The aeroplane factory in Hannover seems never to work for Germany.
I took a look at the files and found the following :
- In the income file, it reads HANNOVER, available from level 3 down to 0
- In the MapAreaGroups, Hannover is a region
- In the MapAreas file (grand campaign) the Hex name is indeed hannover but the tag name is HANOVER with only one N. Thus, the game does not recognize the name HANNOVER from the income sheet and Germany is sheeted from its fifth aeroplane factory.