Originally Posted by 172510
1/2 (Density of the ICAO atmosphere at MSL ) CAS^2 = Total Pressure - Static Pressure
Is it the official definition of what a speed indicator should indicate?
No, it isn't. If you replace CAS by EAS (equivalent airpeed) the formula defines EAS. It is only valid at relatively low speeds, where the the airflow can be considered to be incompressible. The definition of CAS is based on compressible flow, where the relation between total pressure and airspeed is more complicated. At standard sealevel: Pt/Ps = (1+(kCAS/1479.1)^2)^3.5, where kCAS is CAS expressed in knots.
The difference between IAS and CAS is due to errors in the measurement of pitot and static pressures. The difference between the pressure at the static port and the ambient static pressure is called 'position error' and is corrected by applying the PEC (position error correction). The error in the pitot pressure is often negligible but may need to be corrected when the pitot tube has water drains.
But I wondered if there are any certification requirements? To what extent a difference between IAS and CAS is allowed? Is it documented somewhere?
For transport category airplanes, paragraph 25.1323 specifies:
(c) The airspeed error of the installation,
excluding the airspeed indicator instrument
calibration error, may not exceed 3% or five knots,
whichever is greater, throughout the speed range,
from –
(1) VMO to 1·23 VSR1 with wing-flaps
retracted; and
(2) 1·23 VSR0 to VFE with wing-flaps in the
landing position.
(d) From 1·23 VSR to the speed at which stall
warning begins, the IAS must change perceptibly
with CAS and in the same sense, and at speeds below
stall warning speed the IAS must not change in an
incorrect sense. (See AMC 25.1323 (d).)
(e) From VMO to VMO + 2
3 (VDF – VMO) the IAS
must change perceptibly with CAS and in the same
sense, and at higher speeds up to VDF the IAS must
not change in an incorrect sense. (See AMC 25.1323
(e))
(f) There must be no indication of air-speed
that would cause undue difficulty to the pilot during
the take-off between the initiation of rotation and the
achievement of a steady climbing condition.
(g) The effects of airspeed indicating system
lag may not introduce significant takeoff indicated
airspeed bias, or significant errors in takeoff or
accelerate-stop distances.