Today's Aviation Terminology Lesson

Doing my part to bring the wonderful world of Airplanes to Wordaholism, I offer this little nugget.

The speed of sound is one of the key limitations to airplane performance. Exceeding this speed--also called "Mach One"--was once thought to be impossible, and quite a number of people died in pursuit of the goal. The feat was eventually achieved by Chuck Yeager on October 14, 1947 in a Bell X-1 rocket plane dropped like a bomb from the belly of a B-29 at 20,000 feet.

The speed that sound travels through the air is variable depending on air temperature and atmospheric pressure, and at sea level in a so-called "standard atmosphere" it is 761 m.p.h. (or 662 knots). At higher altitudes, it takes a slower indicated airspeed to achieve the speed of sound.

No normal civilian jet aircraft is able to fly into this "supersonic" realm. The Concorde could, which its claim to fame. But for the rest of us, our airspeed indicator--an airplane's speedometer--shows our speed relative to this always-shifting speed of sound. This speed is expressed as a percentage of the speed of sound, and is referred to as our

mach number (mäk)
n. (Abbr. M)
The ratio of the speed of an object to the speed of sound in the surrounding medium.

[After Ernst MACH.]

As an example, a speed of "Mach Eight Zero" (M.80) would be 80% of the speed of sound at a given altitude and pressure. Our speeds are often assigned by Air Traffic Control in this way. "Indicate Mach Eight Four," they might tell us. This is the speed indicated by the magenta ".842" in the upper left corner of the display above.

http://www.answers.com/Mach%20number