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Atlas / eCFR / Part 33 / Appendix

Appendix

Appendix D to Part 33—Mixed Phase and Ice Crystal Icing Envelope (Deep Convective Clouds)

Part 33 — AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES Amended: 2026-04-24 View on eCFR ↗

The ice crystal icing envelope is depicted in Figure D1 of this Appendix.

Source: eCFR ER04NO14.0080 · Original on federalregister.gov ↗

Within the envelope, total water content (TWC) in g/m 3 has been determined based upon the adiabatic lapse defined by the convective rise of 90% relative humidity air from sea level to higher altitudes and scaled by a factor of 0.65 to a standard cloud length of 17.4 nautical miles. Figure D2 of this Appendix displays TWC for this distance over a range of ambient temperature within the boundaries of the ice crystal envelope specified in Figure D1 of this Appendix.

Source: eCFR ER04NO14.0090 · Original on federalregister.gov ↗

Ice crystal size median mass dimension (MMD) range is 50-200 microns (equivalent spherical size) based upon measurements near convective storm cores.

The TWC can be treated as completely glaciated (ice crystal) except as noted in the Table 1 of this Appendix.

Table 1—Supercooled Liquid Portion of TWC
Temperature range—deg C Horizontal cloud length—nautical miles LWC—
g/m3
0 to −20 ≤50 ≤1.0
0 to −20 Indefinite ≤0.5
< −20 0

The TWC levels displayed in Figure D2 of this Appendix represent TWC values for a standard exposure distance (horizontal cloud length) of 17.4 nautical miles that must be adjusted with length of icing exposure.

Source: eCFR ER04NO14.0100 · Original on federalregister.gov ↗

[Amdt. 33-34, 79 FR 65538, Nov. 4, 2014]