Pitot Tube

What a pitot tube actually does

A pitot tube is almost embarrassingly simple as a piece of hardware: a slender open-ended tube pointing straight into the oncoming air. There are no moving parts, no electronics inside the tube itself, no calibration that drifts. Yet it is the instrument an entire airliner trusts with the one number that keeps it flying — its airspeed. The trick is not in the tube but in what happens to air that runs head-first into a dead end.

When air flowing at speed V meets the open mouth of the tube, it has nowhere to go. The column of air inside the tube is already there, so the incoming flow is brought to a complete halt right at the entrance. That point where the velocity drops to zero is called the stagnation point, and the pressure there is the stagnation pressure (also called total or pitot pressure). All of the kinetic energy the moving air carried has been converted into pressure. The faster the flow, the higher the pressure piled up inside the tube.

That pressure alone is not airspeed, because it includes the ordinary atmospheric pressure of the day, which changes with altitude and weather. So a pitot tube is always paired with a static port — a hole that senses the undisturbed ambient pressure where the air slides past without being stopped. The airspeed instrument measures the difference between the two pressures. That difference is the dynamic pressure, and it depends only on how fast the air is moving and how dense it is. Bernoulli's equation converts it straight into a speed.

The physics — Bernoulli in one line

Along a streamline in steady, incompressible, frictionless flow, Bernoulli's equation says the total pressure is constant:

p + ½ρV² = p₀ = constant along a streamline

Here p is the static pressure, ρ is the air density, V is the local flow speed, and p₀ is the stagnation (total) pressure. At the mouth of the pitot tube the air is brought to rest, so V = 0 and the pressure there is the full p₀. At the static port the air is still moving at V, so the pressure there is just p. Subtract one from the other and the static-pressure terms cancel:

p₀ − p = ½ρV²        ← dynamic pressure, q

V = √( 2(p₀ − p) / ρ )

That is the whole instrument. The pressure difference the system measures, divided by density and square-rooted, is the speed. Note the square root: airspeed is not proportional to the pressure difference, it is proportional to its square root. Doubling the speed quadruples the dynamic pressure. This is why airspeed indicators have a cramped, bunched-up scale at the low end and a stretched-out scale at the high end.

Worked example — reading a number off the dial

Take an aircraft at sea level, where the standard air density is ρ = 1.225 kg/m³. Suppose the pitot-static system measures a pressure difference of 766 Pa between the stagnation and static lines.

Given:   p₀ − p = 766 Pa,   ρ = 1.225 kg/m³

V = √( 2 × 766 / 1.225 )
  = √( 1250.6 )
  = 35.4 m/s
  = 127 km/h
  = 68.7 knots

Reverse check — find q at 100 knots (51.4 m/s) at sea level:
q = ½ × 1.225 × 51.4²
  = ½ × 1.225 × 2642
  = 1619 Pa

So at sea level it takes only about 1.6 kilopascals of pressure difference — roughly 1.6% of atmospheric pressure — to register 100 knots. The pressures involved are tiny, which is why the sensing lines must be sealed perfectly and why a single pinhole leak or a smear of water in a drain line can throw the reading off badly.

The altitude trap — indicated versus true airspeed

Here is the single most consequential subtlety of the pitot tube, and the source of more confusion than any other: a pitot tube measures dynamic pressure, not speed. The instrument reads V correctly only if it uses the right density ρ. But the airspeed indicator is a sealed mechanical gauge built around a single fixed density — the sea-level standard 1.225 kg/m³. It cannot know how high you are.

At 35,000 feet the air density is roughly 0.38 kg/m³, less than a third of sea level. The same true airspeed produces less than a third of the dynamic pressure, so the gauge — assuming sea-level density — reads a much lower number. An airliner cruising at 480 knots true airspeed may show only 280 knots indicated.

Quantity	What it is	Depends on	Who uses it
Indicated airspeed (IAS)	Raw gauge reading from q at sea-level density	Dynamic pressure ½ρV²	Pilot — stall and structural limits live here
Calibrated airspeed (CAS)	IAS corrected for position and instrument error	IAS + airframe-specific correction	Performance charts
Equivalent airspeed (EAS)	CAS corrected for compressibility	CAS + Mach correction	Structural / aeroelastic limits
True airspeed (TAS)	Actual speed through the air mass	EAS × √(ρ₀/ρ)	Navigation, fuel planning

Pilots actually want the indicated number, not the true one. Because indicated airspeed tracks dynamic pressure, the speed at which a wing stalls is the same indicated number at sea level or at 40,000 feet — the wing only cares about the dynamic pressure pushing over it, which is exactly what the pitot tube senses. The maximum structural speed (V_NE, the red line) is likewise an indicated value. The air data computer separately combines static pressure and outside air temperature to compute true airspeed for navigation.

The static port — the unglamorous half that makes it work

Without a clean static-pressure reference, the pitot mouth measures nothing useful. The static port has to sample the ambient pressure at a location where the airframe neither speeds up nor slows down the local flow — because any acceleration of the air over the skin lowers the static pressure there (Bernoulli again) and biases the reading. That ideal spot is hard to find on a real airplane, which is why every airframe has a position error correction baked into its calibration.

Two arrangements are common. A combined pitot-static probe (the classic Prandtl tube) puts the total-pressure mouth at the tip and a ring of small static holes around the barrel a few diameters back, where the flow has reattached cleanly. Alternatively the static source is split off to flush static ports on the side of the fuselage, often paired left and right so that yaw or sideslip averages out. Transport aircraft duplicate everything: captain, first officer, and standby each get an independent pitot probe and static source, so a single blockage never removes all airspeed information at once.

