This is a query I have now recieved a couple of times and so will eventually go into a FAQ:
In rule [1.5] Scale: Altitude is in Angels, which are increments of 1000 feet...In the game, Angels are given in 2000 foot intervals.
This seems to have caused a small amount of confusion. 'Angels'--units of 1,000 feet--are a real-world measure. As noted in the historical background, fighter controllers would call Angels when relaying instructions to pilots, so that 'Angels 8' was an instruction to fly to 8,000 feet, 'Angels 15' was an order to fly to 15,000 feet, and so on.
However, in the game we only measure Angels that are a multiple of 2. So units can only fly at Angels 0, 2, 4, 6, 8, 10, 12, 14, 16, 18... etc. up to a ceiling of 34 or so. When there is an altitude change, it is always couched in terms of a multiple of 2 Angels.
Note that the altitude counters are all multiples of 2, so you can't go wrong.