¶ Introduction
The CDI is found in most training aircraft. 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 the right course, the needle is centred (see figure below).
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 (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.
In the next CDI instrument, there are:
0) Course deviation bar (white bar)
- TO/FROM indicator (and marks)
- Course select knob OBS (omnibearing)
- Course deviation scale
- Compass card (manually rotation method for CDI)
- NAV flag warning (NAV and/or GS)
- Glide scope bar or pointer (not used with VOR)
¶ CDI instrument reading
In this chapter, we present the basics of CDI instrument reading and use.
¶ TO/FROM Region
On every VOR instrument, we have a flag named TO/FROM. This flag determines 2 zones in the airspace.
A CDI instrument never displays the aircraft heading !!!
The TO/FROM flag is set to "TO" position The white triangle is pointing to the top 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 TO/FROM flag is set to "FROM" position.
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.
These region definitions are independent from the heading. The TO/FROM indication is not enough to determine if you fly to the beacon or from the beacon.
By centering the needle, the course selector indicates either the course "FROM" the station or the course "TO" the station.
- If the flag displays a "TO", the course shown on the course selector will be flown to the station.
- If the flag displays a "FROM", the course shown on the course selector will be flown away from the station.
¶ CDI reading in function of region
On a CDI instrument, the pilot will turn the OBS knob to the desired radial (070° in our example). We suppose that the aircraft is on the heading 070° (same orientation as the radial). The CDI will react as shown in the figure below in function of the TO/FROM zone.
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.
¶ 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.
¶ Where is the VOR station and shortest distance to join the radial ?
Using a CDI or an HSI, you must know as a pilot where the VOR station is and which is the shortest path to reach it.
The VOR station is located in the sector determined by the TO/FROM flag in the direction of the radial and the shortest path in direction to the radial.
The shortest path can be determined by the direction from the centre of the instrument towards the course deviation bar. On the CDI instrument just read the number displayed on the compass card (±90° of the radial).
In the left figure above, the shortest path to join the VOR radial is 340° (=070°-090°) (blue straight arrow). The VOR is located in the sector between 340° and 070° as we are in the TO region.
In the right figure above, the shortest path to join the VOR radial is 160° (= 070° +090°) (green straight arrow). The VOR is located in the sector between 160° and 250° as we are in the FROM region.
In the left figure above, the shortest path to join the VOR radial is 160° (= 070° +090°) (blue straight arrow). The VOR is located in the sector between 160° and 070° as we are in the TO region.
In the right figure above, the shortest path to join the VOR radial is 340° (=070°-090°) (green straight arrow). The VOR is located in the sector between 340° and 250° as we are in the FROM region.
Pay attention that the reading is in function of the radial course and not the heading of the aircraft. The aircraft can have a very different heading from the radial course!
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