Instrument Rating Study Guide: Intercepting and Tracking Navigational Systems & DME Arcs
Instrument flying requires more than simply knowing where an airway or course is located. Pilots must be able to intercept a desired course, establish themselves on it, and accurately track it while accounting for wind and changing conditions.
These skills become especially important when flying instrument procedures involving VORs, localizers, GPS courses, and DME arcs. Although modern avionics can make navigation considerably easier, instrument pilots still need to understand what the navigation indications mean and how to respond appropriately.
This study guide reviews the fundamentals of intercepting and tracking navigational systems and flying DME arcs in preparation for instrument training and the Instrument Rating practical test.
Understanding Intercepting vs. Tracking
Although the terms are closely related, intercepting and tracking are two different tasks.
Intercepting means maneuvering the aircraft to join a desired course.
Tracking means maintaining that course after it has been intercepted.
During an intercept, the pilot selects a heading that creates an angle between the aircraft’s current position and the desired course. Once approaching the course, the pilot reduces the intercept angle and establishes a heading that will maintain the desired track.
The process can be thought of as:
Determine position → Select an intercept → Capture the course → Establish a wind correction → Track the course
A successful intercept should transition smoothly into accurate course tracking rather than crossing through the course and requiring large corrections.
Choosing an Intercept Angle
There is no single intercept angle that works for every situation.
The appropriate angle depends on several factors, including:
- Distance from the desired course
- Distance from the navigation facility
- Wind direction and velocity
- Aircraft groundspeed
- Sensitivity of the navigation equipment
- ATC instructions
- How quickly the course needs to be intercepted
A larger intercept angle generally produces a faster intercept, while a smaller angle provides a more gradual course capture.
The pilot should anticipate the course approaching center rather than waiting until the CDI is centered before beginning the turn. Waiting too long can cause the aircraft to overshoot the desired course.
Tracking a Course
Once the desired course has been intercepted, the objective changes from intercepting to maintaining the course.
This is where wind correction becomes important.
If there were no wind, maintaining the appropriate course heading could keep the aircraft on the desired track. With a crosswind, however, the aircraft will gradually drift away unless the pilot establishes a correction into the wind.
A useful technique is:
Bracketing.
If the aircraft begins drifting away from the desired course, make a heading correction toward the course. Once the aircraft returns to the desired track, reduce the correction while retaining enough wind correction to prevent the drift from occurring again.
Through progressively smaller corrections, the pilot can determine the approximate heading required to maintain the course.
Avoid Chasing the CDI
One of the most common mistakes during instrument navigation is reacting too aggressively to every movement of the Course Deviation Indicator.
Large corrections can cause the aircraft to repeatedly cross back and forth over the desired course.
Instead, use deliberate corrections and observe the result.
As the aircraft gets closer to the desired course, corrections should generally become smaller. The objective is to maintain a stable flight path rather than constantly reacting to the needle.
Remember:
Intercept with an angle. Track with a correction.
Navigation Becomes More Sensitive Near the Station
When using a ground-based navigation facility such as a VOR, the same angular course deviation represents a smaller lateral distance as the aircraft gets closer to the station.
As a result, CDI movement may appear increasingly sensitive when approaching the facility.
Pilots should anticipate this and avoid making increasingly aggressive corrections simply because the CDI begins moving faster.
Smaller corrections are often appropriate as the aircraft approaches the station.
VOR Course Interception and Tracking
When navigating with a VOR, the pilot should understand the relationship between the selected course, CDI, TO/FROM indication, and aircraft position.
Before beginning an intercept, verify:
- The correct navigation source is selected
- The correct frequency or facility is being used
- The desired course is selected
- The TO/FROM indication is appropriate
- The navigation indication is behaving as expected
Once established on the course, maintain an appropriate heading and use small corrections to compensate for wind.
Instrument pilots should avoid simply “following the needle” without understanding where the aircraft is located relative to the selected course.
GPS Course Tracking
GPS navigation may provide more intuitive information than traditional VOR navigation, but the underlying principles remain similar.
The pilot must still:
- Identify the desired course.
- Determine the aircraft’s position relative to that course.
- Establish an appropriate intercept.
- Capture the course.
- Maintain the desired track.
Modern GPS systems may also provide additional information such as desired track, actual track, cross-track error, and groundspeed.
These tools can improve situational awareness, but pilots should understand what each indication represents rather than simply following the magenta line.
Localizer Interception
Localizer tracking requires particular attention because the localizer becomes increasingly sensitive as the aircraft approaches the runway.
An intercept that might be acceptable when joining an airway could result in an unstable capture when intercepting a localizer.
When approaching the localizer, pilots should anticipate CDI movement, reduce the intercept angle as appropriate, and make increasingly small corrections after becoming established.
Overcontrolling the localizer can quickly lead to a series of corrections back and forth across the final approach course.
Smoothness and anticipation are critical.
What Is a DME Arc?
A DME arc is a curved flight path flown at a specified distance from a DME facility.
Instead of tracking directly toward or away from the facility, the aircraft maintains approximately the same DME distance while flying around it.
For example, a procedure may require an aircraft to fly a 10 DME arc around a navigation facility.
The aircraft’s heading will continuously change as it progresses around the facility, but its distance from the station should remain approximately 10 nautical miles.
