For the people here who say that the drones can't take down an airliner or a small Cessna how many lives are you willing to bet on it? The live of two people typical of a Cessna 172, the lives of a few hundred passengers in a Boeing 777? What we are forgetting is the human factors here too. It is not just a collision that might damage an engine, a wing, or a tail section. It is also the distraction the pilot now has to deal with very close to the ground. Now instead of paying attention to the landing and other aircraft the pilot has to figure out what was that noise that just collided with it. Especially under instrument meteorological conditions. In fact, FAA has regulations for airlines to have a sterile cockpit (100% concentration on the landing and no outside distractions) due to accidents that have happened and loss of life while pilots were distracted with other things even higher than the 400 ft.