The Airspeed Indicator: V-Speeds, the Color Arcs, Why You Fly IAS, and Blockage Failures
Updated: Sep 12
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. But there's real depth to the ASI beyond "it shows your speed": the color arcs are bounded by specific V-speeds that every pilot must know, one critical speed (maneuvering speed) deliberately ISN'T marked on it, and there's a profound reason all the markings are in indicated airspeed rather than true airspeed. Understanding how the ASI works, what its markings mean, and how it fails helps pilots recognize abnormal indications and maintain control.
This post covers the airspeed indicator in practical depth: how it derives airspeed from dynamic pressure, the color arcs and the specific V-speeds at their boundaries, why maneuvering speed isn't marked, the deep reason you fly indicated airspeed, IAS/CAS/TAS, groundspeed vs. airspeed, and the blockage failures.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
How the Airspeed Indicator Works
The ASI is the only flight instrument that uses BOTH pitot and static pressure, comparing them to derive airspeed.
The mechanism:
Pitot (total) pressure is routed into a flexible DIAPHRAGM inside the instrument
Static (ambient) pressure fills the instrument CASE around the diaphragm
The diaphragm expands/contracts based on the DIFFERENCE (pitot − static)
This difference is DYNAMIC PRESSURE
Mechanical linkage translates it to the needle
Dynamic pressure — the key:
Dynamic pressure = pitot (total) − static
At zero airspeed: pitot = static, so dynamic pressure = zero (needle at zero)
As airspeed increases: pitot rises, static stays constant, dynamic pressure grows
The needle rises with dynamic pressure
The driving force
The immediate response:
The ASI responds immediately to acceleration/deceleration (dynamic pressure changes fast)
Unlike the VSI (which lags)
Real-time speed feedback
Responsive
(For the full pitot-static plumbing and the other instruments, see our pitot-static system post.)
The Color Arcs and Their V-Speed Boundaries
Here's the depth the basic treatment gestures at: each color arc is bounded by SPECIFIC V-speeds every pilot must know.
The white arc (flap operating range):
The bottom of the white arc: Vs0 — the stall speed in the LANDING configuration (full flaps, gear down)
The top of the white arc: Vfe — the maximum flap extended speed
So the white arc spans Vs0 to Vfe
The flap range
Vs0 (bottom of white arc):
Stall speed, landing configuration (full flaps)
The lowest speed you can fly with full flaps
The clean-configuration stall (Vs1) is higher
Landing-config stall
Vfe (top of white arc):
Maximum flap extended speed
Don't extend/fly with flaps above this
Flap structural limit
The flap ceiling
The green arc (normal operating range):
The bottom of the green arc: Vs1 — the stall speed in a specified (usually clean) configuration
The top of the green arc: Vno — the maximum structural cruising speed
So the green arc spans Vs1 to Vno
The normal range
Vs1 (bottom of green arc):
Stall speed, clean configuration (flaps/gear up)
Higher than Vs0 (clean wing stalls faster)
The clean stall
Normal-config stall
Vno (top of green arc / bottom of yellow):
Maximum structural cruising speed
Below Vno: safe in normal turbulence
Above Vno: caution (smooth air only)
The normal-ops ceiling
The yellow arc (caution range):
From Vno (bottom) to Vne (top)
Fly here ONLY in smooth air
Turbulence could cause structural damage in this range
The caution zone
The red line (Vne):
Vne — never-exceed speed
Never exceed under any circumstances
Structural failure/flutter risk beyond it
The absolute limit
The arcs summarized:
Arc/Line | From | To | Meaning |
White | Vs0 | Vfe | Flap operating range |
Green | Vs1 | Vno | Normal operating range |
Yellow | Vno | Vne | Caution (smooth air only) |
Red line | — | Vne | Never exceed |
All in IAS:
These markings are based on INDICATED airspeed (not groundspeed or true airspeed)
Read them directly off the ASI
The reason why is below
Indicated
Why Maneuvering Speed (Va) Isn't Marked
A tested detail the basic treatment doesn't mention: one critical speed is deliberately absent from the ASI.
