Definition
A Very High Frequency (VHF) communication radio capable of tuning to channels separated by 8.33 kilohertz, rather than the older 25 kHz spacing. The narrower spacing triples the number of available frequencies in the aviation VHF band, easing congestion in busy airspace. This capability is required for operations in much of European airspace and may be specified by certain procedures or air traffic control assignments.
Plain English
A radio that can tune to extra, more closely-packed channels so controllers have more frequencies to hand out. Without it, the pilot simply cannot select some of the frequencies in use today.
Context Anchor
Seen in aircraft equipment listings and flight planning information when identifying what communication radios the aircraft has available.
Derivation
Hertz (Hz) is the unit of frequency, named after physicist Heinrich Hertz. Kilohertz means thousands of cycles per second. The unusual figure 8.33 comes from dividing the original 25 kHz channel into three equal parts, which was the engineering solution to creating more channels without expanding the radio band itself.
Why Pilots Care
Aircraft without this capability cannot access many ATC frequencies in Europe and may be restricted from certain airspace or airports.
Analogy
Think of a radio dial with very fine click-stops. A radio with 8.33 kHz spacing can stop on more closely spaced channels than a radio with only wider spacing.
Intuition Check
Do not read “8.33 kHz” as the radio’s power or range. It describes how closely the selectable radio channels are spaced.
Example Sentence 1
Because the departure routed them through European upper airspace, the crew confirmed the aircraft was equipped with VHF with 8.33 kHz channel spacing capability before accepting the clearance.
Example Sentence 2
During the preflight check, the crew verified that the radios supported VHF with 8.33 kHz channel spacing capability for the European segment of the trip.