The X-Plane 11 GNS 430/530 rules now match real world Garmin GPS equipment: local U.S. identifiers with numbers are valid without the K, letter only codes are not.
As a result of having only one giant database for the world, the new X-Plane GNS 430/530 does not recognize three-letter US FAA airport codes. To enter a four-letter ICAO code, you have to prefix the three-letter FAA code with the letter ‘K’, which is the country identifier for the US.
The real GNS430/530 came out in 1998 when you could fit about 32MB on a compact flash memory card. It wasn’t possible to fit a database for the whole world onto that even if the creators wanted to. Therefore, you can either get the card with the database for “U.S.” or the one for “Everything in the world without the U.S.” (and also a few smaller breakdowns like “U.S. Eastcoast only” but that’s for budgeting/market segmentation purposes).
The real GPS behaves differently when the U.S. card is inserted than it does for the rest of the world. For one, it allows you to enter FAA identifiers instead of ICAO identifiers. It also behaves differently in what it suggests with it’s Spell’N’Find(TM) autocompletion.
In fact, the logic of the real thing is even weirder: It says you can substitute the FAA identifier for the ICAO identifier if and only if the FAA identifier does contain numbers. That means: you can say 6J0 instead of K6J0, but you cannot say BOS instead of KBOS.
Now, X-Plane does not have this distinction based on different data cards. Our database is a single worldwide one – it includes both the U.S. and everything else. The database, whether it comes from Aerosoft or from Navigraph, relies on global identifiers that work worldwide, and those are the ICAO identifiers, not the FAA ones. Therefore, the X-Plane GPS cannot look at an identifier and think “it has numbers in it, so skip the prefix,” since there is absolutely no one preventing a local aviation authority from issuing a three-digit code that might duplicate another country’s. For example, say the Brazilian Civil Aviation Authority issued the identifier “6J0” for a jungle strip. Is this “6J0” now K6J0 or S6J0?
So the actual issue here is the original piece of hardware not having a worldwide database like we have.
In fact, this didn’t even stop with the switch to SD cards on the G1000. Austin’s plane for example reads SD cards up to 4GB and is equipped with a “U.S. only” database that infers the ‘K’ prefix for any three-letter code with numbers.
To my knowledge, only the real life jet versions now have truly worldwide coverage because it would be really impractical to have to reboot your Citation Mustang halfway across the North Atlantic to switch the SD card.