Va is not on the ASI:
Maneuvering speed (Va) is NOT marked on the airspeed indicator
No arc or line for it
You must know it from the POH (or a placard)
Deliberately absent
Why Va isn't marked:
Va CHANGES WITH WEIGHT (it's lower at lighter weights)
A single mark on the ASI would be wrong at different weights
So it can't be a fixed marking
Weight-dependent
The weight dependence:
At max gross weight, Va is highest
At lighter weights, Va is lower (the square-root relationship)
A fixed ASI mark would only be right at one weight
Hence no marking
Where to find Va:
The POH (often at multiple weights)
Sometimes a placard in the cockpit
You calculate/know it for your weight
Not on the ASI
Other unmarked speeds:
Va (maneuvering speed) — weight-dependent
Vx (best angle of climb), Vy (best rate of climb) — from the POH
Best glide (Vg) — from the POH
These aren't ASI markings (they're POH speeds)
(For maneuvering speed in depth — the weight adjustment and why it protects against a single input — see our maneuvering speed post.)
Why You Fly Indicated Airspeed: The Deep Reason
A profound concept the basic treatment states but doesn't fully explain: WHY the color arcs and limits are in IAS.
The dynamic pressure insight:
The ASI measures DYNAMIC PRESSURE (pitot − static)
Dynamic pressure is what the WING feels (the aerodynamic force)
The wing's behavior (lift, stall) depends on dynamic pressure, not true speed
So IAS (which reflects dynamic pressure) reflects the aerodynamic force
Why stall speed is constant in IAS:
A wing stalls at a certain dynamic pressure (angle of attack aside)
The ASI shows dynamic pressure (as IAS)
So the stall speed is the SAME indicated airspeed at any altitude
Even though the TRUE airspeed at stall is higher at altitude
IAS captures the aerodynamics
The altitude example:
At sea level, the aircraft stalls at, say, 50 KIAS (and 50 KTAS)
At 10,000 feet, it still stalls at 50 KIAS — but the TRUE airspeed is higher (maybe 58 KTAS)
The INDICATED stall speed is constant (50 KIAS)
Because the dynamic pressure at stall is the same
Why this matters:
All the V-speeds (stall, Vfe, Vno, Vne) are constant in IAS across altitudes
So the ASI markings work at any altitude (read IAS directly)
You fly IAS because it reflects the aerodynamic force (the wing's reality)
IAS is the aerodynamically meaningful speed
The unifying point:
The wing "cares about" dynamic pressure (IAS), not true speed
So limits and stall speeds are in IAS (constant, readable)
This is WHY you fly indicated airspeed
The deep reason behind the markings
The Airspeed Types: IAS, CAS, EAS, TAS
The airspeed types, from the ASI's perspective.
Indicated Airspeed (IAS):
The raw reading from the ASI
What you fly by (V-speeds, limits, color arcs)
Reflects dynamic pressure (the aerodynamic force)
The primary flight reference
Calibrated Airspeed (CAS):
IAS corrected for INSTRUMENT and POSITION error
Position error: the static port doesn't perfectly sense ambient pressure (varies with angle of attack/configuration)
The POH airspeed calibration table provides the correction
Refined IAS
The position error detail:
Greatest at high angle of attack (slow flight)
The airflow over the static port changes with the aircraft's attitude
The POH table corrects IAS to CAS
A small but real error
Equivalent Airspeed (EAS):
CAS corrected for compressibility (high speed)
Matters for jets/high speed
For GA, CAS ≈ EAS
A high-speed correction
True Airspeed (TAS):
The actual speed through the air mass
CAS corrected for air density (altitude and temperature)
Increases with altitude for a given IAS
Rule of thumb: TAS ≈ IAS + 2% per 1,000 feet
The real speed
Computing TAS:
Use an E6B (manual or electronic) or a flight computer
Inputs: CAS, pressure altitude, temperature
Output: TAS
Or the rule of thumb (2%/1,000 ft)
Why TAS matters:
For navigation (groundspeed = TAS ± wind)
Performance planning (cruise speed, range)
Not for flying the airplane (that's IAS)
Navigation and planning
Groundspeed vs. Airspeed
A distinction worth clarifying.
