Pitot Tube 101: How Your Airspeed Indicator Works
Understanding the Pitot system could save your life. That’s not an exaggeration. Whether you're lining up for take-off or descending through cloud, knowing your true airspeed is critical. And your airspeed indicator relies entirely on one unassuming bit of kit: the Pitot tube.
In this blog, we’ll break down how the Pitot system works — what it measures, why it matters, and how it feeds information to your airspeed indicator. No complex jargon, just clear explanations that every student pilot needs to know. By the end, you’ll understand not just how the system functions, but also how to identify when something’s gone wrong.
Let’s start with the basics.
Pitot, Static, and Pitot-Static Tubes
Understanding how air data is gathered starts with paying attention to the different types of pressure inputs on an aircraft. Knowing the difference is especially important when diagnosing instrument failures or blockages. This image shows the three primary types of tubing configurations used in aircraft instrumentation:
Simple Pitot Tube. It is a hollow tube bent at a 90-degree angle, measures total pressure (ram air) and is used exclusively by the airspeed indicator. It’s typically mounted on the wing or fuselage, pointing directly into the airflow.
Static Source. Captures ambient atmospheric pressure. This feeds data to the altimeter, vertical speed indicator (VSI), and is one half of the calculation in the airspeed indicator.
Pitot–Static Tube. Combines both total and static pressure sensing in one unit. Common on more advanced or high-performance aircraft, this integrated tube reduces drag and simplifies installation.
Did you know there are 5 different types of altitudes in aviation? Altitude Basics: 5 Types Of Altitude Explained makes them easy to understand, helping you fly safer and more confidently.
What Is a Pitot-Static System?
The pitot-static system is one of the most vital components in any aircraft—it’s the unsung hero behind your airspeed, altitude, and climb/descent information. Simply put, it’s a pressure-based system that uses two sources of air: pitot (dynamic pressure) and static (ambient pressure) to power three primary instruments:
Airspeed Indicator (ASI) – compares dynamic and static pressure.
Altimeter – reads static pressure to determine altitude.
Vertical Speed Indicator (VSI) – measures the rate of change in static pressure.
By combining inputs from the pitot tube and static port, the system gives pilots real-time feedback about how fast they’re flying, how high they are, and whether they’re climbing or descending. It’s a beautifully simple setup with incredibly important consequences—especially when flying in IMC (Instrument Meteorological Conditions).
Understand the key differences between Visual Flight Rules and Instrument Flight Rules. Read our blog Beyond the Clouds: VFR vs. IFR , to learn more.
How Does a Pitot Tube Work?
At its core, the Pitot tube is a simple but genius device that uses air pressure to tell you how fast you’re flying. To understand it properly, we need to quickly touch on two important types of air pressure: dynamic pressure and static pressure.
Static pressure is the pressure of the air that’s just “sitting there” — the natural atmospheric pressure at your current altitude.
Dynamic pressure is the extra pressure created when your aircraft moves through the air and pushes it into the Pitot tube.
Struggling with static and dynamic pressure? Let Chris Keane, our aviation theory expert, guide you through the essentials in our latest video “Static and Dynamic Pressure”.
Bernoulli’s Principle tells us that as the speed of a fluid (like air) increases, its pressure decreases. In aviation terms: when you move through the air faster, dynamic pressure builds up. And we can measure it to figure out how fast you’re going.
While the Pitot tube measures dynamic pressure, the static port measures ambient (static) air pressure. Together, they make the airspeed indicator work.
So, how does the Pitot system use this? The Pitot tube collects the ram air pressure — air that’s being forcibly pushed into the tube because of the aircraft’s motion. The system then compares that ram air pressure to the static pressure gathered by the static port. The difference between these two gives you dynamic pressure, which the airspeed indicator uses to show your airspeed.
A simple equation for dynamic pressure:
q=1/2 × ρ × v²
Where: q = dynamic pressure, ρ (rho) = air density, v = velocity (your true airspeed)
In basic terms: as your speed increases, the dynamic pressure increases, and your airspeed indicator shows a higher speed.
Explore the Standard Atmosphere and its critical elements: air pressure, temperature, and density. Learn how these factors influence lift, aircraft performance, and flight safety. Find out more in ATPL Lessons with Chris Keane: Atmosphere Introduction.
Components of the Pitot-Static System
The pitot-static system is the hidden powerhouse behind three critical flight instruments: your airspeed indicator, altimeter, and vertical speed indicator (VSI). Here's a quick breakdown of the key parts:
Pitot Tube. This collects ram air (air forced in by the aircraft’s motion) and feeds it to the airspeed indicator. It measures dynamic pressure, essential for calculating airspeed.
Static Port. Mounted flush on the fuselage, this port samples undisturbed air pressure outside the aircraft. It’s connected to the airspeed indicator, altimeter, and VSI, providing each instrument with static (non-moving) air pressure.
Pressure Chamber. A chamber connected to the pitot tube where ram air pressure builds up before being directed to the airspeed indicator.
Drain Opening. Allows moisture, like rain or condensation, to escape the system—because water in the lines would mess up pressure readings.
Alternate Static Source. A backup static source located inside the cockpit. If the external static port gets blocked (say, by ice or debris), you can pull a lever and use cabin air pressure instead. Not perfect, but better than no reading at all!
Pitot Heater Switch. Activates heating elements around the pitot tube to prevent it from icing up in cold, moist conditions.
