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Could you please explain to me one moment in relation to IAS and TAS. In all books I read I see that the IAS is highlighted as a reference for stall, take-off, landing and other performance items. So, it seems that regardless of the altitude and meteo- conditions I need to just monitor IAS in order to stay in safe margins of flight performance. And TAS is referenced mainly just to flight planning.

But for example the yellow arc of ASI that indicates caution range is also IAS and in high altitude (where TAS is higher then IAS) it seems pilot can overspeed the aircraft without keeping in mind TAS calculation. So, what is the tactic here - if TAS is only for planning or should be considered during certain phases of flight, etc.? Or only in certain conditions or altitudes?

Question is about GA with standard instruments.

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  • $\begingroup$ Relevant-- last paragraph of this answer -- aviation.stackexchange.com/a/86195/34686 -- the point being that if the aircraft's top allowable speed is limited by concerns about flutter (which is not always the case), then that speed will tend to correspond to constant value of TAS, not IAS. In that case the pilot may need to consult a table to convert to IAS, or his instruments may show a redline on the IAS airspeed display that actually moves, to show the changing relationship between IAS and TAS. This needs to be worked up into a real answer-- if the question is not a duplicate. $\endgroup$ Jan 31, 2022 at 16:13
  • $\begingroup$ Thank you. Will have a look $\endgroup$
    – Sergey
    Jan 31, 2022 at 16:40
  • $\begingroup$ Another good answer-- explains why pilots are generally most interested in IAS/CAS, not TAS -- but does not address the flutter case -- aviation.stackexchange.com/a/58130/34686 -- I know that when sailplanes are flown at high altitudes (e.g. mountain waves), flutter generally is what sets the redline, and the need to reduce the redline (as expressed in terms of IAS) to deal w/ the changing relationship between TAS and IAS at altitude, is a very significant issue. $\endgroup$ Jan 31, 2022 at 16:54
  • $\begingroup$ The flutter issue exists, but I have never seen a limitation in an AFM regarding a TAS limitation. Perhaps I've just missed it. Or do the other limitations (IAS- Vne, max service ceiling, etc.) functionally render the TAS-flutter matter a non-issue? $\endgroup$
    – user22445
    Jan 31, 2022 at 20:43
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    $\begingroup$ @quietflyer: interesting article here: australianflying.com.au/news/vne-and-flutter-explained $\endgroup$
    – user22445
    Jan 31, 2022 at 23:04

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Too long for a comment to Alexander's post, so I post it as my own comment:

CAS is IAS corrected for installation anomalies, mostly around the pitot tube. The correction itself is very minor, so most people just ignore it. Some airplane handbooks have CAS / IAS correction tables in them. At larger speeds (jet planes) CAS vs. IAS becomes more important, but you are still a beginner and therefore won't be flying the Gulfstreams soon.

IAS/CAS measures what "grip" the plane / its wings have in the air. Therefore, for the same speed of a motion object in space, it is lower for thinner air and higher for denser air. It's why you look at IAS for maneuvers and take-off and landing, because you need to know how the plane "feels" the air.

"TAS is used primarily for navigation" Not true. What would TAS have to do with navigation? Why? TAS corrects for non-standard pressure and non-standard temperature if properly done on an E6B.

"if your aircraft generates the same lift, you IAS will be the same". Assuming same pressure, temp, and humidity (very minor factor), that is correct.

Your moon example ... there is no airflow or lift on the moon. Which is why propeller flight has a certain maximum height. I forgot what it was, someone fill my blank here, but it is something like 30 thousand feet or 40 thousand feet. You aren't allowed to fly higher with props, because the props would have no "grip" in the air. It's why space rockets are "jet engines" and not proppies.

"TAS on the other hand has nothing to do with the aircraft performance, it just shows how fast your are moving in a body of air" which has a lot to do with performance :)

"I don't even know if TAS is needed in flight if you use GPS with ground speed. Guys, who know what else is TAS for in flight if you have GPS, please share." TAS was used for decades, long before GPS was around. Oftentimes for a specific flight you want to look at the current ground speed, because that tells you how fast you are going over the specific terrain, therefore is the end result after winds. So GPS is useful here, it is the "end result" after IAS, CAS, TAS, pressure corrections, temp corrections, wind corrections. Just measure the end result instead of computing it with assumptions. But oftentimes you want to know TAS and do not want to include winds. For example to compute some performance data for your plane. You might be interested in figures like distance per gallon, to make performance figures comparable to well-known car figures. So you use a timer, look at the fuel flow (this exercise can be done rich-of-peak or lean-of-peak or ideally both) and compute distance per gallon, or gallons per mile. Here using the GPS to have "auto-wind correction" would be wrong, because the wind would significantly distort the computation. So you use TAS, that is independent of wind direction. Doing this exercise with strong tailwinds or strong headwinds would give the same results, because your TAS isn't impacted by winds. But that means TAS is very useful for performance calculations, contrary to your claim (see previous para).

