Sorry, the split between airspace definitions and area control definitions is so jarring that it's a complete non-starter for us. We'd be trying to use that data for something it isn't strictly supposed to be being used for, by creating a weird mapping that isn't entirely clear, and expecting our data maintainers to keep on top of it, error-free, when the country itself wants us to use the FIS frequencies for any VFR pilots wishing to gain access to those pieces of airspace.

Our best bet is to campaign for the authorities to change that recommendation, if you wish SkyDemon to advise different frequencies than the FIS frequencies.

Yes, the definition of the airspaces are maintained at the beginning of the document. The radio boundaries however, are maintained starting at page 36. Those boundaries are different from the TMA/CTAs and also include general class C, airspace E and G. IFR traffic in Germany never talks to FIS, so they are always in contact with a Radar-Frequency. At night, when FIS is closed, also NVFR traffic talks to Radar.

For example the Situation around Cologne from ENR 2.1:

Airspace classification for Köln/Bonn has been done at page 9. Radio classification has been done at page 67 (EDGGDKA).

For the radio, the lower limit for the boundaries of "EDGGDKA1" is GND and the upper limit is FL145, which already is general class C, however in between, it is class C TMA. The same counts also for "EDGGDKA2" from GND to FL195 and "EDGGDKA3" from GND to FL105.

Another example: The area around Dortmund and Paderborn. The airspace D (TMA) is maintained at page 23 and 24, the radio assignments are maintained at page 72 (EDGGPADL), including the boundaries for the frequencies (EDGGPADL1 - EDGGPADL5).

I hope this brings some light into the tunnel. The DFS makes it really complicated, but the information is there.

Good to know. I was in the expectation that once you have configured the settings, you could read out those frequencies automatically for each upcoming Skydemon chart.

Nevermind, I've found the frequencies also in the German AIP ENR 2.1, starting at page 36. Could that help to maintain the data in the SD-charts, or would that still mean too much manual work for each update?

Possible yes, but that doesn't make it sustainable. If we did as you suggest we would have to check all new IFR approach plates for changes to the frequency for class C/D airspace whenever they were released. That would not be very good value.

Other products are indeed available, but as has been shown over the long term, their data tends to be ill-maintained.

Germany: Yes, they are documented, however, only on the IFR plates of the corresponding airport. I know, this is really tricky from the DFS, but would it be possible to read out that information? For example class C for Köln/Bonn, the Arrival/Departure charts reveal the TMA-frequency: "Langen Radar" on 135,350. For Dortmund, this is "Langen Radar" on 125,225 and for Stuttgart, there are two frequencies: 125,050 and 119,200. Just check the SID and STAR charts. These frequencies are also shown correctly in AirNavPro, so I assume that it would be possible to get them also into Skydemon? Would be a great help!

Switzerland: Sounds great, I really appreciate that! :-)

They're not officially documented anywhere, which is a problem because even if we take it on faith that they work, our data is at risk if they ever change. We rely heavily on the AIP update process to ensure that changes to airspace design are correctly represented in our data when they occur, which is why we try to never add our own interpretation on top of the official data.

Yes, I have done the work myself on the Swiss delta frequencies this morning and you should see that take effect in tomorrow's chart update.