During the Second World War fighter pilots found that their planes became difficult to control when they reached speeds of around 870 kilometres an hour. They were getting close to the speed of sound, know as the 'sound barrier' . Many experts believed that no aircraft could ever go that fast and survive. But it didn't take long for someone to prove that those experts were wrong. These days many aircraft can fly faster than the speed of sound. Most of them military aircraft, but there was one - the Concorde- that carried passengers.