Airnav got very confused with this issue back in May.
http://www.airnavsystems.com/forum/index.php?topic=2673.0There are still to this day many brand new aircraft, straight from the factory where GAS immediately puts up the correct code but when we download from the Airnav server it gives us a spurious non-ICAO derived code.
Offending codes include
ICAO Airnav Server
B77W B773 (note non-ER triple sevens use this code but ER versions also populate with this on Airnav)
F27 FF-2
K35R BKC3
C17 BC-A
C17 BCC7
C25A C525 (Note CJ2 and CJ3 both populate with the C525 code that should only be for CJ1 types)
C25B C525
C56X C560 (both ultras and XL citations populate with the ultra ICAO code)
FA10 F10A
If you search for the incorrect codes in the righthand column, you will find thousands of them in the 80,000 odd records that Airnav give you with the latest 3.13 download.
I am afraid it is manual updates of all these records in the aircraft data tables until Airnav understand the issue, cleanse the servers and baseline dataset given to new users and fix the problem in a future version.