PITOT-STATIC FLIGHT INSTRUMENTS

There are two major parts of the pitot-static system: the impact pressure chamber and lines, and the static pressure chamber and lines. They provide 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 in locations that 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. 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 must be checked during the preflight inspection to assure that they are free from obstructions. A certificate mechanic should clean blocked or partially blocked openings. Blowing into these openings is not recommended because this could damage the instruments.

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.

An alternate source for static pressure is provided in some airplanes in the event that the static ports become blocked. 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.

Consult the Airplane Flight Manual or Pilot's Operating Handbook (AFM/POH) to determine the amount of error.

If the airplane is not equipped with an alternate static source, breaking the glass seal of the vertical speed indicator allows ambient air pressure to enter the static system. This makes the VSI unusable.

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