Hmm, taken all into account what we know about that game I would bet that Knights of the Sky uses the BOB-engine.

They startet the project based on the old Maddox engine, then suddenly switched to an unspecified new one. I think it would make sense if this new engine is also from Maddox games.

Also, have you noticed that the release date of KotS seems to be tied to the one of BoB? Shortly after UBI announced the October 06 date for BoB to be void and Oleg stated that a late 07 date would be realistic KotS was moved back to late 07 as well.
I would guess that it's part of the deal that KotS must hit the shelves only after BoB.