RAS or CAS?

[malaysian eaglet]

On the french side of the JAR channel, the problem has been cleared since a long time. But I have still some difficulties with my english speaking colleagues because I have not yet succeed to get the good reference book, more some questions (feedback or school made ones) are confusing.
let me give my version:
RECTIFIED AIR SPEED (RAS) does not exist anymore officially. As everybody know this speed was not taking into account at all the compressibility effect . This speed was computed from simplified BERNOULLI equation
CALIBRATED AIRSPEED as defined by the JAR 1 is equal to True Air Speed (TAS) in standart at main sea level. It is also given the precision - in the same JAR 1- that Equivalent Air Speed (EAS) is also equal to TAS at sea level in standart. So if CAS = EAS it means that COMPRESSIBILITY is taken into account at sea level and in standard within CAS. If somebody is doubting about that point of view, AP3456 confirms this in its book 3 (Flight Instruments). Our Airspeed indicator must be calibrated using Saint Venant's equation (Subsonic Aircraft) or Lord Raleigh one's (Transonic/Supersonic aircraft). It means that compressibility effect is taken into account at sea level. This not only theory when Concorde begins its climb what is its CAS in the low layers of the atmosphere?
So RAS and CAS are different based on two different equations.
I must also underline that the JAR 25 states that an Air Speed Indicator MUST be calibrated in CAS and that all the stall reference for JAR 23 (general Aviation) are also expressed in CAS.
So, why some people are still talking about RAS and are confusing CAS and RAS?
;)

[Knold]

I'm sorry but I had a hard time choosing between the last two options...

[Dan Winterland]

Technicaly, RAS is IAS corrected for Instrument and Pressure error. CAS is what an Air Data Computer will give you if fitted. The ADC corrects pitot pressure for Instrument, Pressure and any other error that the manufacturer thinks relevant.

So in essence, RAS and CAS are the same, but CAS only applies when an ADC is in the loop.

Yawn.

[malaysian eaglet]

Sorry Dan,
But CAS apply also for independant instruments.
The main difference between RAS and CAS is that compressibility is taken into account at Sea Level in ISA conditions for CAS and that RAS does not take into account compressibility at all. The graduations on the indicator are not exactly computed the same way.
In altitude, it is a correction due to variation of compressibility with altitude which is applied to CAS and not the whole compressibility error it the reason why some countries name this "error"pressure error" and not compressibility.
I have post this topic because some instructors are confusing the 2 definitions and I am not sure than some examiners are cleared on this subject. I have seen some ambiguous questions in the exams there is a need for clarification.

;)

[northwing]

1. The instrument reads Indicated Air Speed (IAS)

2. Take out the instrument error (Eg. the bent needle) and you get Rectified Air Speed (RAS). (The instrument error is normally minimal, although ASIs only have to be good to plus or minus a couple of knots.)

3. Then take out the Pressure Error Correction and you get Calibrated Air Speed (CAS). In modern large aircraft this will be what appears on your primary display and is then synonymous with IAS.

4. Then take out the scale altitude correction and you get Equivalent Air Speed (EAS) which is the speed at sea level which would feel the same to the aircraft.

5. Then apply a density correction and you get the True Air Speed (TAS) which is the actual speed through the air.

6. With TAS and the wind you get True Ground Speed (TGS) and with TAS and the temperature, which governs the speed of sound, you get Mach number.

7. The pilot these days is unlikely ever to need RAS because the instrument error will almost always be negligible or will be taken out in the ADC before he ever sees it. On some older generation aircraft the difference between IAS and CAS can be significant. CAS is the "truth" as far as ATC and other aircraft are concerned. Eg. If two aircraft are in formation they will both be at the same CAS even though their ASIs may be showing different, and possibly both erroneous IASs.

[Private jet]

CAS is simply an alternative name for RAS, regardless of whether an ADC is used or not. Use of RAS or CAS is predicated on which side of the Atlantic you learnt to fly.
RAS/CAS is IAS with correction for pressure error, which is then corrected again for compressibility to give EAS and non standard air density to give TAS.

[BEagle]

Private jet has provided the clearest and best answer!

:ok:

[Genghis the Engineer]

Private jet's answer certainly tallies with my understanding, although I'd be willing to accept that Northwing's is slightly more technically correct.

G

[Engineer]

Did I miss something or is this thread really 2 years old ? :O

[Genghis the Engineer]

Oh no, it's the phantom poll resurrector again.

G