A pitot tube is a small tube pointed forward into the airflow. Air rams into its open end and stops, so the pressure inside it is higher than the pressure of the still air around the aircraft. A static port, a small hole flush with the side of the fuselage, senses that surrounding pressure. The airspeed indicator shows the difference between the two, converted to speed.
I learned the system on an old Navion at our Air Force technical school, which I wrote about in a note on its cockpit. This one works the physics through with numbers.
The equation: total pressure minus static pressure
Bernoulli's equation says that along a streamline, static pressure plus dynamic pressure stays constant. When air is brought to rest inside the tube, all of its dynamic pressure turns into extra static pressure. So the tube reads the total pressure: p_total = p_static + ½ρv².
Subtract the static port's reading and what is left is the dynamic pressure, q = p_total − p_static = ½ρv². Solve for speed: v = √(2q / ρ), where ρ is the air density.
The instrument does not compute this with a processor. In a mechanical airspeed indicator, the pitot pressure inflates a thin diaphragm inside a case filled with static pressure, and the dial is marked so that each deflection reads as the speed this equation gives.
A worked example at sea level
Take standard sea-level air, ρ = 1.225 kg/m³, and a measured difference of q = 1,000 Pa. Then v = √(2 × 1,000 / 1.225) = √1,632.7 = 40.4 m/s. That is 145.5 km/h, or 78.5 knots.
For scale, 1,000 Pa is about one hundredth of atmospheric pressure, the weight of a column of water 102 mm tall. The whole airspeed range of a small aircraft lives in a few thousand pascals on top of roughly 101,000.
The square law
Because q grows with the square of speed, the same instrument at 250 Pa reads 20.2 m/s, 39.3 knots, and at 4,000 Pa reads 80.8 m/s, 157.1 knots. Four times the pressure gives twice the speed.
This is why low speeds are the hardest to read. Near the bottom of the scale, a large change in speed is a tiny change in pressure, so small leaks and small errors matter most exactly where an aircraft is slow, close to the ground.
Why it reads low at altitude
The dial is marked assuming sea-level density. At 3,000 m the standard atmosphere has ρ = 0.909 kg/m³. The same 1,000 Pa now means v = √(2 × 1,000 / 0.909) = 46.9 m/s, or 91.2 knots. The needle still shows 78.5.
The needle shows indicated airspeed. The speed through the air is the true airspeed, here about 16 percent higher. Pilots fly by the indicated value on purpose: the wing's lift also depends on ½ρv², so the needle reports the thing that keeps the aircraft flying, even when it misreports the speed.
One more limit: the equation above treats air as incompressible. Above about Mach 0.3, roughly 100 m/s at sea level, the air compresses noticeably inside the tube and the instrument needs a corrected formula.
How a blocked tube fails
The system fails quietly, because a blocked line does not stop the needle. It changes what the needle measures.
If ice or an insect blocks the pitot inlet and its drain hole too, the air already inside is trapped. The instrument now compares a fixed pressure against the static port. In a climb, static pressure falls, the difference grows, and the needle shows the aircraft speeding up while it may be slowing down. It has become an altimeter.
If the static port is blocked instead, the trapped static pressure stays at the old altitude. In a climb the difference shrinks and the needle reads low; in a descent it reads high. The altimeter freezes at the same time, because it reads the same port.
That is why pitot tubes on many aircraft are heated, why covers go on them on the ground, and why checking the tube and the port is part of walking around an aircraft before flight. The numbers on the dial are only as good as two small holes.