In 1947 Derry joined
de Havilland as a test pilot, working largely on the
de Havilland DH 108 aircraft. He is widely believed to have exceeded the speed of sound on 6 September 1948, when he lost control of his aircraft and the Mach meter supposedly 'briefly showed' supersonic speeds in a shallow dive from 12,195 m (40,000 ft) to 9,145 m (30,000 ft). Despite lack of substantial evidence (the recording apparatus was switched off during the flight), the news was promoted by the British press as having broken the sound barrier. Also in 1948, he was awarded the Gold Medal of the
Royal Aero Club.
Tom Wolfe's book
The Right Stuff mentioned this claim, but referred to another test pilot,
Geoffrey de Havilland Jr., who had been killed in a previous test flight, when his DH 108 broke up at about Mach 0.9. As a demonstration pilot, Derry developed a new type of
aerobatic manoeuvre which became known as the "Derry Turn". It consists of a reversal of
bank during a steep turn which is performed with the aircraft passing through the inverted rather than upright attitude. ==Death==