Learning to fly a plane, whether a light aircraft like a Cessna 172 here at Epic or a Boeing 747 jet, involves mastering many cockpit instruments. However, none is more essential than the Airspeed Indicator (ASI). Indicated Airspeed (IAS) is a fundamental concept in pilot training because it helps pilots measure and manage flight performance, lift-to-drag ratio, and safety-critical V-speeds.
Here, I’ll get into IAS, explain how it works, define related airspeed types, and show you how to read and correct airspeed for better accuracy. Let’s get started!
Quick Navigation to Indicated Airspeed
The Basics
IAS, TAS, and CAS
Pilot Training and IAS
What is indicated airspeed (IAS)?
A good definition for indicated airspeed is the speed shown directly on the aircraft’s ASI without correction for instrument errors, altitude, or air density. It reflects the dynamic pressure of air flowing into the aircraft’s pitot-static system.
How does an airspeed indicator work?
The ASI operates using two primary pressure sources:
- Pitot pressure (ram pressure and static pressure from the pitot tube)
- Static pressure (ambient air pressure from static ports)
The difference between these pressures determines airspeed. This system works on the principle of barometric pressure and atmospheric pressure affecting air density.
The pitot tube measures the total air pressure caused by the aircraft’s movement, which includes both static pressure and ram (impact) pressure. The airspeed indicator shows the indicated airspeed by comparing this total pressure (from the pitot tube) with the static pressure (from the static port). Inside the instrument, a diaphragm expands or contracts based on the difference between the two pressures.
Analog vs Digital Airspeed Indicators
Most cockpit instruments in light aircraft are still analog. You’ll see a round dial and colored arcs. However, modern jets and military aircraft may feature digital displays or use an app for training simulations. Regardless of format, understanding the color markings is key:
- White arc: Flap operating range (starts at Vso, ends at Vfe)
- Green arc: Normal operating range
- Yellow arc: Caution range (fly in smooth air only)
- Red line: Vne – Never Exceed Speed
Types of Airspeeds
To understand flight dynamics and instrumental corrections, it’s essential to learn the types of airspeeds and how they differ:
| Type of Airspeed | Description |
| IAS | What you see on the ASI (uncorrected) |
| CAS | Calibrated Airspeed: IAS corrected for instrument errors |
| EAS | Equivalent Airspeed: CAS corrected for compressibility at high speeds |
| TAS | True Airspeed: EAS corrected for density altitude and temperature |
| Ground Speed | TAS corrected for wind correction (headwind or tailwind) |
IAS vs TAS: What’s the Difference?
True airspeed vs. indicated airspeed is important. Understanding the difference matters, especially in jets since often fly at high altitudes near the transition altitude, where Mach speed becomes relevant.
- IAS reflects what the aircraft “feels” through the air.
- TAS shows your actual horizontal speed over the air mass.
- At high altitude, low pressure makes the air thinner, so TAS increases while IAS may stay the same.
Fun fact: What does the speed of sound have to do with IAS? The speed of sound plays a key role in understanding airspeed, especially at high altitudes and high-performance flight.
- IAS is a measure of dynamic pressure, not your proximity to the speed of sound.
- At high altitudes, you can reach transonic speeds at relatively low IAS.
- The speed of sound decreases with altitude, making Mach number more useful for high-speed flight.
Airspeed Calibration and Error Correction
All aircraft flight manuals include an airspeed calibration chart that allows pilots to convert IAS to CAS, correcting for indicator errors and installation errors.
To convert CAS to TAS, you can use:
- A flight calculator
- An aviation app
- The TAS formula: TAS = CAS × sqrt(air density at sea level / air density at altitude). Another way to write it is: TAS = CAS + (2% per 1,000 ft above sea level).
- Below I’ll share a step-by-step example to apply this formula. We’ll begin with this basic information:
CAS = 120 knots. Altitude = 5,000 feet.
Apply the TAS formula:
Step 1: Calculate the altitude factor.
Divide the altitude by 1,000. 5,000 divided by 1,000 = 5.