Failure modes — how pitot systems lie

A pitot tube is an open hole pointed into the weather, and that openness is its weakness. The classic failures all stem from a blockage somewhere in the plumbing, and each one produces a distinct, diagnosable signature.

• Pitot mouth blocked, drain hole open. Ice or debris seals the mouth but the small drain at the bottom of the probe stays clear. The trapped pressure bleeds out through the drain to ambient, so the dynamic pressure collapses and indicated airspeed falls toward zero — even in level flight at cruise speed. A sudden, unexplained drop toward zero airspeed is the textbook symptom.
• Pitot mouth and drain both blocked. Now the trapped air is sealed. Its pressure stays constant, but the static side keeps tracking the changing atmosphere. The instrument behaves like an altimeter: indicated airspeed rises as the aircraft climbs (static pressure falls, so the difference grows) and falls as it descends — utterly disconnected from real speed.
• Static port blocked. The mirror image. With static pressure frozen at the value where the blockage occurred, the airspeed indicator over-reads in a descent and under-reads in a climb, and the altimeter freezes at the blockage altitude.
• Pitot icing. The dominant in-flight cause. Supercooled water freezes on the probe within seconds. The cure is an electrically heated element — 200 to 500 watts per probe — switched on before entering cloud. The 2009 Air France 447 loss began when the probes iced over and the three airspeed channels disagreed, dropping the autopilot and confusing the crew; it is the canonical case study in why redundancy and crew interpretation of disagreeing instruments matters.
• Covers and insects. On the ground, mud-dauber wasps famously build nests inside pitot tubes within hours, and removable protective covers are sometimes left in place. Both produce a dead airspeed indication on the takeoff roll — which is why airspeed alive at a sensible speed is a mandatory takeoff callout.

When Bernoulli is not enough — compressible and supersonic flow

The clean √(2q/ρ) relation assumes the air is incompressible, which is fine below roughly Mach 0.3 (about 100 m/s at sea level). Above that, the air piling up at the stagnation point compresses noticeably and heats up, and the simple formula starts to over-read. Subsonic high-speed systems therefore use the compressible form of Bernoulli, which adds Mach-number terms.

Above Mach 1 the situation changes qualitatively. A detached bow shock forms ahead of the pitot mouth, and the air must cross that shock — an abrupt, irreversible jump in pressure, density, and temperature — before it stagnates. The relation between the stagnation pressure measured behind the shock and the conditions ahead of it is the Rayleigh pitot formula. Supersonic air data systems are calibrated with it so a single probe can read sensibly from low subsonic through transonic and into supersonic flight, as on fighters and the retired Concorde.

Beyond aircraft — where else the pitot principle shows up

• Formula 1 and motorsport. The thin probe sticking up ahead of an F1 nose is a pitot tube feeding the car's speed and the engine's air-mass logic. Wind tunnels are wall-to-wall pitot rakes.
• Industrial flow measurement. An averaging pitot tube (Annubar) spans a pipe diameter with several sensing holes to read the average velocity of gas or liquid, giving a low-pressure-drop alternative to an orifice plate.
• HVAC and ventilation testing. A handheld pitot tube traverses a duct to map the velocity profile and compute volumetric airflow during commissioning.
• Hydraulics and marine craft. A pitot log on a boat hull, or a pitot gauge on a fire-hose nozzle, applies the identical stagnation-minus-static idea to water.
• Five-hole and multi-hole probes. Adding angled holes around the tip lets the probe resolve flow direction as well as speed — used to measure angle of attack and sideslip, and ubiquitous in turbomachinery research.

Pitot tube versus the alternatives

The pitot tube is one of several ways to turn a flow into a number. Each trades accuracy, pressure loss, cost, and robustness differently.

Property	Pitot / pitot-static	Orifice plate	Venturi tube	Hot-wire anemometer
Measures	Point velocity (q = ½ρV²)	Volumetric flow (Δp across plate)	Volumetric flow (Δp at throat)	Point velocity (heat loss)
Governing relation	Bernoulli, √(2q/ρ)	Bernoulli + discharge coeff.	Bernoulli + discharge coeff.	King's law (convective cooling)
Permanent pressure loss	Very low	High (40–80% of Δp)	Low (10–20% of Δp)	Negligible
Moving parts	None	None	None	None (fragile wire)
Frequency response	Slow (line + cavity lag)	Slow	Slow	Very fast (turbulence)
Typical use	Aircraft airspeed, F1, ducts	Process pipe metering	Pipe metering where loss matters	Lab turbulence research
Main weakness	Blockage, icing, position error	Large unrecoverable loss	Bulky, costly casting	Delicate, drift, needs calibration

Common pitfalls when using pitot data

• Forgetting the density correction. The gauge reads as if density were sea-level standard. Treating indicated airspeed as true airspeed at altitude under-reads true speed by 50% or more. Always convert through the air data computer or an E6B before navigating.
• Misaligning the probe. Pitot tubes are sensitive to angle of attack. A few degrees of misalignment is tolerable, but beyond about 15–20° the captured pressure falls off and the reading drops — which is exactly why high-alpha aircraft use self-aligning or multi-hole probes.
• Ignoring the line lag. The pneumatic tubing between probe and gauge is a low-pass filter. Rapid manoeuvres or gusts arrive at the instrument blurred and delayed; never use raw pitot pressure for fast control loops without accounting for it.
• Bad static placement. Putting the static port where the airframe accelerates the local flow gives a chronic position error that grows with speed. This is why every airframe needs an individual calibration curve, not a generic one.
• Skipping the heater check. An unheated or failed-heater probe will ice in seconds in visible moisture below freezing. Pitot heat on, and confirmed drawing current, is a standard pre-flight and pre-cloud item.