Just a thought but when they eny cap gets really high and you can only up planes with 20 or greater eny value, how about the ability to still up planes with a perk value but with an extra fee?
Say the eny is at 23, so you can't up any 20 or less eny planes. An F4U-4 costs 60 perks to up but you can't take it because it has an eny value of 5 and eny cap is at 23. You just pay an extra fee (let's just say 100 perks) and you can now take the F4U-4 out for a spin. You still get all your perks back if you land, and if you die you not only loose the 60 perks for the plane, but an extra 100 to up it with eny in effect.