XPLM_API void       XPLMRegisterFlightLoopCallback(
                         XPLMFlightLoop_f     inFlightLoop,
                         float                inInterval,
                         void *               inRefcon);

This routine registers your flight loop callback. Pass in a pointer to a flight loop function and a refcon (an optional reference value determined by you). inInterval defines when you will be called. Pass in a positive number to specify seconds from registration time to the next callback. Pass in a negative number to indicate when you will be called (e.g. pass -1 to be called at the next cylcle). Pass 0 to not be called; your callback will be inactive.

(This legacy function only installs pre-flight-loop callbacks; use XPLMCreateFlightLoop for more control.)