differential when it is displayed on a linear scale. Analog  airspeed indicators come with a correction chart that allows  cross-referencing of indicated airspeed to calibrated airspeed  for various flight conditions. These differences are typically  very small and often are ignored. Digital instruments have  these corrections performed in the ADC. calculations performed so a true airspeed value can be  digitally sent to the cockpit for display. Refer to  Figure 10-34 for the display of airspeed information on the primary flight  display on a light aircraft. Note that similar to its position  in the standard T configuration of an analog cockpit, the  airspeed indication is just left of the artificial horizon display.  Refer to  Figure 10-4  showing T configuration. More importantly, indicated airspeed does not take into  account temperature and air pressure differences needed to  indicate true airspeed. These factors greatly affect airspeed  indication. True airspeed, therefore, is the same as indicated  airspeed when standard day conditions exist. But when  atmospheric temperature or pressure varies, the relationship  between the ram air pressure and static pressure alters. Analog  airspeed instruments often include bimetallic temperature  compensating devices that can alter the linkage movement  between the diaphragm and the pointer movement. There can  also be an aneroid inside the airspeed indicator case that can  compensate for non-standard pressures. Alternatively, true  airspeed indicators exist that allow the pilot to set temperature  and pressure variables manually with external knobs on the  instrument dial. The knobs rotate the dial face and internal  linkages to present an indication that compensates for non- standard temperature and pressure, resulting in a true airspeed  indication.  [Figure 10-43] Complications continue when considering airspeed  indications and operating limitations. It is very important to  keep high-speed aircraft from traveling faster than the speed  of sound if they are not designed to do so. Even as an aircraft  approaches the speed of sound, certain parts on the airframe  may experience airflows that exceed it. The problem with  this is that near the speed of sound, shock waves can develop  that can affect flight controls and, in some cases, can literally  tear the aircraft apart if not designed for supersonic airflow.  A further complication is that the speed of sound changes  with altitude and temperature. So, a safe true airspeed at sea  level could put the aircraft in danger at altitude due to the  lower speed of sound.  [Figure 10-44] In order to safeguard against these dangers, pilots monitor  airspeed closely. A maximum allowable speed is established  Digital flight instrument systems perform all of the  calculations for true airspeed in the ADC. Ram air from the  pitot tube and static air from the static vent(s) are run into the  sensing portion of the computer. Temperature information  is also input. This information can be manipulated and  10-25 Figure 10-44.  As temperatures fall at higher altitudes, the speed  of sound is reduced.  Figure 10-43.  An analog true airspeed indicator. The pilot manually  aligns the outside air temperature with the pressure altitude scale,  resulting in an indication of true airspeed.  120 130 140 150 2 4 6 MPH TEMP 30 30 0 + - AIRSPEED TRUE SPEED KNOTS I80 40 60 I60 80 I00 I20 I40 40 I40 I20 60 80 100 Altitude (feet) Temperature (°F) Speed of sound (knots) Sea level 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000 22,000 24,000 26,000 28,000 30,000 32,000 34,000 36,000 38,000 40,000 42,000 44,000 46,000 48,000 50,000 59 52 48 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –55 –62 –69 –70 –70 –70 –70 –70 –70 –70 661 657 652 648 643 638 633 629 624 619 614 609 604 599 594 589 584 579 574 574 574 574 574 574 574 574 Standard Altitude, Temperature, and the Speed of Sound