A pitot tube is a simple instrument that measures the speed of a fluid, most commonly air, by comparing two types of pressure. You’ll find one on nearly every aircraft in the world, where it provides the data that drives the airspeed indicator in the cockpit. The device is small (about 10 inches long and half an inch in diameter on a typical airplane) but essential: without it, a pilot has no reliable way to know how fast the plane is moving through the air.
How a Pitot Tube Works
The core idea behind a pitot tube comes from Bernoulli’s principle, which describes the relationship between a fluid’s speed and its pressure. When air flows faster, its pressure drops. When air slows down or stops, its pressure rises. A pitot tube exploits this by capturing air at two different conditions and measuring the gap between them.
A center hole at the front of the tube points directly into the oncoming airflow. Air rams straight into this opening and comes to a stop, creating what’s called total pressure (sometimes called stagnation pressure or ram air pressure). This total pressure reflects both the ambient air pressure and the extra pressure created by the aircraft’s forward motion.
Several smaller holes are drilled around the outside of the tube, perpendicular to the direction of travel. Because these holes sit at right angles to the airflow, they only pick up the ambient air pressure, known as static pressure. This is the baseline pressure of the surrounding atmosphere with no motion factored in.
Inside the tube, these two pressure readings are kept completely separate and fed to a device called a pressure transducer. The transducer measures the difference between total pressure and static pressure using a strain gauge on a thin sensing element. That pressure difference is directly related to airspeed: the faster the aircraft moves, the more air rams into the center hole, and the larger the gap between total and static pressure becomes.
The Math Behind It
Once you have the pressure difference, calculating velocity is straightforward. Bernoulli’s equation says that static pressure plus half the air density times velocity squared equals total pressure. Rearranging that, velocity squared equals twice the pressure difference divided by air density. So the instrument needs two inputs: the pressure difference (measured directly by the transducer) and the local air density (determined from temperature and pressure readings). Plug those in, and you get airspeed.
This is why airspeed readings can shift with altitude and temperature even at the same true speed. Air density changes as you climb, and the instrument’s raw pressure reading has to be corrected to reflect actual conditions.
What It Powers in the Cockpit
The pitot tube is part of a broader system called the pitot-static system, which feeds data to three critical flight instruments. The airspeed indicator is the only one that uses both total and static pressure, comparing them to display how fast the aircraft is moving through the air. The altimeter uses static pressure alone to determine altitude, since atmospheric pressure drops predictably with height. The vertical speed indicator also relies on static pressure, measuring how quickly that pressure changes to show whether the aircraft is climbing, descending, or flying level, displayed in feet per minute.
The static ports that supply ambient pressure to the altimeter and vertical speed indicator are sometimes built into the pitot tube itself, sometimes mounted separately on the fuselage. Either way, they’re part of the same interconnected system, and a problem with any component can cascade into misleading readings across multiple instruments.
Uses Beyond Aviation
Pitot tubes aren’t exclusive to aircraft. They’re a low-cost, reliable way to measure airflow in HVAC and ventilation systems, where engineers use them to check air velocity inside ducts. Mining operations use them to monitor ventilating-duct airflows underground. Wind tunnels rely on pitot tubes to calibrate and measure airflow over scale models and test sections.
In industrial piping, engineers can insert a pitot tube directly into an existing pipe to measure fluid velocity at a specific point within the cross-section. The tubes can be made very small relative to the pipe diameter, so they barely disturb the flow they’re measuring. In practice, pitot tubes in industrial settings are often used for preliminary flow-rate testing to help specify what permanent flow-measuring equipment a pipeline needs, rather than serving as the long-term monitoring solution themselves.
Why Pitot Tubes Fail
The biggest threat to a pitot tube in flight is ice. Under certain conditions, ice can form inside the tube and trap whatever total pressure was present at that moment. Two common causes are an inoperative heating system and high liquid water content in the atmosphere, where the heater can’t evaporate water fast enough before it enters the tube.
The consequences are deceptive rather than obvious. If ice blocks the total pressure port while the aircraft stays at the same altitude, the airspeed indicator may freeze in place and show no changes regardless of what the aircraft actually does. If the pilot climbs, the indicated airspeed will falsely increase because the trapped pressure inside the tube becomes larger relative to the dropping static pressure outside. If the pilot descends, the opposite happens: indicated airspeed drops even if the plane is actually speeding up. These phantom readings can lead to dangerous decisions.
In one documented incident, an aircraft climbing through 16,000 feet at 305 knots saw its airspeed and vertical speed indicators suddenly increase with no change in engine power. In another case, a blocked probe made the airspeed read erroneously low during descent, which caused the fly-by-wire flight control system to apply gains that were far too aggressive for the plane’s actual speed. Snow and ice buildup in pitot-static ports has also caused erroneous airspeed readings during attempted takeoffs on the ground.
Heating and Maintenance Requirements
To prevent icing, most aircraft pitot tubes include built-in electric heating elements. U.S. federal aviation regulations require that if a pitot heating system is installed, the cockpit must have an amber warning light in clear view of the flight crew. That light must activate in two situations: when the heating system is switched off, and when it’s switched on but any heating element has failed. This ensures the crew always knows whether the pitot tube is protected against ice.
Preflight inspections include checking the pitot tube and static ports visually. Insect nests, dirt, and even protective covers accidentally left in place have all caused blockages. The instrument is simple, but its placement on the outside of the aircraft exposes it to every environmental hazard the plane encounters.