8.33 refers to the kilohertz (kHz) channel spacing for VHF communications radios required in certain types of airspace. Normal channel spacing for VHF comm radios is 25 kHz. Currently, 8.33 kHz channel spacing is only required above FL195 (19500 ft.) in the ICAO European region
as of 15 March 2007.
8.33 channel spacing was introduced and implemented to help reduce VHF communication overlap and congestion in high traffic areas in Europe. With dividing 25 kHz by 3 (thus 8.33), many new communication frequencies were created. Only very specific radio equipment is capable of channeling, transmitting, and receiving in 8.33 kHz increments. Some of the more popular systems include the Rockwell Collins VHF-21/22/422/C/D and Bendix/King KX-165A (-0201), KY-196B. The Garmin GNS-430/530 series systems are capable of 8.33 also.