8.33 kHz Voice Channel Spacing

On Thursday 6 December 2018 8.33 kHz Air-Ground Voice Channel Spacing (AGVCS) has been implemented below FL195 in the Brussels FIR. In this blog post you will discover what has happened and why it was necessary.

Radio communication is the primary way of communication between pilots and air traffic controllers. Most of it takes place on amplitude modulated (AM) frequencies in the Very High Frequency (VHF) band for air traffic control (117.975 MHz - 137.000 MHz). This is also the band that is currently used to simulate radio communication on the IVAO network. In this VHF band a limited amount of distinct VHF voice communication channels is available for radio communication. Each VHF voice communication channel is represented by an AM frequency. The exact amount of available channels mainly depends on the spacing between adjacent frequencies. The more spacing in between, the less distinct frequencies and thus voice channels that are available and the other way around. This article does not cover the technical aspects of frequency spacing as it would lead us too far into details for the intended purpose. More information about this subject is available on the internet.


The birth of 8.33 kHz voice channel spacing

When voice communication channels were initially regulated, it was agreed that every VHF frequency representing a voice channel needed to be spaced 25 kHz from its adjacent frequencies, representing other voice channels. Below you can see an example of a sequence of available frequencies with 25 kHz spacing in between each frequency.


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This worked well and still continues to do so in several areas on earth, including Europe. However, with the growing air traffic in Europe over the last decades the need for additional voice communication channels has increased as well. With 25 kHz spacing not enough frequencies were available to facilitate the air traffic growth. As such a solution had to be found to establish additional frequencies in the same VHF band without interfering with the 'old' 25 kHz spaced voice channels.

A solution was found and it was decided to start implementing voice channels with a spacing of only 8.33 kHz instead of the earlier used 25 kHz. Essentially this decision makes it possible to almost triple the amount of available voice communication channels in the VHF band. Below you can see an example of the additional frequencies becoming available with 8.33 kHz frequency spacing. Note that the three frequencies from the example above are still in there, although now as 8.33 kHz spaced frequencies.


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With these 8.33 kHz spaced voice channels you might notice that in several cases you need a fourth decimal to specify the frequency, which means you need seven digits in total. This is quite inconvenient in an operational context compared to the maximum of six digits that you need to indicate a 25 kHz spaced frequency. Besides it also increases the chances on confusion for and errors made by pilots and air traffic controllers.

To solve this issue, a smart solution was put in place. Instead of indicating a newly created 8.33 kHz spaced voice channel with its real frequency, a six-digit channel number similar to this frequency was chosen. Furthermore the new 8.33 kHz radio equipment was designed in such a way that dialing the six-digit channel number would make the radio tune the mapped 8.33 kHz spaced frequency. Below you can see an example of the same 8.33 kHz spaced frequencies with for each frequency the mapped channel number that needs to be dialed in the 8.33 kHz radio.


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The big advantage of the 8.33 kHz channel mapping is that communication and phraseology can continue in the exact same way as with 25 kHz spaced voice channels. Every channel, either 25 kHz or 8.33 kHz spaced, can now be indicated with six digits. In case of a 25 kHz spaced voice channel these six digits represent the real AM frequency on which the communication takes place. In the case of an 8.33 kHz spaced voice channel these six digits don't represent the real AM frequency, but a channel number that is mapped to this AM frequency. Below you can find a reminder about the only correct ICAO phraseology to indicate voice communication channels.


When indicating a VHF voice communication channel all six digits have to be pronounced, except when the last two digits are both zeros. In that case pronouncing the first four digits is sufficient. Please check the following examples.




