90 80 70 60 50 40 4 3 6 TAS 64KT G Y X R General cruising speed Best rate of climb speed Best angle of climb speed Rotation speed Flap range Airborne airspeed Ground airspeed 40 30 20 10 40 30 20 10 050  060  070  080  090  100  110  120 1040  B Figure 2-3. Attitude indicator with symbols to assist in recovery  from unusual attitude. Figure 2-2. Vertical airspeed (tape type) indicator. Primary Flight Instrument Systems Navigation Instruments Figure 2-3 shows an attitude indicator that presents red  symbols to assist in recovery from unusual attitudes. The  symbols on the display recommend a lower pitch attitude.  Some PFDs offer enhancements to the primary flight  instruments.  Figure 2-2 shows an airspeed indicator that  displays reference speeds (V‑speeds) and operating ranges  for the aircraft. Operating ranges are depicted using familiar  color coding on the airspeed indicator. One negative human  factor concerning this type of presentation should be  remembered: while most of the displays are intuitive in that  a high indication (such as climb pitch or vertical speed) is  corrected by lowering the nose of the aircraft, the situation  with the usual airspeed vertical tape is the opposite. In most  current displays, the lower speeds are at the lower side of the  airspeed indicator, while the upper or higher speeds are in the  top portion of the airspeed display area. Therefore, if a low  airspeed is indicated, you must lower the nose of the aircraft  to increase, which is counterintuitive to the other indications. Enhancements to the Primary Flight Instruments Other valuable enhancements include trend indicators, which  process data to predict and display future performance. For  example, some systems generate “trend vectors” that predict  the aircraft’s airspeed, altitude, and bank angle up to several  seconds into the future. The primary flight instruments that appear on a PFD are driven  by instrument sensor systems that are more sophisticated than  conventional instrument systems. The attitude of the aircraft  may be measured using microelectronic sensors that are more  sensitive and reliable than traditional gyroscopic instruments.  These sensors measure pitch, roll, and yaw movements away  from a known reference attitude. Aircraft heading may be  determined using a magnetic direction‑sensing device such  as a magnetometer or a magnetic flux valve. Attitude and heading systems are typically bundled together as  an attitude heading reference system (AHRS), which contains  not only the sensors used to measure attitude and heading,  but also a computer that accepts sensor inputs and performs  calculations. Some AHRSs must be initialized on the ground  prior to departure. The initialization procedure allows the  system to establish a reference attitude used as a benchmark  for all future attitude changes. As in any navigation system,  attitude heading reference systems accumulate error over  time. For this reason, AHRSs continually correct themselves,  using periods of stable flight to make small corrections to the  reference attitude. The system’s ability to correct itself can  be diminished during prolonged periods of turbulence. Some  AHRSs can be reinitialized in flight, while others cannot. The  pilot must become familiar with the operating procedures and  capabilities of a particular system. Information on altitude and airspeed is provided by sensors  that measure static and ram air pressure. An air data computer  (ADC) combines those air pressure and temperature sensors  with a computer processor that is capable of calculating  pressure altitude, indicated airspeed, vertical speed, and  true airspeed. An air data attitude heading reference system  (ADAHRS) combines all of the systems previously described  into one integrated unit. PFDs and multi‑function displays (MFDs) typically combine  several navigation instruments into a single presentation.  The instrument appearing at the bottom of the PFD in  Figure 2‑1 contains two navigation indicators: a course  deviation indicator and a bearing pointer. These instruments  can be displayed in a variety of views, and can be coupled  to many of the navigation receivers (e.g., instrument landing  system (ILS), global positioning system (GPS), very high  frequency (VHF) omnidirectional range (VOR)) available  2-3