That's a whole flock of birds though.

It only really takes one to whack an engine. The "flock" bit was particularly interesting there because it got both engines at once. A single drone would presumably only take out a single engine. The airplane would likely survive the encounter, since they're built to survive an engine failure. But it would still do a hefty amount of damage (I bet the drone owner doesn't want to be on the hook for the cost of a new engine!) and survival, while likely, is far from guaranteed in the event of a single engine failure.

The fact that people need to be convinced to keep drones away from planes is a little disconcerting. People just aren't going to be convinced until there's an accident. I wrote the apology a couple days ago. Bookmark it:

https://h4labs.wordpress.com/2015/07/19/dangerous-drones-dai...

We're discussing it again 2 days later as the #1 HN story. Has everyone seen Groundhog Day?

A whole "flock" of drones is not hard to imagine.