A DME arc is an imaginary circle defined by a constant DME distance; the aircraft maintains this distance while flying a curved track that is part of the instrument approach. Because DME measures slant range, the arc will have at least one minimum altitude, and its protected airspace normally extends 4 miles either side of the published DME distance. The procedure is used to transition an aircraft from the enroute setting to the final approach course; controllers refer to it as a ‘right arc’ when the turn is clockwise. To stay on the arc, pilots fly a series of short legs that repeatedly correct toward the stated distance, so distance information from the DME continuously determines aircraft position. Short legs maintain the arc until the aircraft intercepts the inbound course.
What is a DME arc in aviation?
A DME arc is a procedure used to transition an aircraft from the enroute setting to an instrument approach, which involves flying a circular course around a VOR/DME or VORTAC station at a specified distance. The radius is defined by a DME distance from the VOR, and the protected area around an arc is 4 miles either side of the DME distance. The DME arc will have at least one minimum altitude.
A DME arc in aviation is a circular course at specified distance measured in nautical miles from a Distance Measuring Equipment co-located with VOR or ILS/LOC. Distance Measuring Equipment determines aircraft position relative to navigational facility, and the arc is usually an initial approach segment charted as a heavy black line that helps aircraft transition from enroute setting to instrument approach.
In IFR operations, a DME arc serves as a defined track where the pilot keeps the station on the right wingtip for a right arc, adjusting aircraft heading to correct for distance while the bearing pointer is allowed to move 5 -10 behind wingtip. VOR provides omnidirectional course information and DME, with VOR transmitted over VHF, enabling precise guidance during the arc.
On an instrument approach, the arc is always part of the approach, guiding the aircraft along a circular course around VORTAC station. Although arcs associated with a procedure are not always based on the facility which is the navaid for the approach, the arc entry is located after the initial fix, establishing a seamless flow into the final approach.
When VOR is paired with DME, the VOR gives omnidirectional course information while DME displays continuous slant-range distance, letting the pilot fly the arc by maintaining the published distance and turning approximately 90 degrees as required to keep the prescribed track. Crosswind drifts aircraft closer, so the pilot must turn out to regain the correct arc.
In a holding pattern based on a DME arc, holds use standard turns or non-standard turns as a classification. The pilot notes aircraft heading at the left or right side of VOR, then executes a lead turn by 0.5 NM with GS < 150 knots to roll onto the inbound course. GPS substitution allows the same lead turn by 0.5 NM with GS < 150 knots, while a simple turn approximately 90 degrees completes hold entry.
What is the purpose of a DME arc?
A DME arc is an initial approach segment flown on a curved track kept at constant distance around a VORTAC, VOR/DME, NDB/DME ground-based facility. Because the facility offers omnidirectional course information and DME simultaneously, the arc shortens time to arrive at the IAF from the en-route segment while it determines the aircraft position relative to the navigational facility. The arc usually leads to the final approach course, so it becomes part of the approach that lets the pilot adjust aircraft heading to correct for distance and maintain situational awareness. Inside the arc the pilot turns approximately 90 degrees as a general method to stay in the protected area. Although an arc will be used in other ways, it will rarely be used in a STAR, and the same curved segment will be generated with GPS where the avionics determine the aircraft position relative to a waypoint, still giving the same lead-in to the final approach.
How to fly an arc DME?
Flying an arc has three phases - joining, maintaining, and exiting. When flying an arc the pilot begins by setting the GPS to the correct station, then rolls into a 90 degree intercept heading until the DME reads the exact published distance. As the arc is captured, the bearing pointer on the RMI lies on the wingtip reference. The pilot maintains that heading and allows the pointer to drift 5 to 10 degrees behind the wingtip, signalling the start of the Maintaining the arc phase. If no wind exists, the aircraft is kept on track by holding a relative bearing of 90 or 270 degrees. Any crosswind that drifts the aircraft closer prompts a gentle turn-out until the pointer again sits behind the wingtip, while drift away demands a turn-in until the pointer creeps ahead of the wingtip. The continuous, small corrections keep the DME distance constant without chasing the indicator. When the outbound course is reached, the final phase, Exiting from the arc, is executed: the pilot keeps heading 240 degrees until the needle on the VOR gauge gets back to center, then rolls smoothly onto the assigned track, leaving the arc.
How to fly an unpublished DME arc?
Controllers sometimes let you fly an arc that is not published on the chart when weather blocks the published path. These improvised arcs are created based upon local need. Ask for vectors, or simply listen for a general direction from the station, then tune the VOR station for the procedure and review any notes that state the capability needed. Inside each segment, maintain heading until, approaching the turn to final, the CDI or RNAV box flashes: NEXT DTK 257, after which the ATC IFR altitude assignment of 2,000 replaces the planned 2,700 as an authoritative numbers change. The unpublished arc, flown with these cues and the customary one-mile check, will keep you on track.
