KIAS in Aviation: What Knots Indicated Airspeed Means, and Why It Isn’t Your Real Speed

KIAS stands for knots indicated airspeed: the speed shown on an aircraft’s airspeed indicator, in knots. It is not how fast the airplane is moving over the ground. It is a measure of how hard the air is pushing into the pitot tube, which is exactly why pilots fly by it.

That gap between the needle and reality is the whole story. An airplane holding 120 KIAS at 12,000 feet is moving through the air at about 144 knots, and a jet cruising at 280 KIAS at 35,000 feet is really doing about 473 knots.

Pilots use five speeds, each one a correction of the one before it. Here they are side by side.

What Does KIAS Mean?

Break the abbreviation in two. The K is knots, and IAS is indicated airspeed, so 120 KIAS reads as “120 knots indicated.”

The FAA defines indicated airspeed as the direct reading from the airspeed indicator, uncorrected for air density, installation error or instrument error. In other words, it is whatever the needle or the speed tape says, taken at face value.

The knot is a nautical mile per hour. One knot is exactly 1.852 km/h and about 1.15 mph, so 120 KIAS works out to roughly 138 mph indicated (222 km/h). Our guide to knots vs mph explains why aviation and shipping kept the nautical unit, and the aviation unit converter does the math for you.

How the Airspeed Indicator Measures KIAS

The airspeed indicator is a differential pressure gauge. It compares two pressures: ram air from the pitot tube, which points forward into the airflow, and still air from the static port on the side of the fuselage.

On the ground in calm air the two are equal and the needle sits at zero. Once the airplane moves, pitot pressure rises above static pressure, a diaphragm inside the instrument expands, and mechanical linkage swings the needle.

That is the key point: the instrument does not measure speed at all. It measures pressure, and the dial is simply marked in knots.

It is the one instrument in the six-pack that uses both the pitot and the static system. On a glass-cockpit primary flight display the same reading moves to a scrolling speed tape, and the FAA’s example display adds a small true airspeed box beside it.

Why Pilots Fly by Indicated Airspeed

If KIAS is not the real speed, why is it the number pilots watch most? Because the pressure the instrument reads is the same air pressure the wings feel.

The FAA puts it plainly: manufacturers use indicated airspeed as the basis for aircraft performance. Takeoff, landing and stall speeds in the airplane’s flight manual are indicated airspeeds, and they do not normally vary with altitude or temperature.

Those indicated limits are painted right on the dial. Airplanes of 12,500 pounds (5,670 kg) or less, built after 1945 and certificated by the FAA, must carry a standard color code on the airspeed indicator.

• White arc: the flap operating range. Its bottom is VS0, the stall speed in the landing configuration, and its top is VFE, the highest speed with flaps extended.
• Green arc: the normal operating range. It runs from VS1, the clean stall speed, to VNO, the maximum structural cruising speed.
• Yellow arc: the caution range, flown only in smooth air.
• Red line: VNE, the never exceed speed.

Other limits, such as maneuvering speed and the landing gear speeds, are not marked on the dial. They are on placards and in the flight manual.

KIAS vs KCAS: Fixing the Instrument’s Errors

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument error and position error. Position error, which the FAA also calls installation error, comes from where the pitot tube and static port are mounted on the airframe.

The FAA notes the combined error can reach several knots at certain speeds and flap settings, and it is greatest at low airspeeds. In cruise, IAS and CAS are about the same.

Pilots find the correction in the airspeed calibration chart in the flight manual. For most everyday flying the difference is small enough that pilots treat KIAS and KCAS as one number.

KIAS vs KTAS: Why Altitude Changes Everything

True airspeed (TAS) is the airplane’s actual speed through the air around it. The FAA defines it as calibrated airspeed corrected for pressure altitude and nonstandard temperature.

Air thins as you climb. To make the same pressure in the pitot tube, the airplane has to move faster, so at a fixed indicated airspeed, true airspeed rises with height.

There are two ways to get from KIAS to KTAS. The quick one is the FAA’s rule of thumb: add 2 percent to calibrated airspeed for every 1,000 feet of altitude.

The accurate one is a flight computer, mechanical or electronic, fed with calibrated airspeed, pressure altitude and outside air temperature. As the chart shows, the rule of thumb runs a little fast at higher altitudes on a standard day, so it is a cross-check, not a replacement.

True airspeed is the speed pilots plan with and the one they write on a flight plan. It also explains why airliners cruise so high: thin air lets them cover more ground for the same indicated speed.

Equivalent Airspeed: The Correction Only Fast Airplanes Need

Equivalent airspeed (EAS) is calibrated airspeed corrected for compressibility. At high speed the air piles up and compresses as it rams into the pitot tube, which makes the indicator read a little high.

At light-airplane speeds the effect is negligible. At 280 knots calibrated and 35,000 feet, though, the standard-atmosphere math puts equivalent airspeed near 264 knots, about 16 knots lower.

EAS matters most to engineers: the FAA rules for transport-category airplanes state their design airspeeds as equivalent airspeeds.

Ground Speed: The Only Speed That Gets You There

Ground speed (GS) is true airspeed adjusted for wind. A headwind takes knots away and a tailwind adds them.

It is the speed that sets your arrival time and how much fuel the trip burns. A jet at 473 knots true into a 100-knot headwind covers the ground at only 373 knots, while the airspeed indicator never notices the wind at all.

One Jet, One Climb: KIAS, KTAS and Mach Side by Side

Here is how far the needle drifts from reality as a jet climbs to cruise. The figures are a standard-day calculation with no wind, treating indicated as calibrated airspeed.

Notice the cruise row. The needle shows a lower number at 35,000 feet than it did at 20,000, yet the airplane is moving about 73 knots faster through the air.

Where KIAS Is the Law: Speed Limits and ATC

US speed limits in the sky are written in indicated airspeed. Under 14 CFR 91.117, no aircraft may fly faster than 250 knots indicated (288 mph) below 10,000 feet MSL unless the FAA authorizes it.

Tighter limits apply near airports. The cap is 200 knots indicated (230 mph) at or below 2,500 feet above the surface within 4 nautical miles of the primary airport of Class C or Class D airspace, and in the airspace beneath a Class B shelf.

Controllers speak the same language. The Aeronautical Information Manual says ATC assigns speeds in knots of indicated airspeed in 5- or 10-knot steps, and pilots are expected to hold within 10 knots of the assigned speed.

Why Jets Switch From KIAS to Mach Up High

High up, the limit that matters stops being indicated airspeed and becomes Mach number, the speed relative to the speed of sound. That is why the same AIM paragraph lets controllers assign speeds as Mach numbers, in 0.01 steps, at or above FL240.

Transport jets carry a paired limit for the same reason. The FAA’s maximum operating limit speed, VMO/MMO, is an airspeed or a Mach number, whichever is critical at a particular altitude.

Down low, indicated airspeed is the tighter limit. Climb high enough and the Mach limit takes over, and near a jet’s ceiling that high-speed limit closes in on the low-speed one, the squeeze explained in our guide to the coffin corner.