separate display unit in the cockpit. Both analog and digital  instruments make use of this method.  [Figure 10-3] The relaying of important bits of information can be done  in various ways. Electricity is often used by way of wires  that carry sensor information into the cockpit. Sometimes  pneumatic lines are used. In complex, modern aircraft,  this can lead to an enormous amount of tubing and wiring  terminating behind the instrument display panel. More  efficient information transfer has been accomplished via the  use of digital data buses. Essentially, these are wires that share  message carrying for many instruments by digitally encoding  the signal for each. This reduces the number of wires and  weight required to transfer remotely sensed information for  the pilot’s use. Flat-panel computer display screens that can  be controlled to show only the information desired are also  lighter in weight than the numerous individual gauges it  would take to display the same information simultaneously.  An added bonus is the increased reliability inherent in these  solid-state systems. It is the job of the aircraft technician to understand and  maintain all aircraft, including these various instrument  systems. Accordingly, in this chapter, discussions begin  with analog instruments and refer to modern digital  instrumentation when appropriate. There are three basic kinds of instruments classified by the  job they perform: flight instruments, engine instruments,  and navigation instruments. There are also miscellaneous  gauges and indicators that provide information that do not  fall into these classifications, especially on large complex  aircraft. Flight control position, cabin environmental systems,  electrical power, and auxiliary power units (APUs), for  example, are all monitored and controlled from the cockpit  via the use of instruments systems. All may be regarded as  position/condition instruments since they usually report the  position of a certain moveable component on the aircraft, or  the condition of various aircraft components or systems not  included in the first three groups.  Engine instruments are those designed to measure operating  parameters of the aircraft’s engine(s). These are usually  Original analog flight instruments are operated by air  pressure and the use of gyroscopes. This avoids the use of  electricity, which could put the pilot in a dangerous situation  if the aircraft lost electrical power. Development of sensing  and display techniques, combined with advanced aircraft  electrical systems, has made it possible for reliable primary  and secondary instrument systems that are electrically  operated. Nonetheless, often a pneumatic altimeter, a gyro  artificial horizon, and a magnetic direction indicator are  retained somewhere in the instrument panel for redundancy.  [Figure 10-5] The instruments used in controlling the aircraft’s flight  attitude are known as the flight instruments. There are basic  flight instruments, such as the altimeter that displays aircraft  altitude; the airspeed indicator; and the magnetic direction  indicator, a form of compass. Additionally, an artificial  horizon, turn coordinator, and vertical speed indicator are  flight instruments present in most aircraft. Much variation  exists for these instruments, which is explained throughout  this chapter. Over the years, flight instruments have come to  be situated similarly on the instrument panels in most aircraft.  This basic T arrangement for flight instruments is shown in  Figure 10-4.  The top center position directly in front of the  pilot and copilot is the basic display position for the artificial  horizon even in modern glass cockpits (those with solid-state,  flat-panel screen indicating systems). Flight Instruments Engine Instruments Classifying Instruments 10-3 Figure 10-4.  The basic T arrangement of analog flight instruments. At the bottom of the T is a heading indicator that functions as  a compass but is driven by a gyroscope and not subject to the  oscillations common to magnetic direction indicators. Figure 10-3.  There are two parts to any instrument system—the  sensing mechanism and the display mechanism.  30.0 29.9 29.8 Sensor + Indication Sensor Indication Direct-sensing  instrument system Remote-sensing  instrument system