14 CFR § 27.1323

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.

The airspeed indicating system must be calibrated in flight at forward speeds of 20 knots and over.

At each forward speed above 80 percent of the climbout speed, the airspeed indicator must indicate true airspeed, at sea level with a standard atmosphere, to within an allowable installation error of not more than the greater of—

±3 percent of the calibrated airspeed; or

Five knots.