Mach Numbers

Go to the large window at 12 o'clock to input the temperature. Be extra careful to set

the green hairline at the intersection of the MACH spiral (black line running left to

right) and the temperature wiggley (black line running top to bottom) representing -

20. Now follow the green line down to the TAS scale and read the value, in this case,

272 kts. (Figure 4.4-7)

Figure 4.4-7 TAS 2 TEMP/TAS/MACH SPIRAL

MACH NUMBER

As an airplane flies, velocity and pressure changes create sound waves in the airflow

around the airplane (Figure 4.4-8). Since these sound waves travel at the speed of sound,

an airplane flying at subsonic airspeeds will travel slower than the sound waves and allow

them to dissipate. However, as the airplane nears the speed of sound, these pressure

waves pile up forming a wall of pressure, called a shock wave, which also travels at the

speed of sound. As long as the airflow velocity on the airplane remains below the local

speed of sound (LSOS), the airplane will not suffer the effects of compressibility.

Therefore, it is appropriate to compare the two velocities. Mach Number (M) is the ratio of

the airplane's True Airspeed to the local speed of sound.

TAS

LSOS

4.6-100