More than 20 aircraft across Europe have been targeted by bomb threats. It's the second time this week, which the UK has called a "malicious hoax".
BBC Verify has been analysing the messages sent by an aircraft to find out what happens when a flight receives a bomb hoax - as Thomas Copeland explains.
Additional reporting by Emma Pengelly. Produced by Aisha Sembhi. Graphics by Mesut Ersoz and Sally Nicholls.
