20 0 -20 -40 -60 40 60 100 140 120 80 F 20 0 -20 -40 40 60 C Total air temp + TAT °C OFF TAT °C + 85.0 96.1 TO 10 2 6 0 4 8 85.0 96.1 10 2 6 0 4 8 450 EGT 10 2 6 0 4 8 450 10 2 6 0 4 8 N1 TAT - 15 c 50 OIL ORESS 50  105 OIL TEMP 100 97.0  N2  97.0 20 OIL QTY  20 8.4  FF 8.4 1.9 N2 VIB 1.9 FAN Mode annunciation Function selector push button Failure flag Balanced bridge indicator  Servo driven indicator LCD indicator Full digital display Figure 10-76.  Different cockpit TAT displays.  10-43 Figure 10-78  shows the earth and the magnetic field that  surrounds it. The magnetic north pole is very close to the  geographic North Pole of the globe, but they are not the same.  An ordinary permanent magnet that is free to do so, aligns  itself with the direction of the earth’s magnetic field. Upon  this principle, an instrument is constructed that the pilot can  reference for directional orientation. Permanent magnets are  attached under a float that is mounted on a pivot so it is free  to rotate in the horizontal plane. As such, the magnets align  with the earth’s magnetic field. A numerical compass card,  usually graduated in 5° increments, is constructed around  the perimeter of the float. It serves as the instrument dial.  Having an instrument on board an aircraft that indicates  direction can be invaluable to the pilot. 14 CFR part 91,  section 91.205 requires that aircraft with standard category  airworthiness certificates have a magnetic direction indicator  for VFR flight during the day. The magnetic compass is a  direction finding instrument that has been used for navigation  for hundreds of years. It is a simple instrument that takes  advantage of the earth’s magnetic field. A myriad of techniques and instruments exist to aid the  pilot in navigation of the aircraft. An indication of direction  is part of this navigation. While the next chapter deals with  communication and navigation, this section discusses some of  the magnetic direction indicating instruments. Additionally, a  common, reliable gyroscopic direction indicator is discussed  in the gyroscopic instrument section of this chapter. Close attention is paid to airflow and materials conductivity  during the design phase. Some TAT sensors channel bleed  air through the units to affect the flow of outside air, so that  it flows directly onto the platinum sensor without gaining  added energy from the probe heater.  TAT sensor/probe design is complicated by the potential of  ice forming during icing conditions. Left unheated, a probe  may cease to function properly. The inclusion of a heating  element threatens accurate data collection. Heating the  probe must not affect the resistance of the sensor element.  [Figure 10-77]  More complex systems use signal correction technology  and amplified signals sent to a servo motor to adjust the  indicator in the cockpit. These systems include closely  regulated power supply and failure monitoring. They often use  numeric drum type readouts but can also be sent to an LCD  driver to illuminate LCD displays. Many LCD displays are  multifunctional, capable of displaying static air temperature  and true airspeed. In fully digital systems, the correction  signals are input into the ADC. There, they can be manipulated  appropriately for cockpit display or for whichever system  requires temperature information.  [Figure 10-76] Simple TAT systems include a sensor and an indicator with  a built-in resistance balance circuit. Air flow through the  sensor is designed so that air with the precise temperature  impacts a platinum alloy resistance element. The sensor is  engineered to capture temperature variations in terms of  varying the resistance of the element. When placed in the  bridge circuit, the indicator pointer moves in response to the  imbalance caused by the variable resistor.  Magnetic Compass Direction Indicating Instruments