14 CFR § 29.1323 - Airspeed indicating system.
For each airspeed indicating system, the following apply:
(a) Each airspeed indicating instrument 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 system error excluding airspeed instrument error. This calibration must be determined -
(1) In level flight at speeds of 20 knots and greater, and over an appropriate range of speeds for flight conditions of climb and autorotation; and
(2) During takeoff, with repeatable and readable indications that ensure -
(i) Consistent realization of the field lengths specified in the Rotorcraft Flight Manual; and
(ii) Avoidance of the critical areas of the height-velocity envelope as established under § 29.87.
(c) For Category A rotorcraft -
(1) The indication must allow consistent definition of the takeoff decision point; and
(2) The system error, excluding the airspeed instrument calibration error, may not exceed -
(i) Three percent or 5 knots, whichever is greater, in level flight at speeds above 80 percent of takeoff safety speed; and
(ii) Ten knots in climb at speeds from 10 knots below takeoff safety speed to 10 knots above VY.
(d) For Category B rotorcraft, the system error, excluding the airspeed instrument calibration error, may not exceed 3 percent or 5 knots, whichever is greater, in level flight at speeds above 80 percent of the climbout speed attained at 50 feet when complying with § 29.63.
(e) 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.
(f) Each system must have a heated pitot tube or an equivalent means of preventing malfunction due to icing.