What does a Mach number of 2 signify about the aircraft?

A Mach number of 2 indicates that the aircraft is traveling at twice the speed of sound. The Mach number is a dimensionless unit used to represent the speed of an object in relation to the speed of sound in the surrounding medium, which is influenced by temperature and altitude. At sea level under standard conditions, the speed of sound is approximately 343 meters per second (or about 1,125 feet per second). Therefore, a Mach number of 2 means the aircraft is flying at approximately 686 meters per second, or 1,350 knots, depending on the specific atmosphere conditions.

In contrast, the other options do not accurately reflect the meaning of a Mach number of 2. Flying at cruising altitude or descending does not inherently relate to a specific Mach number; rather, it depends on the aircraft’s altitude and atmospheric conditions. Similarly, being within controlled airspace pertains to flight regulations and air traffic management, not speed. Thus, the definition of a Mach number provides a clear understanding that the aircraft's speed is twice that of sound, making the first choice the only correct one.