airport. Traditional navigation instruments include a clock  and a magnetic compass. Along with the airspeed indicator  and wind information, these can be used to calculate  navigational progress. Radios and instruments sending  locating information via radio waves have replaced these  manual efforts in modern aircraft. Global position systems  (GPS) use satellites to pinpoint the location of the aircraft  via geometric triangulation. This technology is built into  some aircraft instrument packages for navigational purposes.  Many of these aircraft navigational systems are discussed in  chapter 11 of this handbook.  [Figure 10-8] To understand how various instruments work and can be  repaired and maintained, they can be classified according to  the principle upon which they operate. Some use mechanical  methods to measure pressure and temperature. Some utilize  magnetism and electricity to sense and display a parameter.  Others depend on the use of gyroscopes in their primary  workings. Still others utilize solid state sensors and computers  to process and display important information. In the following  sections, the different operating principles for sensing  parameters are explained. Then, an overview of many of the  engine, flight, and navigation instruments is given. A number of instruments inform the pilot of the aircraft’s  condition and flight situations through the measurement of  pressure. Pressure-sensing instruments can be found in the  flight group and the engine group. They can be either direct  reading or remote sensing. These are some of the most critical  instruments on the aircraft and must accurately inform the  pilot to maintain safe operations. Pressure measurement  involves some sort of mechanism that can sense changes  in pressure. A technique for calibration and displaying the  information is then added to inform the pilot. The type of  pressure needed to be measured often makes one sensing  The Bourdon tube is the internal mechanism for many  pressure gauges used on aircraft. When high pressures need to  be measured, the tube is designed to be stiff. Gauges used to  A Bourdon tube is illustrated in  Figure 10-9.  The open end  of this coiled tube is fixed in place and the other end is sealed  and free to move. When a fluid that needs to be measured is  directed into the open end of the tube, the unfixed portion  of the coiled tube tends to straighten out. The higher the  pressure of the fluid, the more the tube straightens. When  the pressure is reduced, the tube recoils. A pointer is attached  to this moving end of the tube, usually through a linkage  of small shafts and gears. By calibrating this motion of the  straightening tube, a face or dial of the instrument can be  created. Thus, by observing the pointer movement along the  scale of the instrument face positioned behind it, pressure  increases and decreases are communicated to the pilot. mechanism more suited for use in a particular instance.  The three fundamental pressure-sensing mechanisms used  in aircraft instrument systems are the Bourdon tube, the  diaphragm or bellows, and the solid-state sensing device. 10-5 XPDR 5537 IDNT LCL23:00:34 270 ° 2 1 1 2 4300 4200 4100 3900 3900 3800 4300 3600 3500 3400 3300 3200 3100 20 80 4000 4000 130 120 110 90 80 70 1 100 9 TAS 100KT OAT 7°C ALERTS NAV1 117.60 117.90 NAV2 117.90 117.60 132.675 120.000 COM1 118.525 132.900 COM2 $_{WPT }$_ _ _ _ _ _$_{ DIS }$_ _ ._ $_{NM}$ $_{DTK }$_ _ _$_{°}$ TRK 360° N-S E -W NAV1 117.60 117.90 NAV2 117.90 117.60 132.675 120.000 COM1 118.525 132.900 COM2 $_{WPT }$_ _ _ _ _ _$_{ DIS }$_ _ ._ $_{NM}$ $_{DTK }$_ _ _$_{°}$ TRK 360° MAP - NAVIGATION MAP VOR 1 DME TUNING DME MODE NAV1 NAV controls COM frequency window NAV frequency window COM controls Com section audio panel NAV section audio panel DME tuning window Glideslope indicator Moving map Pointer linkage attaches here Spur gear Pressure entrance Pressure in Sector gear Free end Fixed end Bourdon tube Pressure Measuring Instruments Figure 10-9.  The Bourdon tube is one of the basic mechanisms for  sensing pressure. Figure 10-8.  Navigation instruments.