## 1 Introduction  
For decades, the development and application of non-destructive testing (NDT) ensure the safety and reliability in many fields such as civil engineering, transportation and the energy sector. The increasing technical know-how, the growth of personnel costs and the advancement of hard- and software tools are the main drivers for this phenomena.  
On the one hand, modern materials and structural concepts come with new and tailored monitoring systems that are potentially embedded in the structure during the manufacturing process. On the other hand, classic fatigue phenomena of \"old-school\" materials like concrete, steel and aluminium alloys undergo advanced monitoring and testing methods. Many of these are empowered by the developments of new sensors and the (on-line) data processing. This work is positioned within the latter field and applies new as well as common technologies to a fuselage panel of a commercial aircraft.  
Since the beginning of civil aviation, aluminium alloy sheets have been important building blocks of the fuselage, and hence, its assembling, usage and fatigue behavior is well understood. In aircraft fuselage panels, riveted lap joints build the connection between neighboring aluminium alloy sheets established by different kinds of rivets. As in most structural components, joints within the fuselage are critical areas. Hence, avoiding rivet failure plays an important role in the save and reliable operation. As all critical parts, also riveted joints are regularly checked to ensure the safe operation of the aircraft.  
Nowadays, the inspection procedure to detect loose and sheared rivets in critical areas requires a lot of manpower of certified and specially trained personnel. The manual visual inspection is performed with the naked eye and comes with some inevitable drawbacks. One major aspect is that the inspection and its outcome is subjective and depends on the individual inspector. For this purpose, the comparison between inspections of the same area at different times and objective documentation of the temporal development of each rivet is impossible. Also, the inspection procedure requires a high concentration level of the inspector, therefore regular breaks are required and mandatory to minimize the likelihood of human errors.  
Due to the aforementioned technical developments, the authors believe that automated rivet inspection is possible with state-of-the-art NDT techniques enhancing safety as well as the economic efficiency of civil aviation.  
This work puts forward a proof of concept to detect and localize loose and damaged rivets in a \"real aircraft fuselage panel\" with advanced NDT methods. Such a study is necessary since many investigations are performed on test samples with parameters not always representative for real aircraft fuselage. This work presents a novel thermal inspection procedure which measures the surface temperature of the specimen while an in-plane heatwave is propagating through the plate-like structure. The disturbances of this in-plane heatwave by irregularities in heat conductivity reveals subsurface flaws and damages. This inspection method will be tested on a commercial aircraft fuselage panel with loose, damaged and intact fasteners. Besides this new technology, measurements with a Scanning Laser Doppler Vibrometer (SLDV) which are realized with a commercial off-the-shelf device and Ultrasound Lock-In Thermography (ULT) which combines a vibrational excitation of the specimen with a thermal detection of flaws will be investigated. By testing all methods on one sample, a comparison provides benefits and drawbacks for each technology.