Figure 8-1.  Pitot-static system and instruments.  precipitation. Both openings in the pitot tube must be checked  prior to flight to ensure that neither is blocked. Many aircraft  have pitot tube covers installed when they sit for extended  periods of time. This helps to keep bugs and other objects  from becoming lodged in the opening of the pitot tube.  The one instrument that utilizes the pitot tube is the ASI. The  total pressure is transmitted to the ASI from the pitot tube’s  pressure chamber via a small tube. The static pressure is  also delivered to the opposite side of the ASI, which serves  to cancel out the two static pressures, thereby leaving the  dynamic pressure to be indicated on the instrument. When  the dynamic pressure changes, the ASI shows either increase  or decrease, corresponding to the direction of change. The  two remaining instruments (altimeter and VSI) utilize only  the static pressure that is derived from the static port.  The static chamber is vented through small holes to the  free undisturbed air on the side(s) of the aircraft. As the  atmospheric pressure changes, the pressure is able to move  freely in and out of the instruments through the small lines  that connect the instruments to the static system. An alternate  static source is provided in some aircraft to provide static  pressure should the primary static source become blocked.  The alternate static source is normally found inside the flight  The pitot tube has a small opening at the front that allows  the total pressure to enter the pressure chamber. The total  pressure is made up of dynamic pressure plus static pressure.  In addition to the larger hole in the front of the pitot tube,  there is a small hole in the back of the chamber that allows  moisture to drain from the system should the aircraft enter  When the wind blows from an angle less than 90° off the  nose of the aircraft, dynamic pressure can be depicted on the  ASI. The wind moving across the airfoil at 20 knots is the  same as the aircraft moving through calm air at 20 knots.  The pitot tube captures the dynamic pressure, as well as the  static pressure that is always present.  The pitot tube is utilized to measure the total combined  pressures that are present when an aircraft moves through  the air. Static pressure, also known as ambient pressure, is  always present whether an aircraft is moving or at rest. It is  simply the barometric pressure in the local area. Dynamic  pressure is present only when an aircraft is in motion;  therefore, it can be thought of as a pressure due to motion.  Wind also generates dynamic pressure. It does not matter if  the aircraft is moving through still air at 70 knots or if the  aircraft is facing a wind with a speed of 70 knots, the same  dynamic pressure is generated.  Static Pressure Chamber and Lines  Impact Pressure Chamber and Lines  30.0 29.9 29.8  Altimeter  Heater (35 watts)  Static port  Vertical speed indicator (VSI) Airspeed indicator (ASI)  Pitot heater switch  Drain hole  Pressure chamber  Alternate static source  Static hole  Heater (100 watts)  Pitot tube  Baffle plate  Static chamber  Ram air  8-2 