Aircraft that are flown in instrument meteorological  conditions (IMC) are equipped with instruments that provide  attitude and direction reference, as well as navigation  instruments that allow precision flight from takeoff to landing  with limited or no outside visual reference. The instruments discussed in this chapter are those required  by Title 14 of the Code of Federal Regulations (14 CFR)  part 91, and are organized into three groups: pitot-static  instruments, compass systems, and gyroscopic instruments.  The chapter concludes with a discussion of how to preflight  these systems for IFR flight. This chapter addresses additional  avionics systems such as Electronic Flight Information  Systems (EFIS), Ground Proximity Warning System  (GPWS), Terrain Awareness and Warning System (TAWS),  Traffic Alert and Collision Avoidance System (TCAS),  Head Up Display (HUD), etc., that are increasingly being  incorporated into general aviation aircraft. Pitot pressure, or impact air pressure, is sensed through an  open-end tube pointed directly into the relative wind flowing  around the aircraft. The pitot tube connects to the ASI or an  air data computer depending on your aircraft's configuration.  Static pressure is also used by the ASI as well as the other  pitot static instruments for determining altitude and vertical  speed. Static pressure may be sensed at one or more locations  on an aircraft. Some may be flush mounted on the fuselage or  integrated into the electrically heated pitot tube.  [Figure 5-1] These ports are in locations proven by flight tests to be in  undisturbed air, and they may be paired, one on either side of  the aircraft. This dual location prevents lateral movement of  the aircraft from giving erroneous static pressure indications.  The areas around the static ports may be heated with electric  heater elements to prevent ice forming over the port and  blocking the entry of the static air. Three basic pressure-operated instruments are found in  aircraft instrument panels flown under IFR. These are the  ASI, sensitive altimeter, and vertical speed indicator (VSI).  All three instruments receive static air pressure for operation  with only the ASI receiving both pitot and static pressure.  [Figure 5-2] Errors in the ASI and VSI almost always indicate a blockage  of the pitot tube, the static port(s), or both. Moisture  (including ice), dirt, or even insects can cause a blockage in  both systems. During preflight, it is very important to make  sure the pitot tube cover is removed and that static port  openings are checked for blockage and damage. If the pitot tube drain hole becomes obstructed, the pitot  system can become partially or completely blocked. When  dynamic pressure cannot enter the pitot tube opening, the ASI  no longer operates. If the drain hole is open, static pressure  equalizes on both sides of the diaphram in the ASI and the  Blockage of the Pitot-Static System Blocked Pitot System Pitot/Static Systems Static Pressure 30.0 29.9 29.8 Ram air Pitot tube Static port Baffle plate Drain hole Heater (100 watts) Static hole Static hole Static chamber Heater (35 watts) Pitot pressure chamber Pitot heater switch Alternate static source Drain hole Figure 5-1.  A typical electrically heated pitot-static head. 5-2