Last Day on Earth: How to make Airdrops appear
In short, to make Airdrops appear you have to waste energy. From early days of the game, developers told that events are more likely appear on low energy. It is to encourage players to spend real money on refilling energy.
Every day you get 3 air drops. First one appears when you have 70 energy left. Last two are bit trickier. Their guaranteed appearance is around when you have 20 energy left, but they can also appear much earlier. I have seen it appear on 38 energy left. This hints that there is algorithm, where 20 energy left is 90-100% chance for it to appear and 40 energy left is like 1% chance it to appear. We don't know what exact numbers are, but its safe to say that close you are to 20, higher the chances it to appear.