How a Pitot Tube Measures an Aircraft's Airspeed
A pitot tube compares the pressure of air rammed into its nose with the surrounding air pressure to find airspeed. See how it works and why blockages matter.
In this article
A pitot tube measures how fast an aircraft is moving through the air by comparing two pressures: the pressure of air that is forced to a stop inside the tube, and the pressure of the still air around the aircraft. The bigger the difference, the faster the aircraft is moving. This article explains the physics, the other instruments that share the system, and what happens when part of it is blocked.
Two pressures, one speed
NASA's Glenn Research Center explains that a pitot tube has a central opening that faces the direction of travel and small holes on its sides. The side holes sense static pressure, which comes from the random motion of the air. The central hole also picks up the pressure from the ordered motion of air flowing into it, so its reading is higher. This higher reading is called total, or stagnation, pressure.
Wikipedia gives the relationship as: stagnation pressure equals static pressure plus dynamic pressure. Dynamic pressure is the part that comes from motion. Subtract the static pressure from the total pressure and you are left with dynamic pressure, which depends on speed.
The formula
For air moving at modest speeds, Bernoulli's equation turns the pressure difference into a speed. NASA writes it as static pressure plus half the air density times speed squared, equal to total pressure. Solving for speed gives: speed equals the square root of twice the pressure difference divided by the air density.
Because speed depends on the square root of the difference, small pressure changes at low speed are hard to measure, and NASA notes the method is not accurate at low speeds. At supersonic speeds, shock waves break the assumptions behind Bernoulli's equation, so corrections are needed. In practice, an aircraft's instruments or air data computer apply such corrections.
A short history
Wikipedia records that Henri Pitot, a French engineer, invented the device while working on aqueducts, and published it in 1732. Henry Darcy modified it into its modern form in 1858. A pitot-static tube, sometimes called a Prandtl tube, combines both pressure measurements in a single probe.
The pitot-static system
On an aircraft the pitot tube is one part of a larger set. Wikipedia's article on the pitot-static system lists a pitot tube that measures ram air pressure, one or more static ports that measure static pressure, and the instruments connected to them. Air data computers, flight data recorders and cabin pressure controllers may also draw from the same sources.
- Airspeed indicator: uses the difference between pitot and static pressure.
- Altimeter: a sealed aneroid barometer vented to the static port.
- Machmeter: shows speed as a ratio of the speed of sound.
- Vertical speed indicator: a diaphragm vented through a calibrated leak, showing climb or descent.
Only the airspeed indicator, and the Machmeter, need the pitot tube. The altimeter and vertical speed indicator rely only on the static port. This split explains why different blockages produce different symptoms.
When something blocks the system
Wikipedia describes a blocked pitot tube as affecting only the airspeed indicator. If the drain hole is also blocked, the indicator shows a higher airspeed in a climb and a lower airspeed in a descent, even if true airspeed stays constant. Ice, water or insects can cause the blockage.
A blocked static port is described as more serious because it affects all of the pitot-static instruments. The altimeter freezes at the altitude where the blockage happened, the vertical speed indicator reads zero, and the airspeed indicator reads low in a climb and high in a descent. Wikipedia says many unpressurised aircraft have an alternate static source selectable from the cockpit.
Many pitot tubes carry a heating element to prevent icing, and the article says aircraft certificated for instrument flight must have a heated pitot tube, with an exception for experimental amateur-built aircraft.
Real incidents
Wikipedia's article on the pitot tube lists several accidents involving pitot-static failures. The French investigation authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean. For Birgenair Flight 301, the failure was suspected to involve an insect nest, probably from a mud dauber wasp. For Aeroperú Flight 603, a cleaning crew left the static port covered with tape.
Not only for aircraft
The same principle appears elsewhere. Wikipedia says industry uses pitot tubes to measure flow in ducts and pipes, helping to estimate the amount of air supplied to a space, and some weather stations use a modified form as an anemometer. The idea of measuring speed through pressure travels well, but on an aircraft it is tied to safety, so the probe and its heater are among the more closely watched parts of the system.
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