How to intercept a DME arc?
To intercept a DME arc, first tune the DME to the correct station, identify the nav aid and, for a simple entry, approach from the inbound course. Arc is intercepted at any point along the published arc, but the cleanest technique is to arrive on the inbound course, then turn approximately 90 degrees left or right from the inbound course. This 90 degree arc-interception turn places the airplane tangent to the arc. Upon completion of the 90 degree arc-interception turn you center the CDI needle (FROM) and immediately set OBS to radial 10 degrees ahead (FROM) so that the next course change is prepared.
You join the arc by anticipating turn radius and the actual turn begins when DME shows the lead distance, not the published distance. A standard-rate 90 degree heading change takes 30 seconds, so roll-out occurs exactly on the DME arc value. Once established you fly a series of short legs: twist the OBS 10 degrees each time the CDI centers, then turn 10 degrees to stay aligned, keeping slightly inside the curve. Three phases are maintaining the arc - track, lead, and turn - and you must watch for flag switches or full deflections because VOR provides omnidirectional course information that fluctuates. By keeping the CDI centered after each 10 degree adjustment and always twisting in the inbound course to your next fix, you remain on the arcing approach until the final lead-out turn is initiated.
When to start a DME arc?
A DME arc begins at an IAF placed at the appropriate DME distance from the VOR on an inbound airway. Turn to the arc starts at 10.5 miles (16.9 km), 0.6 miles (0.97 km) before the arc. Anticipation distance equals ground-speed divided by 200. At 109 knots this gives 0.55 NM. A standard-rate 90 degrees heading change onto the arc takes 30 seconds.
Can you fly a DME arc with GPS?
Yes, you can fly a DME arc with a GPS. DME arcs associated with instrument approaches are flown using GPS distance provided the DME transmitter is identified in the GPS database and the GPS navigation database is current.
To use GPS for instrument flight you need a TSO-C129, TSO-C196, TSO-C145, or TSO-C146 compliant GPS. When these conditions are met, GPS distance substitutes DME distance for DME arcs. The GPS calculates your distance across the ground and the display shows the lat/long distance measured between two points. Above Flight Level 240, aircraft utilizing IFR GPS in lieu of DME are not required to be equipped with DME, because IFR GPS satisfies the requirement for DME at and above Flight Level 240.
What is the DME arc turn 10 twist 10?
Turn 10 twist 10 is a method to fly the DME arc. The pilot turns 10 degrees along the arc and twists the OBS or HSI another 10 degrees in the direction of the turn. When the CDI centers, the cycle repeats. This turn 10 twist 10 procedure keeps the DME constant within 1 NM while the aircraft follows the curved track at a fixed distance around the VOR. It is used to maintain accuracy while joining the arc, while on the arc, while flying inbound to an entry radial, while intercepting radials, while exiting the arc, and while transitioning to the final approach course. For greater accuracy, some pilots prefer the turn 20 twist 20 rule of thumb, turning 20 degrees and twisting 20 degrees all the way around, especially when outside the arc. Inside the arc the aircraft maintains heading and makes smaller corrections: if the DME indicator shows 11.7 NM, meaning the aircraft is drifting away from the facility, the pilot adds more than 10 degrees during the turn. If it shows 11.2 NM, meaning drifting toward the facility, the turn is less than 10 degrees. Although the technique adds to an already high workload situation, it remains a reliable way to stay on the arc when GPS navigation is not available.
How to fly a DME arc with an HSI?
To fly a DME arc with an HSI, before turning, you verify HI or HSI aligned with the magnetic compass, then set the desired DME value on the computer and note the inbound radial that defines the arc. Pick a heading 20 degrees ahead of the arc and twist HSI 10 degrees off that heading. Remember you fly FROM the station throughout the arc. You are matching the displayed bearing to the chosen DME distance rather than chasing the to-from flag. When the DME limit is reached, stop turning and let the airplane fly until HSI centers and this keeps you on the imaginary ring. Let the amber digital distance read-out control your bank instead of rigid timing: roll out when the numbers match, then start the next 10-degree bite.
The heading bug is moved only in small increments so that the pilot twists HSI 10 degrees each time the DME hits the desired mileage, producing a segmented but smooth curve. These HSI hints translate easily if you use RMI for backup. With an RMI, the same 10-degree rotation of the card is mentally executed each time the single needle touches a pre-drawn mark on the bezel. Whether viewed on glass or the older mechanical card, maintain a picture of the airplane moving around the inside of the circle so that arc join is not 90 degrees but the next intercept of the final approach course remains precise.
What does Lead Radial (LR) mean on a DME arc?
Lead radials are points on the arc and must be charted when intercept is more than 90 degrees or when mandated by a flight check. Jeppesen denotes lead radials identically to radials defining intersections.