Flight Instruments Connected: Airspeed Indicator — uses both pitot (ram) pressure and static pressure. Altimeter — uses static pressure only. Vertical Speed Indicator (VSI) — uses static pressure only.
A blocked pitot tube or static port can cause critical errors in flight instrument readings. Recognizing and troubleshooting these pitot-static failures is a fundamental skill for all pilots. We will delve into this essential knowledge in the following section.
From air movement to cooling surfaces, the formation of fog varies greatly. Learn the science behind the 6 key types in our blog post, Fog Alert: 6 Types Every Pilot Should Know.
If the Pitot Tube Gets Blocked
Blockages can lead to inaccurate airspeed readings—one of the most dangerous forms of instrument error a pilot can face. There are two main types of pitot blockage scenarios:
1. Pitot Tube Blocked, Drain Hole Open
If the front of the pitot tube gets blocked but the drain hole remains open (see diagram), the pressure inside the system vents out.
Result: The Airspeed Indicator (ASI) will fall to zero. Why? Because it is now only sensing static pressure, just like the altimeter and VSI, and no longer registering any dynamic (ram) air pressure.
2. Pitot Tube Blocked, Drain Hole Blocked
If both the pitot tube and the drain hole are blocked (for example by ice, insects, or debris), the trapped air is sealed inside the system.
Result: The Airspeed Indicator will act frozen or show erroneous values depending on climbs and descents. Why? Because it holds onto the last pressure reading, and as altitude changes, it can cause false airspeed indications (e.g., airspeed increases in a climb or decreases in a descent, dangerous and misleading).
Quick Note:This is why pitot heat is critical in icing conditions. Always switch on pitot heat when flying through visible moisture below freezing temperatures.
How do TWR and Drag dictate your climb performance? Find out in our educational blog post, “It's All Connected: TWR, Drag, and the Climb Rates.”
If the Static Port Gets Blocked
If the static port becomes blocked, it can cause misleading readings not only on the Airspeed Indicator but also on the Altimeter and Vertical Speed Indicator.
Here’s what happens when the static port is blocked:
1. Airspeed Indicator (ASI) will still operate, but incorrectly. In a climb: It will indicate lower than actual airspeed. In a descent: It will indicate higher than actual airspeed. Why? Because without updated static pressure, the ASI is referencing incorrect pressure differences.
2. Altimeter will freeze at the altitude where the blockage occurred. No matter if you climb or descend, the reading won’t change, because it’s stuck sensing the last known static pressure.
3. Vertical Speed Indicator (VSI) will show zero, even if you are climbing or descending. VSI relies on changes in static pressure over time. If no new pressure data comes in, it shows no vertical movement.
Pilot Tip: Most aircraft are equipped with an Alternate Static Source inside the cockpit. If you suspect a static port blockage, switching to the alternate static source can restore (at least) partial instrument functionality. Always know where it is and how to use it.
Remember PUDSOD
When the static source is blocked, your instruments can mislead you — typically showing the opposite of what’s actually happening. PUDSOD is a simple mnemonic to help you remember how instrument errors behave when the static port gets blocked:
Mnemonic | Meaning |
Pitot | still working (only affected if blocked separately) |
Up (climb) | makes the altimeter read Down (lower altitude) |
Down (descent) | makes the altimeter read Up (higher altitude) |
Slow | indicated airspeed reads Slower than actual in a climb |
Overread | indicated airspeed reads Over (higher) in a descent |
Down | VSI shows zero (no vertical trend) |
Recognising Pitot-Static System Failures in Flight
Sometimes, you won’t get a flashing warning light screaming "Blocked Pitot!" — instead, you’ll need to spot the signs based on unusual instrument behaviour. Here’s what to watch for:
Signs of Pitot Tube Blockage:
Airspeed Indicator behaves oddly. Freezing at one speed regardless of pitch or power changes. Showing a climbing airspeed in a descent (or vice versa).
Altimeter and VSI normal. If only the ASI is acting up, it points towards a pitot tube issue.
Signs of Static Port Blockage:
Altimeter freezes, even though you’re climbing or descending.
VSI shows zero: No vertical movement indicated.
Airspeed Indicator errors:Reads too high in descent or too low in climb.
Beyond the Routine: Understanding Aviation Checklists covers the importance of checklists and how to use them effectively. Discover best practices for checklist management, learn how to avoid common errors, and understand why these procedures are vital for every flight.
5 Practical Actions to Take
A blocked pitot-static system is an inconvenience, not a crisis — if you know what to look for and how to fly by fundamentals, you’ll handle it like a pro. If you detect a pitot-static system anomaly mid-flight, stay calm and:
Cross-check your instruments: If only one instrument seems wrong, use the others to back it up.
Use your alternate static source (if equipped): Open it to restore altimeter and VSI indications.
Rely on attitude and power settings: Fly known pitch and power combinations that correspond to safe airspeeds.
Communicate: Inform ATC of your situation early. They can provide support and even vectoring if necessary.
Plan a safe, normal landing: No need for crazy manoeuvres — just a well-controlled approach based on attitude and power.
Airhead's Takeaway
Though it may look like just a small metal tube on the outside of the aircraft, the Pitot-Static System plays a massive role in keeping you informed and safe in the cockpit. It powers key instruments that tell you how fast you're going, how high you are, and whether you're climbing or descending. Misreading any of those could lead to serious trouble. That’s why it’s vital to understand how this system works, recognise its vulnerabilities, and always treat even the simplest-looking components with the respect they deserve. After all, in aviation, the little things often make the biggest difference.
