Request answer for this question on airspeed indicator

[aviator82]

Q Compressiblity error

A Causes overreading of the ASI at all levels whenever TAS exceeds 300kt.

B Causes overreading of the ASI whenever CAS exceeds 300kt.

C is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and CAS exceeds 300kt.

D is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and TAS exceeds 300kt.

As per my understanding answer is one of A or D as compressibility error occurs at TAS more than 300kts, also airspeed indicator is calibrated at seal level for density error, but I am not sure if its A or D.
Please list the answer and the reason also. Thanks alot in advance for the help :)

[Get-a-Grip]

Hi,

This is my understanding of compressibility......

It has to be taken into consideration when Either;

1) TAS > 300kts
2) CAS > 200Kts
3) Mach > 0.4m

The ASI is calibrated for standard ISA condidtions (hence why at sea level in ISA conditions if you fly at SLOW speed IAS/CAS = EAS = TAS).

The ASI gives you a reading of IAS/CAS - therefore you would not want to 'calibrate' out compressibility - as if you did you are then showing EAS, which is not what the purpose of the ASI is.

Hope this helps!

G-A-G

[saurabh sharma]

Compressebility error always causes air speed indicator to over read at a TAS of 300 knots or above. This is because at such high speed air is brought to rest at pitot tube and is compressed . Due to which total pressure which is measured by pitot tube increases and thus causing air speed indicator to over read.

Hope this clears your doubt

[selfin]

...except that modern airspeed indicators are designed for compressible flow and the whole argument about 'compressibility corrections' is largely irrelevant here.* The solution derived from Euler's dynamic equations and the equation of state by Jean Claude Barré de Saint-Venant (used in modern FAA/EASA-approved ASIs) is valid below M 1.0, and reasonably accurate until about M 1.3, after which the Rayleigh 'supersonic' Pilot tube formula should be used.

* See Old Smokey's 2006-01-12 post on dynamic -v- impact pressure here: http://www.pprune.org/questions/205710-help-dynamic-pressure-formula.html#post2318100

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See also NACA Technical Report 837 (http://naca.central.cranfield.ac.uk/report.php?NID=2451) (Langley, 1946).

[Pugilistic Animus]

Oh Yeah Selfin that was a true Classic:D:D:D
I have that post in my 'notes':)
PA