View Full Version : A320 IAS correction
skywaytoheaven
22nd Oct 2007, 07:15
The speed we fly is mainly refered to by airbus as IAS. So is it correct to assume that the IAS we read on the PFD speed scale is infact CAS/EAS, having been corrected by the ADR?
Tail-take-off
22nd Oct 2007, 09:38
Yes spot on
longobard
22nd Oct 2007, 12:40
no, it should be the IAS
If it's speed is corrected for position, instrument and compressibility errors before displayed on the PFD it is EAS. And it probably is, right?
skywaytoheaven
22nd Oct 2007, 14:54
Yes it probably is, since we dont fly around holding correction charts, but could not find a definite set in stone answer.
Thanks anyway for the help.
ChristiaanJ
22nd Oct 2007, 15:04
Excuse question from oldie.
Does the A320 still have one of those round clock-like thingies directly connected to the pitot, aka as a standby ASI?
whoop.whoop
22nd Oct 2007, 15:15
The new 320s have a Standby EFIS display with Speed and Altimeter Tapes.
http://http://www.jetphotos.net/viewphoto.php?id=6081116&nseq=8
ChristiaanJ
22nd Oct 2007, 15:45
The new 320s have a Standby EFIS display with Speed and Altimeter Tapes.Thanks for the pic, whoop.whoop !
I suppose even the standby now has a little brain, so it probably shows EAS as well, not raw IAS.
Mach trim
25th Oct 2007, 15:35
It is the actual airspeed
FE Hoppy
25th Oct 2007, 15:42
Check your AFM. if no correction charts then its CAS.
on the E-jets 2 systems give CAS the third is IAS with correction charts in the AFM.
downsouth
25th Oct 2007, 16:32
Must be IAS.... Or maybe CAS but not EAS...
ChristiaanJ
25th Oct 2007, 20:34
IAS : raw data
CAS : IAS corrected for instrument calibration and position errors
EAS : CAS corrected for compressibility errors.
Or that's what I've been taught.
Old Smokey
25th Oct 2007, 22:49
ChristiaanJ is absolutely correct. The PFD reads CAS, the Standby Airspeed Indicator PROBABLY reads IAS.
At no time, excepting operations at Sea Level ISA conditions, does the PFD display EAS.
Regards,
Old Smokey
skywaytoheaven
26th Oct 2007, 09:08
So why are no errors associated with compressibility effects in the Pitot tube experienced? Surely if it was just CAS you would get compressibility errors, but you dont, so then it must be EAS????
Old Smokey
26th Oct 2007, 09:42
There is always compressibility error at all levels, instrument calibration removes the compressibility error for ISA / Sea Level. At other levels in the atmosphere, only that degree of CAS Vs EAS error is removed which is attributable to Sea Level, leaving "residual" compressibility effect. Up to about 10,000 feet, the effect is quite small, even at high speed, but at typical jet cruise levels, the "residual" effect is considerable.
As an example, for an aircraft cruising at M0.80 at 36,000 feet, CAS is 265 Kt, whereas EAS is 251 Kt, a 14 knot difference.
Regards,
Old Smokey
longobard
9th Nov 2007, 15:13
As i can remind Airbus AFM reports position error corrections so PFD airdspeed should be IAS.
Old Smokey
10th Nov 2007, 10:00
With respect to the last post, the Air Data Computer (ADC) receives IAS from the raw Pitot and Static information, and then removes position errors to deliver CAS (or RAS if you call it that) to the PFD.
To avert the possibility of ADC error, "raw" IAS is supplied to the standby IAS, thus the standby ASI reads IAS.
In the calibration process (as opposed to the removal of position error), the effects of compressibility according to ISA / Sea Level conditions are removed, thus, except at ISA / Sea Level, the PFD airspeed display does NOT read EAS, but CAS with all of the attendant compressibility errors.
Regards,
Old Smokey