Radios and Transponder

pkg/avionics sets the user aircraft’s radios through the simulator’s key events. Each event is mapped on first use; call Reset after a reconnect.

r := avionics.New(client, 0)  // client: the engine or a manager instance
r.SetCOMStandby(1, 134.560)   // COM_STBY_RADIO_SET_HZ
r.SwapCOM(1)                  // COM1_RADIO_SWAP
r.SetCOMActive(2, 118.105)    // COM2_RADIO_SET_HZ (8.33 kHz channels too)
r.SetSquawk("4521")           // XPNDR_SET, BCD16 (SquawkBCD)

Events (MSFS 2024 “Aircraft Radio Navigation Events”):

Action	COM1	COM2	COM3
Standby, in Hz	`COM_STBY_RADIO_SET_HZ`	`COM2_STBY_RADIO_SET_HZ`	`COM3_STBY_RADIO_SET_HZ`
Active, in Hz	`COM_RADIO_SET_HZ`	`COM2_RADIO_SET_HZ`	`COM3_RADIO_SET_HZ`
Swap	`COM1_RADIO_SWAP`	`COM2_RADIO_SWAP`	`COM3_RADIO_SWAP`

XPNDR_SET sets the squawk; MSFS supports one transponder.

Errors: ErrBadRadio (COM 1–3 only), ErrBadFrequency (118.000–136.990 MHz) and ErrBadSquawk (four digits 0–7) are returned before anything is sent.

Checked live in MSFS 2024 on the Fenix A319, with the aircraft powered:

Squawk: SetSquawk takes effect.
Standby: SetCOMStandby(1, …) sets the RMP 1 standby. Read it back from L:N_PED_RMP1_STDBY (kHz, the systems profile’s com1Standby), not from COM STANDBY FREQUENCY:1, which does not follow the RMP.
Swap: the stock COM1_RADIO_SWAP swaps the simulator’s own pair, which the RMP’s standby never reached. On the Fenix the swap must be its transfer key, L:S_PED_RMP1_XFER. COM ACTIVE FREQUENCY:1 then follows.
Dark aircraft: with the aircraft dark the RMP is unpowered and nothing changes. The first test was dark and wrongly read as “the Fenix ignores the event”.

Per-model actions. Use(profile.Actions) takes a model’s actions from its systems profile (pkg/systems). On the Fenix the profile gives "com1Swap": {"press": "L:S_PED_RMP1_XFER"}, so SwapCOM presses that key (1, then 0 after 300 ms) instead of sending the event. Pressing needs a client that can set variables (Presser: the engine and the manager both can). Aircraft without such an action keep the key events.

p := systems.For(aircraft)
r := avionics.New(client, 0)
r.Use(p.Actions)
r.SetCOMStandby(1, 121.805)
r.SwapCOM(1) // the Fenix: its RMP transfer key

Stock aircraft have not been checked yet.

The call sign for ATC

SetFlight(client, defBase, airline, number) sets the user aircraft’s ATC AIRLINE (“Czech Air Force”, the call sign as said) and ATC FLIGHT NUMBER (“007”); "" leaves one as it is (#680). Measured in MSFS 2024: both are settable and read back as set (ATC ID, the registration, is separate). An app can offer to use the flight plan’s call sign in the sim.