# Course deviation indicator > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Course_deviation_indicator > Markdown URL: https://mediated.wiki/source/Course_deviation_indicator.md > Source: https://en.wikipedia.org/wiki/Course_deviation_indicator > Source revision: 1273228477 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Avionics instrument}} thumb|right|250px|A mechanical VOR display A '''course deviation indicator''' ('''CDI'''){{cite book|author1=FAA|title=Pilot's Handbook of Aeronautical Knowledge|date=2016|page=23|url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/18_phak_ch16.pdf|chapter=16. Navigation|quote=The CDI is found in most training aircraft. It consists of an omnibearing selector (OBS) sometimes referred to as the course selector, a CDI needle (left-right needle), and a TO/ FROM indicator.}} is an [avionics](/source/avionics) instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a course to or from a radio navigation beacon. If the location of the aircraft is to the left of this course, the needle deflects to the right, and vice versa. ==Use== The indicator shows the direction to steer to correct for course deviations. Correction is made until the vertical needle centres, meaning the aircraft has intercepted the given course line. The pilot then steers to stay on that line. Only the receiver's current ''position'' determines the reading: the aircraft's heading, orientation, and track are not indicated. The deflection of the needle is proportional to the course deviation, but sensitivity and deflection vary depending on the system being used: * When used with a [VOR](/source/VHF_omnidirectional_range) or VORTAC, the instrument can be referred to as an "omni bearing indicator" ("OBI").{{cite book|title=Radio Navigation & Instrument Flying|date=2008|publisher=Air Pilot Publishing Ltd|isbn=978-1-84336-069-8}} The course line is selected by turning an "omni bearing selector" or "OBS" knob usually located in the lower left of the indicator. It then shows the number of degrees deviation between the aircraft's current position and the "radial" line emanating from the signal source at the given bearing. This can be used to find and follow the desired radial. Deflection is 10° deviation at full scale (each side), with each dot on the CDI representing 2°. (See [Using a VOR](/source/VHF_omnidirectional_range) for usage during flight.) * When used with a [GPS](/source/Global_Positioning_System), or other [RNAV](/source/RNAV) equipment, it shows actual distance left or right of the programmed course line. Sensitivity is usually programmable or automatically switched, but {{convert|5|nmi|km}} deviation at full scale is typical for en route operations. Approach and terminal operations have a higher sensitivity frequently up to {{convert|0.3|nmi|km}} at full scale. In this mode, the OBS knob may or may not have an effect, depending on configuration.{{cite web|url=http://static.garmin.com/pumac/143_InstallationManual.pdf|title=400 Series Installation Manual|page=5.10|access-date=2016-11-23|archive-url=https://web.archive.org/web/20161124024608/http://static.garmin.com/pumac/143_InstallationManual.pdf|archive-date=2016-11-24|url-status=dead}} * When used for instrument approaches using a [LDA](/source/localizer_type_directional_aid) or [ILS](/source/instrument_landing_system) the OBS knob has no function because the course line is usually the runway heading, and is determined by the ground transmitter. A CDI might incorporate a horizontal needle to provide vertical guidance when used with a precision ILS approach where the glideslope is broadcast by another transmitter located on the ground. A CDI is not used with an [automatic direction finder](/source/automatic_direction_finder) (ADF), which receives information from a normal [AM radio](/source/AM_radio) station or an [NDB](/source/non-directional_beacon). ==Operation== The CDI was designed to interpret a signal from a VOR, LDA, or ILS receiver. These receivers output a signal composed of two [AC](/source/alternating_current) voltages. When used with a VOR, a converter decodes this signal, and, by determining the desired heading or radial from a [resolver](/source/resolver_(electrical)) connected to the OBS knob, provides a 150mV control signal to drive the CDI needle left or right. Most older units and some newer ones integrate a converter with the CDI. CDI units with an internal converter are not compatible with GPS units. More modern units are driven by a converter that is standalone or integrated with the radio. The resolver position is sent to the converter which outputs the control signal to drive the CDI. For digital units, the desired position of the needle is transmitted via a serial [ARINC 429](/source/ARINC_429) signal from the radio or GPS unit, allowing the CDI design to be independent of the receiver and by multiple receiver types. ==See also== * [Acronyms and abbreviations in avionics](/source/Acronyms_and_abbreviations_in_avionics) * [Horizontal situation indicator](/source/Horizontal_situation_indicator) * [Index of aviation articles](/source/Index_of_aviation_articles) ==References== {{reflist}} ==External links== * [http://www.luizmonteiro.com/Learning_VOR_Sim_1.aspx Flash VOR type Course Deviation Indicator Simulator] {{Flight instruments}} {{Aircraft components}} Category:Aircraft instruments Category:Avionics Category:Radio navigation --- Adapted from the Wikipedia article [Course deviation indicator](https://en.wikipedia.org/wiki/Course_deviation_indicator) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Course_deviation_indicator?action=history)). 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