8.2 VOR Intercepting, Tracking & Radial Orientation
Key Takeaways
- A standard Course Deviation Indicator (CDI) features a 5-dot deflection scale on each side of center, where full-scale deflection represents 10° (2° per dot); off-course linear displacement follows the Rule of 60.
- The TO/FROM indicator flag is determined strictly by aircraft geographic position relative to the 90° line of ambiguity perpendicular to the course selected on the OBS, and is completely unaffected by aircraft heading.
- Reverse sensing occurs on conventional CDIs when flying toward a station with a FROM flag or away from a station with a TO flag; setting the desired course matching the direction of flight prevents reverse sensing.
- Standard course interception uses an intercept angle between 30° and 45°: determine current radial, calculate bearing difference, set desired inbound/outbound course on the OBS, turn to intercept heading, and track inbound.
- Station passage over a VOR is defined by the first complete, positive reversal of the TO/FROM indicator, accompanied by rapid CDI oscillations inside the Cone of Confusion.
VOR Intercepting, Tracking & Radial Orientation
Quick Answer: A conventional Course Deviation Indicator (CDI) has a full-scale deflection of 10° (representing 2° per dot on a 5-dot scale). The TO/FROM flag depends entirely on the aircraft's physical location relative to the 90° line of ambiguity perpendicular to the Omni Bearing Selector (OBS) setting and is independent of aircraft heading. To prevent reverse sensing on a traditional CDI, always select the inbound course with a TO flag when flying toward the station, and the outbound radial with a FROM flag when flying away from the station. Course intercepts are executed by turning 30° to 45° toward the needle, and station passage is confirmed upon the first positive reversal of the TO/FROM indicator as the aircraft exits the Cone of Confusion.
Accurate VOR navigation requires a solid understanding of instrument geometry. Pilots must mentally visualize the aircraft's position relative to ground radials, interpret the Course Deviation Indicator (CDI), and apply aerodynamic wind correction angles to track courses in instrument meteorological conditions (IMC).
Course Deviation Indicator (CDI) Anatomy & Angular Sensitivity
The standard cockpit VOR display combines an Omni Bearing Selector (OBS) knob, a rotating azimuth scale, a vertical Course Deviation Indicator (CDI) needle, and a TO/FROM/NAV status indicator.
[ Standard VOR Indicator / CDI ]
030
. . | . .
330 060
. .
300 090
. +-----+-----+ .
| | |
W -------|--o--|--o--|------- E
| | |
. +-----+-----+ .
240 [ TO ] 120
. .
210 150
. . | . .
210
[ OBS Knob (O) ]
Angular vs. Linear Deviation (The Rule of 60)
The VOR CDI displays angular deviation, not linear distance from course:
- Full-scale deflection: 10° off the selected radial/course.
- Deflection scale: 5 dots on each side of center $\rightarrow$ 2.0° per dot.
- Half-scale deflection: 5° (2.5 dots).
Because deflection is angular, the needle becomes progressively more sensitive as the aircraft nears the ground station. The actual nautical miles off centerline is calculated using the Rule of 60:
| Distance from VOR | 1 Dot Deflection (2°) | Full Scale Deflection (10°) |
|---|---|---|
| 60 NM | 2.0 NM off centerline | 10.0 NM off centerline |
| 30 NM | 1.0 NM off centerline | 5.0 NM off centerline |
| 15 NM | 0.5 NM off centerline | 2.5 NM off centerline |
| 3 NM | 0.1 NM (600 ft) | 0.5 NM (3,000 ft) |
The TO/FROM Flag & The Line of Ambiguity
The TO/FROM flag informs the pilot whether the selected OBS course will take the aircraft toward or away from the VOR station.
Selected Course: 360° (OBS = 360)
360° Radial
^
|
"FROM" ZONE | "FROM" ZONE
(Aircraft is | (Aircraft is
North of line) | North of line)
|
============================[ VOR STATION ]==========================
90° / 270° LINE OF AMBIGUITY (Perpendicular Line)
|
"TO" ZONE | "TO" ZONE
(Aircraft is | (Aircraft is
South of line) | South of line)
|
180° Radial
Fundamental Geometric Rules of the TO/FROM Flag
- The 90° Line of Ambiguity: A line drawn perpendicular (90°) to the OBS course setting through the center of the VOR station divides the airspace into two halves:
- TO Hemisphere: The half containing the tail of the OBS course arrow (the reciprocal course direction).
- FROM Hemisphere: The half containing the head of the OBS course arrow (the selected radial direction).
- Independence from Heading: The TO/FROM flag and CDI needle position depend strictly on aircraft geographic location relative to the station and the OBS setting. Turning the aircraft on a heading of 090°, 180°, 270°, or 360° in place does not alter the CDI needle or flag in any way.
Reverse Sensing: Mechanisms & Prevention
Reverse sensing occurs when the CDI needle deflects in the direction opposite to the actual course location, causing the pilot to fly away from the desired centerline when attempting to "fly toward the needle."
