Aero-Terms!

Aero-Terms are designed to be a daily reminder of the terms, names, acronyms and explanations of the unique language that populates the aviation world. Aerospace, sport aviation, fixed wing, helo, you name it... it's all fair game.

Aero-Terms should serve as a quick but intriguing reminder of the terms you may use every day, or an introduction to an aspects of the Aero-World you may not yet be familiar with. ANN also encourages readers to go beyond the FMI link, and further research any intriguing terms.

Suggestions for future Aero-Terms are ALWAYS welcome, as are additions or discussion of the explanations given for each Aero-Term.

Mach Number

Unit of speed, named after the Austrian physicist Ernst Mach (1838-1916), equal to the ratio of the speed of a moving object to the speed of sound in the surrounding medium under ambient conditions. The actual speed of sound varies depending on the altitude above sea level because sound travels at slightly different speeds at different temperatures, and the temperature varies according to altitude. At sea level, the speed of sound, known as Mach 1.0, is about 761 miles per hour (1,225 km/h). At 20,000 ft (6,096 m), the speed of sound is 660 mph (1,062 km/h). If an aircraft is traveling at half the speed of sound, it is said to be traveling at Mach 0.5. A speed of Mach 2.0 is twice the speed of sound. Because the speed of sound varies, a particular speed at sea level expressed as a Mach number would be faster than the same speed at 30,000 ft (9,144 m), which would be faster than the same speed at 40,000 ft (12,192 m). In other words, Mach 2 at sea level is a greater number of miles per hour (or kilometers per hour) than Mach 2 at 30,000 ft, which is a greater number of miles per hour than Mach 2 at 40,000 ft.

FMI: www.grc.nasa.gov/WWW/K-12/airplane/mach.html