###### Abstract
For stress-free materials with pre-existing internal damages, such as the case of fuselage and wing skin of grounded aircraft undergoing inspection, active thermography is used in order to identify damage by capturing and analyzing the thermal energy dissipated from the surface while it is being thermally excited by an external thermal energy source. Independent of the technique used (active or passive IRT), the severer the damage in the material, the more intense the thermographic fingerprint of the damaged area, which appears on a thermogram as warmer or brighter color than pristine/unaffected areas. IRT is particularly applicable to composite materials, which are currently trending to replace more than 50% of the metallic aircraft fuselage and wing parts [18; 19]. The technique has been highly successful in assessing the delamination damage typical of impact incidents that are found in aircraft-grade composites [20; 21]. Most importantly, the technique is extremely versatile, as it can operate as stand-alone or complementary to other inspection methods [22; 23]. The impact damage detectability of IRT depends on the thickness of the material, shape, dimensions, type of damage, and camera hardware sensitivity [24]. Although few previous studies have investigated combined the IRT and PA methods to characterize impact damages in CFRP composites [25; 26; 27; 28], measurements in such studies have been performed with heavy equipment, which is unsuitable for deployment in the field and robotic applications or related to different engineering domains [29]. Moreover, automated in-service inspection of aircraft using combined PA and IRT methods have received little attention. The limitations in available IRT techniques restricted their ability to perform fast and, at the same time, reliable quantitative inspections [30]. The recent development of an advanced variant of IRT, termed Pulsed Phase-informed Lock-in Thermography (PPI-LT), lifted this limitation [20]. As a two-step approach, PPI-LT relies on the fast initial assessment of defect presence, by pulsed-phase thermography (PPT) and the subsequent quantitative investigation of the area by lock-in thermography (LT). However, although PPI-LT can be particularly rapid and effective in imaging a wide material area (depending on sensor resolution and the instantaneous field of view-IFOV) from afar in a single acquisition and detecting the presence of defects therein, it is not equally powerful in the accurate quantitative assessment of the defects' full geometrical and dilatational characteristics, under rapid wide-area mode. In other words, although rapid IRT could be used to scan a whole aircraft fuselage for damage in a cost-effective manner, the collected data would not contain sufficient defect sizing and depth profiling information for a reliable repair. Phased Array, on the other hand, is a slower and extremely accurate technique requiring contact-full and coupling; it is particularly powerful in achieving high-resolution three-dimensional (3D) geometrical assessment of superficial and subsurface defects. Although it would not cost-effectively scan a whole aircraft, it could provide extremely detailed information on size, shape, and depth profiling of a defect at a known fixed location. The present paper reports the development of a novel synergistic strategy for the fast, accurate, and reliable detection, with full geometrical representation of damage in aircraft composites, by the sequential application of PPI-LT and PA. The strategy is suitable for automated inspection of impact damage in composite aircrafts and it consists of two sequential inspection steps: firstly, the entire composite structure of the aircraft is rapidly inspected with a newly-developed state-of-the-art, compact, and powerful IRT module operating under PPI-LT mode with custom control and analysis software (presented in [20]) mounted on a Vortex Robotic (VR) platform to detect, identify, and mark all of the impact damage locations.