An aircraft health and usage monitoring system (HUMS) (10) comprising one or more monitoring sensors (30) arranged to be coupled to an aircraft subassembly (102, 104, 106, 108) to monitor the health of one or more parts of the subassembly and a trigger subsystem. The trigger subsystem comprises a sound transducer (20) and a processor (101), the processor being coupled to the sound transducer to receive and process sound signals from the sound transducer to extract sound information and being coupled to the monitoring sensors to provide control commands to the monitoring sensors. The processor is 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.