What Is Indicated Airspeed? IAS, CAS, TAS Untangled

Indicated airspeed (IAS) is the speed shown on the face of your airspeed indicator — the raw, uncorrected reading the pitot-static system produces by comparing the ram air pressure caught by the pitot tube against the ambient static pressure sensed at the static port. It is not your speed across the ground, and at altitude it is not even your true speed through the air. But it is the airspeed you actually fly by.

That last point trips up a lot of students. The indicated airspeed on your panel is the most important number you have, even though it is the “least accurate” of the bunch. Your wing does not care how fast you cross the ground or how thin the air is — it cares about dynamic pressure, and IAS is your direct read on that. Stall speed, flap limits, best-climb speed, maneuvering speed: the airplane’s whole personality is written in indicated airspeed. This article untangles the four airspeeds the FAA tests — indicated, calibrated, equivalent, and true — plus groundspeed, so you know which number to fly and why.

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What Is Indicated Airspeed?

Indicated airspeed is the value shown on the face of your airspeed indicator — the uncorrected number the instrument produces from the pitot-static system. The system measures the difference between ram air pressure (caught by the pitot tube as you move forward) and static air pressure (sensed at the static port). That pressure difference, called dynamic pressure, is what the instrument translates into a speed.

The key word is uncorrected. Indicated airspeed has not been adjusted for the small errors built into any real airplane — the instrument’s own imperfections and the way air flows around the static port. The FAA’s own definition, carried in the Pilot’s Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25C), is “the direct instrument reading obtained from the airspeed indicator, uncorrected for variations in atmospheric density, installation error, or instrument error.”

Here is why that matters: every speed on your checklist, your placards, and the indicator’s colored arcs is an indicated (or closely related calibrated) airspeed. When your instructor says “rotate at 55,” that is 55 knots indicated.

How Does the Airspeed Indicator Actually Work?

The airspeed indicator is a differential pressure gauge. Ram air from the pitot tube fills a small expandable diaphragm inside the instrument, while static pressure from the static port fills the sealed case around it. As you speed up, ram pressure rises faster than static pressure, the diaphragm expands, and a mechanical linkage drives the needle. Slow down and the diaphragm contracts and the needle falls. No electricity, no GPS — just two air pressures fighting each other.

According to PHAK (FAA-H-8083-25C), Chapter 8, the airspeed indicator is the only flight instrument that uses both the pitot (ram) source and the static source; the altimeter and vertical speed indicator use static pressure alone. Because it measures dynamic pressure, it is really measuring how hard the air is hitting the airplane — the same thing your wing feels. The needle is a dynamic-pressure gauge with a knots label painted on it. That also tells you its weak points: block the pitot tube and the diaphragm cannot fill; block the static port and the case loses its outside reference. More on each below.

What Is the Difference Between IAS, CAS, EAS, and TAS?

The four airspeeds form a chain, and each step removes one source of error. Indicated airspeed (IAS) is the raw reading. Calibrated airspeed (CAS) is IAS corrected for instrument and position error. Equivalent airspeed (EAS) is CAS corrected for compressibility. True airspeed (TAS) is EAS corrected for air density (altitude and temperature). In light trainers at low altitude, CAS and EAS are nearly identical, so most students work straight from IAS to CAS to TAS.

A few honest notes. For a Cessna 172, the IAS-to-CAS gap is small through most of the range — often a knot or two — but it grows near the stall, which is exactly where your flight manual publishes a calibration table. Compressibility (the EAS step) is negligible below about 200 knots, so as a private pilot you can usually treat CAS and EAS as the same. The big mover is the last step: true airspeed climbs above indicated as you go higher and warmer, because thinner air gives less dynamic pressure for the same true speed. That is the correction that changes your flight plan.

Why Do You Fly Indicated Airspeed Instead of True Airspeed?

You fly indicated airspeed because the wing responds to dynamic pressure, and IAS is your direct measure of it. A wing stalls at a certain angle of attack, which corresponds to a certain dynamic pressure for a given weight and configuration — and that always shows up at the same indicated airspeed, no matter your altitude. Your stall speed in IAS does not change when you climb, even though your true airspeed does.

Think about what that buys you. Your published stall speed, best-rate-of-climb speed (V_Y), maneuvering speed (V_A), and flap-extension limit (V_FE) all stay valid in indicated airspeed at any altitude you will fly as a private pilot. You memorize one set of numbers and they work everywhere. If you flew true airspeed instead, your stall speed would creep up as you climbed, and you would be doing density-altitude math while trying not to spin.

So here is the clean version: aerodynamic limits are fixed in indicated airspeed; true airspeed is a navigation number. When you manage the aircraft — approach speed, climb speed, staying below V_A in turbulence — you live entirely in indicated airspeed. True airspeed earns its keep on the cross-country side, where you combine it with wind to get groundspeed, time en route, and fuel burn.

How Do You Convert Indicated Airspeed to True Airspeed?

To convert indicated airspeed to true airspeed, first correct IAS to calibrated airspeed using your flight manual’s table, then correct CAS for air density using pressure altitude and outside air temperature. In practice you do this in the airspeed-correction window on the calculator side of a mechanical E6B, or you read it straight off a glass cockpit. The result, TAS, is your real speed through the air mass.

For cockpit mental math, the FAA-taught rule of thumb is worth memorizing: true airspeed increases by roughly 2% over indicated airspeed for every 1,000 feet of altitude. So if your Cessna 172 indicates 110 knots at 6,000 feet, your true airspeed is about 110 + (6 × 2% × 110) ≈ 123 knots — a 13-knot difference you would otherwise plan around wrong. The rule assumes near-standard temperature; on a hot day, with thinner air, TAS runs even higher. That is just density altitude wearing a different hat — the same physics that hurts takeoff and climb performance.

