Unless operations at night in icing conditions are prohibited by an operating limitation, CS 25.1403 requires that a means be provided, during flight at night, to illuminate or otherwise determine ice formation on parts of the wings that are critical from the standpoint of ice accumulations resulting from Appendix C and Appendix O icing conditions. For showing compliance with the CS-25 certification specifications relative to SLD icing conditions represented by Appendix O, the applicant may use a comparative analysis. AMC 25.1420(f) provides guidance for comparative analysis. a. If the flight crew cannot see the wings, one acceptable means of compliance with this regulation would be to install an ice evidence probe in a position where the flight crew can observe ice accumulation. The applicant should substantiate that formation of ice on this device precedes formation of ice on the wings or occurs simultaneously with it. Consideration should be given to the need for illuminating the ice evidence probe. b. Wing icing detection lights should be evaluated both in and out of clouds during night flight to determine that the component of interest is adequately illuminated without excessive glare, reflections, or other distractions to the flight crew. These tests may be accomplished during the aeroplane certification flight tests. Typically, aeroplane-mounted illumination has been used to comply with this regulation. Use of a hand-held flashlight has not been considered acceptable because of the associated workload. The appropriate manual should identify the ice characteristics which the flight crew is expected to observe as well as the action the flight crew must perform if such ice is observed.
[Amdt 25/16]
[Amdt 25/18]