The impact sensitivity of GFRP and CFRP sandwich structures with honeycomb cores requires reliable NDT methods after fabrication and in service. The DLR part of the EU-project AISHA II (Aircraft integrated structural health assessment) is focused on impact detection of a 3.5 m long tail-boom of the helicopter EC 135 with Guided waves. Guided waves are able to propagate over large areas in components with a small attenuation and interact with defects. Especially for the complex design of the tail boom these interactions are difficult to predict and to compute. However their knowledge is important for the development of a structural health monitoring system. At DLR the non-contact ultrasonic imaging techniques are successfully used for a complementary NDT method and for the visualisation of the guided wave propagation. Experimental investigations showed that impacts and other stiffness discontinuities provide mode conversions. Furthermore usable modes only propagate at excitation frequencies below 30 kHz. For So-mode excitation a special actuator based on piezocomposite transducers has been developed. This transducer is a combination of piezoceramic materials with ductile polymers and flexible electrodes to form a reliable and damage tolerant actuator. At delaminations the S-mode with a wavelength of about 185 mm is converted to an Ao-mode. The paper describes the possibility of a defect selective SHM system and shows the capability of impact detection.