FAA-H-8083-15, Instrument Flying Handbook -- 1 of 2 - US-PPL

Mach NumberAs an aircraft approaches the speed of sound, the air flowingover certain areas of its surface speeds up until it reaches thespeed of sound, and shock waves form. The indicated airspeedat which these conditions occur changes with temperature.Therefore airspeed, in this case, is not entirely adequate towarn the pilot of the impending problems. Mach number ismore useful. Mach number is the ratio of the true airspeed ofthe aircraft to the speed of sound in the same atmosphericconditions. An aircraft flying at the speed of sound is flyingat Mach 1.0.Most high-speed aircraft are limited as to the maximum Machnumber they can fly. This is shown on a Machmeter as adecimal fraction. [Figure 3-11] For example, if the Machmeterindicates .83 and the aircraft is flying at 30,000 feetwhere the speed of sound under standard conditions is 589.5knots, the airspeed is 489.3 knots. The speed of sound varieswith the air temperature, and if the aircraft were flying atMach .83 at 10,000 feet where the air is much warmer, itsairspeed would be 530 knots.Figure 3-12. A maximum allowable airspeed indicator has amovable pointer that indicates the never-exceed speed, whichchanges with altitude to avoid the onset of transonic shock waves.Airspeed Color CodesThe dial of an airspeed indicator is color coded to alert you,at a glance, of the significance of the speed at which theaircraft is flying. These colors and their associated airspeedsare shown in figure 3-13.Figure 3-11. A Machmeter shows the ratio of the speed ofsound to the true airspeed the aircraft is flying.Figure 3-13. Color codes for an airspeed indicator.Maximum Allowable AirspeedSome aircraft that fly at high subsonic speeds are equippedwith maximum allowable airspeed indicators like the one infigure 3-12. This instrument looks much like a standard airspeedindicator, calibrated in knots, but has an additionalpointer, colored red, checkered, or striped. The maximumairspeed pointer is actuated by an aneroid, or altimeter mechanism,that moves it to a lower value as air density decreases.By keeping the airspeed pointer at a lower value than themaximum pointer, the pilot avoids the onset of transonicshock waves.Vertical Speed Indicators (VSI)The vertical speed indicator (VSI) in figure 3-14 is also calleda vertical velocity indicator (VVI) and was formerly knownas a rate-of-climb indicator. It is a rate-of-pressure changeinstrument that gives an indication of any deviation from aconstant pressure level.3–8