CAS Calculator

Q: Where do I find my aircraft's position error correction?

Look in your AFM or POH. Most manuals have an airspeed calibration table or chart. It lists IAS in one column and CAS in the other. The difference between them is your position error correction (ΔV pc ). Some manuals give one chart for flaps up and another for flaps down. Use the one that matches your setup.

Q: Should I enter the position error as a positive or negative number?

It depends on your chart: If the manual shows CAS lower than IAS, enter a negative number. If the manual shows CAS higher than IAS, enter a positive number. Quick check: 250 IAS with −3 gives 247 CAS. 250 IAS with +3 gives 253 CAS.

Q: Do I need Full Conversion mode, or is IAS → CAS enough?

Use IAS → CAS when you only need to check a limit speed like V FE or V NE , or a stall speed. Use Full Conversion when you are planning a leg and need true airspeed for time and fuel numbers.

CAS Calculator

Introduction

This CAS calculator turns your indicated airspeed (IAS) into calibrated airspeed (CAS). Your airspeed indicator is never perfect. The pitot-static system reads a little high or a little low, and that gap changes with speed and angle of attack. Your aircraft's AFM or POH lists the fix, called the position error correction. Add it to your IAS and you get CAS, the airspeed number you can trust.

The tool has two modes. IAS → CAS does the simple fix: enter your indicated airspeed and the correction from your book, and you get CAS. Full Conversion goes further. Add your pressure altitude and outside air temperature, and it also gives you equivalent airspeed (EAS), true airspeed (TAS), Mach number, ISA deviation, and density altitude. It uses real air math, including compressibility, so the numbers hold up high and fast.

You can work in knots, mph, or km/h, and in feet or metres. The chart shows how CAS, EAS, and TAS change as you climb. CAS stays flat. TAS grows, because thin air lets you move faster for the same feel on the gauge. That picture makes the whole idea click fast.

How to use our CAS Calculator

Enter your indicated airspeed and your aircraft's position error, and the calculator gives you calibrated airspeed (CAS). In Full Conversion mode, add pressure altitude and temperature to also get EAS, TAS, Mach number, and density altitude.

Mode tabs: Pick IAS → CAS for a quick position error fix. Pick Full Conversion to work all the way out to true airspeed.

Units: Click the Units button to choose knots, mph, or km/h for speed, and feet or metres for altitude. The labels and results change right away.

Indicated Airspeed (IAS): Type the speed shown on your airspeed indicator.

Position Error Correction (ΔV_pc): Enter the correction from your AFM or POH chart. Use a negative number when the pitot-static system reads low.

Pressure Altitude: In Full Conversion mode, enter your altitude with the altimeter set to 1013.25 hPa (29.92 inHg).

Outside Air Temperature (OAT): Enter the air temperature in degrees Celsius from your outside air temperature gauge.

Calculate and Reset: Press Calculate to see your results and the airspeed chart. Press Reset to return to the default values.

What Is Calibrated Airspeed (CAS)?

Calibrated airspeed is the speed shown on the airspeed indicator after you fix the errors in the pitot-static system. The airspeed indicator measures air pressure from a small tube on the outside of the plane. The shape of the plane, the angle of the wings, and the spot where the tube sits can all make that reading a little off. That error is called position error. When you correct for it, you get CAS.

How CAS Is Found

The math is simple:

CAS = IAS + ΔV_pc

IAS is indicated airspeed, the raw number on the dial. ΔV_pc is the position error correction. You get that number from the aircraft flight manual (AFM or POH), usually from a table or chart. It can be positive or negative, and it changes with flap setting, weight, and angle of attack. The error is biggest at slow speeds, like takeoff and landing.

The Four Airspeeds

IAS – what the gauge shows.
CAS – IAS with position and instrument error removed.
EAS – CAS with the compressibility error removed. Air squeezes as it enters the pitot tube, and this matters more at high speed and high altitude.
TAS – how fast the plane really moves through the air. Air gets thinner as you climb, so TAS grows above CAS. A rough rule: TAS rises about 2% for every 1,000 feet of altitude.

Why CAS Matters

Most of the key speeds in a flight manual are listed as CAS. Stall speed, flap limit speed (V_FE), gear speed (V_LO), and never-exceed speed (V_NE) all come from CAS. Using CAS keeps you inside safe limits no matter how high you fly, because the wing cares about the pressure of the air hitting it, not the ground speed. Pilots use CAS for performance planning, and use TAS for navigation and flight plan times.

Things That Change the Numbers

Pressure altitude and outside air temperature (OAT) both change TAS. Warm air is thin air, so on a hot day the plane moves faster through the sky at the same CAS. That is also why density altitude goes up on hot days, and why takeoff rolls get longer. A blocked or iced-over pitot tube or static port will make all of these readings wrong, so pilots check them before every flight.

