What is the Difference Between True Air Speed and Indicated Air Speed?

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An aircraft’s Indicated Airspeed and True Airspeed may be very different. Image by Helen Krasner

You might think that an aircraft’s speed through the air would only be defined by one number.

However, if you start reading about it, you may find books talking about different versions of airspeed, in particular Indicated Airspeed and True Airspeed, but also sometimes Calibrated Airspeed or Rectified Airspeed.

So what are these and what are the differences between them?

True Airspeed (TAS)

TAS is exactly what it says – the speed of the aircraft through the air.  However, this should not be confused with groundspeed.

Groundspeed is the actual speed covered by the aircraft over the ground, but airspeed is affected by factors such as head and tail winds, and is really a measurement of the rate at which air is passing over the wings.

So with a headwind, when wind is blowing from the front, TAS will be more than groundspeed; with a tailwind, where wind blows from behind, TAS will be less than groundspeed.

Indicated Airspeed (IAS)

IAS is airspeed as measured by the aircraft’s Airspeed Indicator (ASI).  It is always less than TAS.  The reason for this is that the ASI actually measures the dynamic pressure, or the pressure of the air moving over the wings.  However, the dynamic pressure varies at different altitudes, and is proportional to the air density (Dynamic Pressure = half air density x velocity squared).  The air is thinner at altitude, so the dynamic pressure will be less for the same airspeed, which means IAS will reduce as you climb, regardless of the rate of movement, while TAS will be consistent.  Put perhaps more simply, the ASI measures how many molecules of air move over the wing in a given time.  At height, at the same speed, there will be less moving air molecules for the same speed, so the ASI will under-read, giving an IAS less than the TAS.

Calibrated Airspeed (CAS) or Rectified Airspeed (RAS)

Like all pressure instruments, the ASI suffers from a number of errors, namely instrument error, time lag, position error, and manoeuvre induced error.  If IAS is corrected for instrument and position error, the resulting airspeed is known as CAS or RAS.  The aircraft’s Pilot’s Operating Handbook will have a table or graph showing the magnitude of these errors, which are most significant at low airspeeds.

Airspeed: Other Points of Interest

At low altitudes, such as those usually used by private pilots, TAS and IAS are very similar, but they can vary quite a lot as aircraft fly higher.  As an approximate rule of thumb, the difference is about 2% per 1000 ft up to about 10,000 ft, so an IAS of 150 kts equates to a TAS of around 180 kts at 10,000 ft.

Note that the best climbing airspeed is obtained at a particular TAS, not IAS.  However – and this is important – the aircraft will still stall at the same IAS, regardless of altitude.

The difference between TAS and IAS can be critical to a safe flight, so make sure you’ve got them straight before you get in the cockpit!

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