Understanding the Geometry of a DME Arc
The easiest way to understand a DME arc is to picture a circle around the navigation facility.
The facility is at the center of the circle, and the specified DME distance represents the radius.
If the procedure requires a 10 DME arc, the aircraft is essentially attempting to remain on the circumference of a circle located 10 nautical miles from the facility.
Because the aircraft is moving around the circle, it cannot simply maintain one heading. The pilot must periodically change heading to remain approximately the same distance from the station.
Entering a DME Arc
Before reaching the arc, the pilot should determine:
- The required DME distance
- The direction the arc will be flown
- The appropriate lead for the turn
- The navigation source being used
- The course or radial associated with leaving the arc
The turn onto the arc should normally begin before reaching the exact DME distance because the aircraft will continue traveling forward during the turn.
If the pilot waits until reaching the published DME before beginning the turn, the aircraft may overshoot the arc.
The amount of lead required depends on factors such as groundspeed and bank angle.
Flying the Arc
Once established on the arc, the pilot’s objective is to maintain approximately the published DME distance.
A traditional technique is often described as:
Turn 10, twist 10.
Using this technique, the pilot makes small heading changes—often approximately 10 degrees at a time—as the aircraft progresses around the arc, while also adjusting the selected VOR radial to maintain awareness of position.
The exact technique will depend on the equipment installed in the aircraft.
The important concept is that the pilot is continuously making small corrections to remain at approximately the same distance from the facility.
Correcting Inside or Outside the Arc
If the DME begins decreasing below the desired distance, the aircraft is moving inside the arc, or closer to the facility.
The pilot should adjust the heading slightly away from the station.
If the DME begins increasing above the desired distance, the aircraft is moving outside the arc, or farther from the facility.
The pilot should adjust the heading slightly toward the station.
Think of it simply as:
DME decreasing → move away from the station
DME increasing → move toward the station
Small corrections are generally preferable to large heading changes.
Wind Correction on a DME Arc
Wind adds another challenge because the required correction changes as the aircraft travels around the facility.
A wind that initially pushes the aircraft toward the station may later push it away as the aircraft’s heading changes.
This means the pilot cannot necessarily establish one wind correction angle and maintain it throughout the entire arc.
Instead, monitor the DME continuously and adjust the heading as necessary.
As with normal course tracking, the goal is to recognize a trend early and make a small correction before a large deviation develops.
Leading the Turn Off the Arc
Just as the pilot should anticipate entering the arc, the pilot must anticipate leaving it.
Waiting until the aircraft reaches the desired radial or course before beginning the turn may result in overshooting the outbound or inbound course.
The pilot should identify the radial or course used to leave the arc and begin preparing for the transition before reaching it.
Good DME arc flying is largely about staying ahead of the aircraft.
Know what is coming next before the navigation indication reaches it.
Using Modern Avionics
GPS and integrated flight displays can make DME arc navigation considerably easier by providing a graphical representation of the aircraft’s position relative to the procedure.
However, pilots should not allow the moving map to replace their understanding of the procedure.
Be able to explain:
- What distance you are maintaining
- Which facility defines the arc
- Which direction you are traveling
- Where you will leave the arc
- What course or segment comes next
Technology should improve situational awareness—not replace it.
Common Errors
During intercepting, tracking, and DME arc operations, watch for common errors such as:
- Selecting the wrong navigation source
- Using an unnecessarily large intercept angle
- Failing to anticipate course capture
- Chasing the CDI
- Making excessive heading corrections
- Failing to account for wind
- Misinterpreting TO/FROM indications
- Losing awareness of the active navigation source
- Entering a DME arc without adequate lead
- Making large corrections while flying the arc
- Failing to monitor DME trends
- Waiting too long to prepare for the next segment
- Becoming overly dependent on the moving map
Good instrument flying requires staying ahead of the navigation system rather than reacting after a deviation has already developed.
Checkride Preparation
For the Instrument Rating practical test, applicants should be prepared to demonstrate an understanding of how to intercept and track courses using the navigation equipment installed in the aircraft.
More importantly, be prepared to explain why you are making a particular heading correction.
If the CDI is moving away from center, know what that tells you about the aircraft’s position and track. If the DME is increasing while flying an arc, understand what that means and how to correct it.
The examiner is evaluating more than your ability to keep a needle centered. You should demonstrate that you understand the navigation information being presented and can use it to maintain situational awareness.
Final Takeaway
Intercepting and tracking navigational systems is a fundamental instrument skill built around three concepts: position, anticipation, and correction.
Determine where the aircraft is relative to the desired course, select an appropriate intercept, anticipate the course capture, and then establish the wind correction necessary to remain on course.
DME arcs apply those same principles to a curved flight path. Instead of maintaining a single course, the pilot continually adjusts the aircraft’s heading while maintaining a specified distance from the navigation facility.
Whether using traditional VOR equipment or modern GPS avionics, the goal remains the same: understand what the navigation system is telling you, stay ahead of the aircraft, and make small, deliberate corrections before minor deviations become major ones.
Study Tip: When practicing, don’t focus only on keeping the CDI centered or holding an exact DME number. Continually ask yourself: Where am I? Where is the course or facility? Where am I going next? If you can answer those three questions throughout the procedure, you’re developing the situational awareness that instrument flying requires.
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