A320 IAS correction
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A320 IAS correction
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?
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The new 320s have a Standby EFIS display with Speed and Altimeter Tapes.
http://http://www.jetphotos.net/view...6081116&nseq=8
http://http://www.jetphotos.net/view...6081116&nseq=8
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The new 320s have a Standby EFIS display with Speed and Altimeter Tapes.
I suppose even the standby now has a little brain, so it probably shows EAS as well, not raw IAS.
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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
At no time, excepting operations at Sea Level ISA conditions, does the PFD display EAS.
Regards,
Old Smokey
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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????
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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
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
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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
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