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
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.