It was clear that something had gone seriously wrong with the thousand-strong swarm of drones twinkling above Darling Harbour during the Vivid Sydney festival last month. Many suddenly started flying out of formation. Almost 90 fell from the sky and into the dark water below. Thankfully no one was injured. Yet the drone show failure, which has been blamed on radio interference, highlighted a challenge facing all autonomous aircraft: what happens when things go wrong? This is an important