In an aircraft, a fuel return valve (FRV) controls a flow of fuel, used to cool an oil feeding a propulsion engine and an oil of an integrated drive generator (IDG). A first sensor supplies measurements of the oil temperature at an output port of the IDG and a second sensor supplies measurements of oil temperature of the propulsion engine. An FRV operation monitoring system: determines, for each flight of the aircraft, an oil temperature maximum at an output port of the IDG and/or a maximum oil temperature of the propulsion engine; assesses a trend of one and/or the other of the oil temperature maxima over several flights; and generates an alarm when the trend of one and/or the other of the oil temperature maxima shows that a predetermined threshold has been exceeded.