Multiply the result by 2%. 5 divided by 2% = 10%.Step 2: Calculate the airspeed increase.
Multiply the CAS by the altitude factor.
120 divided by 10% = 120 x 0.1 = 12Step 3: Calculate the TAS.
Add the airspeed increase to the CAS.
120 + 12 = 132
So, the true airspeed is approximately 132 knots.
Understanding zero-lift drag, compressibility, and density altitude is critical in high-performance aircraft and supersonic or transonic regimes (both subsonic and supersonic).
Understanding V-Speeds
1. V-Speeds in KIAS
V-speeds are specific airspeeds vital to flight safety:
| V-Speed | Meaning |
| Va | Maneuvering Speed |
| Vne | Never Exceed Speed |
| Vno | Maximum Structural Cruising Speed |
| Vso | Stall Speed in Landing Configuration |
| Vs1 | Stall Speed in Clean Configuration |
| Vx | Best Angle of Climb |
| Vy | Best Rate of Climb |
V-speeds are defined in the unit of knots (KIAS – Knots Indicated Airspeed). However, you may use knots to mph conversion when learning.
2. Performance Charts
All performance calculations use KIAS as a baseline, including:
- Takeoff and landing distances
- Climb performance (e.g., FPM (feet per minute) at specific KIAS)
- Cruise speeds
- Fuel consumption at specific KIAS and power settings
- These charts help pilots fly the aircraft within safe and optimal performance limits.
3. Limitations and Markings
- KIAS is used to define airspeed limits, like maximum flap extension speed (Vfe) or turbulence penetration speed (Va).
- These speeds are matched to airspeed indicator markings (e.g., green arc, white arc).
4. Important Notes About KIAS
The POH often includes disclaimers such as:
“All airspeeds are shown in Knots Indicated Airspeed (KIAS) unless otherwise noted.”
This ensures pilots don’t confuse KIAS with TAS (True Airspeed) or GS (Ground Speed), which can vary significantly due to altitude, temperature, and wind.
Watch Our Six-Minute Video on Airspeed!
Bridging Classroom Training to Flight Training
In our ground school classes we rely on diagrams to teach a lot of concepts, such as:
- The pitot-static system
- Location of the Kollsman window (used for setting pressure altitude)
- ASI markings
- The relationship between altitude, air density, and airspeed
- PFD/MFD pilot perspective diagram
- Airspeed calibration chart
PFD/MFD Pilot Perspective Diagram
Found in pilot training materials, aircraft POHs, and flight simulator guides, this visual layout that shows how a pilot views and interacts with the Primary Flight Display (PFD) and Multi-Function Display (MFD) from the cockpit.
PFD (Primary Flight Display) shows critical flight data:
- Attitude indicator (artificial horizon)
- Airspeed (KIAS)
- Altitude
- Vertical speed (FPM)
- Heading
- Flight director cues
MFD (Multi-Function Display)shows supplementary data:
- Moving map/GPS navigation
- Engine data
- Traffic/weather information
- Checklists and system status
“Epic flight instructors are trained to watch for common errors by student pilots, such as confusing IAS and TAS. If they do this, they may think they are flying faster than they actually are. We also remind them not to fixate on the instrument or even ignore the instrument.” –Ray Altmann, Chief Flight Instructor, Epic Flight Academy
Common Errors and Safety
As Chief Flight Instructor, I’ve learned that one common error student pilots might make regarding indicated airspeed (IAS) is misinterpreting it as true airspeed (TAS). This is especially at higher altitudes, so it’s something our instructors watch for. This matters for several reasons:
- IAS measures dynamic pressure and is affected by air density.
- As altitude increases, air density decreases, so for the same TAS, the IAS reads lower.
- Students often overestimate their performance (e.g., thinking they’re flying faster than they are). This can affect navigation, fuel planning, and flight timing.
