XPLM_API void       XPLMScheduleFlightLoop(
                         XPLMFlightLoopID     inFlightLoopID,    
                         float                inInterval,    
                         int                  inRelativeToNow);    

This routine schedules a flight loop callback for future execution. If inInterval is negative, it is run in a certain number of frames based on the absolute value of the input. If the interval is positive, it is a duration in seconds.

If inRelativeToNow is true, ties are interpretted relative to the time this routine is called; otherwise they are relative to the last call time or the time the flight loop was registered (if never called).