Altitude levels trade with speed and fuel: steep dives add speed, steep climbs lose it.
How it works
Diving costs 1 less fuel, climbing 1 more. A dive of 2+ levels adds one speed next turn; a climb of 2+ loses one, and speed 0 tests for spin. A dive of 2+ levels permits a Zoom Climb at +0 fuel and the Climbing Immelmann. Over-diving beyond the safe rate draws a blue damage marker per extra level to the wings. Medium/long bursts by rigid guns at higher or lower targets force a climb or dive next turn.
An aircraft which dives 2 or more altitude levels during a movement phase may elect to “Zoom Climb” the following movement phase.