One trap: do not confuse true airspeed with groundspeed. TAS is your speed through the air; groundspeed is TAS adjusted for wind. With a 20-knot headwind, an airplane making 123 knots true crosses the ground at about 103. The wing still feels 123; the GPS still reads 103. Both are correct — they answer different questions.

Want the full pitot-static picture, the E6B walkthrough, and every airspeed question the FAA can throw at you, explained so it sticks? That is exactly what we build in the Private Pilot Ground School — taught for understanding, not memorization.

What Do the Colored Arcs on the Airspeed Indicator Mean?

The colored markings on your airspeed indicator are standardized by 14 CFR § 23.1545 and represent calibrated airspeed values. They turn the instrument into an instant limits reference: at a glance you can see whether you are in the normal range, the caution range, the flap range, or pressing toward never-exceed. You do not have to remember the numbers — the airplane shows you the safe zones in color.

The logic is intuitive once you see it. The bottom of the green arc is your clean stall speed; the bottom of the white arc is your stall speed with flaps down (lower, because flaps add lift). The yellow arc, above the green, is for smooth air only — turbulence at those speeds can overstress the airframe. The red line is V_NE, never-exceed.

Notice what is not a fixed color line: maneuvering speed, V_A. It changes with aircraft weight, so it lives in your flight manual and on a placard rather than painted on the gauge — lighter airplane, lower V_A. That is a favorite checkride question.

What Happens When the Pitot Tube or Static Port Is Blocked?

When the pitot tube or static port is blocked, the airspeed indicator gives a false reading in a predictable pattern. A blocked pitot tube makes the indicator behave like an altimeter; a blocked static port makes it read backward from what you expect during climbs and descents. Pitot heat exists specifically to prevent the most common cause: ice.

Start with the pitot tube. If the ram-air inlet and drain both block — the classic full pitot blockage — the air is trapped and the indicator stops responding to airspeed. Per PHAK (FAA-H-8083-25C), Chapter 8, it then acts like an altimeter: as you climb, the trapped pressure looks higher against the falling static pressure, so the airspeed reads higher even if you are slowing down. That false high reading has killed pilots who pitched up to “slow down” and stalled.

Now the static port. If it blocks, the case reference pressure freezes where it was. As you climb, the airspeed reads lower than actual; as you descend, it reads higher than actual — and the altimeter and vertical speed indicator are affected too, since they share the static source. The fix in many aircraft is the alternate static source, a valve that draws static pressure from inside the cabin.

The takeaways are simple. Turn on pitot heat in visible moisture near freezing, confirm the pitot cover is off and the static ports are clear on every preflight (an insect nest in a pitot tube is a real, recurring problem), and cross-check: if airspeed disagrees with power, pitch attitude, and GPS groundspeed trend, suspect the pitot-static system before you trust one gauge.

A Story From a Cold Alaska Morning

Let me tell you about a January preflight in Homer that rewired how I think about the airspeed indicator.

It was one of those clear, brutally cold Kenai Peninsula mornings — single digits, the kind of air that feels like it has edges. I was getting the 172 ready for a short hop. Pitot cover off, I glanced at the pitot tube, saw the opening, and started to move on. But something nagged at me. The night before had brought freezing drizzle, then a hard temperature drop. So I went back, crouched down, and actually put a flashlight into the tube instead of just confirming the cover was gone. There it was: a thin, almost invisible glaze of ice across the inlet, right where the ram air needs to enter.

If I had flown it, here is how it would have played out. The airspeed might have looked fine on the roll. But as I climbed into colder air with that inlet partly blocked, the trapped pressure would have read against the falling static pressure, and the needle would have crept upward even as I slowed. I would have seen a number that said “plenty of speed” while the wing quietly ran out of it — the exact trap the textbook describes, and it does not feel like an emergency until the stall horn.

I warmed the inlet, confirmed it was clear, turned on pitot heat, and watched the airspeed come alive correctly before I committed. The flight was uneventful, which is the whole point. Aviation education since 2006, a CFI since 2017, and I will still crouch down with a flashlight on a cold morning. The airspeed indicator is the most honest instrument you have — but only if the air can get to it.

PLT Study Guide

The FAA written exam covers indicated airspeed and its cousins under several learning statement codes — the PLT codes that show up on your Airman Knowledge Test Report. These are the codes whose official FAA wording actually matches this topic, translated into plain-English study points.

Do not study just one of these in isolation. PLT007 covers the calculation side, but a complete grasp of airspeed also lives in PLT132 (definitions and markings), PLT337 (the pitot-static plumbing), and PLT123 (the altitude and temperature performance effect). The test mixes them, so study all four together. As you review, anchor on three ideas: the IAS → CAS → EAS → TAS chain and what each correction fixes; the 2%-per-1,000-feet rule of thumb as a TAS sanity check; and the blocked-pitot (reads high in a climb) versus blocked-static (reads low in a climb) failure patterns.

Frequently Asked Questions

FROM CHRIS

Once indicated airspeed clicks, the pitot-static system stops being a memorization chore and becomes a story you can read off the panel: ram pressure against static pressure, dynamic pressure on the wing, limits fixed in IAS, true airspeed earned back through the air. Get that picture in your head and you will fly the number with confidence instead of fear — day one, not just checkride day.

Throttle On!

Chris Palmer

Founder & Chief CFI, Angle of Attack — Two-Time Master Aviation Educator and Gold Seal CFI