A Course Deviation Indicator (CDI) is an avionics instrument used in aircraft navigation to display lateral deviation from a selected course. Acting as a standalone indicator, it works with VOR, LORAN, GPS, or other area-navigation systems and can switch between GPS and NAV sources like a VOR or localizer. The instrument consists of an omnibearing selector (course selector), a left-right CDI needle, and a TO/FROM indicator. Together they show the pilot whether the airplane is left or right of the desired track and its position relative to the chosen navigation beacon, enabling precise course guidance.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a course deviation indicator?
The course deviation indicator is an avionics instrument used in aircraft navigation, which determines an aircraft's lateral position in relation to a course to or from a radio navigation beacon. The CDI shows the pilot the relationship of the airplane to the selected course and indicates the direction to steer to correct for course deviations, acting somewhat like a course line. The entire needle is the course select needle and its deflection shows the pilot how far left or right the aircraft is from the desired line. Correction is made until the vertical needle centres.
The CDI scale is the angular ruler painted on the instrument face. GPS coupling defines CDI full-scale limits: when the course deviation exceeds the maximum deviation on the scale, the crosstrack error is displayed. System changes CDI scaling accordingly when required conditions occur, so sensitivity increases as the aircraft nears the facility.
During an ILS approach, the glideslope deviation pointer indicates relation of aircraft to glideslope while the vertical needle continues to show localizer displacement. The system automatically switches from GPS to LOC navigation source when an ILS approach is loaded, assuring that the pilot steers to stay on course line as the course arrow head points to the selected facility.
What are the types of CDI in aviation?
There are two types of CDI in aviation: the conventional Course Deviation Indicator and the Horizontal Situation Indicator. The conventional Course Deviation Indicator is built around a single CDI needle that shows left-right deviation from the selected track. A step above this is the Horizontal Situation Indicator, often called HSI, which is a more advanced instrument since it gives more information in one place. The HSI also provides visual display of aircraft position, attitude and heading, making it pivotal to effective navigation.
How does a course deviation indicator work?
A course deviation indicator works by showing how far the aircraft has deflected from the course. The course deviation bar operates with a VOR/LOC or GNSS navigation receiver to indicate left or right deviations from the selected course. The needle moves to the right when the aircraft is left of the course and deflects to the left when the aircraft is right of the course.
The angular movement of the bar shows lateral distance in miles for a GNSS-sourced CDI and angular degrees for a VOR/LOC CDI. Full-scale deflection equals a 10 degree deviation on each side. When the needle centers, the aircraft has intercepted the given course line and correction is made until the vertical needle centres. The pilot then steers to stay on that line until the next deviation appears. The bar and its deflection thus give continuous lateral guidance along a selected track.
A course deviation indicator works by giving an optical response that shows how far the aircraft has drifted from the wanted course. When my aircraft strayed somewhat off way, the needle turned proportionally to the port or right. The needle stayed centered only while I retained the proper direction along the selected radial.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the sensitivity of a course deviation indicator?
The sensitivity of a course deviation indicator has a range. During en-route flight, WAAS-capable receivers present 2 NM full-scale deflection, whereas non-WAAS units keep the older 5 NM on either side of the centerline. Beginning 2 NM prior to the FAWP, the full-scale CDI sensitivity will smoothly change from 1 NM to 0.3 NM at the FAWP. Without arming, the receiver will not change from en-route CDI and RAIM sensitivity of 5 NM to 1 NM terminal sensitivity. In the approach mode, full-scale deflection of the CDI left or right represents 0.3 nautical miles left or right of the centerline. Each dot equals two degrees (0.0349 radians) for VOR, 0.5 (0.0087 radians) for localizer, and, for some GPS displays, simple math dictates 70 feet (21.34 meters) of deviation for each dot of deflection on the CDI.
What is the difference between CDI and OBS in aviation?
The OBS is the control side of the same instrument, while the CDI is the display side that tells the pilot how far left or right he is from the selected line. The OBS (Omni-Bearing Selector) knob is on the CDI instrument itself. The pilot twists the OBS knob to pick the radial he wants to follow. A CDI is a single-needle course deviation indicator whose needle moves as the pilot rotates the OBS knob. In a standalone CDI, the selected radial appears only through the OBS. Changing the knob re-zeros the needle to the new course.
What is the difference between CDI and HSI in aviation?
An HSI is an aircraft instrument that displays VOR guidance along with heading information. HSI combines a CDI with a heading indicator, incorporating heading and course information into a single instrument. The HSI eliminates the need to cross-check multiple instruments and reduces pilot workload. HSI provides better situational awareness than a standalone CDI. HSI shows both present heading and lateral position in relation to a VOR radial or GPS course line. HSI includes a TO/FROM indicator and a course select knob and has a fixed aircraft symbol and a rotating compass card. It eliminates reverse sensing during ILS approaches. HSI can be electronic and included into glass cockpit systems. HSI is deemed better because it combines a CDI with a heading indicator and provides a comprehensive view of aircraft position relative to selected navigation courses.
How to use CDI in aviation?
While using a CDI, the pilot selects the source-VOR or GPS-and the CDI needle swings to show lateral displacement from the chosen course. When paired with VOR, the needle centers when the aircraft is on the selected radial; a left or right deflection commands a corresponding turn to intercept. With GPS, the same needle indicates cross-track error, letting the pilot maintain the desired track by keeping the bar centered with gentle aileron pressure and heading corrections.
I found myself depending on the Course Deviation Indicator. While heading via a VOR radial, I steered onto a terminal close path for an ILS, and a thin air current started to deviate my direction. I continually observed the CDI pointer, which allowed me to monitor my direction. I retained my direction by maintaining the pointer at the center, making quick tiny rectifications until the needle was recentered.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How to read a CDI in aviation?
For reading a CDI, the pilot begins by noting the position of the vertical needle that moves laterally. When that needle is centered, the aircraft is on course. Any deflection shows the direction and size of the deviation in degrees. The TO/FROM indicator mounted in the same navigator head eliminates half of all possible headings to the station by announcing ‘ahead’ or ‘behind’. With both presentations seen together, the pilot reads a single picture: steer toward the needle to center it, and confirm the arrow points TO the station, not FROM it.
The device offered me a clean picture of my direction comparative to the wanted path. Each point on the scale comprised a particular angle variance, and the CDI pointer was fully blocked until I moved onto the exact bearing. I observed the needle start to move somewhat to the right as I veered onto the exact bearing, showing I was heading toward a VOR facility. My allotted radial was 090 degrees, and this showed I was being moved to the direction of my planned path by an air current.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
