how a course deviation indicator works

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

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.

How does a slaved HSI work?

The ST-180 Horizontal Situation Indicator (HSI) system combines a magnetically slaved gyroscopic compass with a VOR/Localizer and glideslope display. The resulting instrument display provides the pilot with a pictorial of the aircraft position and heading relative to the selected VOR/GPS or Localizer course.

How do you use HSI instruments?

3:5812:30Notice 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 does a VOR indicator work?

The VOR Indicator The direction of deflection tells the pilot where he is relative to the radial, while the dots on the instrument's face tell the pilot how many degrees he is off his course. Each dot represents a 2-degree deflection from the desired course.

How many degrees is each dot on an HSI?

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

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 read an HSI presentation?

0:074:08So 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 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 does a vertical speed indicator work?

The VSI uses the aircraft pitot-static system to determine the vertical speed and depicts the result on a conventional needle and circular scale instrument, or on a ribbon at the side of an Electronic Flight Instrument System EADI. Two typical VSI indications are depicted below.

How does a VOR know to or from?

The VOR is indicating that the aircraft is on the 360° course (north) to the VOR station (i.e. the aircraft is south of the VOR station). If the To/From indicator were showing "From" it would mean the aircraft was on the 360° radial from the VOR station (i.e. the aircraft is north of the VOR).

How does VOR DME work?

In radio navigation, a VOR/DME is a radio beacon that combines a VHF omnidirectional range (VOR) with a distance-measuring equipment (DME). The VOR allows the receiver to measure its bearing to or from the beacon, while the DME provides the slant distance between the receiver and the station.

Are VORs magnetic or true?

VOR degrees are magnetic, not true, so you can read your magnetic course for that location right from the VOR rose. Again, the difference between the true course you've drawn on your chart and the magnetic course that runs through the VOR rose is the magnetic variation.

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.

Course Deviation Indicator (CDI)

The Course Deviation Indicator (CDI) is an avionics instruments used in aircrafts navigation systems to determine the aircrafts lateral position in relation to the course direction.

How It Works

The OBS Selector is rotated to select a desired radial over which the aircraft is flying. The needle will indicate the position radial relative to the aircraft. When the deviation needle is centered the magnetic course “FROM” or “TO” the station is determined.

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.