I am watching aircraft come and go from the private airport LL40 but FlightAware says this is an "Unknow or Invalid " airport code. This is the former C18 Frankfort Airport in Illinois. Why is this showing as an Invalid airport?
Are the airplanes flying IFR? Only IFR aircraft will be tracked.
The FlightAware database still accepts C18 as a valid code, though it probably will not show current traffic who would probably be using the new, correct code.