A subsonic aircraft is a machine, such as an airplane, helicopter, glider or dirigible, that is capable of atmospheric flight and its speed is less than the speed of sound. A supersonic aircraft's overall speed may be less than the speed of sound but they are able to break the sound barrier. On the other hand, the subsonic aircraft are not able to do this, their speed is always below the speed of sound i.e. mach number is below 1. Some examples are Airbus A300, Airbus A310, Airbus A318, Airbus A319, Airbus A320, Airbus A321, Airbus A330, Bristol Britannia, Cessna 208, Boeing 747, Boeing 757, Boeing 777 and Boeing 787.