With the introduction of the new GPS/NAV/COM units we started getting bug reports like this:
New GNS430 COM Frequence not fine adjustable
COM Frequence adjustment sep between xxx.x15 to xxx.x25 is 0.010 and not 0.005 as in other range.
ATIS Frequence at airport CYUL is 120.82 and cannot be tuned in by new GNS430.
or
8.33khz tuning is wrong
The GNS430 actually does 5khz tuning not 8.33khz!
from users and authors wondering why tuning certain frequencies wouldn’t work.
Most of the confusion stems not from 8.33kHz spacing, but from failing to understand the old 25kHz spacing in the first place.
Most COM radios tune in steps of 25kHz, therefore the frequencies available starting from 118.000MHz are 118.025MHz, 118.050MHz, 118.075MHz and so on. Most radios only have five digit-displays so they can only display 5 instead of the actual 6 significant digits of the frequency.
Take for example the ATIS frequency at CYUL, which is given as 120.82. It is important to understand that the decimal value is simply cut off because most mechanical and digital radios only display two decimal digits. The frequency, however is actually 120.825 MHz and not 128.820MHz!
So to tune the frequency that on old radios read 120.82 on one of the new radios you need to tune 120.825 on the three digit display.
Some third-party aircraft allowed tuning settings like 120.81 on their radios. This is WRONG because that is not a valid COM frequency. X-Plane will snap to the nearest available multiple of 25kHz, so if you try to set a frequency of 120.81 via a plugin, using any of the COM radio datarefs, X-Plane will tune 120.800MHz which is the nearest 25kHz frequency.
With a useable COM frequency every 25kHz, the COM frequency range (the “Airband”) from 118.000MHz to 136.975MHz allows for 760 different settings (“channels”), with 100kHz on either side of the emergency frequency of 121.500MHz not assigned for safety. Now, with increasing air traffic density especially in Europe, a different, more fine-grained channel spacing was introduced to allow for even more frequencies: Instead of having a communication channel every 25kHz modern radios have a channel every 8.33kHz, tripling the number of available channels to 2280. With that kind of radio, the available frequencies become 118.000MHz, 118.0083MHz, 118.0166MHz, 118.0250MHz, 118.0333MHz and so on…
As you can see, displaying those frequencies on a radio display would be very inconvenient, because you’d need 7 digits, and reading such a frequency assignment back to an air traffic controller would be a nightmare. Therefore, 8.33kHz spaced communication channels are referred to by a channel number rather than the actual frequency. For the previously existing 25kHz-spaced channels, the channel number is identical to the frequency, so nothing changes here. For the 8.33kHz spaced channels in between that didn’t exist before, the channel is just a number, that doesn’t actually coincide with the frequency! But that does not matter at all, because charts will tell you the channel number and ATC will tell you the channel number, so if you are handed off to “132.010” you simply read back “132.010” and dial in “132.010” on your shiny new radio. In the background, it will actually tune 132.0083MHz, but you don’t care.
X-Plane deals only with channels, as do the controls of any modern radio. In X-Plane, dialling the 132.010 channel means just that – just like your charts or the ATC controller we do not care what the actual frequency is, that’s a detail left to the HF-technicians.
The same holds true for our new datarefs (introduced with X-Plane 10.30) like sim/cockpit2/radios/actuators/com1_frequency_hz_833 the dataref corresponds to what you see on your radio. It is not necessarily a frequency anymore, but a COM channel number. Only for the old 25kHz spaced channels frequency and channel number are identical.
So THAT is the full explanation why tuning one of the new COM radios looks like a “5kHz spacing, but with strange jumps here and there” and why 118.020 is not tuneable, because it is neither a valid frequency nor a valid channel number.
Still not convinced? There is a pretty good explanation by the U.K. Light aircraft association in their magazine “Light Aviation” that really explains everything, complete with a nice little table and some James Bond jokes.