The purpose of the present invention is to cool the brakes of the landing gear of an aircraft. For this purpose, the air of the air-conditioning system of the aeroplane is used. Pre-conditioned air from the outside can be supplied to the air-conditioning system of the aeroplane through an air inlet in the lower part of the aeroplane fuselage, an external device being connected to the inlet. The air inlet contains a check valve that prevents the air from escaping to the outside. The valve must be bypassed to extract the air from the air-conditioning system when the aeroplane is on the ground. For this purpose, a tool has been designed, which bypasses the valve from the outside, extracting and directing the air towards the brakes by means of connectors and tubes, thereby cooling the brakes for the next take-off.