Single European Sky
8.33 kHz channel Spacing.
Under the EU regulation No. 1079/2012, all voice communications in the VHF band (117.975 kHz to 137 MHz), carried out in the ICAO EUR region must be done with radio equipment with 8,33 channel spacing kHz.
This legislation has the date of implementation on 1 January 2018. It requires many aircraft flying in European airspace to have all their radios with channel spacing of 8.33 kHz installed before December 31, 2017; however a mandate published by the Spanish Ministerio de Fomento on 2/12/16 allows certain light aircraft flying in VFR conditions to not comply until 31/12/2022.
In the following link you will find the complete legal information.
Why is necessary to change the spacing between channels?
This legislation is a direct cause of the initiative called Single European Sky Eurocontrol. This initiative, among others, aims to increase the capacity of European air traffic. Making all aircraft use a channel spacing of 8.33 kHz instead of 25 kHz (conventional), this will triple the number of communication channels available.
The reason why it is necessary that all aircraft have to change their radios with 25 kHz spacing is that the radios will interfere with the new closed spacing even if the aircraft are a long distance apart. Radios equipped with channel spacing of 25 kHz have much lower selectivity, thus can interfere with adjacent channels, which can be used by the radios with channel spacing of 8.33 kHz.
Therefore the use of radios with 25 kHz spacing can interfere with communications of other aircraft, which can be dangerous.
The selectivity of a radio is the ability of exactly selecting a particular frequency. In other words, is the ability to discriminate between signals on adjacent channels and get exactly the information you want.
Due to the limitations of selectivity characteristics in the equipment, a spacing of 25 kHz channels was established, this permits select frequencies without interfere with other frequency range. Over time the quality of the radios has improved a lot, and therefore its selectivity characteristics are much better, which makes it possible to reduce the channel spacing without risk any interference.