A lead radial is the radial at which the turn from the arc to the inbound course is started, and it is marked on the chart with LR followed by the magnetic heading like LR-083 or LR-070. These designations give the pilot over two miles of warning measured along the arc, allowing time to anticipate the intercept. Lead radials are published only when the DME source is not on the extended final approach course, when the intercept angle exceeds 90 degrees, or when mandated by a flight check. Because the lead varies with groundspeed, the bigger the arc the closer these radials appear to the inbound course, assuring that the pilot starts thinking about the turn before the actual intercept.
How to calculate lead radial DME arc?
To determine the inbound turn point on a DME arc, calculate the lead radial by converting the outbound distance to angular displacement. First, find Turn Radius (TR) from the indicated Mach number: TR = (IMN X 10) - 2. Next convert outbound distance, expressed in nautical miles, to the number of radials: radials = outbound distance = 60 X TD, where TD is the turn-in degrees required. A convenient rule of thumb is to multiply TR (in nautical miles) by 60 and the product equals the number of radials to lead the turn. Thus, Offset in degrees = TD X 60. For example, at 0.8 Mach on a 16 DME arc: TR = (0.8 X 10) - 2 = 6 NM.
When can you descend on a DME arc?
Descent while on DME arc is allowed if altitude is not below the published minimum, but clearance does not give the pilot permission to descend prior to or once established on the arc. Descent is authorized after crossing the 17 NM arc and is then authorized to the lower charted altitudes. If the words ‘cleared for the approach’ are included, the pilot descends to the altitude specified on the chart on a published segment of the approach, yet descent is not initiated until the airplane is actually established on the final approach course.
What are common errors when flying a DME arc?
Overshooting the intercept angle or DME arc entry point is a common error that forces abrupt corrections. Not twisting the inbound course early enough during arc exits delays the rollout and invites overshoot. Forgetting wind correction during course tracking produces steady lateral drift that requires ever-larger heading inputs, which in turn trigger needle chasing. Making large heading corrections to fix that drift exaggerates the problem, because each big swing overshoots the center and provokes another swing in the opposite direction. Failure to identify the nav aid before entry undermines every later step and creates surprise flag switches or full deflections at the worst moment.
The instrument itself introduces a systematic slant-range error: the DME readout always exceeds the true horizontal distance because it measures the hypotenuse to the station. The higher the aircraft, the larger the excess. The rule is that if you are at least 1 NM away for every 1,000 ft AGL (304.8 m AGL) the error is negligible. Pilots who do not allow for this inflation roll out early and fly an arc that is actually inside the charted distance. Combining that bias with wind drift, late inbound twists, and last-second overcorrections produces a tracking picture that looks busy but never settles, wasting altitude, airspeed, and situational awareness.
How does wind correction affect a DME arc?
Wind correction angle constantly changes throughout the arc. As the arc progresses, the aircraft position relative to the wind changes, so the pilot must constantly correct heading to counteract the cross-wind effect. One expects increasing or decreasing wind correction angles, because the cross-wind pushes the aircraft inside or outside the desired track. Adjust aircraft heading to compensate for wind or to correct for distance and correction adds or subtracts the desired number of degrees.
Discrepancies develop as wind affects aircraft position. If DME shows 11.2 NM you are drifting closer because of the right cross-wind; lower NM indicates right cross-wind and higher NM indicates left. Wind is used to return to the desired track when slant-range measurements exceed planned distances.
What is the distance along a DME arc?
The distance along a DME arc is normally Arc length 5-15 NM, with 10 NM preferred. The protected area is 4 miles (6.44 km) either side of the DME distance, so pilots are expected to remain within one mile (1.61 km) of the specified arc distance. Arc radius is 7-30 NM.
Which airports have a DME arc approach?DOUGLAS/BISBEE, Arizona (KDUG) and OGDEN/HINCKLEY, Utah field instrument approaches that contain a DME arc. CHAMPAIGN/URBANA, Illinois (KCMI) uses an arc on runway 22R, while DANVILLE/VERMILION COUNTY, Illinois (KDNV) arcs are published for runways 3 and 21. BATTLE MOUNTAIN, Nevada (KBAM) and FOND DU LAC, Wisconsin include DME-arcs in their VOR/DME procedures. In the United Kingdom, Sheffield airport is used routinely for DME-arc training.
Across the Atlantic, Norway and Denmark each supply examples of active DME-arc approaches, Denmark's being hosted by an uncontrolled aerodrome. Within the United States system, the VOR/DME-15 approach at KMTN (Baltimore-Martin State) is built around an arc that forms the entire final segment, and KWVI (Watsonville, California) offers a VOR/DME-A that is flown as an arc. At KDRO (Durango-La Plata County, Colorado) the ILS to runway 2 carries two successive DME arcs, giving pilots repeated practice in arc technique on one approach. In South America, SKMZ Manizales, Colombia, presents a distinctive approach that employs the DME-arc geometry.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
