True Air Speed (TAS)

ASI 0-120kts smallI regularly hear and take part in discussions about True Air Speed (TAS) – I sell aeroplanes and a common question is: “What’s it TAS at then?”

The resulting discussion indicates strongly that there is not much real understanding of how the speed indicated on the Air Speed Indicator (ASI) can vary widely from the true or actual airspeed. And since the calculation of TAS is quite complex, based on indicated air speed (IAS), temperature, barometric pressure and altitude, many pilots can’t be bothered to work it out and assume IAS and TAS are much the same. Or at most a couple of knots different.

In general terms, the higher and warmer you are, the higher the TAS compared with the IAS. While this may help aeroplane sales people who can say that the ‘real’ air speed is greater than indicated on the ASI, there is in fact a very real use for being aware of TAS.

For example – say your aeroplane has a manoeuvring speed limit of 90 knots, ie the maximum speed for full control deflections or cruise in rough air. At 5,000 feet on a warm 25º Celsius day at 1013 Mb QNH your ASI indicates 85 knots – are you under the speed limit? Or are you running the risk of damaging the airframe through over-speeding?

When you work it out, your TAS is actually 95 knots and if you hit a big thermal, you run the risk of structural damage to your aircraft. That’s why calculating your TAS is important.

Many of today’s digital instrument panels calculate TAS for you, provided you set the barometric pressure (often called QNH) correctly.

For those of you without the benefit of one of these panels, here’s a link to a website that will calculate TAS for you: TAS Calculator