#### § 25.1323 Airspeed indicating system.

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 flaps and power settings corresponding to the values determined in the establishment of the takeoff path under [§ 25.111](/on/2024-09-19/title-14/section-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 three percent or five knots, whichever is greater, throughout the speed range, from—

(1) *V**MO* to 1.23 *V**SR*1, with flaps retracted; and

(2) 1.23 *V**SR*0 to *V*FE with 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.

(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.

(f) There must be no indication of airspeed that would cause undue difficulty to the pilot during the takeoff 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.

(i) Each system must have a heated pitot tube or an equivalent means of preventing malfunction in the heavy rain conditions defined in Table 1 of this section; mixed phase and ice crystal conditions as defined in part 33, Appendix D, of this chapter; the icing conditions defined in [Appendix C of this part](/on/2024-09-19/title-14/part-25/appendix-Appendix%20C%20to%20Part%2025); and the following icing conditions specified in [Appendix O of this part](/on/2024-09-19/title-14/part-25/appendix-Appendix%20O%20to%20Part%2025):

(1) For airplanes certificated in accordance with [§ 25.1420(a)(1)](/on/2024-09-19/title-14/section-25.1420#p-25.1420(a)(1)), the icing conditions that the airplane is certified to safely exit following detection.

(2) For airplanes certificated in accordance with [§ 25.1420(a)(2)](/on/2024-09-19/title-14/section-25.1420#p-25.1420(a)(2)), the icing conditions that the airplane is certified to safely operate in and the icing conditions that the airplane is certified to safely exit following detection.

(3) For airplanes certificated in accordance with [§ 25.1420(a)(3)](/on/2024-09-19/title-14/section-25.1420#p-25.1420(a)(3)) and for airplanes not subject to [§ 25.1420](/on/2024-09-19/title-14/section-25.1420), all icing conditions.

Table 1—Heavy Rain Conditions for Airspeed Indicating System Tests

| Altitude range | | Liquid water content | Horizontal extent | | Droplet MVD |
| --- | --- | --- | --- | --- | --- |
| (ft) | (m) | (g/m3) | (km) | (nmiles) | (μm) |
| 0 to 10 000 | 0 to 3000 | 1 | 100 | 50 | 1000 |
| | | 6 | 5 | 3 | 2000 |
| | | 15 | 1 | 0.5 | 2000 |

(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.

[Doc. No. 5066, [29 FR 18291](https://www.federalregister.gov/citation/29-FR-18291), Dec. 24, 1964, as amended by Amdt. 25-57, [49 FR 6849](https://www.federalregister.gov/citation/49-FR-6849), Feb. 23, 1984; Amdt. 25-108, [67 FR 70828](https://www.federalregister.gov/citation/67-FR-70828), Nov. 26, 2002; Amdt. 25-109, [67 FR 76656](https://www.federalregister.gov/citation/67-FR-76656), Dec. 12, 2002; Amdt. 25-140, [79 FR 65526](https://www.federalregister.gov/citation/79-FR-65526), Nov. 4, 2014]