From X-Plane SDK
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. 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.