SMS: Today, OO-CQD left for IFR Flight with no Fuel cap on the fuel cap. The fuel cap was discovered 10 min after OOCQD Take Off. Action was immediately taken by calling LFQQ and MERVILLE and EBKT towers to try to contact the aircraft. Finally after 40 min LFQQ was able to see the aircraft on radar flying CIV towards EBKT. EBKT Tower contacted OOCQD to inform the pilots and suggest to check their fuel indicator. We know now that no fuel is lost due to side filling of the tank (which we where not sure) Remember that, flying with a fuel cap on upper side of the wing will result in rapid lost of fuel due to vacuum over the wing. This was sent to all ALFAKO pilots and instructors and to the BCAA as well.