As the title suggest, aircraft item will be destoyed when your aircraft is shot down or crashed. I am not sure if this is a new feature, but since it is not noted in the changelog, I will assume it is a bug. This started happen since version 1.3.0 when bamboo hopper is added. In versions before, the aircraft item will drop upon being destoyed, allowing the player to use it again. This problem is most problematic when you have expensive upgrade in your aircraft, which will also vanish when your aircraft is destoyed.