Text in force at CS25-28: from CS25-27, effective 7 Dec 2021.
(See AMC 25.1323) For each airspeed indicating system, the following apply:
(a) Each airspeed indicating instrument must be approved and must be calibrated to indicate true airspeed (at sea-level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) Each system must be calibrated to determine the system error (that is, the relation between IAS and CAS) in flight and during the accelerated takeoff ground run. The ground run calibration must be determined –
(1) From 0·8 of the minimum value of V1, to the maximum value of V2, considering the approved ranges of altitude and weight; and
(2) With the wing-flaps and power settings corresponding to the values determined in the establishment of the take-off path under CS 25.111 assuming that the critical engine fails at the minimum value of V1.
(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 + ⅔ (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. (h) Each system must be arranged, so far as practicable, to prevent malfunction or serious error due to the entry of moisture, dirt, or other substances. (See AMC 25.1323(h).) (i) Reserved (j) Where duplicate airspeed indicators are required, their respective pitot tubes must be far enough apart to avoid damage to both tubes in a collision with a bird.
ED Decision 2021/015/R · Source