Getting Established on the Arc
Each DME Arc will be defined by a radial from the navigation station.
As can be seen in our example below, the DME Arc begins at RACGO intersection.
RACGO is defined as the 10 DME from the OGD VOR on the 331 Radial.
A DME Arc is executed by flying short, straight segments around the arc.
That being said, we must know the first heading we must turn to in order to get established.
As seen below, out first turn will always be to a heading that is 90 degrees from the arc’s defining radial.
This initial heading will be the same regardless of where the pilot joins the arc.
This initial heading will be the same regardless of which direction the pilot is flying when joining the arc.
When should the pilot begin the turn to the initial heading?
We don’t want to wait until we are at 10 DME to begin the turn to the initial heading.
This would put us too close or too far from the VOR (depending on our direction of flight).
With that being said, a pilot should begin the turn to the initial heading at ½ his/her groundspeed prior to reaching the desired DME.
If flying at 100 KIAS, this would mean the pilot would begin the turn 0.5 NM prior to the desired DME.
Now that the pilot is established on the arc at the desired DME, he/she will fly a series of short straight lines to maintain the desired DME and intercept each 10 degrees of radial at a 90 degree angle.
Turn 10, Twist 10
The pilot can fly these straight lines and maintain the desired DME by using “Turn 10, Twist 10.”
This means, after intercepting each 10 degrees of radial, the pilot will turn the airplane 10 degrees and then twist the HIS/OBS needle 10 degrees to set up the new intercept radial.
Completing the Arc
To complete the arc, the pilot will turn inbound on the established radial to continue the approach.
Adjusting for Crosswinds
A crab angle must be established during crosswind situations to maintain the desired DME Arc path.
If the proper crab angle is not established and maintained, we will experience the situation depicted below.
In this situation, the pilot may elect to only Turning 5 Degrees instead of 10 degrees when performing the “Turn 10, Twist 10 procedue.”
Instrument Rating ACS Standards
Tune and identify the correct navigation facility.
Determine airplane position in relation to the navigation facility.
Set and correctly orient the course to be intercepted.
Maintain altitude +/- 100’.
Maintain airspeed +/- 10 KIAS.
Maintain headings +/- 5 degrees.
Maintain a distance of +/- 1NM while flying the DME arc.
FAA Sources Used for This Lesson
Instrument Flying Handbook
Instrument Airmen Certification Standards (ACS)