## 4 Conclusions  
A synergistic strategy that was based on infrared thermography and phased array was presented for the rapid and, at the same time, accurate and reliable detection of damage in aircraft composites. Therein, a newly developed powerful IRT variant, termed PPI-LT, was adopted for the initial rapid, wide-area scanning of the structure for the identification and localization of damages. A thorough dilatational characterization of damages is pursued by the preferential application of an advanced PA inspection approach, only to areas previously identified as damage suspected by IRT. The synergistic strategy minimizes the timescale that is required for reliable aircraft inspection, as it eliminates the requirements of scanning the complete structure by detailed and time-consuming PA inspection. This lifts the current trade-off between speed and reliability of aircraft inspection and unravels the unprecedented potential for reduction of airplane accidents and inspection timescales, while, at the same time, maximizing the airworthiness and cost-effectiveness of tactical inspections, such as C-checks and D-checks. The strategy also offers highly reliable damage data for improved aircraft performance and effective repair. Most importantly, both of the inspection techniques were developed for field measurements, with custom state-of-the-art lightweight modules mountable on vortex robot platforms for fully automated NDT inspection of aircraft in the context of a Future and Emerging Technologies (FET) Horizon 2020 research project (Compinnova).  
The strategy was successfully benchmarked across full-spec aircraft composite coupons, on selected case-study laminates with typical damages and repaired sections on a real aircraft composite panel with artificially induced barely visible impact damages (BVID). The panel was equipped with standard counter-lighting strike copper-mesh lamina and stiffening stringers on the back-face. The specimens and panels were finished in aircraft-grade paint. All of the impact-inflicted damages were successfully captured by IRT and further characterized by PA ultrasonic. The smallest defect identified by the resolution-limiting step of the approach, fast initial PPI-LT, was of the order of 35.2 mm\({}^{2}\) and met current FFA specifications for minimum defect detectability in tactical C- and D-checks.  
It is important to highlight that, provided a smooth and IRT/PA-active surface, no other part of the proposed strategy imposes significant limitations on the type and nature of the inspected surfaces, which unfolds the potential for expansion to other types of engineering and industrial applications.  
Conceptualization, J.P.M., L.Z.F., I.P., K.G.D.; methodology, J.P.M., S.F., D.E.; software, I.P., L.Z.F., D.E.; validation, J.P.M., S.F., D.E.; formal analysis, J.P.M., S.F.; resources, A.T., I.G., T.E.M., K.G.D.; data curation, S.F., D.E.; writing--original draft preparation, J.P.M., K.G.D.; writing--review and editing, J.P.M., L.Z.F., K.G.D.; supervision, J.P.M, I.P., L.Z.F. project administration, J.R., I.G., K.G.D.; funding acquisition, I.G., T.E.M., K.G.D. and A.T. All authors have read and agreed to the published version of the manuscript.  
This research was fully funded by the European Union's H2020 Framework Program, call FET-OPEN under REA grant agreement number 665238: ComplInnova. [[http://compinnova.net/](http://compinnova.net/)]([http://compinnova.net/](http://compinnova.net/)).  
Not applicable.  
We would like to thank [PERSON] and [PERSON] from the research team of [PERSON], Laboratory of Applied Mechanics and Vibrations, University of Patras, Greece for manufacturing composite coupons and laminates and [PERSON] of Optimal Structural Solutions, Portugal for processing and providing the stiffened composite panel. We would also like to thank the wider ComplInnova consortium for the support in carrying out this research.  
The authors declare no conflict of interest.