Okay but that wouldn't produce the signal "all of the planes have suddenly taken off at the same time." No one who knows Manhattan's going to be nuked on Tuesday is going to be in Manhattan on Tuesday counting on their plane to get them out.
Assuming that apocalyptic event is something which would impact the elites, is reliably predictable by the elites, escapable by flying, and has a window of opportunity where everyone will be trying to escape at the same time.
In reality, look at things like the 9/11 attacks, the 2004 tsunami, the Covid lockdowns, the Maduro raid: the elites didn't fly out all at once shortly ahead of time before the plebs knew what was going to happen. No doubt you can think of good reasons why it didn't happen in those circumstances, but trying to come up with a scenario where it actually would happen is quite a bit more challenging.