XPLM MSG AIRPLANE COUNT CHANGED
From X-Plane SDK
#define XPLM_MSG_AIRPLANE_COUNT_CHANGED 105This message is sent whenever the user adjusts the number of X-Plane aircraft models. You must use XPLMCountPlanes to find out how many planes are now available. This message will only be sent in XP7 and higher because in XP6 the number of aircraft is not user-adjustable.