The Pitot Static System in Aviation

Overview

• The Pitot Static System is integral to aircraft instrumentation.
• Connected to three main instruments:
  • Altimeter
  • Airspeed Indicator
  • Vertical Speed Indicator
• Measures atmospheric pressure to determine altitude, airspeed, and rate of climb or descent.

Understanding Atmospheric Pressure

• Atmospheric pressure varies with altitude.
• Air molecules are denser at Earth's surface due to gravity.
• 50% of air molecules are within the first 18,000 feet above the surface.
• Pressure decreases with altitude, allowing pitot-static instruments to measure altitude and speed.

Components of the Pitot Static System

• Pitot Tube: Measures dynamic air pressure during flight.
  • Located typically under the wing.
  • Features a drain hole to prevent water ingress.
  • Can be heated to prevent ice formation.
• Static Port: Measures static air pressure.
  • Located variably depending on aircraft design (e.g., Cessna 172 has it on the left forward fuselage).

Functionality of Pitot Static Instruments

Altimeter

• Displays aircraft altitude.
• Key Components:
  • Aneroid wafers: Expand/contract with pressure changes.
  • Mechanical linkages: Convert wafer movement to altitude readout.
• Operation:
  • As altitude increases, static pressure decreases, causing wafers to expand.
  • Descending increases static pressure compressing wafers.
• Adjustment:
  • Uses a Kolsman window to adjust for current atmospheric pressure.

Vertical Speed Indicator (VSI)

• Measures rate of climb/descent in feet per minute.
• Key Components:
  • Diaphragm: Connects to static port, matches atmospheric pressure.
  • Calibrated leak: Slows pressure change in case.
• Operation:
  • Compares current with previous pressure to determine vertical speed.
  • Experiences a slight lag in reading changes.

Airspeed Indicator

• Uses both static port and pitot tube inputs.
• Key Components:
  • Diaphragm: Expands with increased ram pressure.
  • Static port: Fills surrounding case with static air to adjust readings.
• Operation:
  • Displays airspeed by measuring the difference between dynamic and static pressure.
• Speed Ranges:
  • Green arc: Normal operations.
  • White arc: Flaps can be extended.
  • Yellow arc: Limited to smooth air.
  • Red line: Maximum speed.

Conclusion

• Understanding the Pitot Static System and its components is crucial for accurate readings of altitude, airspeed, and vertical speed in aviation.