Forget 'aircraft factories'.
Give every player a personal hangar. The hangar comes stocked with x of each plane(x depends on popularity and the planes ma abilities, and you can never have more than x planes) that takes y minutes(of flight time) to resupply each one. And a perk cost of z. The less common(say over 40eny) planes would have no limitation.
Whenever you lose a plane, you lose one from your hangar. When you have none of that type in your hangar, you need to either pay the perk cost, or switch planes. The time it takes to resupply aircraft is divided by the current state of a countries strat. (So with strat half dead, each plane takes twice as long to resupply)
Using a system like this opens up the opportunity for semi-perk planes. Allowing players to fly them for free, but only a limited number of times in a certain time period. (THEN they have to pay the perks)
Perk planes could be given very long timers, and still allow newer pilots the opportunity to fly them for free once in a while, without causing too many to be around.
Then again, it's a relativly complex system, which probably is way too much work to be worth even a full design of. But what the hell, I'm bored.