A “pilot tube” typically refers to one of two things: a Pitot tube (pronounced “pee-TOH”), which is an instrument that measures the speed of air or fluid flow, or a pilot tube in a gas furnace, which is the small metal tube that delivers gas to the pilot light. Both are common searches, so here’s what you need to know about each.
The Pitot Tube: Measuring Air and Fluid Speed
A Pitot tube is a slender, open-ended tube that measures how fast air or another fluid is moving. It was invented by Henri Pitot in the early 1700s and later refined by Henry Darcy in the mid-1800s. You’ll find Pitot tubes on nearly every commercial aircraft, in wind tunnels, and inside HVAC ductwork. The core idea is simple: the device converts the energy of moving air into a pressure reading, then uses that pressure reading to calculate speed.
When air flows into the forward-facing opening of the tube, it slows to a stop right at the entrance. That “stopped” air creates what’s called stagnation pressure, which is higher than the pressure of the air moving freely around the tube. The difference between these two pressures tells you how fast the air was moving before it was stopped. The greater the difference, the faster the flow.
How a Pitot Tube Is Built
Most Pitot tubes used in practice are actually Pitot-static tubes, meaning they measure two pressures at once. According to NASA’s Glenn Research Center, the design has a center hole drilled down the axis of the tube that points directly into the oncoming airflow. Several smaller holes are drilled around the outside of the tube, perpendicular to the flow direction.
The center hole captures total pressure (the combination of the air’s movement energy and its baseline atmospheric pressure). The outer holes, because they sit sideways to the flow, only pick up the baseline atmospheric pressure, called static pressure. Each set of holes connects to opposite sides of a pressure transducer, a sensor that measures the difference between the two pressures. That difference is all you need to calculate velocity using Bernoulli’s principle: velocity equals the square root of twice the pressure difference divided by air density.
Pitot Tubes in Aviation
On aircraft, the Pitot tube is the primary instrument behind the airspeed indicator on the cockpit dashboard. The tube is mounted on the fuselage or wing, always pointing forward into the oncoming air. As the plane speeds up, more air rams into the center hole, raising the total pressure. The transducer compares this to the static pressure from the outer ports and feeds the result to the airspeed gauge.
The reading you see on the dial is called indicated airspeed. It’s not identical to your actual speed over the ground because air density changes with altitude and temperature. At higher altitudes, where the air is thinner, the same true airspeed produces a lower pressure difference. Pilots and onboard computers account for this with corrections, but the raw Pitot tube measurement remains the starting point for all of them.
Because Pitot tubes are so critical to safe flight, aircraft typically carry multiple units as backups. Blockages from ice, insects, or debris can produce dangerously inaccurate readings, which is why most planes have heated Pitot tubes that prevent ice from forming over the openings.
Pitot Tubes in HVAC and Industry
Outside of aviation, Pitot tubes are a standard tool for measuring airflow inside ducts. HVAC technicians pair a Pitot tube with a digital manometer to check whether ventilation fans, kitchen range hoods, and dryer vents are moving enough air. The Building America Solution Center notes that this setup works accurately in any duct 4 inches or larger in diameter.
This matters for home safety, not just comfort. Measuring dryer duct airflow, for example, helps determine whether a combustion appliance might backdraft, pulling exhaust gases back into the house instead of venting them outside. ASHRAE Standard 62.2, the main ventilation guideline for homes, requires measurement of ventilation fans, making the Pitot tube a practical, everyday tool for building inspectors and HVAC professionals.
The Furnace Pilot Tube
If you searched “pilot tube” while troubleshooting a gas furnace or water heater, you’re likely looking at something completely different. A furnace pilot tube is a thin copper or aluminum line that carries a small amount of gas from the combination gas valve to the pilot light assembly. When you press the button on a gas valve to light the pilot, gas flows through this tube and exits at the pilot termination head, where it ignites.
Once lit, the pilot flame heats a thermocouple, a safety sensor that tells the gas valve, “The flame is on, keep the gas flowing.” If the thermocouple doesn’t detect heat, a spring-loaded plunger inside the valve pops up and shuts off gas to both the pilot tube and the main burner. This prevents unburned gas from filling your home.
Common Pilot Tube Problems
A weak or absent pilot flame is one of the most frequent furnace complaints, and the pilot tube itself is often the cause. Over time, dust, rust flakes, or spider webs can partially clog the narrow tube, starving the flame of fuel. A kinked tube creates the same problem. In either case, the flame may be too small to properly heat the thermocouple, causing the valve to shut off and the pilot to go out repeatedly.
Fixing a clogged pilot tube is straightforward. You disconnect the tube from the pilot termination head and blow compressed air through it to clear the obstruction. If the tube is kinked, it usually needs to be replaced. These are inexpensive parts, and the repair is one of the more common DIY furnace fixes, though working with gas lines does carry risk if you’re unfamiliar with the process.