Calibrated Air Speed

bqmassey

bqmassey

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The Instrument Flying Handbook, page 5-9, says this about Calibrated Airspeed: "CAS is the speed at which the aircraft is moving through the air".

This sounds like a mistake to me. Thoughts?
 
That sounds wrong.

From memory, is not CAS just IAS adjusted for instrument and position error?

The definition given would be for TAS.

Right!
 
If you post the entire quote, it states: "CAS is the speed at which the aircraft is moving through the air, which is found by correcting IAS for instrument and position errors."
 

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FastEddieB said:
But that's still not right.

TAS is the TRUE speed at which an aircraft moves through the air.
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"TAS is CAS corrected for nonstandard temperature and pressure. TAS and CAS are the same in standard atmosphere at sea level."
 
FastEddieB said:
But that's still not right.

TAS is the TRUE speed at which an aircraft moves through the air.
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but air is thinner at altitude and lower pressures. your pitot tube will feel less resistance even though the wind is hitting it just as fast. you have to calibrate the readings to get a more accurate number.
 
If by "speed through the air" we mean how many air molecules we pass per second then the Handbook is correct.
 
HighFlyingA380 said:
"TAS is CAS corrected for nonstandard temperature and pressure. TAS and CAS are the same in standard atmosphere at sea level."
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True, but that's not what the passage you quoted says.

The book, even with your added verbiage, is still wrong IMO.
 
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There is no shortage of mistakes in the FAA books.
 
alfadog said:
If by "speed through the air" we mean how many air molecules we pass per second then the Handbook is correct.
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Sorry, but that's redefining or reinterpreting it to the point of confusion.

1) Take my Cirrus panel:

12907715084_c8cf9e7897.jpg


The IAS is 147k. The CAS should be very close - most planes try to have IAS=CAS in the range of normal cruise regime, usually drifting off at either extreme.

But the plane's actual speed through the air (molecules notwithstanding ;)) is 176k. In one hour it will travel 176 nm within the mass of air that it finds itself. That is, by definition, it's TRUE airspeed.

2) There's a subtle difference between defining something and the mechanics of how to find it.

For instance, if I ask a student to define Pressure Altitude, and he or she answers, "It's what you get when you put 29.92 in as your altimeter setting", I know they're on the right track, but I would probably follow up with, "Well, that's how you determine it, but what is it exactly?" I then try to lead them to something like, "Well, it's your actual height above the level in the atmosphere where there's standard pressure", or something involving the "Standard Datum Plane" if they want to sound really smart.

Similar here - TAS is defined as your actual speed through the air - very simple. How it relates to IAS and CAS (or even EAS) is a slightly different matter.

Anyway, I still hold the manual is wrong - and confusing.
 
timwinters said:
True, but that's not what the passage you quoted says.

The book, even with your added verbiage, is still wrong IMO.
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With the exception of accidentally switching " pressure" and "temperature," it's verbatim per the most recent edition (FAA-H-8083-15b) dated Nov. 26, 2012.
 

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kgruber said:
Flight testing.
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Hell, no.

It's calculable from basic physics. The formula is a bit of a mess, not something you'll use with a calculator in flight. But is absolutely sufficient to build into an E-6B. See http://williams.best.vwh.net/avform.htm#Mach for an example.

The IAS/CAS correction requires testing (not necessarily in flight, but that may be the simplest test setup), as it's sensitive to airframe geometry, especially placement of the static port(s) and variation with angle of attack.
 
FastEddieB said:
Similar here - TAS is defined as your actual speed through the air - very simple. How it relates to IAS and CAS (or even EAS) is a slightly different matter..
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This image below is the explanation of TAS from page 7-9 of the Pilot's Handbook of Aeronautical Knowlege (PHAK).

It says that TAS is used for flight planning and, of interest to me, that there are two methods to find TAS, both of which start with CAS.

How many of you use a CAS-based method to get TAS? If not, what do you do?

FYI, for the LSA that I trained in, I was taught to use a POH table that gave me a TAS based on altitude (__ ft ISA, density altitude I think) and power setting (RPMs).

tas.jpg
 
MAKG1 said:
Hell, no.

It's calculable from basic physics. The formula is a bit of a mess, not something you'll use with a calculator in flight. But is absolutely sufficient to build into an E-6B. See http://williams.best.vwh.net/avform.htm#Mach for an example.

The IAS/CAS correction requires testing (not necessarily in flight, but that may be the simplest test setup), as it's sensitive to airframe geometry, especially placement of the static port(s) and variation with angle of attack.
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This is more of what I was curious about. Thank you! Not every aircraft has a POH and correction table.
 
My point of view: (Please correct if wrong)

IAS = What your air speed indicator reads.
CAS = IAS corrected for the pitot not being perfectly aligned with the airflow (slow flight, climbs, etc.), essentially more accurate IAS
TAS = This is effectively ground speed + the winds aloft. That's really all this is. If you subtract/add the appropriate head/tail wind from your TAS then you get your ground speed.

