First you need to difine what you mean by aircraft. If you mean heavier than air aircraft then this should be in the post 1900 section. It was more than just the lack of a power source that prevented earlier flight, but also the need to develope light weight structures able to sustain themselves and their payloads aloft as well as means of control and an accurate way of calculating the lift generated by the machines wings.
I would argue that OTL events were for the large part technically possible.
Steam-powered heavier-than-air flight was achieved successfully at least twice in OTL before 1900. The light-weight structure thing was a practical limitation on internal combustion HTA flight, yes, but wasn't really the issue for steam.
The reason steam didn't take off was about engine size and control surfaces. You could only build a steam turbine so small. While that could and did generate more than enough power to put anything even vaguely aerodynamic in flight (despite
serious weight), it wasn't enough merely to fly. You also need control. Achieving controlled flight was much more difficult with steam aircraft because they were too big and powerful. The Wright brothers got around this with an engine that gave a much weaker thrust and the aforementioned light building materials. That made it possible to control direction with methods that would have been insufficient for a steam craft.
But there was nothing impossible about working out some control surfaces - just financially difficult.