How to use cdi aviation

Check out this manual from the FAA on flight instruments

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:

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):

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.

How to use cdi aviation

In navigation days of old, a pilot was lucky to have an ADF or possibly a VOR to get around. Other than that it was true IFR – “I Follow Roads.” As technology has greatly improved over the past decades, ADFs have been all but phased out, and even VORs are on the beginning of the end of their time here. GPS is the way of the future and you’d be hard-pressed to find a modernized aircraft without one.

But one instrument in particular has been effective, adaptable, and extremely effective both through the VOR era (which isn’t over yet) and into the GPS era, and that is the HSI or Horizontal Situation Indicator. I did not know how much I loved and needed one of these instruments until I bought an airplane that had one, and I will never go back.

The HSI can admittedly be confusing at first because there is a lot of information packed into that one instrument. Not to mention there are two different input knobs on it (versus a VOR with just one). So it’s a very common question: what’s the difference between an HSI (Horizontal Situation Indicator), CDI (Course Deviation Indicator), OBS (Omnibearing Selector), and Heading Selector? 

A standalone CDI (Course Deviation Indicator) is an instrument that shows your deviation from a VOR radial that is selected using the OBS (Omnibearing Selector) knob located on the instrument itself. An HSI (Horizontal Situation Indicator) is an entirely separate instrument that combines a CDI and a heading indicator. Whereas a traditional CDI will only show your deviation to a given VOR radial, an HSI will show both your present heading and lateral position in relation to a VOR radial or GPS course line and doesn’t have the potential for reverse sensing.

I think part of the reason these terms get a bit confusing is that we refer to an HSI and CDI as the same thing when in reality they are not. A CDI can be its own standalone instrument used for VOR navigation, or it can be part of an HSI which also integrates a heading indicator. When part of an HSI, the CDI is more just the actual course deviation needle on the instrument rather than the entire instrument itself.

Below we’ll incorporate some pictures to hopefully clear some of this up.

CDI (Course Deviation Indicator) Explained

The reason the CDI can be a little confusing is because it can either be its own standalone instrument (used for VOR navigation) or it can be a component within an HSI (Horizontal Situation Indicator).

Below is a picture of a CDI used for VOR navigation (not all of them have the cross hairs that will also show you vertical guidance for an ILS… some just have the vertical needle referred to as the CDI needle). You’ll notice it has an OBS (omnibearing selector) knob in the bottom left that will change the outer ring of the CDI which will determine what VOR radial to analyze your position against.

How to use cdi aviation

The standalone CDI will not display any information about your current heading, only the VOR radial you have tuned with the OBS. It will essentially tell you what your deviation from the tuned radial is in that moment. It takes a good amount of interpretation to understand where you are in relation to that CDI needle since there is no heading information displayed within the instrument.

Because of this there is a real downside to the standalone CDI instrument and that is you can have reverse sensing. Remember that the instrument is only telling you your deviation to the given radial you have selected with the OBS, and at any given point you can tune two different radials of the same VOR and center the needle – one would have a “to” indication (radial TO the VOR) and one would have a “from” indication (radial FROM the VOR).

If you are tracking with a “FROM” indication, you will have correct sensing (meaning that if the needle is deflected to the right, you are left of course and must correct to the right). But if the CDI shows a “TO” indication, you have reverse sensing (meaning that if the needle is deflected to the right, you are actually right of course and must correct to the left). It isn’t a malfunction of the instrument, just something you have to be keenly aware of particularly when flying IFR.

As you can tell (or as you might know from personal experience), the potential to have reverse sensing in the cockpit can lead to some major issues. That’s where the HSI (Horizontal Situation Indicator) comes in – a simple but significant improvement to the standalone CDI.

HSI (Horizontal Situation Indicator) Explained

The HSI is an improvement on the standalone CDI instrument in that it takes into consideration your actual heading. The old CDI would just have a radial set in the OBS and show your deviation (which could have reverse sensing depending on which radial you tuned) in relation to that radial. The HSI serves as a heading indicator as well as a CDI and be coupled to a VOR or a GPS.

The two inputs of the HSI are the radial or course you want to fly off of a navigational aid (controlled by the OBS – note this doesn’t move the outer ring like a CDI but instead moves the course needle within the HSI to the heading you select) as well as the heading bug (this doesn’t move the heading or the outer ring, but rather just your heading “target” that you can select).

The entire HSI instrument will rotate as you turn the airplane and show your updated position in relation to the radial or course you have selected with the CDI needle (controlled by the OBS).

One of the greatest benefits of this is that you cannot have reverse sensing with an HSI. While there is still a to/from indication for the VOR, the HSI will correct for any reverse sensing and show where you are in relation to that radial. So if the CDI needle is deflected to the right, you are always left of course (regardless of the to/from indicator).

How to use cdi aviation

HSI’s Showing GPS Course

HSI’s can also be very adaptable to both legacy navigational technology (like VORs) and even newer navigational technology (GPS). For example, I have an Aspen Evolution 1000 (pictured below) in my Cessna 182 that contains a digital (versus analog which is pictured above) HSI. It is coupled to both a VOR receiver as well as my Garmin 430 GPS, meaning that the HSI can read and display information if I am tracking a VOR or ILS, or it can also display CDI information based on a GPS course that I have created in my Garmin. It’s incredibly flexible because of this and I never have any reverse sensing in the cockpit.

How to use cdi aviation

The heading bug does not control anything in the HSI per se, but is more of a “target” you can set if you are assigned a heading by ATC or if you are trying to remember your wind correction angle when tracking a VOR. If you have an autopilot that is coupled to your HSI, the heading bug can be used to set the heading of the airplane and if you change the heading with the heading selector knob, the airplane will turn to that heading.

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