Combining 25 kHz and 8.33 kHz voice channel spacing

Below you can see a combination image from the images above. Note that the earlier 25 kHz spaced voice channels can now also be used as 8.33 kHz spaced channels. E.g. if you dial 118.250 or 118.255, your 8.33 kHz capable radio will tune the physical frequency of 118.2500 MHz. The essential difference though is that when dialing 118.250 your radio will behave like a 25 kHz 'old' radio while dialing 118.255 will make your radio behave like an 8.33 kHz 'new' radio. This is a very important and necessary feature for 8.33 kHz capable radios because that way they are backwards compatible with the older 25 kHz voice channel spacing. This is necessary because both 25 kHz and 8.33 kHz spaced voice communication channels are used in Europe.


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Implementation in real aviation

The implementation of 8.33 kHz spaced voice channels in Europe (ICAO EUR region) happened in three main phases:

  • 1999: mandatory carriage of 8.33 kHz radio equipment and implementation of several 8.33 kHz spaced voice channels above FL245 in about 30 States of the ICAO EUR region
  • 2007: initial regulation about mandatory carriage of 8.33 kHz capable radio equipment in airspace below FL245 and implementation of 8.33 kHz spaced voice channels above FL195
  • 2012: final regulation mandating implementation of 8.33 kHz spaced frequencies below FL195 by the end of 2018

Before AIRAC cycle 1813 (effective 06 DEC 2018) the Brussels FIR below FL195 was regulated with 25 kHz voice communication channels only. Since then most of these channels have been replaced by 8.33 kHz spaced voice channels. Some of them remain 25 kHz spaced though. This is necessary in real life to facilitate communication with state aircraft that might not be equipped with an 8.33 kHz capable radio (state aircraft are exempted from the rule of mandatory 8.33 kHz capable radio carriage).


Implementation on the IVAO network

For the changes in the airspace below FL195 the BeLux division has followed the real life changes. This means that since Thursday 6 December 2018 several voice communication channels have a new channel number. All applicable documentation has been updated and is available on this website.

ATS Unit Callsign Before 06 DEC 2018 From 06 DEC 2018 Remark
EBBU_W_CTR Brussels Control 131.100 131.100
EBBU_E_CTR Brussels Control 128.200 128.200 Secondary position
EBBU_FSS Brussels Information 126.900 126.900
EBGL_CRC_CTR Efflux 130.575 130.580 8.33 kHz CH Secondary position
EBSZ_MIL_CTR Belga Radar 129.325 129.325 Secondary position
EBAW_TWR Antwerpen Tower 135.200 135.205 8.33 kHz CH
EBBR_DEL Brussels Delivery 121.950 121.955 8.33 kHz CH
EBBR_N_GND Brussels Ground 118.050 118.055 8.33 kHz CH
EBBR_S_GND Brussels Ground 121.875 121.880 8.33 kHz CH Secondary position
EBBR_N_TWR Brussels Tower 120.775 120.780 8.33 kHz CH
EBBR_S_TWR Brussels Tower 118.600 118.605 8.33 kHz CH Secondary position
EBBR_APP Brussels Approach 118.250 118.255 8.33 kHz CH Callsign Brussels Arrival if EBBR_DEP is online
EBBR_F_APP Brussels Final 120.100 120.105 8.33 kHz CH Secondary position
EBBR_DEP Brussels Departure 126.625 126.630 8.33 kHz CH Secondary position
EBCI_GND Charleroi Ground 121.800 121.805 8.33 kHz CH
EBCI_TWR Charleroi Tower 121.300 121.305 8.33 kHz CH
EBLG_GND Liège Ground 121.925 121.930 8.33 kHz CH
EBLG_TWR Liège Tower 118.125 118.130 8.33 kHz CH
EBLG_APP Liège Approach 119.275 119.280 8.33 kHz CH
EBOS_GND Oostende Ground 121.975 121.980 8.33 kHz CH Secondary position
EBOS_TWR Oostende Tower 118.175 118.180 8.33 kHz CH
EBOS_APP Oostende Approach 120.600 120.600
ELLX_TWR Luxembourg Tower 118.100 118.105 8.33 kHz CH
ELLX_APP Luxembourg Approach 118.900 118.905 8.33 kHz CH
EBBE_GND Beauvechain Ground 121.850 121.855 8.33 kHz CH
EBBE_TWR Beauvechain Tower 130.725 130.730 8.33 kHz CH
EBBE_APP Beauvechain Approach 122.825 122.830 8.33 kHz CH
EBBE_PAR_APP Beauvechain PAR 119.625 119.630 8.33 kHz CH Secondary position
EBFS_GND Florennes Ground 122.100 122.100
EBFS_TWR Florennes Tower 125.875 125.880 8.33 kHz CH
EBFS_APP Florennes Approach 124.375 124.380 8.33 kHz CH
EBFS_PAR_APP Florennes PAR 123.300 123.300 Secondary position
EBBL_GND Kleine-Brogel Ground 122.100 122.100
EBBL_TWR Kleine-Brogel Tower 134.100 134.105 8.33 kHz CH
EBBL_APP Kleine-Brogel Approach 122.500 134.480 8.33 kHz CH
EBBL_PAR_APP Kleine-Brogel PAR 123.300 123.300 Secondary position
EBFN_TWR Koksijde Tower 122.100 122.100
EBFN_APP Koksijde Approach 122.500 121.055 8.33 kHz CH
EBFN_PAR_APP Koksijde PAR 123.300 123.300 Secondary position


