Infinite Dreams has added a new feature to the game in the latest update. Cards are like bonus achievements for your plane. I can't find any pattern when or how they appear (if one exists). Did anyone ever find something like a pattern on their appearance?
There is absolutely no pattern, and has been discussed at great length on other forums. I will tell you this, when they introduced cards, they were dropping like crazzy. But around 6 months ago, they NERFED these card drops dramatically.
Not confiremd But I've heard, but I doubt that it's true, that people farm cards on Stage 5 on different difficulties because the card is in one of the first 3 crates normally. If they don't find the card, they start the map over, and do this one a day.