The use of instruments as an aid to flight enables the pilot to operate the airplane more precisely, and therefore, obtain maximum performance and enhanced safety. This is particularly true when flying greater distances. Manufacturers have provided the necessary flight instruments; however, it is the pilot’s responsibility to gain the essential knowledge about how the instruments operate so that they can be used effectively.

 This chapter covers the operational aspects of the pitot-static system and associated instruments: the vacuum system and associated instruments; and the magnetic compass.


 There are two major parts of the pitot-static system: (1) impact pressure chamber and lines; and (2) static pressure chamber and lines, which provides the source of ambient air pressure for the operation of the altimeter, vertical speed indicator (vertical velocity indicator), and the airspeed indicator.

Impact Pressure Chamber and Lines

 In this system, the impact air pressure (air striking the airplane because of its forward motion) is taken from a pitot tube, which is mounted either on the leading edge of the wing or on the nose, and aligned to the relative wind. On certain aircraft, the pitot tube is located on the vertical stabilizer. These locations provide minimum disturbance or turbulence caused by the motion of the airplane through the air. The static pressure (pressure of the still air) is usually taken from the static line attached to a vent or vents mounted flush with the side of the fuselage. Airplanes using a flush-type static source, with two vents, have one vent on each side of the fuselage. This compensates for any possible variation in static pressure due to erratic changes in airplane attitude.
 The openings of both the pitot tube and the static vent should be checked during the preflight inspection to assure that they are free from obstructions. Clogged or partially clogged openings should be cleaned by a certificated mechanic. Blowing into these openings is not recommended because this could damage any of the three instruments. [Figure 3-1]

 As the airplane moves through the air, the impact pressure on the open pitot tube affects the pressure in the pitot chamber. Any change of pressure in the pitot chamber is transmitted through a line connected to the airspeed indicator which utilizes impact pressure for its operation.

Static Pressure Chamber and Lines
The static chamber is vented through small holes to the free undisturbed air, and as the atmospheric pressure increases or decreases, the pressure in the static chamber changes accordingly. Again, this pressure change is transmitted through lines to the instruments which utilize static pressure as illustrated in figure 3-1.
Figure 3-1.—Pitot-static system with instruments.
An alternate source for static pressure is provided in some airplanes in the event the static ports become clogged. This source usually is vented to the pressure inside the cockpit. Because of the venturi effect of the flow of air over the cockpit, this alternate static pressure is usually lower than the pressure provided by the normal static air source. When the alternate static source is used, the following differences in the instrument indications usually occur: the altimeter will indicate higher than the actual altitude, the airspeed will indicate greater than the actual airspeed, and the vertical speed will indicate a climb while in level flight.