Your next para shows a misunderstanding of CAS. With CAS we want to correct IAS for installation error of the pitot tube.

Rule of thumb for TAS: add 2% to IAS for every thousand feet of altitude. For example, at 10 thousand feet your IAS (proxy for CAS) is 120 knots, you would add 20% of IAS and get 120 + 24 =144 knots TAS. This ignores temperature error, which is much less of a factor than pressure. It is actually 1.6 % for every ten thousand feet, but people round it to 2% to make the math a tad simpler. But be aware: this rule of thumb ignores temperature error!

Regarding the yellow arc question from Sergey: From the book: "Yellow arc—caution range. Fly within this range only in smooth air and then only with caution" It is still a speed that has to do with the airflow over the plane, therefore IAS is used, not TAS. We don't fly in the yellow are when we are in gusts, because a gust can significantly affect the wing loading. Thus we don't care about how fast we fly through space, but how fast we fly through the surrounding air. It's IAS that tells us how the plane "feels" the air surrounding it.

I recommend you read some the FAA's manuals, the AFH and PHAK are excellent resources that explain all that. From your questions I glean you have more "ground school" ahead of you. The FAA's manuals are here: https://www.faa.gov/regulations_policies/handbooks_manuals

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I'm a new pilot, so it's possible I have misunderstanding here, but what I figured out (at least for general aviation) is that IAS/CAS are used to monitor aircraft behavior/performance characteristics, and TAS is used primarily for navigation.

Imagine two scenarios when you are flying in dense air and in thin air: To generate the same lift, the aircraft has to move faster in thin air so TAS will be different, but in my understanding if your aircraft generates the same lift, you IAS will be the same. As such regardless of TAS, IAS/CAS show you how your aircraft is currently preforming as if it would be flying with the same IAS at sea level at standard temperature, which includes stall speed, flaps retraction speed, etc.

Imagine if you'd manage to fly your Cessna on Moon with no air pressure at all:

Because the air density is extremely low, almost zero, but there are just a few molecules of gas on the entire moon no matter how fast you're moving, your IAS would be showing 0 because there is no pressure on the pito, there would be no lift as well. No matter how fast you're moving against those few molecules of gas (which is TAS), you plane would fall like a brick, and IAS would be telling exactly that, as it would be showing 0. This is why you use IAS/CAS on takeoffs, landings, because you need to know when it's going to stall.

TAS on the other hand has nothing to do with the aircraft performance, it just shows how fast you are moving in a body of air and by applying wind corrections you calculate GS for navigation planning in calculating time in flight, checkpoints, etc. I don't even know if TAS is needed in flight if you use GPS with ground speed.


The one thing I don't really get is what actually has more impact on aircraft performance: IAS or CAS? Let's say on Cessna with one static port on the right your IAS will be noticeably different between left and right slips because if your aircraft is turned to the right, it's left side with the static port will be more exposed to upcoming airflow which will be pushing on static port reducing differential pressure and as such IAS will be lower, and if it's turned to the right, the opposite will happen: left side will be hidden from the airflow and will be in the thin air increasing diff pressure and showing higher IAS. So aircraft performs the same or very similar in the left and right slips, but IAS will be very different.

And I don't remember how flaps effect on IAS/CAS difference, but something tells me that CAS is more accurate speed that shows aircraft performance then IAS.

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  • $\begingroup$ Too long for a comment, see my own post $\endgroup$ Nov 20, 2023 at 8:27
  • $\begingroup$ Welcome to Aviation! Well written first answer, however, if you'll take the tour, you'll note that this is a Q&A site, not a general discussion forum. The part below the line is asking another question, which is inappropriate in the "Answer" section. Please edit this out (you can copy it), then ask a whole new question. $\endgroup$
    – FreeMan
    Dec 20, 2023 at 16:37

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