Which of the following best describes the function of a Mach meter?

The Mach meter serves a specific purpose in aviation by indicating airspeed relative to the speed of sound. This measurement is essential for pilots to understand how their aircraft is performing in relation to the speed of sound, particularly because aerodynamic characteristics change significantly as an aircraft approaches and exceeds this critical speed.

When flying at high altitudes, especially in jet aircraft, the Mach number becomes especially relevant. As temperature and pressure change with altitude, the speed of sound also varies, making it crucial for pilots to gauge their speed in Mach rather than just in knots or miles per hour. By knowing their Mach number, pilots can avoid potentially dangerous situations such as compressibility effects or shock waves that occur near and beyond Mach 1.

The other options focus on different flight parameters not related to the specific function of a Mach meter, such as fuel consumption or altitude, which fall under the purview of other instruments. Thus, the main function of the Mach meter as a critical aviation instrument is accurately captured in the correct option.