The map of Sart-Tilman is available here
We recommend to use the train - Airport City Express (every twenty minutes) which travels between Brussels-National-Airport and Brussels-North Station. It takes about 15 minutes to get from Brussels National Airport to Brussels-North Station.
Connection Map from Brussels National Airport
to Brussels North Station"
At Brussels-North Station change for the train to Liège-Guillemins Station. The ticket should be bought at the National Airport Station, the cost of the ticket is about ±12 EUR (in second-class) or ±18 EUR (in first-class). The trains to Liège are every half hour, with a fast train every hour.
Timetable bus 48
Timetable bus 58
Royal Meteorological Institute of Belgium