pilot’s flight instruments and the copilot’s flight instruments.  This is designed to ensure that there is always one set of flight  instruments operable in case of a malfunction. or in a separate unit designed for this function. Then, all  calculation and compensations are performed digitally by  the computer. Outputs from the ADC are electric to drive  servo motors or for use as inputs in pressurization systems,  flight control units, and other systems. DADC outputs are  distributed to these same systems and the cockpit display  using a digital data bus.  There are numerous benefits of using ADCs. Simplification  of pitot-static plumbing lines creates a lighter, simpler,  system with fewer connections, so it is less prone to leaks and  easier to maintain. One-time compensation calculations can  be done inside the computer, eliminating the need to build  compensating devices into numerous individual instruments  or units of the systems using the air data. DADCs can run a  number of checks to verify the plausibility of data received  from any source on the aircraft. Thus, the crew can be alerted  automatically of a parameter that is out of the ordinary.  Change to an alternate data source can also be automatic so  accurate flight deck and systems operations are continuously  maintained. In general, solid-state technology is more reliable  and modern units are small and lightweight.  Figure 10-29 shows a schematic of how a DADC is connected into the  aircraft’s pitot-static and other systems.  The basic flight instruments are directly connected to  the pitot-static system on many aircraft. Analog flight  instruments primarily use mechanical means to measure and  indicate various flight parameters. Digital flight instrument  systems use electricity and electronics to do the same.  Discussion of the basic pitot-static flight instruments begins  with analog instruments to which further information about  modern digital instrumentation is added.  An altimeter is an instrument that is used to indicate the height  of the aircraft above a predetermined level, such as sea level  or in the case of a radio/radar altimeter, the height of terrain  beneath the aircraft. The most common way to measure this  distance is rooted in discoveries made by scientists centuries  ago. Seventeenth century work proving that the air in the  atmosphere exerted pressure on the things around us led  Evangelista Torricelli to the invention of the barometer. Also  in that century, using the concept of this first atmospheric  air pressure measuring instrument, Blaise Pascal was able  to show that a relationship exists between altitude and air  pressure. As altitude increases, air pressure decreases. The  amount that it decreases is measurable and consistent for any  given altitude change. Therefore, by measuring air pressure,  altitude can be determined.  [Figure 10-30]  Essentially, all pressures and temperatures captured by  sensors are fed into the ADC. Analog units utilize transducers  to convert these to electrical values and manipulate them in  various modules containing circuits designed to make the  proper compensations for use by different instruments and  systems. A DADC usually receives its data in digital format.  Systems that do not have digital sensor outputs will first  convert inputs into digital signals via an analog-to-digital  converter. Conversion can take place inside the computer  High performance and jet transport category aircraft  pitot-static systems may be more complicated. These  aircraft frequently operate at high altitude where the  ambient temperature can exceed 50 °F below zero. The  compressibility of air is also altered at high speeds and at  high altitudes. Airflow around the fuselage changes, making  it difficult to pick up consistent static pressure inputs. The  pilot must compensate for all factors of air temperature and  density to obtain accurate indications from instruments.  While many analog instruments have compensating devices  built into them, the use of an air data computer (ADC) is  common for these purposes on high-performance aircraft.  Moreover, modern aircraft utilize digital air data computers  (DADC). The conversion of sensed air pressures into digital  values makes them more easily manipulated by the computer  to output accurate information that has compensated for the  many variables encountered.  [Figure 10-28] Figure 10-28.  Teledyne’s 90004 TAS/Plus air data computer (ADC)  computes air data information from the pitot-static pneumatic  system, aircraft temperature probe, and barometric correction  device to help create a clear indication of flight conditions.  Altimeters and Altitude Pitot-Static Pressure-Sensing Flight Instruments Air Data Computers (ADC) and Digital Air Data  Computers (DADC) 10-16