An aircraft landing gear assembly comprising an aircraft health and usage monitoring system, HUMS, characterised by the HUMS comprising: one or more monitoring sensors coupled to the aircraft landing gear assembly (102, 104, 108) to monitor the health of one or more parts of the landing gear assembly; and a trigger subsystem comprising: a sound transducer mounted on the aircraft landing gear; and a processor, the processor being coupled to the sound transducer to extract sound information from sound signals generated by the sound transducer and being coupled to the monitoring sensors to provide control commands to the monitoring sensors, the processor being configured to: provide a first control command to the monitoring sensors in response to a first criteria having been met, the first criteria comprising registering first sound information, wherein the first command is an activation command which causes the monitoring sensors to start monitoring the condition of the aircraft landing gear assembly and wherein the first sound information comprises one or more of: a sound signature corresponding to one of a plurality sound signatures templates, each sound signature template being associated with the operational noise of an aircraft system; and a frequency spectra corresponding to one of a plurality frequency spectra templates, each frequency spectra template being associated with the operational noise of an aircraft system, the sound signature or frequency spectra corresponding to the sound of one or more of the following aircraft events: landing gear bay door(s) opening or closing; landing gear up-lock or down-lock engaging; aircraft touchdown; aircraft taxiing; aircraft coming to rest; aircraft engines on or off; towing vehicle manoeuvring; aircraft take-off; landing gear retracted. one or more monitoring sensors coupled to the aircraft landing gear assembly (102, 104, 108) to monitor the health of one or more parts of the landing gear assembly; and a trigger subsystem comprising: a sound transducer mounted on the aircraft landing gear; and a processor, the processor being coupled to the sound transducer to extract sound information from sound signals generated by the sound transducer and being coupled to the monitoring sensors to provide control commands to the monitoring sensors, the processor being configured to: provide a first control command to the monitoring sensors in response to a first criteria having been met, the first criteria comprising registering first sound information, wherein the first command is an activation command which causes the monitoring sensors to start monitoring the condition of the aircraft landing gear assembly and wherein the first sound information comprises one or more of: a sound transducer mounted on the aircraft landing gear; and a processor, the processor being coupled to the sound transducer to extract sound information from sound signals generated by the sound transducer and being coupled to the monitoring sensors to provide control commands to the monitoring sensors, the processor being configured to: provide a first control command to the monitoring sensors in response to a first criteria having been met, the first criteria comprising registering first sound information, wherein the first command is an activation command which causes the monitoring sensors to start monitoring the condition of the aircraft landing gear assembly and wherein the first sound information comprises one or more of: a sound signature corresponding to one of a plurality sound signatures templates, each sound signature template being associated with the operational noise of an aircraft system; and a frequency spectra corresponding to one of a plurality frequency spectra templates, each frequency spectra template being associated with the operational noise of an aircraft system,