The disclosed embodiments relates to a method and an apparatus for determining the interfering field strength in an aircraft and the impairment of an electric system in the aircraft including cables between the outer shell and the interior panelling of the fuselage for transmitting signals within the aircraft. In order to enable direct and reliable determination of interfering field strength in an aircraft and assessment of the electromagnetic vulnerability of the communications system in the aircraft depending on the results of the determination, either a predetermined transmission signal is fed in at least one leaky line between outer shell and panelling of the fuselage and the reception signal that is irradiated by the leaky line is received with a receiver, or a predetermined transmission signal is transmitted with a transmitter and the reception signal is extracted from the leaky line. Subsequently the amplitudes of the transmission and the reception signals are compared with each other.