Abstract: A reservoir for aircraft, such as a helicopter, containing a fluid and having at least orifices in its side wall close to its bottom and to which a fluid feed and return ducts can be connected. The reservoir has to allow the aircraft to effect temporary flight phases under negative load factor, a plate having a passage, for retarding the movement of the fluid contained in the reservoir, so that the fluid, moving from the bottom to the top of the reservoir during the temporary flight phases and passing through the passage of the retarding means, remains in communication with the orifice for the duration of the flight phases, while avoiding the entry of air into the duct.