Airspeed:
Speed through the AIR MASS
What the ASI shows (IAS; TAS is the true version)
Reflects aerodynamic performance
Independent of wind
Groundspeed:
Speed over the GROUND
TAS adjusted for WIND
Groundspeed = TAS ± wind component
What matters for navigation timing
The relationship:
Tailwind: groundspeed = TAS + tailwind (faster over the ground)
Headwind: groundspeed = TAS − headwind (slower over the ground)
The wind is the difference
Air vs. ground
Why the ASI shows airspeed, not groundspeed:
The airplane flies through the air (aerodynamics depend on airspeed)
The ASI measures air pressure (airspeed)
Groundspeed comes from GPS (or dead reckoning)
Different measurements
The practical point:
Fly by airspeed (IAS) — the aerodynamics
Navigate by groundspeed (GPS) — the timing
Don't confuse them
Both matter, for different things
Blocked Pitot Tube: The ASI Failures
The blocked-pitot failures, from the ASI's perspective (reinforcing the pitot-static post).
The ASI is the affected instrument:
A blocked pitot affects ONLY the ASI (the only pitot instrument)
The altimeter and VSI are unaffected
Isolated to airspeed
The ASI's vulnerability
Ram blocked, drain open:
Pitot pressure escapes through the drain
Drops to static pressure
The ASI reads ZERO (or toward zero)
Airspeed lost
Ram and drain both blocked:
Pitot pressure is TRAPPED (constant)
The ASI acts like an ALTIMETER:
Climb: reads HIGHER (falsely — static drops, trapped pitot looks relatively higher)
Descent: reads LOWER (falsely)
Responds to altitude, not speed
The danger:
In a climb with both blocked, airspeed reads increasing (falsely)
A pilot might lower the nose to "reduce" it — actually accelerating/diving
Recognize the false indication (cross-check attitude and power)
The trap
Causes:
Ice (the main one — use pitot heat)
Insects, debris, cover left on
Preflight and pitot heat prevent it
Preventable
Blocked Static Port: The ASI Effect
The static blockage effect on the ASI.
The ASI is affected (via static):
A blocked static affects all three instruments (all use static)
The ASI becomes unreliable
Along with the frozen altimeter and zero VSI
Widespread
The ASI behavior (static blocked):
Climb: the ASI reads LOWER than actual (trapped static is higher than ambient, reducing the difference)
Descent: reads HIGHER than actual
Opposite of the both-pitot-blocked case
Reversed errors
The alternate static source:
Opens the static system to cabin air (backup)
Cabin static is lower → ASI (and altimeter) read HIGHER than actual
Account for the error
The backup (with a correction)
(For the full blockage details and the alternate static source, see our pitot-static system post.)
Pitot Heat and Pilot Responsibilities
The preflight and protection, expanded.
Pitot heat:
Protects the pitot tube from icing
Use in visible moisture or cold conditions
Test it during preflight (feel it warm)
Ice prevention
Preflight checks:
Inspect the pitot tube (clear, cover removed)
Check the static ports (clear)
Test pitot heat (if required)
Verify the drain holes
The cross-check:
Cross-check airspeed with ATTITUDE and POWER
If the airspeed doesn't match the expected attitude/power, suspect an error
A key technique (especially for blockages)
Attitude + power = expected airspeed
The "airspeed alive" call:
On the takeoff roll, verify the airspeed is increasing ("airspeed alive")
Catches a blocked pitot before liftoff
Standard callout
Early detection
The Airspeed Indicator in Modern Aircraft
The glass-cockpit reality, brief.
In glass cockpits:
Pitot and static pressures are sensed electronically (by the Air Data Computer)
The airspeed is displayed as a digital tape (usually)
The color arcs are shown on the tape
Same principles, digital display
The tape display:
A vertical airspeed tape (instead of a round dial)
The color arcs are shown along the tape
May include trend vectors (predicting speed)
Modern presentation
The failures still apply:
A blocked pitot/static still produces errors (the ADC gets bad data)
Glass cockpits may alert to disagreements
But the failures are the same
Understand the fundamentals
Why Understanding the ASI Matters
The stakes, expanded.
