IAS, CAS or EAS on the PFD
Thread Starter
Join Date: Nov 2006
Location: Tropics
Posts: 359
Likes: 0
Received 0 Likes
on
0 Posts
IAS, CAS or EAS on the PFD
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!
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!
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by dream747
......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.
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
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Operational speeds
Originally Posted by dream747
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 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 ...
Last edited by HazelNuts39; 13th Aug 2010 at 13:17. Reason: FAR/JAR quote
Moderator
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.
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.
