Course Deviation Indicator (CDI) – In-Depth Guide

A Course Deviation Indicator (CDI) is a fundamental cockpit instrument for pilots navigating under instrument flight rules (IFR) or using ground- or satellite-based navigation aids. It visually displays lateral deviation from a selected course—helping pilots stay precisely on track, whether following a VOR radial, intercepting a localizer for an instrument approach, or flying a route programmed in a GPS. Mastery of CDI interpretation is a cornerstone of instrument flying and required for safe, compliant operations in controlled airspace.

Key Terms

• Course Deviation Indicator (CDI): Instrument displaying aircraft’s left/right position relative to a selected navigation course.
• VHF Omnidirectional Range (VOR): Ground-based radio navigation system transmitting radials.
• Global Positioning System (GPS): Satellite-based navigation providing digital course data.
• Omni-Bearing Selector (OBS): Knob to set desired course or radial on CDI/navigation system.
• Course Needle: Moving vertical bar indicating lateral deviation.
• To/From Indicator: Shows whether course leads toward or away from navigation aid.
• Navigation Aids (NAVAIDs): Ground/space-based systems providing navigation data.
• Horizontal Situation Indicator (HSI): Advanced display combining CDI and heading info.
• Instrument Landing System (ILS): Precision approach system using localizer and glideslope.
• Flag Annunciation: Visual warning for invalid/unreliable navigation signals.

How the CDI Works

VOR Navigation

• Pilot tunes navigation radio to VOR frequency and sets desired radial using OBS.
• CDI needle moves left or right—each dot typically = 2° deviation.
• Centered needle: exactly on course; deflected needle: off course—fly toward the needle to correct.
• To/From flag clarifies if the course leads toward or away from the station.
• At greater distances, each dot represents a larger ground distance—pilots must compensate accordingly.

Localizer (ILS) Navigation

• CDI displays lateral position relative to localizer beam (runway centerline).
• Much higher sensitivity: each dot = 0.5°, full-scale = 2.5° or more.
• Some CDIs have a horizontal glideslope needle for vertical approach guidance.
• Used for precision approaches.

GPS Navigation

• CDI receives digital course info from GPS.
• CDI scale auto-adjusts per phase of flight (±5 NM enroute, ±1 NM terminal, ±0.3 NM approach).
• To/From function managed by GPS; CDI reflects active leg.
• Modern CDIs can drive autopilots and, in WAAS-equipped systems, provide vertical guidance.

Signal Flags

• CDI displays warning flag if navigation signal is lost or unreliable.
• Pilots must ignore CDI and revert to alternate navigation per procedure.

CDI Sensitivity Table

CDI Components and Features

Face and Needle

• Vertical course line with left/right dots (2° per dot for VOR, 0.5° per dot for ILS).
• Needle pivots horizontally. Fly toward the needle to return to course.
• Some CDIs have a separate horizontal glideslope needle for ILS.

OBS (Omni-Bearing Selector)

• Rotary knob to set desired course/radial.
• In digital/glass cockpits, may be touchscreen or virtual knob.
• Correct setting ensures proper CDI indications; errors can cause reverse sensing.

To/From Indicator

• Flag or pointer showing whether selected course leads to or from the navigation aid.
• Crucial for confirming intended track.

Signal Flags

• Red/white/black warning panels.
• Indicate loss or unreliability of navigation signal.
• Pilots must ignore CDI when flag is visible.

CDI Scale and Markings

• Physical or digital dots/ticks.
• VOR: 2° per dot, ILS: 0.5° per dot.
• GPS: auto-adjusts per flight phase.
• Essential for interpreting magnitude of deviation.

Practical Use Cases

VOR Radial Tracking – Step-by-Step

1. Tune and Identify: Set VOR frequency, confirm Morse code ID.
2. Set Course: Rotate OBS to desired radial.
3. Check To/From: Confirm directionality for intended path.
4. Interpret Needle: Centered = on course; left = right of course, fly left; right = left of course, fly right.
5. Fly Toward Needle: Always steer toward needle for correction.
6. Monitor Flags: Disregard CDI if signal flag appears.

GPS Navigation with CDI

• Select route/waypoint on GPS.
• CDI reflects position relative to programmed course.
• CDI scale auto-adjusts for phase of flight.
• Fly toward needle to regain course.
• Signal integrity monitored by system; warning issued if unreliable.

CDI in Modern Cockpits

• CDI often combined with HSI (Horizontal Situation Indicator) for enhanced situational awareness.
• Glass cockpits present CDI info digitally, integrating with GPS, VOR, and ILS data.
• Autopilots use CDI signals for lateral navigation.
• ICAO and FAA standards ensure CDI uniformity and reliability worldwide.

Regulatory Framework

• ICAO Annex 10 and FAA TSO-C40 set standards for CDI operation, scale, annunciations, and integration.
• Pilots must demonstrate competent CDI use for instrument ratings.
• CDI remains essential even as VORs are gradually replaced by GNSS in many regions.

Training and Safety

• Correct CDI interpretation is a core skill for instrument-rated pilots.
• Training emphasizes understanding scaling, wind correction, intercept angles, and signal integrity.
• Regular proficiency checks and simulator training reinforce proper CDI usage.

Summary

A Course Deviation Indicator (CDI) remains a critical navigation tool for pilots, providing clear, real-time feedback on lateral position relative to a selected course. Whether using legacy VORs, modern GPS, or precision ILS approaches, the CDI simplifies complex navigation tasks and enhances both safety and efficiency in the cockpit. Knowledge of CDI operation, scaling, and failure modes is essential for every instrument pilot.

Additional Resources

• FAA Instrument Flying Handbook
• ICAO Annex 10 – Aeronautical Telecommunications
• AOPA: How to Use a CDI
• EASA PBN Manual

See Also:
VOR Navigation | HSI | GPS Aviation

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