Other potential IAS-related mistakes include:
- Ignoring V-speed markings (e.g., flying in the yellow arc in turbulence)
- Not cross-checking with other instruments (e.g., using IAS to judge descent rate instead of also monitoring the VSI)
- Failing to correct for instrument errors using the airspeed calibration chart in the POH (Pilot’s Operating Handbook)
- Relying solely on IAS during performance calculations, when CAS or TAS is required
- Fixating on the instrument or even omitting the instrument
We endeavor to address mistakes like this very early in training so our pilots build strong, safe flying habits.
Accidents Involving IAS
Because the pitot tube is crucial to providing accurate IAS, it is imperative to keep it clean and free of debris, ice, and anything else that could clog it. Several aviation accidents have been linked to errors in indicated airspeed readings. An accurate measurement is critical and cannot be overemphasized. For example:
- Air France 447 (2009): The pitot tube became clogged with ice, creating inconsistencies in airspeed measurement. All three airspeed indicators ultimately failed. This Airbus A330 crashed into the Atlantic Ocean killing all onboard.
- Northwest Orient Airlines Flight 6231 (1974): Erroneous airspeed readings caused the fatal crash of this Boeing 727. No one turned on the pitot tube heater prior to takeoff, and the pitot tube became clogged with ice leading to a stall and rapid descent.
Types of Indicator Errors
| Error Type | Effect on IAS | Corrected By |
| Instrument Error | Minor inaccuracies | Calibration → CAS |
| Position Error | Disturbed airflow leads to small errors | CAS (via POH chart) |
| Density Error | IAS ≠ TAS at altitude | Apply TAS correction |
| Lag/Hysteresis | Delayed needle movement | Pilot awareness |
| Pitot/Static Blockage | Faulty or frozen reading | Inspection & alternate sources |
Apps That Function as an Airspeed Indicator
Although you might want to use an app to determine IAS, to get true IAS, you need a pitot-static system, which mobile devices don’t have. Here are a few apps our students have explored:
1. ForeFlight Mobile
- Platform: iOS
- Type: Professional EFB (Electronic Flight Bag)
- Features: Displays GPS-based groundspeed and synthetic airspeed-style instruments
- Use: For actual flight and training (FAA-approved in many cockpits)
- Learn more about ForeFlight.
2. Infinite Flight or X-Plane Mobile
- Platform: iOS, Android
- Type: Flight simulator
- Features: Realistic airspeed indicator (ASI) in cockpit views
- Use: Excellent for training and understanding airspeed behavior under different flight conditions
- Learn more about Infinite Flight and X-Plane Mobile.
3. Avare
- Platform: Android
- Type: Free aviation GPS app
- Features: Basic GPS-derived speed, location, and navigation
- Use: Light EFB functions with limited ASI-like features
- Learn more about Avare.
4. Airspeed Indicator – Sim Instrument Panel Apps
- Platform: iOS/Android
- Type: Educational/training tools
- Features: Emulates traditional six-pack instruments, including ASI
- Use: For training, demos, or cockpit simulator builds
- Learn more about some of these:
- Flight Instruments (iOS – shows ASI, altimeter, etc.)
- Aviation Tools Free (Android)
5. GPS HUD Speedometer Apps
- Platform: iOS, Android
- Type: General-purpose speed apps (not aircraft-specific)
- Features: Show groundspeed, not true airspeed
- Use: Can mimic airspeed for rough practice or fun, but not reliable for training
- Learn more about GPS HUD.
Note: GPS-based apps show groundspeed, not indicated airspeed. These apps can be helpful for training and simulation, but should not be used for actual in-flight IAS unless paired with certified hardware.
Why IAS Matters
Whether flying a 172, 747, or military jet, understanding indicated airspeed is essential to safe, efficient flying. It tells you how the aircraft responds to aerodynamic forces, allowing for instantaneous decisions in takeoff, climb (FPM – Feet Per Minute), cruise, and descent.
If you’re just learning to fly, start by getting comfortable reading the ASI, using the flight manual, and applying simple error corrections. You’ll be well on your way to understanding how to fly accurately, safely, and confidently.
