I'm interested in a system that would allow me to conduct night bombing raids and implement night fighters. The idea is to create unit promotions for fighters and bombers allowing night missions. Mechanics are simple. When receiving a night mission promotion, an air unit loses some of its strength (both combat strength and bomb rate) in exchange for which they become "unterceptable" by normal units. The latter is the problem. How does one make an air unit "uninterceptable" unless the appropriate promotion is available (still interceptable by night fighters)?