The Avia Factory is first available beginning from HQ2 allows one to build Cyclone, Vertex and Thor.

Even though it is the most durable production building that can be placed on land, it should still be placed in a very secure location of the base to keep enemies from sniping it.

It needs to be noted, that both the second and the third level of this factory need Airfields to work properly, as both Vertex and Thor can't be built without them.

It is a common strategy to build a level 2 Avia Factory while upgrading the HQ to level 3 to get out Vertex faster