Good question. I first thought that the answer would be obvious, but when looking at the rules, it’s not so clear.
If your aircraft is in an adjacent territory, then yes you do need an airbase to scramble to defend the facility or the attacked territory (Rule 8.6) .
However, if your aircraft is already in the territory, the table for air combat mentions :
« 2. The Defender decides if he wants to scramble Fighters.
3. If the Defender has scrambled Defending Fighters, there is one round of interception combat […] »
The use of « Scramble » takes us back to rule 8.6 which requires an airbase. It does not say anything about scrambling planes without an airbase in the same territory.
It feels a bit wrong that your territory can be better defended by placing aircraft in an adjacent territory with an airbase rather than where you think the attack will happen.
I have always played with the ability to use on-site aircraft to intercept, but I now find that the rules do not say that you can do so.
Maybe one for the FAQ/Errata ?