 The biggest continuing effect is on air travel, after carriers cancelled thousands of flights with planes and crews being stranded in the wrong place. But at some of Europe's biggest airports, operations are more or less returning to normal. At the heart of the massive disruption is CrowdStrike, a cyber security firm providing software to companies worldwide. CrowdStrike says the outage happened when it deployed a faulty update, noting the issue was not a security incident or a cyber attack. Mimi Montgomery, London.