The ASI directly affects:
Stall prevention (flying above stall speed)
Structural protection (staying below Vno/Vne)
Takeoff and landing safety (correct speeds)
Performance management (climb, cruise speeds)
The IMC danger:
In IMC, you can't see the horizon
Misinterpreting airspeed (or a blockage) is dangerous
Cross-check with attitude and power
Critical in instrument conditions
The V-speed knowledge:
Know the color arcs and V-speeds
Fly the correct speeds for each phase
Respect the limits (Vfe, Vno, Vne)
Foundational
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 and static pressure to derive dynamic pressure, it gives real-time feedback on how the airplane is flying through the air. Its color arcs mark the critical V-speeds — Vs0 and Vfe (white), Vs1 and Vno (green), the caution range to Vne (yellow), and the never-exceed red line — all in indicated airspeed, because IAS reflects the dynamic pressure the wing actually feels. And one crucial speed, maneuvering speed (Va), isn't marked at all, because it changes with weight.
Understanding how the ASI works, what its markings mean, why you fly indicated airspeed, and how it fails (a blocked pitot can make it act like an altimeter) allows pilots to recognize abnormal indications, cross-check intelligently, and maintain safe control. In aviation, airspeed isn't just a number — it's a cornerstone of safe flight.
On the Written Test and Checkride
The airspeed indicator appears on tests and checkride orals. The most commonly tested topics:
How the ASI works (dynamic pressure = pitot − static)
The color arcs and their V-speed boundaries (Vs0, Vfe, Vs1, Vno, Vne)
Why Va isn't marked (weight-dependent)
Why you fly IAS (reflects dynamic pressure / aerodynamic force)
IAS, CAS, TAS
Blocked pitot and static failures
Quick Reference
How the ASI Works:
Uses BOTH pitot and static pressure
Pitot into the diaphragm, static in the case
Difference = dynamic pressure = airspeed
Responds immediately
The Color Arcs and V-Speeds:
Arc/Line | From | To |
White (flap range) | Vs0 (landing-config stall) | Vfe (max flap extended) |
Green (normal range) | Vs1 (clean stall) | Vno (max structural cruising) |
Yellow (caution) | Vno | Vne |
Red line | — | Vne (never exceed) |
Key V-Speeds:
Vs0: stall, landing configuration (bottom of white)
Vs1: stall, clean configuration (bottom of green)
Vfe: max flap extended (top of white)
Vno: max structural cruising (top of green / bottom of yellow)
Vne: never exceed (red line)
Va (NOT marked):
Maneuvering speed is NOT on the ASI
Because it changes with WEIGHT
Find it in the POH (or placard)
Lower at lighter weights
Why You Fly IAS:
The ASI shows dynamic pressure (what the wing feels)
Stall speed is constant in IAS at any altitude (same dynamic pressure)
The color arcs work at any altitude (read IAS directly)
IAS = the aerodynamically meaningful speed
Airspeed Types:
IAS: raw reading (fly by this)
CAS: IAS + instrument/position error correction (POH table)
EAS: CAS + compressibility (high speed)
TAS: CAS + density (≈ IAS + 2%/1,000 ft)
Groundspeed vs. Airspeed:
Airspeed: through the air (ASI)
Groundspeed: over the ground (TAS ± wind, from GPS)
Fly by airspeed; navigate by groundspeed
Blocked Pitot (ASI only):
Ram blocked, drain open → reads ZERO
Both blocked → acts like an altimeter (climb: reads high; descent: reads low)
Causes: ice, insects, cover left on → use pitot heat
Blocked Static (all three):
ASI: climb reads LOW, descent reads HIGH (reversed from both-pitot-blocked)
Altimeter frozen, VSI zero
Alternate static source: reads HIGH (cabin static lower)
Preflight:
Pitot clear, cover removed
Static ports clear
Test pitot heat
"Airspeed alive" on takeoff
Cross-check airspeed with attitude and power
Key Principle:
The airspeed indicator derives airspeed from dynamic pressure (pitot − static). Its color arcs mark the V-speeds — Vs0/Vfe (white), Vs1/Vno (green), yellow to Vne, red line at Vne — all in indicated airspeed, because IAS reflects the dynamic pressure the wing feels (so stall speeds are constant in IAS at any altitude). Maneuvering speed (Va) isn't marked because it changes with weight. A blocked pitot can make the ASI act like an altimeter (reading high in a climb) — cross-check with attitude and power.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.