Formulas used

Calibrated Airspeed from Indicated Airspeed

V_{CAS} = V_{IAS} + \Delta V_{pc}

Impact (dynamic) pressure from CAS

q_c = P_0\left[\left(1 + 0.2\left(\frac{V_{CAS}}{a_0}\right)^2\right)^{3.5} - 1\right]

ISA pressure and temperature (troposphere, h \le 11 km)

T = T_0 - L h,\qquad P = P_0\left(\frac{T}{T_0}\right)^{\frac{g_0}{L R}}

ISA pressure above the tropopause (h > 11 km)

P = P_{11}\,e^{-\frac{g_0 (h - 11000)}{R\,T_{11}}},\qquad T = T_{11}

Mach number from impact and static pressure

M = \sqrt{5\left[\left(\frac{q_c}{P} + 1\right)^{\frac{2}{7}} - 1\right]}

True airspeed from Mach number and OAT

V_{TAS} = M\sqrt{\gamma R T}

Equivalent airspeed from TAS and air density

V_{EAS} = V_{TAS}\sqrt{\frac{\rho}{\rho_0}},\qquad \rho = \frac{P}{R\,T}

Density altitude

h_{DA} = \frac{T_0}{L}\left[1 - \left(\frac{\rho}{\rho_0}\right)^{\frac{R L}{g_0 - R L}}\right]

Frequently asked questions

Where do I find my aircraft's position error correction?

Look in your AFM or POH. Most manuals have an airspeed calibration table or chart. It lists IAS in one column and CAS in the other. The difference between them is your position error correction (ΔV_pc).

Some manuals give one chart for flaps up and another for flaps down. Use the one that matches your setup.

Should I enter the position error as a positive or negative number?

It depends on your chart:

If the manual shows CAS lower than IAS, enter a negative number.
If the manual shows CAS higher than IAS, enter a positive number.

Quick check: 250 IAS with −3 gives 247 CAS. 250 IAS with +3 gives 253 CAS.

What if my CAS and IAS are the same?

That is normal. Many aircraft have almost no position error in the cruise range. Just enter 0 for the correction. The error usually grows at slow speeds, like on approach or during takeoff, so check your chart at those speeds too.

When are CAS, EAS, and TAS all the same?

At sea level on a standard day (15 °C, 1013.25 hPa) and at slow speed, all three are nearly equal. As you climb or speed up, they split apart. TAS goes up the most, EAS drops a little below CAS because of compressibility.

What does the Mach number in the results mean?

Mach is your true airspeed divided by the speed of sound at your altitude. Mach 0.50 means you are flying at half the speed of sound. It matters at high altitude, where the speed of sound drops and jets watch a Mach limit instead of a CAS limit.

What does ISA+ or ISA− mean in the results?

It shows how far the air temperature is from the standard day. ISA+10 means the air is 10 degrees warmer than standard for that altitude. ISA−5 means 5 degrees colder. Warmer air is thinner, so TAS and density altitude both go up.

Do I need Full Conversion mode, or is IAS → CAS enough?

Use IAS → CAS when you only need to check a limit speed like V_FE or V_NE, or a stall speed. Use Full Conversion when you are planning a leg and need true airspeed for time and fuel numbers.

What do I enter if I do not know my pressure altitude?

If your altimeter is set to 1013.25 hPa (29.92 inHg), the altitude you read is pressure altitude. On the ground or at low level with a local setting, work it out first with a pressure altitude calculator, then bring that number here.

Why do I see the message that CAS is zero or negative?

Your position error is bigger than your indicated airspeed, or it has the wrong sign. Check that a −3 correction is really −3 and not −300. Also check you are using the same speed unit for both boxes.

Why do I get a supersonic error message?

The math here only works below the speed of sound. If your speed and altitude push past Mach 1, the tool stops instead of showing a wrong answer. Lower the airspeed or check that your unit setting is right.

Can I use miles per hour or kilometres per hour?

Yes. Click the Units button and pick knots, mph, or km/h. Altitude can be feet or metres. The input labels, results, and chart all switch right away. Just make sure your position error uses the same unit as your IAS.

What is the highest altitude I can enter?

You can enter up to 51,000 ft. That covers airliners and most business jets. The chart draws from sea level to 45,000 ft so you can see how EAS and TAS spread out as you climb.

Why is EAS lower than CAS?

Air squeezes as it enters the pitot tube, and that makes the gauge read a bit high. EAS takes that squeeze out. At 250 kt down low the gap is tiny. At high speed and high altitude it can be several knots, which is why fast jets care about it.

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