We address the problem of health monitoring of a structure using normal operating conditions: the excitation is natural and non controlled,  not measured, and can be sometimes even nonstationary -- this is typically the case when the excitation is turbulent. We present subspace-based methods using output-only measurements.  Capabilities and limitations of the method are discussed, and it is explained how the method can be used in industrial environments. An application to the early detection of a fatigue in a sports car is reported.

Keywords : structural monitoring, in operation, output-only, subspace algorithms.