A Pitot-static tube is an instrument used to measure fluid flow velocity, most commonly the airspeed of aircraft. It consists of two concentric tubes: the inner tube faces directly into the airflow to capture total (stagnation) pressure, while the outer tube has small holes perpendicular to the flow to measure static pressure. The difference between these two pressures is the dynamic pressure (q), which is directly related to airspeed via Bernoulli's principle: v = √(2q/ρ), where ρ is the air density.

Bernoulli's principle states that for an incompressible, inviscid fluid flowing along a streamline, an increase in velocity corresponds to a decrease in pressure (and vice versa). For airspeed measurement, the principle is expressed as: P_total = P_static + ½ρv². The Pitot tube measures P_total at its forward-facing opening (where air comes to a complete stop, converting all kinetic energy to pressure), while the static ports measure P_static. The term ½ρv² is the dynamic pressure, representing the kinetic energy per unit volume of the moving air.

IAS (Indicated Airspeed) is the raw reading from the airspeed indicator, calculated assuming standard sea-level air density (ρ₀ = 1.225 kg/m³). CAS (Calibrated Airspeed) is IAS corrected for instrument and installation errors. TAS (True Airspeed) is the actual speed of the aircraft through the air, corrected for density variations due to altitude and temperature. The relationship is: TAS = IAS × √(ρ₀/ρ). At high altitudes where air is thinner, TAS is significantly higher than IAS for the same dynamic pressure.

Dynamic pressure is typically measured in Pascals (Pa), inches of water (inH₂O), or millimeters of water (mmH₂O). In aviation, 1 inH₂O ≈ 249.1 Pa. Airspeed is commonly expressed in knots (nautical miles per hour, 1 knot ≈ 0.5144 m/s), m/s, km/h, or mph. At sea level with standard density, an airspeed of 50 m/s corresponds to a dynamic pressure of approximately 1,531 Pa (about 6.15 inH₂O).

As altitude increases, air density decreases according to the International Standard Atmosphere (ISA) model. At 10,000 meters, air density is only about 34% of sea-level density (≈0.413 kg/m³ vs 1.225 kg/m³). This means that for the same true airspeed, the dynamic pressure (and thus indicated airspeed) is much lower at high altitude. Pilots must be aware that IAS under-represents TAS at altitude—a critical factor for navigation, stall margins, and flight envelope protection. This simulator uses the ISA model to calculate actual air density based on your altitude input.

A blocked Pitot tube is a serious aviation hazard. If the total pressure inlet is blocked (e.g., by ice, insects, or debris), the airspeed indicator may read zero or give erratic readings. If both the Pitot inlet and the drain hole are blocked, the trapped pressure can cause the airspeed indicator to behave like an altimeter—showing increasing speed during climb and decreasing speed during descent. This has contributed to several aviation accidents, including Air France Flight 447. Modern aircraft have Pitot tube heating systems to prevent ice blockage.

The Pitot tube was invented by French engineer Henri Pitot in 1732. His original design was a simple L-shaped glass tube used to measure the flow velocity of the River Seine. In 1858, Henry Darcy modified the design into the modern Pitot-static tube we use today, adding the static pressure ports. The instrument has since become fundamental to aviation, meteorology, and fluid dynamics research.