I 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
Very handy Peter. I have always felt TAS hard to quote as so dependent on variables, and Foxbats have been known to travel a tad lower than some. But I also had not thought of exceeding max manoeuvring due hot and high. Another handy source is Sporty’s E6B, and cloud base calcs available on it.
Excellent article Peter. For those wanting more detail google Ken Krueger’s article on TAS versus IAS
at Vans aircraft.
Jim Barry
Hi Peter,
I hope I am not missing something obvious here but I don’t believe this is correct. The white, green and blue lines on the ASI for flap, normal operating (Vno) and manouevring speed are in IAS, not TAS. In other words, 95 KTAS is the same as 85 KIAS as far as the forces on the airframe are concerned, which is what those limits are meant to indicate right ? 85 kts at 0′ creates the same force differential between the static and dynamic pitot lines as 95 kts does at 5000′ – and hence the same forces on the airframe.
The situation is different for Vne because that takes account of the potential for “flutter” which is a harmonic response and which IS related to air velocity rather than soley to air pressure (it’s explained better here: http://www.vansaircr…df/hp_limts.pdf).
As an aside: For most of us who fly below 10,000 I reckon the old rule of thumb of an increase of 2% in TAS for every 1000′ pressure altitude is accurate enough for navigation planning. One can of course get an idea of the pressure altitude by temporarily setting the altimeter to 1013.2 mb and noting the difference to what it was when we set airfield altitude before taking off – but in many most cases even that is not necessary (i.e. I can’t fly accurately enough to notice a difference !).
Please don’t hesitate to slap me down if I have this all wrong !
Cheers,
AM
Hi Andrew – thanks for your comment. I’m not really sure of your point. I agree, the markings on the ASI are indicated air speed (IAS). What I was trying to say was that it’s important to know the TAS so that you don’t exceed the design limits of the airframe. Thus an indicated IAS under the load speed limit may in fact be over the limit when you calculate the TAS. As an example, on the early A22L Foxbats, the manoeuvering & rough air limit is 83 knots. It is quite possible to exceed an 83 knot IAS in that aircraft at most altitudes up to 5,000 feet – by which time the TAS will be well over 90 knots, significantly higher than the airframe limit. Which means you could damage the airframe in turbulence. Similarly, you could easily exceed the A22L Vne of 115 knots (TAS) in a dive where the IAS shows ‘only’ 110 knots. Then there’s the question of navigation and calculating speeds….
Thanks for the quick response Peter,
I ‘m sorry, I probably confused matters with the first line. Let me try again:
What we need to be concerned about are the forces on the airframe. Those forces are a function of both the speed of the air past the aeroplane and the air density. My understanding Is that a line on the ASI at 85 Kias for Vma says “this is the speed which will give the limiting force in a standard atmosphere”. If I climb to 5000′ the line still tells me that force because the air is going about 10% faster past the airframe (TAS) but it is also 10% less dense. Same force on the pitot, therefore same reading of 85 on the ASI and same aerodynamic force on the airframe. So I don’t need to worry about TAS as far as the flap extension, normal operating or manoeuvring speeds are concerned.
Another way to think of it: at a certain weight in level flight lift = weight. At 5000′ in level flight lift still = weight, despite the higher speed of air because the air is less dense. The lift force and all other aerodynamic forces are the same.
I DO need to think in terms of TAS for the red line, the VNE. This is because this limit is often determined not by the pressure forces but by the risk of harmonic flutter, which relates to speed of air only.
And yes, I need to calculate TAS for navigation, 2% addition to IAS for every 1000’ altitude will do it nicely.
Cheers
AM