popee
Topic Author
Posts: 151
Joined: Fri Feb 23, 2001 10:26 pm

### IAS To CAS, How The Two R Equalled?

Talking about MCP of B 737, the speed i/p given to A/T is by rotating a knob which reads speed in IAS (Indicated Air Speed). Whereas the ASI reads CAS (Calibrated Air Speed----given by DADC). How IAS Equals CAS ?

Guest

### RE: IAS To CAS, How The Two R Equalled?

CAS is IAS corrected for position error and compressibility.

avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

### RE: IAS To CAS, How The Two R Equalled?

IAS is lower than actual airspeed at altitude due to lower air density. This is also factored into CAS.

Guest

### RE: IAS To CAS, How The Two R Equalled?

I beg to disagree with Ambasaid (even though I respect him  ). The difference between IAS and CAS is just the instrument error correction and the position error correction. For this reason, the MD-11 IAS and CAS is identical because the computers take out the instrument and position errors.

EAS (equivalent air speed) is the one that includes scale altitude correction (compressibility).

Regards,
Nut

Guest

### RE: IAS To CAS, How The Two R Equalled?

Morning Nut,

Dont know what i did to get on your respected list, but i guess that i better answer this properly to keep myself there.......

ASIR = Airspeed Indicator Reading, the uncorrected reading of the airspeed indicator.
IAS= Indicated Airspeed, the ASIR corrected for instrument error only.
RAS = Rectified Airspeed, the IAS corrected for position error.
EAS = Equivalent Airspeed, IAS corrected for position error and compressibility error.
CAS = Calibrated Airspeed, An Amercian term for EAS at sea level only.
TAS = True airspeed, the true speed of an aircraft through undisturbed air.

Position Error = The total error in the pressure instrument readings due to the position and limitations of the pitot static system.

Compressibility Error, the error in pressure instrument readings due to Mach Number effect.....

These are taken from my ATP notes, so basically IAS is corrected for instrument error. CAS and EAS are the same thing and equate to IAS corrected for position error and compressibility.

Does this keep me on your list...

Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

### RE: IAS To CAS, How The Two R Equalled?

Just to throw in my two cents, more and more transport aircraft are at the point where pitot-static measurements are made by an Air Data Computer before they are presented to the crew. In these cases, CAS is referred to as "computed airspeed." It is esentially the same as "calibrated airspeed," but the distinction exists nevertheless.

Guest

### RE: IAS To CAS, How The Two R Equalled?

Ambasaid,
I'm afraid I still do not agree with your first post. Don't worry you are still on my respected list for correcting me on a 777 post a while back.

I'm afraid I disagree with your notes (are they straight from a book? I have trouble believing this, that Americans and Europeans are using the same in different context!) regarding CAS as an American term to indicate only EAS at sea level. According to my aerodynamics books and all my years as an flt test (aero) engineer in the US, CAS is always IAS corrected for position and instrument error. EAS is CAS further corrected for compressibility...So CAS and EAS is not at all the same at different altitudes. Even my basic private pilot book (Glaeser,Gum, Walters, An Invitation to Fly) has it right.

I'm not familiar with European terms but I can't imagine any differences significant as this. Give me your email, I can mail you the stuff I got, complete with formulas and all  .

Best Regards,
Nut

Guest

### RE: IAS To CAS, How The Two R Equalled?

Nut,

My explanation was taken from Irish ATP course notes, the following is taken from the MD11 Operational Engineers course notes.

Quote

CAS = IAS + Delta V Where Delta V is the instrument and position correction.

(Flight tests are conducted to develop airspeed calibration curves to convert IAS to CAS)

EAS = CAS - Delta V Where Delta V is compressible correction

Note: At low altitudes and slow speeds the compressible correction is small and for these conditions EAS is approximately equal to CAS.

End Quote

By 2nd guessing the author of my ATP notes, I would say that he has taken a similar note verbatim and is actually WRONG.

The Boeing Jet Transport Performance Methods manual has an excellent formula for converting CAS = EAS which I cant repeat here, but they also have a compressibility correction chart which shows that low and slow the correction is negligible but at M85 / FL350, the correction is 16 kts.

I do apologize for giving you a bum steer.

I'm actually pretty upset as the author of my notes was also the person who set AND corrected Irish pilot exams for over the last 10 years, therefore he was expecting everyone to give a wrong answer!!!!!!

popee
Topic Author
Posts: 151
Joined: Fri Feb 23, 2001 10:26 pm

### RE: IAS To CAS, How The Two R Equalled?

Thanx for a healthy discussion.
I think I failed to put this question in exact frame that I intended to ask.
May I re-post my question.
Lets say we want to maintain a/c speed at 250 kts ( IAS ). We turned the speed knob on MCP to read 250 (IAS). The speed bug on ASI moves to 250 kts ( CAS ) and the aircraft speed is maintained. Question is that the ASI is reading and maintainig 250 CAS ( as ASI is getting signal from DADC which has corrected Position Error ) where as we wanted to maintain 250 kts ( IAS ). So how come 250 CAS equalled 250 IAS ?

I hope I am clear now .

Guest

### RE: IAS To CAS, How The Two R Equalled?

Popee, if what you said is true then the displayed speed on the indicator is NOT IAS in the definition of the word. Rather it is CAS that is displayed on the indicator. In this case CAS is the same as IAS for the aircraft and they can be used interchangeably.

CAS is what you wanted to be displayed in the first place. In older airplanes, pilots have to consult placards or flip chart to mentally correct for position and instrument errors (if any). The ADCs take that laborious chore away by having the errors stored and computed so the calibrated airspeed is displayed instead. This is the case for the most newer aircraft designs, especially those with glass cockpits.

Regards,
Nut

popee
Topic Author
Posts: 151
Joined: Fri Feb 23, 2001 10:26 pm

### RE: IAS To CAS, How The Two R Equalled?

Hi Nut, Thanx.

Thats the main thing I wanted to ask. ASI is reading CAS, whereas MCP knob gives IAS as o/p. Can we really say that IAS = CAS ? Or is it a wrong cocept which is accepted correct worldwide ?

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