Flying a DME Arc

Flying a DME Arc

A DME arc is a curved approach segment that maintains a constant distance from a ground-based navigation facility, typically a VOR/DME or VORTAC. While GPS has made DME arcs less common in everyday flying, they remain part of the instrument rating practical test standards and appear on numerous published instrument approaches. Understanding how to fly them smoothly demonstrates solid instrument scan technique and situational awareness.

Understanding the Geometry

A DME arc defines a circular path around a navigation facility at a specified distance, usually between 10 and 30 nautical miles. The arc serves as a transition from the en route environment to the final approach course, allowing ATC to vector multiple aircraft efficiently while maintaining terrain and obstacle clearance.

The key to flying a DME arc is recognizing that your aircraft is constantly turning. Unlike a straight course where you make heading corrections and then fly straight, an arc requires continuous small adjustments to maintain the proper radius. The turn rate depends on your groundspeed—faster aircraft need a steeper bank angle to maintain the same radius turn.

The Lead Radial Technique

The most reliable method for intercepting and flying a DME arc uses lead radials spaced every 10 or 20 degrees around the arc. Here's how it works:

When approaching the arc, turn to intercept at approximately a 90-degree angle. As you near the specified DME distance (typically 0.5 nm before), begin your turn to parallel the arc. Your initial heading should be roughly perpendicular to the radial you're on—if you're on the 090 radial flying eastbound, turn to approximately 180 degrees (assuming no wind).

As you fly the arc, monitor both your DME distance and radial position. Every 10 degrees of radial change, adjust your heading by 10 degrees in the direction of the turn. If you're flying a clockwise arc and cross from the 090 to the 100 radial, turn your heading 10 degrees right. This technique naturally creates the curved path while keeping your corrections manageable.

Managing Distance Deviations

Despite your best efforts, you'll drift inside or outside the prescribed arc. The key is making prompt, appropriate corrections. If you drift 0.5 nm inside the arc, turn 20-30 degrees toward the station (tightening your turn) until the DME increases back to the desired distance, then resume your arc heading. If you drift outside, turn 20-30 degrees away from the station (widening your turn) until you return to the proper distance.

The common mistake is over-correcting. Small, timely adjustments work better than large, delayed corrections. Think of it as bracketing—make a correction, observe the result, and refine. Your DME readout updates frequently, giving you excellent feedback.

Wind Correction and Practical Considerations

Wind significantly affects DME arc flying. A headwind or tailwind changes your groundspeed, affecting your turn rate. Crosswinds push you toward or away from the arc. The solution is the same as any instrument flying: establish a heading, observe the results, and adjust.

In practice, you'll often fly slightly inside or outside the arc—that's normal. The protected airspace extends well beyond the depicted arc line. Focus on maintaining a smooth, stabilized flight path rather than chasing the exact DME reading. Altitude control matters more than perfect distance tracking.

Modern aircraft with GPS moving maps make DME arcs significantly easier by providing a visual reference of your position relative to the arc. However, you should be able to fly the arc using raw data—DME distance and VOR radials—since that's what the approach procedure is based on and what you'll be tested on during your instrument checkride.

Transitioning to Final

The arc typically ends at a specific radial where you intercept the final approach course. Plan this intercept carefully. Most procedures specify a lead radial for the turn—usually 5 degrees before the final approach course. At this point, you transition from arc-flying technique to standard course interception, turning to intercept the inbound course at an appropriate angle (usually 30-45 degrees).

Set up your navigation radios early. Have your inbound course set on your VOR head or HSI before you reach the final approach radial. This allows a smooth transition from the arc to the final approach segment without task saturation at a critical phase of flight.

--- *Reference and study only — not for operational use. Always follow your POH/AFM, current regulations and a qualified instructor.*