Pitot-Static Pressure Measurement: Static, Total, and Dynamic Pressure

The measurement of pressure is fundamental to flight instrumentation. The Pitot-Static System provides the raw pressure data needed to determine airspeed, altitude, and vertical speed.

Principles of Pressure

• Static Pressure ($P_s$): The atmospheric pressure surrounding the aircraft. It acts on all surfaces and decreases with altitude.
• Total (Pitot) Pressure ($P_t$): The sum of static pressure and dynamic pressure created by the aircraft's forward motion.
• Dynamic Pressure ($q$): The pressure determined by airflow velocity and density ($q = 1/2 \rho V^2$).
  • Relationship: $P_t = P_s + q$

System Components

1. Static Ports: Vents located on the fuselage (often paired to cancel sideslip errors) where airflow is undisturbed. They supply the Altimeter, VSI, and ASI.
2. Pitot Tube: An open-ended tube facing the relative airflow. It measures Total Pressure and supplies ONLY the ASI (and Machmeter/ADC).

Blockages and Errors

• Static Blockage:
  • Altimeter: Freezes at the altitude where blockage occurred.
  • VSI: Indicates zero.
  • ASI: Behave like an altimeter in reverse (Under-reads in Climb / Over-reads in Descent).
  • cockpit static vent: If used as an alternate source, the lower pressure in the cockpit (due to aerodynamic suction) causes the Altimeter and Airspeed to over-read and the VSI to indicate a momentary climb.
• Pitot Blockage:
  • ASI: Acts like an altimeter (Over-reads in Climb / Under-reads in Descent).