How the Airspeed Indicator Works: Understanding Its Role in the Pitot-Static System
- wifiCFI
- Dec 22, 2025
- 4 min read
The airspeed indicator (ASI) is one of the most critical instruments in an airplane cockpit. It tells pilots how fast the aircraft is moving through the air—information essential for safe takeoff, climb, cruise, approach, and landing. Unlike groundspeed, which depends on wind, airspeed reflects the airplane’s true aerodynamic performance.
The airspeed indicator works by using the pitot-static system, a pressure-based system that compares two different air pressures. Understanding how the ASI functions—and how it can fail—helps pilots recognize abnormal indications and maintain control when conditions aren’t ideal.
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The Pitot-Static System: A Quick Review
The pitot-static system supplies pressure information to three primary flight instruments:
Airspeed Indicator
Altimeter
Vertical Speed Indicator (VSI)
Of these, the airspeed indicator is unique because it uses both pitot pressure and static pressure. The altimeter and VSI rely only on static pressure.
Pitot Pressure vs. Static Pressure
To understand the airspeed indicator, it’s important to distinguish between the two pressure sources it uses.
Pitot Pressure (Total Pressure)
Collected by the pitot tube, which faces directly into the relative wind
Increases as airspeed increases
Represents the combination of static pressure plus the pressure caused by the airplane’s forward motion
Static Pressure (Ambient Pressure)
Collected through static ports on the side of the fuselage
Represents the surrounding atmospheric pressure
Decreases with altitude and changes with weather conditions
How the Airspeed Indicator Works
Inside the airspeed indicator is a flexible diaphragm connected to a mechanical linkage and display needle.
Here’s what happens step by step:
Pitot pressure is routed directly into the diaphragm
Static pressure fills the instrument case surrounding the diaphragm
The diaphragm expands or contracts based on the difference between pitot and static pressure
This pressure difference—called dynamic pressure—moves the needle
The needle displays airspeed, calibrated in knots or miles per hour
The greater the difference between pitot and static pressure, the higher the indicated airspeed.
Dynamic Pressure: The Key to Airspeed
Dynamic pressure is what truly drives the airspeed indicator.
At zero airspeed, pitot and static pressure are equal
As the airplane accelerates, pitot pressure increases
Static pressure remains relatively constant (at a given altitude)
The increasing difference causes the ASI needle to rise
This is why the airspeed indicator responds immediately to acceleration and deceleration.
Types of Airspeed Indications
The airspeed indicator does not show a single “speed”—it displays different values depending on corrections applied.
Indicated Airspeed (IAS)
The raw reading from the airspeed indicator
Used for almost all flight operations and limitations
Referenced for stall speeds, maneuvering speed, and flap limits
Calibrated Airspeed (CAS)
IAS corrected for instrument and position errors
Found in the aircraft’s performance charts
True Airspeed (TAS)
CAS corrected for altitude and temperature
Increases with altitude for the same IAS
While pilots primarily fly using IAS, understanding these distinctions is important for performance planning.
Color Coding on the Airspeed Indicator
Most airspeed indicators include standardized color arcs:
White arc – Flap operating range
Green arc – Normal operating range
Yellow arc – Caution range (smooth air only)
Red line – Never-exceed speed (Vne)
These markings are based on indicated airspeed, not groundspeed or true airspeed.
Common Airspeed Indicator Errors and Failures
Blocked Pitot Tube
If the pitot tube becomes blocked:
Airspeed may read zero
Or behave like an altimeter if the drain hole is also blocked
This can occur due to:
Ice
Insects
Debris
Pitot cover left installed
Blocked Static Port
If the static port is blocked:
Airspeed readings become unreliable
Altimeter and VSI are also affected
Many airplanes include an alternate static source to mitigate this risk.
Pitot Heat and Pilot Responsibilities
To protect the pitot tube from icing:
Aircraft are equipped with pitot heat
It should be used in visible moisture or cold conditions
Pilots must:
Check pitot tube condition during preflight
Verify pitot heat operation when required
Cross-check airspeed with attitude and power settings
Airspeed Indicator in Modern Aircraft
In glass cockpits:
Pitot and static pressures are sensed electronically
Data is processed by air data computers
The display may be digital, but the principles remain unchanged
A blocked pitot or static source still produces dangerous errors—even with advanced avionics.
Why Understanding the ASI Matters
The airspeed indicator directly affects:
Stall prevention
Structural protection
Takeoff and landing safety
Aircraft performance management
Misinterpreting airspeed or trusting a faulty indication can quickly lead to unsafe flight conditions, especially in IMC.
Conclusion
The airspeed indicator is a simple but powerful instrument that translates pressure differences into one of the most important numbers in aviation. By comparing pitot pressure and static pressure, it gives pilots real-time feedback on how the airplane is flying through the air.
Understanding how the ASI works—and how it can fail—allows pilots to:
Recognize abnormal indications
Cross-check instruments intelligently
Maintain safe control when systems malfunction
In aviation, airspeed isn’t just a number—it’s a cornerstone of safe flight, and the pitot-static system makes it possible.
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