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What are the differences between an OBS, a CDI, a Heading indicator, and an HSI? As far as I can tell they all use VORs. Why would 4 different version of the same instrument be invented? Is the HSI the most advanced of the 4 because it shows the most detail?

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  • $\begingroup$ Some are GPS or glideslope coupled and do not necessarily use just VOR's. For example the second one also has a glideslope indicator and is GPS coupled (Garmin). The third one does not use a VOR/GPS or glideslope, its a gyro-driven heading indicator device. $\endgroup$
    – Ron Beyer
    Mar 10, 2016 at 20:19
  • $\begingroup$ just say "yes" to backcourse localizer approaches with an HSI $\endgroup$
    – rbp
    Mar 10, 2016 at 22:07
  • $\begingroup$ That's not exactly 4 different versions of the same instrument: The third one in your image is just a directional gyro (heading indicator) - it has no VOR/ILS functionality. $\endgroup$
    – voretaq7
    Mar 10, 2016 at 22:20
  • $\begingroup$ You forget dual needle RMI $\endgroup$
    – Max Power
    Aug 25, 2020 at 7:59

2 Answers 2

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Check out this manual from the FAA on flight instruments

Heading Indicator: Heading Indicator

The heading indicator is fundamentally a mechanical instrument designed to facilitate the use of the magnetic compass. Errors in the magnetic compass are numerous, making straight flight and precision turns to headings difficult to accomplish, particularly in turbulent air. A heading indicator, however, is not affected by the forces that make the magnetic compass difficult to interpret.

Heading Indicators are not VOR related. They simple provide an alternative to the compass and are generally used when the compass reading may be inaccurate (non straight and level flight). Due to their functionality they need to be periodically aligned with the compass.

Course Deviation Indicator: CDI

A course deviation indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a course. If the location of the aircraft is to the left of course, the needle deflects to the right, and vice versa.

The OBS (Omni Bearing Selector) knob is on the CDI, it is not its own instrument. The knob allows the selection (via the outer ring) of the radial (course) you wish to track from a VOR ground station. When tracking a radial you only track left/right deviation and thus only a single needle is required (the first picture you have). For instrument flying you also need to track the glide slope for an approach and thus vertical deviation may also be displayed on a CDI. When on an instrument approach the CDI is used to track the left/right deviation from the runway (not a VOR).

Horizontal Situation Indicator (HSI): HSI

Is a CDI and Heading Indicator packed into one unit. it provides both left right deviation along with glide slope information as well as heading indication in turns. Many HSI's are coupled to auto pilots and may allow course selection via a heading knob if they are connected to do so. The HSI is not any more advanced than the sum of its counter parts but it is more complex and to a new pilot may be hard to read.

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    $\begingroup$ You should add the first image from the question too. It is also a CDI, but without glide-slope (and probably older design). $\endgroup$
    – Jan Hudec
    Mar 11, 2016 at 8:11
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These 3 things all make use of a Very High Frequency Omnidirectional radio fix.(a VOR). A VOR may or may not have a DME (Distance Measuring Equipment) different symbols on the map.

  1. CDI (course deviation indicator) is like the gauge found in the Cessna that locks on to a fix when you tune your NAV radio to that particular fix.
  2. HSI Horizontal Selection Indicator does exactly the same thing but the design of the instrument is different like in a Beechcraft Baron. It still works off of the NAV radio
  3. OBS button on a GPS unit Omnidirectional Bearing Selector turns the GPS into a VOR tracker just like above but you do not have to tune the radio as it works off of the GPS. All three only work in the horizontal plane. To lock onto a glidescope you need an ILS (Instrument landing System) fix which does vertical and horizontal navigation.
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  • $\begingroup$ You are overlooking GPS vertical guidance. RNAV/LNAV do not use any ground based input, with lateral & vertical guidance down to 200 feet above the runway. With ILS one can go even lower. HSI in my small plane is GPS driven. $\endgroup$
    – CrossRoads
    Sep 13, 2018 at 14:43
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    $\begingroup$ it is LNAV/VNAV and the vertical guidance is only a nice calculation so you can smooth out the step-down fixes, the pressure altimeter is still the controlling primary instrument. To get a real GPS vertical glidepath, and 200 feet agl with GPS you need LPV performance which requires either WAAS (wide area augmentation system), or GBAS(LAAS) which is a local ground based augmentation system that corrects GPS signal errors. The LPV and LNAV/VNAV designations are more than signal accuracy, they are totally different approaches with different lateral and vertical obstacle clearance surfaces. $\endgroup$
    – Max Power
    Aug 25, 2020 at 8:13

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