Avoid Errors in Coding ILS and VOR frequencies

Many of you who have years of experience with coding add-on scenery for FSX and MSFS may already know this, but some of you might not - especially if you have been struggling with intermittent problems coding ILS's and VOR's. This is embarrassing to admit, because I ran FSX for at least 15 years, and now FS2020 and FS2024 since 2020: I didn't know that the frequency band for VOR and ILS radio navaids was split up into specific pairs of frequencies. In other words, if you are coding an ILS, you can't just throw in any frequency between 108.10 and 111.95 mHz - there are 40 specific frequencies available, and they aren't contiguous. If you tell the MSFS SDK to use, say, 111.45 mHz for the Rwy 24 ILS at your airport, the SDK will do so, and when you Build that Project, the compiler isn't going to throw an error code. But when you actually fly the approach to that runway, your autopilot isn't going to capture either the Localizer or the Glideslope in NAV or APPR modes: because 111.45 mHz is a VOR frequency, not an ILS frequency!

If you are using Jon Masterson's (Scruffy Duck's) Airport Design Editor, and you select your ILS frequency from the provided pull-down menu, it will only show the 40 authorized frequencies for ILS's - this is how I found out that there are only 40 channels for ILS's, out of the 200 available 50 kHz channels between 108.00 MHz and 118.00 MHz. I went searching on the Internet, to try and find out why ADE only provides 40 ILS channels, and found this document: https://wiki.radioreference.com/index.php/Instrument_Landing_System_(ILS)_Frequencies

I've clipped out just the section for ILS's as a JPEG if anyone might find it useful to have: https://drive.google.com/file/d/1ld16kChkjF6NIqKRdN9A86ZyuyFtG8Ag/view?usp=sharing

So when you are designing custom scenery that's going to include a VOR/DME or a runway with an ILS, you need to be careful to not use VOR frequencies for ILS's, or ILS fequencies for VOR/DME's. The SDK will allow you to do this and the Compiler won't throw an error code, but in-sim, the game engine does seem to be aware of the difference between VOR's and ILS's, and they won't behave properly if you used a wrong frequency - especially, the NAV and APPR autopilot functions probably won't work properly, or maybe not at all.

In order for the autopilot in your plane to fly a glideslope, you need to manually add an APPROACH element to your XML file; for some reason, Asobo's SDK doesn't seem to include a tool to do this. I've covered this issue in a couple of separate postings on AVSIM, but to reiterate briefly, your ILS needs to have a couple of Approach Legs and Missed Approach legs, with a minimum of 3 terminal waypoints that define them. In order for your ILS to function properly, the Approach Legs and Waypoints have to be laid out in a specific geometry and with specific tags; it's a pretty complex chunk of code, and if your runway is on a heading that isn't exactly 0, 90, 180, or 270 degrees, computing the lat/long coordinates for the Waypoints is a heavy chore. It's easiest to let ADE do this, by starting an ADE project, putting in just a runway with the correct location, heading, and length/width, adding one or both  ILS's to it, then Export the XML code, Copy and Paste the code for the Approach from ADE's XML file to the XML file you are building in the Asobo SDK, and Build it there. ADE20's compiler appears to be broken - I've never been able to get it to actually Build an ADE20 project into a completed Package, and ADE20 was left as an incomplete Alpha build when Jon retired, so it will probably never be "finished". Anyway, this is a extra hoop you have to jump through just to get a functional ILS, but I know of no easier way to generate the XML code for a basic ILS Approach, and without it, the autopilot in your plane can't capture the Localizer or the Glideslope.

BTW, If anyone knows "why" the tools to build an Approach are missing from the Asobo SDK, when an Approach is a requirement for an ILS to work properly with an Autopilot even in the smallest GA planes like the NX Cub and C-172, I'd be interested to know. Good luck! Coding custom airports is a bewildering job at first, especially learning the ins and outs of XML coding, but the rewards are a lot of fun. I imagine building a small gravel runway in a meadow next to a lake somewhere in Alaska or NWT in Canada, and populating the scenery with some camp chairs, a campfire, a tent (yes, these are available in some add-on scenery packs), and imagine myself sitting in that chair and watch the sun set while I make some S'Mores over the campfire.