The Static System

The Static System Overview

  • The Pedo Static System connects to the airplane's critical flight instruments:

    • Altimeter

    • Airspeed Indicator

    • Vertical Speed Indicator

  • These instruments provide essential data regarding an airplane's altitude, speed, and rate of climb or descent, which are determined through atmospheric pressure measurement.

Understanding the Atmosphere

  • Column of Air: A vertical column of air extending from earth's surface into space reflects the density changes in air molecules with altitude.

    • Air molecules at ground level are more densely packed compared to those higher up due to gravity.

    • Air's weight results in pressure changes with altitude.

    • Greater atmospheric pressure exists at sea level than at high elevations, such as mountain tops.

  • Pressure Change Measurement:

    • Approximately 50% of the atmosphere's molecules reside within the first 18,000 feet above sea level.

    • As altitude increases, atmospheric pressure diminishes, enabling pitot static instruments to measure this pressure and calculate altitude and speed.

The Pitot Static System

  • The system derives its data from:

    • Pitot Tube: Measures dynamic air pressure as the airplane moves.

    • Usually mounted under a wing for direct airflow measurement.

    • May feature a pitot mast in some aircraft, but functionality remains the same.

    • Contains a drain hole on the rear to expel any accumulated moisture.

    • Heating capabilities to prevent ice blockages.

    • Static Port: Measures static air pressure devoid of dynamic airflow interference.

    • Placement varies by aircraft; for instance, on a Cessna 172, it is located on the left side of the fuselage.

    • Connection to Instruments:

    • All three instruments (altimeter, VSI, airspeed indicator) connect to the static port.

    • Only the airspeed indicator connects to the pitot tube.

Details of Pitot Static Instruments

Altimeter

  • Functionality: Displays the airplane's altitude based on static pressure readings.

    • Includes Aneroid Wafers:

    • Expand and contract according to air pressure variations.

    • Air inside the wafers remains trapped, while the external air adjusts with static pressure.

    • Altitude Readings:

    • Ascending: As altitude increases, static pressure decreases; air escapes through the back, causing aneroid wafers to expand until pressures equalize, reflecting (higher) altitude on the gauge.

    • Descending: Increases static pressure leads to air entering the instrument case, compressing the wafers to lower the altitude reading on the gauge.

    • Mechanical Linkages:

    • A system consisting of gears and levers transfers the movement of the wafers to the dial, indicating altitude.

  • Indicators:

    • Equipped with hands indicating different altitude scales:

    • 10,000 feet hand

    • 1,000 feet hand

    • 100 feet hand

  • Sensitive Altimeters: Called "sensitive" because they can be adjusted per current atmospheric pressure (altimeter setting).

    • Calibration Process:

    • Pilots adjust the instrument by rotating a dial to select the current pressure displayed as the altimeter setting in the Colesman window, recalibrating the instrument for accurate readings.

Vertical Speed Indicator (VSI)

  • Functionality: Indicates the vertical speed of the aircraft in feet per minute.

  • Operational Mechanism:

    • Measures current pressure differences against previous readings with a diaphragm inside.

    • The diaphragm connects directly to the static port, reflecting outside atmospheric pressure.

    • Calibrated Leak: A small opening controls the pressure change in connection with the diaphragm, resulting in a delay (lag) in the indication of vertical speed.

  • Example Scenario:

    • Cessna at 3,000 feet:

    • Diaphragm and case pressure equalizes initially indicating zero vertical speed.

    • Upon climbing, diaphragm pressure decreases faster than case pressure, creating a pressure differential; leads to the needle indicating a climb.

    • When leveling off, pressures equalize again and vertical speed returns to zero.

Airspeed Indicator

  • Functionality: Displays the speed of the airplane.

  • Mechanism:

    • Utilizes both pitot tube and static port connections.

    • Ram Pressure: The dynamic pressure from the pitot tube is utilized to assess airspeed.

    • Increased speed leads to heightened ram pressure sent to a diaphragm in the instrument.

  • Static Pressure Inclusion:

    • The static port fills the instrument case with static air, allowing the airspeed reading to account for altitude variations influencing static pressure—ensuring consistency across altitudes.

  • Speed Display: The instrument face shows color-coded ranges:

    • Green Arc: Indicates normal operational speeds.

    • White Arc: Represents speeds allowable for flap extension.

    • Yellow Arc: Denotes speeds confined to smooth air use only.

    • Red Line: Marks the maximum speed limit the aircraft should not exceed.