So, from that, I would agree that CAS is the speed at which you are moving through the air. TAS is CAS corrected so that the air at your altitude and temp is corrected to a standard block of air at MSL so that you can then calculate everything relative to the ground.

TAS = "indicated" ground speed
CAS = Airspeed
 
N659HB said:
This is more of what I was curious about. Thank you! Not every aircraft has a POH and correction table.
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New at this, but I think the POH correction tables go from IAS to CAS under different indicated speeds and flap conditions, not from CAS to TAS.

Isn't MAKG1 talking about going from CAS to TAS?
 
tehmightypirate said:
My point of view: (Please correct if wrong)

IAS = What your air speed indicator reads.
CAS = IAS corrected for the pitot not being perfectly aligned with the airflow (slow flight, climbs, etc.), essentially more accurate IAS
TAS = This is effectively ground speed + the winds aloft. That's really all this is. If you subtract/add the appropriate head/tail wind from your TAS then you get your ground speed.

So, from that, I would agree that CAS is the speed at which you are moving through the air. TAS is CAS corrected so that the air at your altitude and temp is corrected to a standard block of air at MSL so that you can then calculate everything relative to the ground.

TAS = "indicated" ground speed
CAS = Airspeed
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No, true airspeed is not ground speed. It's the speed you're actually moving through the air. i.e. if I'm on a 200 NM XC and I'm doing 200 kts bucking a 20kt headwind, then I'll still be about 20 NM short of my destination after an hour of flight.

Calibrated airspeed = true airspeed on a standard day at sea level.
 
register@teamandras.com said:
Isn't MAKG1 talking about going from CAS to TAS?
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Yes. It's the same for every airplane, though it's frequently approximated as non-compressing (works fine for spam cans, not so good for jets).

True airspeed is what the name says, as Tim is saying. It's what you would get with a direct measurement of speed as the air flows over the wings (at least, far enough away that the flow isn't disturbed by the wings). Wind is not relevant.

We measure airspeed with pitot tubes, which are also sensitive to air density, usually parametrized through pressure and temperature.

CAS is the airspeed that determines aircraft performance. With the same CAS, you get the same performance at any altitude (however, getting that same CAS may be problematic). It is an equivalent airspeed, not an actual airspeed. In effect, it removes the effect of altitude, temperature, and pressure from aircraft performance.

In a nutshell, you use CAS when trying to predict performance like sink rate to a landing, stall speeds, etc. Pretty much anything localized to the aircraft. You use TAS for navigation, usually with the winds added in to get ground speed. TAS itself is not relevant, but ATC uses it as a substitute for ground speed when the winds may or may not be known.
 
tehmightypirate said:
TAS = This is effectively ground speed + the winds aloft. That's really all this is. If you subtract/add the appropriate head/tail wind from your TAS then you get your ground speed.
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Bringing groundspeed into it in any way only confuses the matter. Airplanes fly through the air, and all the speeds we've been discussing are airspeeds.

So, from that, I would agree that CAS is the speed at which you are moving through the air.
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Absolutely not. Though it may be the case on a standard day at sea level.

In my Cirrus example, I'm going through the air at 176k (calculated by the Avidyne) with a CAS of 149k (IAS of 147k corrected for 2k error as below):

12909588825_107ab3f33a.jpg


It should be clear there's normally a huge discrepancy between CAS and TAS, again allowing they may be the same under very tightly limited instances.
 
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timwinters said:
No, true airspeed is not ground speed. It's the speed you're actually moving through the air. i.e. if I'm on a 200 NM XC and I'm doing 200 kts bucking a 20kt headwind, then I'll still be about 20 NM short of my destination after an hour of flight.

Calibrated airspeed = true airspeed on a standard day at sea level.
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Fair enough. Though, note that I'm not saying TAS is groundspeed in anyway, I'm specifically addressing what we use these speeds for.

In the end all airspeeds are "the speed through the air" they're simply in relation to different references. My point was (poorly made) that CAS is the most useful term in relation to speed through the air and without relation to anything involving ground, altitude, pressure or temperature.
 
bqmassey said:
The Instrument Flying Handbook, page 5-9, says this about Calibrated Airspeed: "CAS is the speed at which the aircraft is moving through the air".

This sounds like a mistake to me. Thoughts?
Click to expand...

A-hem, my 2 cents: Indicated airspeed could be thought of as "indicated air pressure" and calibrated airspeed thought of as indicated pressure that has the reading scale "calibrated" to correct for angular errors at the mouth of the pitot tube. Since air is thinner above us, reading the same "pressure" up there means you're truly going faster (get it, heh, heh), ie, reading/calculating "true airspeed". So, the book is right because pressure is being converted to speed.

dtuuri
 
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dmspilot said:
There is no shortage of mistakes in the FAA books.
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Not surprising.

Another product of AFS-800. My friend was working there and they gave him the Rotorcraft Handbook (rewrite) to proofread. The errors in it were horrible, then he found out the guy that rewrote it had zero experience with helicopters. :rolleyes2:
 
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