Originally Posted by
DownUnderFlyer
No idea about nationality but if you are a German citizen then you can get a German passport even if you hold other citizenship. The first "if" is the important one.
You are correct regarding German citizens. But since different countries apply different rules/laws regarding their nationality there is no universal rule of thumb.
If a baby is born in a car of the US embassy (=US territory) driving through Afghanistan, the kid will be a US citizen (territorial principle). If the baby's parents are German, the kid will be able to opt for dual citizenship because the German laws offer this option.
Originally Posted by
DownUnderFlyer
While there is no legally binding definition of the vertical extend of a countries boarders, nobody doubts that the cruising altitude of a commercial jet is still within the territorial space of the country underneath.
AFAIK this is not quite correct. Like the US, a lot of countries (but not all of them) treat the airspace above their country as their territorial space.
Some other countries simply don't do that. In this case it doesn't matter if the plane was flying over Germany (both countries do not see airspace as their territory; correct me if I am wrong). Only if the first landing of an British airplane after the baby's delivery is, let's say, in Germany, the baby of British parents might opt for dual citizenship.