IAS, CAS or EAS on the PFD

[dream747]

Hi everyone,

I remember reading a thread regarding this before some time ago but I just can't seem to find it now matter how I tried to search for it.

Anyway my question is, does the PFD on our modern jets today display IAS, CAS or EAS? Most people told me that it should be IAS or CAS but not EAS. Correct me if I am wrong, I understand that CAS is IAS corrected for position and instrument errors, and EAS is CAS corrected for compressibility error. Our Vspeeds are in CAS.

Which leads me to the question, say for example, our aircraft has a stalling speed of 200kts. Suppose we're flying at FL350 and should the PFD display IAS/CAS and is reading 220kts (for example), the error due to compressibility is not corrected for and EAS is actually 200kts. Wouldn't this be a danger as the pilot would think that he's 20kts away from stalling the airplane without knowing that he's closer to the stall than he actually thinks?

Many thanks for any inputs!:}

[PBL]

......does the PFD on our modern jets today display IAS, CAS or EAS? ....... I understand that CAS is IAS corrected for position and instrument errors, and EAS is CAS corrected for compressibility error.

Your airspeed indicator displays IAS by definition. The sensors don't sit in the pure freestream. The CAS is what they would display if they did. Modern kit corrects for the difference in the air data computation system before display, so often IAS=CAS.

EAS is not anything corrected for compressibility error. Say the dynamic pressure is Q_actual. EAS is the velocity your aircraft would have if it were to experience Q_actual at sea level.

PBL

[HazelNuts39]

Which leads me to the question, say for example, our aircraft has a stalling speed of 200kts. Suppose we're flying at FL350 and should the PFD display IAS/CAS and is reading 220kts (for example), the error due to compressibility is not corrected for and EAS is actually 200kts. Wouldn't this be a danger as the pilot would think that he's 20kts away from stalling the airplane without knowing that he's closer to the stall than he actually thinks?

The pilot normally doesn't have to consider the "stalling speed". He operates on the basis of minimum speeds recommended for operation such as V1; VR; V2; VREF; VFTO; VLS. These speeds provide a safe margin above the stall speed and are given to him in CAS for electronic displays driven by an Air Data Computer which corrects for errors in the pressure sensing system, or IAS for pneumatic indicators.

The stall speed at FL350 differs considerably from that at sealevel due to Mach effects other than CAS/EAS difference, see discussion on other threads.

regards,
HN39
EDIT:: Quote from FAR/JAR 25.103(a): The reference stall speed VSR is a calibrated airspeed ...

[john_tullamarine]

The pilot normally doesn't have to consider the "stalling speed".

Yet, in abnormal situations, the pilot might well find him/herself there anyway. Best he/she has some knowledge about the situation and, more particularly, how best to get away from there .. without undue embarrassment.

[dream747]

Thanks guys for the responses!:ok: