An aircraft (11) with at least an inflatable device (41) installed in a cavity (21) in its external surface which is equipped with a door (25) for opening/closing said cavity (21) is provided. The inflatable device (41), comprising a cushion (45) and a gas generator is configured for filling the cavity (21) with the cushion (45) when it is inflated with the gas supplied by said gas generator, without protruding from the cavity (21) for reducing the downstream air flow disturbances generated by said cavity (21) when the door (25) is open. The inflatable device (41) includes control means for inflating the cushion (45) in predetermined circumstances, being the door (25) open. The invention is particularly applicable to the cavities of the Main Landing Gear (31) for avoiding power losses to a Ram Air Turbine (51) placed behind it.