from FAA (2005)
TSO C16, Air-speed tubes (electrically heated), requires compliance
to the performance specifications of SAE Aerospace Standard AS393. SAE
AS393A includes a test to demonstrate deicing and anti-icing capability, but only temperature and airspeed are specified. Liquid water content is not specified but it influences heat requirements. Although functioning of pitot probes are evaluated in natural icing conditions during certification test programs, there is no requirement to flight test at the Appendix C or X icing limits because the low probability of finding those conditions imposes a burden. The airframe manufacturer is responsible for showing the pitot heat is adequate for the Appendix C and Appendix X icing conditions. If not obtained in flight test, analysis or icing tunnel test data should be submitted.
Although Appendices C and X of part 25 only consider the liquid
water content of icing conditions, recent cloud characterization research has indicated that approximately 40 percent of icing condition events consist of liquid water drops and ice crystals (mixed-phase icing conditions). The ice crystal environment may be more critical than liquid water for thermal systems since more energy is required to evaporate the ice crystals.