Many digital data displays are available from numerous  manufacturers as original equipment in new aircraft, or  as retrofit components or complete retrofit systems for  older aircraft. Approval for retrofit displays is usually  accomplished through supplementary type certificate (STC)  awarded to the equipment manufacturer.  Early digital displays presented scale indication in digital  or integer format readouts. Today’s digital data displays  are analogous to computer screen presentations. Numerous  aircraft and flight instrument readouts and symbolic  presentations are combined with communication and  navigational information on multifunctional displays (MFD).  Often a display has a main function with potential to back-up  another display should it fail. Names, such as primary flight  display (PFD), secondary flight display, navigational display  (ND), etc., are often used to describe a display by its primary  use. The hardware composition of the displays is essentially  the same. Avionics components and computers combine to  provide the different information portrayed on the displays. indicator display on the lower half. Numerous ancillary  readouts are integrated or surround the electronic attitude  indicator and the horizontal situation indicator (HSI). On full  glass cockpit PFDs, all of the basic T instrument indications  are presented and much more, such as communication and  navigation information, weather data, terrain features, and  approach information. Data displays for engine parameters,  hydraulics, fuel, and other airframe systems are often displayed  on the secondary flight display or on an independent display  made for this purpose.  [Figure 11-72] As with other avionics components, repair and maintenance  of the internal components of digital data displays is reserved  for licensed repair stations only. Controls on the instrument panel or on the display unit itself  are used for selection. Some screens have limited display  capability because they are not part of a totally integrated  system; however, they are extremely powerful electronic  units with wide capability.  [Figure 11-71] Numerous communication and navigation devices are  described in the following sections of this chapter. Many  of these use radio waves and must be tuned to a desired  frequency for operation. As a flight progresses, retuning  and changing from one piece of equipment to another can  occur frequently. An audio panel or digital tuner consolidates  various communication and navigation radio selection  controls into a single unit. The pilot can select and use, or  select and tune, most of the aircraft’s avionics from this one  control interface.  [Figure 11-73] The basis of the information displayed on what is known as a  PFD, is usually an electronic flight instrument system (EFIS)  like representation of the aircraft attitude indicator in the  upper half of the display, and an electronic horizontal situation  Much of aviation communication and navigation is accomplished  through the use of radio waves. Communication by radio was  the first use of radio frequency transmissions in aviation. Radio Communication Digital Tuners and Audio Panels Figure 11-71.  retrofit digital data display.  11-30 M E N U R N G + - R E V _ _ _ _ _ MODE SYNC MODE SYNC 160 6500 6400 6200 6100 150 140 130 110 100 90 TAS 134 kt GS 119 kt 148°/16 kt OAT 11°C 29.92 in 120 1 9 6280 6500 6560 8500 105 kt 10 10 20 10 10 20 MIN 5520 V$_{le}$ V$_{a}$ V$_{lo}$ V$_{lg}$ ILS1 CRS HDG VLOC1 164° 110.90 CRS HDG −580 FPM 199° ILS1 TRFC 10nm 163° RW16R KANO VERDI -01 +02 21 24 W 30 33 N 3 6 E 12 4SD TAKLE 15 S .5 2 − 2 L T N G N X R D T R F C N R M 1 2 M E N U R N G + - R E V _ _ _ _ _ MODE SYNC MODE SYNC 160 6500 6400 6200 6100 150 140 130 110 100 90 TAS 134 kt GS  119 kt 148°/16 kt OAT 11°C 29.92 in 120 1 9 6280 6500 6560 8500 105 kt 10 10 20 10 10 20 MIN 5520 V$_{le}$ V$_{a}$ V$_{lo}$ V$_{lg}$ ILS1 CRS HDG VLOC1 164° 110.90 CRS HDG −580 FPM 199° ILS1 TRFC 10nm 163° RW16R KANO VERDI -01 +02 21 24 W 30 33 N 3 6 E 12 4SD TAKLE 15 S .5 2 − 2 L T N G N X R D T R F C N R M 1 2