how does a course deviation indicator work

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.

Full Answer

How do you determine the course deviation of an aircraft?

If the course selector is rotated until the deviation needle is centred, the radial (magnetic course “FROM” the station) or its reciprocal (magnetic course “TO” the station) can be determined. The course deviation needle also moves to the right or left if the aircraft is flown or drifting away from the radial which is set in the course selector.

Where should the course deviation bar be on the instrument?

As you can see in the CDI, if the aircraft follows the desired radial heading, the radial direction is represented by the location of the course deviation bar on the instrument. If now the aircraft is exactly on the radial, the course deviation bar should be centered on the instrument.

What is course deviation needle?

The course deviation needle also moves to the right or left if the aircraft is flown or drifting away from the radial which is set in the course selector. In this chapter, we present the basics of CDI instrument reading and use.

How do I use the courseline on the indicator?

The courseline 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.

How do you read a course deviation indicator?

The CDI acts somewhat like a course line, showing the pilot the relationship of the airplane to the selected course. If you're flying westbound, for example, and the OBS is properly tuned and the CDI is to the left (or south), you know the airplane is north of the desired course.

What is the difference between HSI and CDI?

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.

How do you use HSI instruments?

3:5812:30Horizontal Situation Indicator Explained | HSI Test QuestionsYouTubeStart of suggested clipEnd of suggested clipNotice the green course selector arrow it's pointed at 0 9 0.. Also notice the white triangleMoreNotice the green course selector arrow it's pointed at 0 9 0.. Also notice the white triangle pointed the same direction as the green arrow marking the two indication.

How do you read CDI aircraft?

2:448:28Simple Way to Determine Aircraft Position Using a VOR, CDI, and OBSYouTubeStart of suggested clipEnd of suggested clipSo to do that the easiest way to illustrate it is we draw a perpendicular line to the three six zeroMoreSo to do that the easiest way to illustrate it is we draw a perpendicular line to the three six zero radial anytime. We're south of that perpendicular.

What is the purpose of the course deviation bar in the HSI?

The HSI has a fixed aircraft symbol and the course deviation bar displays the aircraft's position relative to the selected course. The TO/FROM indicator is a triangular pointer: When the indicator points to the head of the course select pointer, the arrow shows the course selected.

What does a course deviation bar illustrate?

The course deviation bar operates with a VOR/Localizer (VOR/LOC) or GPS navigation receiver to indicate left or right deviations from the course selected with the course select pointer; operating in the same manner, the angular movement of a conventional VOR/LOC needle indicates deviation from course.

How do you read an HSI presentation?

0:074:08Easiest Way To Read an HSI - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo if in your mind's eye you drag those white lines in the direction of that arrow you see that yourMoreSo if in your mind's eye you drag those white lines in the direction of that arrow you see that your crosshairs will end up looking something like this.

How many degrees are each dot on an HSI?

Each division (dot) represents 1.25 degrees deviation from the ideal ILS flightpath approach.

How does HSI correct reverse sensing?

Reverse sensing occurs on the back course using standard VOR equipment. With a horizontal situation indicator (HSI) system, reverse sensing is eliminated if it is set appropriately to the front course.

How many degrees is each dot on a CDI?

2°Deflection is 10° deviation at full scale (each side), with each dot on the CDI representing 2°. (See Using a VOR for usage during flight.)

How far off course is 1 degree?

60 milesExperts in air navigation have a rule of thumb known as the 1 in 60 rule. It states that for every 1 degree a plane veers off its course, it misses its target destination by 1 mile for every 60 miles you fly. This means that the further you travel, the further you are from your destination.

What is the difference between OBS and CDI?

OBS vs CDI The OBS (Omni-Bearing Selector) is the knob that the pilot adjusts. On the other hand, the CDI (Course Deviation Indicator) is the needle in the window that shows your desired course. So the pilot turns and sets the OBS but sees their course by looking at the CDI needle.

What is a CDI needle?

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 (ADF), which receives information from a normal AM radio station or an NDB .

What is CDI in training?

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. ^ Radio Navigation & Instrument Flying. Air Pilot Publishing Ltd. 2008. ISBN 1-84336-069-1.

What is CDI in aircraft?

From Wikipedia, the free encyclopedia. Jump to navigation Jump to search. 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 to or from a radio navigation beacon. If the location of the aircraft is to the left of this course, ...

What is the indicator on a plane?

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 courseline. 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.

How many nautical miles is a full scale en route?

Sensitivity is usually programmable or automatically switched, but 5 nautical miles (9.3 km) deviation at full scale is typical for en route operations. Approach and terminal operations have a higher sensitivity up to frequently 0.3 nautical miles (0.56 km) at full scale.

What happens when the course selector is rotated?

If the course selector is rotated until the deviation needle is centred, the radial (magnetic course “FROM” the station) or its reciprocal (magnetic course “TO” the station) can be determined. The course deviation needle also moves to the right or left if the aircraft is flown or drifting away from the radial which is set in the course selector.

How does a mobile compass rotate?

A mobile compass is present and can be rotated by using the omnibearing knob usually written as ”OBS”: When the course selector is rotated, the instrument moves the needle to indicate the position of the radial relative to the aircraft . If the course selector is rotated until the deviation needle is centred, the radial ...

What is radial indicator on CDI?

In the CDI instrument, the radial indicator is a needle which is clearly visible on the top of the instrument. If the location of the aircraft is to the left off course, the needle deflects to the right. If the location of the aircraft is to the right off course, the needle deflects to the left. If the location of the aircraft is on ...

What does the yellow triangle on a compass card mean?

The white triangle is pointing to the bottom of the instrument. The yellow triangle represents the selected radial . Do not mix up the selected RADIAL on the compass card with aircraft HEADING. The two regions are dependent from the radial selected in the aircraft.

What is CDI display?

CDI display is independent from the aircraft heading. Note that in the CDI instrument, the display is independent from the aircraft heading. This can be strange for novice pilots to instrument navigation. The instrument displays the information in the perspective of an aircraft with course equals heading.

What is HSI in music?

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 ...

What is CDI in navigation?

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.

Why is CDI confusing?

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).

Can you reverse sensing a CDI needle?

Because of this there is a real downside to the standalone CDI instrument and that is you can have reverse sensing.

Does the heading bug control HSI?

It’s incredibly flexible because of this and I never have any reverse sensing in the cockpit. 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.

Is reverse sensing a malfunction?

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.