+-------------------------------------------------------------------------+
| REVERSE SENSING CAUSE AND EFFECT |
| |
| Standard (Command) Sensing: |
| - Flying INBOUND with OBS set to INBOUND course (TO flag visible) |
| - Flying OUTBOUND with OBS set to OUTBOUND radial (FROM flag visible) |
| - Needle points TOWARD the course: "Fly toward the needle" |
| |
| Reverse Sensing: |
| - Flying INBOUND toward VOR with OUTBOUND radial set on OBS (FROM flag)|
| - Flying OUTBOUND away from VOR with INBOUND course set on OBS (TO flag)|
| - Needle points AWAY from the course: Flying toward needle INCREASES |
| cross-track error! |
+-------------------------------------------------------------------------+
The Horizontal Situation Indicator (HSI) Advantage
In modern or advanced glass cockpits, the Horizontal Situation Indicator (HSI) integrates the magnetic compass heading card with the VOR course pointer. Because the entire compass card rotates dynamically as the aircraft turns, an HSI provides automatic visual orientation without reverse sensing, provided the course pointer is set to the intended front course.
CONVENTIONAL CDI HORIZONTAL SITUATION INDICATOR (HSI)
(Fixed Card) (Rotating Compass Card)
030 330
. | . . | .
300 060 300 | 000
[ | ] <-- Reverse sensing if [---\|/---] <-- Needle auto-aligns
210 150 flying reciprocal heading 210 | 030 with aircraft heading
| |
Systematic Course Interception Procedures
Interception of a specific VOR radial or inbound course follows a structured five-step method:
DESIRED INBOUND COURSE: 360°
^
| (Course Centerline)
|
| <--- 45° Intercept Angle
| / (Heading 315°)
| /
| /
| /
Aircraft [ > ]--+ (Currently on 030° Radial)
The 5-Step Intercept Method
- Tune & Identify: Tune the VHF frequency, verify the 3-letter Morse code identifier, and check for the absence of NAV warning flags.
- Determine Current Position: Turn the OBS until the CDI needle centers with a FROM flag. The number at the top of the index is your current radial.
- Set Desired Course: Rotate the OBS to set the desired course:
- Inbound Intercept: Set the inbound course (reciprocal of the assigned radial) at the top index $\rightarrow$ verify a TO flag.
- Outbound Intercept: Set the outbound radial at the top index $\rightarrow$ verify a FROM flag.
- Determine Intercept Angle & Heading:
- Note the side to which the CDI needle has deflected.
- Standard intercept angles range from 30° to 45° (up to 90° for close-in rapid intercepts).
- Formula: $\text{Intercept Heading} = \text{Desired Course} \pm \text{Intercept Angle}$ (turn toward the needle).
- Example: Desired inbound course is 360°, needle is deflected LEFT $\rightarrow$ Intercept heading $= 360° - 45° = 315°$.
- Fly the Heading & Lead the Turn: Fly the intercept heading until the CDI needle moves off the peg ("needle alive"). Initiate a standard-rate turn onto the desired course before the needle fully centers (leading the turn by $1°–2°$ of deflection).
Wind Drift Correction & Bracket Tracking
Once established on the desired course, crosswinds will cause the aircraft to drift off centerline unless a proper Wind Correction Angle (WCA) is maintained. Pilots use bracketing to establish the exact correction:
+-------------------------------------------------------------------------+
| BRACKET TRACKING TECHNIQUE STEPS |
| |
| 1. Initial Centerline On-Course: |
| Fly the exact magnetic course heading (e.g., Heading 360°). |
| |
| 2. Recognize Drift: |
| If the CDI needle drifts right, a crosswind from the right is |
| pushing the aircraft left of course. |
| |
| 3. Establish Initial Intercept Re-entry: |
| Turn 20° into the wind (Heading 020°) to re-intercept course. |
| |
| 4. Halve the Bracket Angle: |
| When the needle re-centers, turn to a heading that maintains half |
| the initial correction: 10° right (Heading 010°). |
| |
| 5. Fine-Tune: |
| If the needle drifts again, adjust by 3° to 5° increments until |
| the CDI needle remains locked on center. |
+-------------------------------------------------------------------------+
Station Passage & The Cone of Confusion
Directly above a VOR ground antenna, radio energy radiates outward rather than vertically upward, creating an inverted conical volume known as the Cone of Confusion (or Zone of Ambiguity).
[ CONE OF CONFUSION GEOMETRY ]
\ / High Altitude (Wide Cone)
\ /
\ /
\ / Mid Altitude
\ /
\ / Low Altitude (Narrow Cone)
\ /
\ /
===================[ VOR STATION ]===================
Characteristics During Station Passage
- Needle Sensitivity: The CDI needle fluctuates rapidly from side to side.
- Flag Activity: The TO/FROM flag may flutter or temporarily display the red NAV/OFF warning flag.
- Formal Station Passage Definition: Station passage is recognized upon the first complete, positive reversal of the TO/FROM indicator (the flag flips decisively from TO to FROM).
- Pilot Action: Maintain the established magnetic heading through the cone; do not chase needle oscillations. Once the FROM flag settles, re-engage tracking on the outbound radial.
A pilot is tracking inbound to a VOR on the 060° radial. To fly directly toward the station with standard (command) CDI sensing, how should the OBS be set?
While tracking inbound on a VOR course of 180° with a TO flag, the CDI needle is deflected 2 dots to the left. At a distance of 30 NM from the VOR, what is the aircraft's approximate lateral distance from course centerline?
How is positive station passage officially identified when flying over a VOR ground facility?