To reduce emissions from civil aviation, research is being conducted in lean premixed hydrogen combustors as part of the APPU project at the TU Delft. Next to mitigating CO2 emissions completely, these technologies have the potential to also reduce NOx emissions greatly, allowing for the future of civil aviation to become sustainable. However, the swirling flow of such combustors and the high reactivity of hydrogen lead to increased flashback propensity which can damage the combustor and the engine all together. To counteract this, a concept called Axial Air Injection (AAI) is under investigation. In this technique, an air jet is injected in the mixing tube on its centreline, in an attempt to push the flame more downstream to prevent flame flashback from occurring. The effect of this concept on flashback propensity has been investigated on an axial swirl-stabilized methane combustor, though a parametric variation of swirl intensity and AAI level. The flowfield inside the combustor of each operating point has been visualized by conducting both non-reacting and reacting PIV, which has given insight in their intrinsic differences on flashback propensity for such a combustion system. The results have shown that the non-reacting flowfields prove to be a good indicator of flashback propensity in reacting flowfields for low and high levels of AAI. For in between AAI values, the interaction of heat release, swirling flow and AAI creates a larger deviation which has been shown to be dependent on the presence of circumferential vorticity in non-reacting conditions. It is envisaged that these experimental results will contribute to the body of knowledge in the hydrogen combustor development as part of the APPU project.