These changes require some extra attention from both pilots and air traffic controllers. Please make sure to download and work with the latest applicable documentation in order to avoid confusion and making errors. Below you can find some information about the software capabilities regarding 8.33 kHz spacing.

Software Status Remarks
IVAO Natively capable for 8.33 kHz channel spacing 25 kHz and 8.33 kHz spaced channels can be selected using LSK 1L or the .c command (see IvAp manual for detailed information).
FSX/P3D Not natively capable for 8.33 kHz channel spacing Some aircraft addon radios are capable for 8.33 kHz voice channel selection in the radio module itself. If not, IvAp can be used instead.
X-Plane 11 Natively capable for 8.33 kHz channel spacing 8.33 kHz voice channels can be selected in the radio module itself, provided that it is a simulated 8.33 kHz capable radio. If not, X-IvAp can be used instead.
Carriage of 8.33 kHz capable radio equipment is mandatory (see AIP GEN 1.5 §2). It needs to be announced in the flightplan by adding equipment code Y in field 10a. (X-)IvAp can be considered as an 8.33 kHz capable radio and this one can always be used for tuning the required channel. Please do NOT attempt to dial an 8.33 kHz spaced voice channel using a radio not capable for 8.33 kHz channel selection. This might cause communication problems, which on its turn could lead to disrupted traffic flows and dangerous situations.


If you don't have an 8.33 kHz capable radio installed you can't dial 8.33 kHz channels from inside the aircraft. (X-)IvAp will need to be used instead. (X-)IvAp is capable for both 25 kHz and 8.33 kHz spaced channel selection. You shall select the required channel in IvAp using LSK 1L or the .c command (for detailed info please refer to (X-)IvAp manual).

CAUTION with using hardware radio modules
In case they can't be overridden by (X-)IvAp, we do not recommend using hardware radio modules unable for 8.33 kHz channel selection. The reason is that in this case your hardware radio will keep tuning to a 25 kHz channel even if you dialed an 8.33 kHz channel in (X-)IvAp. As a result of this you will not be able to get in the correct teamspeak channel and therefore will not be able to communicate with the controller on voice. If you are unaware about this potential issue, this can lead to delay for yourself and other aircraft. Therefore, please check this in advance and if you are in this situation, please consider flying without the hardware radio module until it supports 8.33 kHz voice channel spacing.



Below you can find some additional documentation, an example and a movie regarding the 8.33 kHz voice channel spacing. Should you have any questions or remarks about it, don't hesitate to contact us on Discord or by using the CONTACT FORM!