#### § 33.68 Induction system icing.

Each engine, with all icing protection systems operating, must:

(a) Operate throughout its flight power range, including the minimum descent idle rotor speeds achievable in flight, in the icing conditions defined for turbojet, turbofan, and turboprop engines in Appendices C and O of [part 25 of this chapter](/on/2024-09-19/title-14/part-25), and [Appendix D of this part](/on/2024-09-19/title-14/part-33/appendix-Appendix%20D%20to%20Part%2033), and for turboshaft engines in Appendix C of [part 29 of this chapter](/on/2024-09-19/title-14/part-29), without the accumulation of ice on the engine components that:

(1) Adversely affects engine operation or that causes an unacceptable permanent loss of power or thrust or unacceptable increase in engine operating temperature; or

(2) Results in unacceptable temporary power loss or engine damage; or

(3) Causes a stall, surge, or flameout or loss of engine controllability. The applicant must account for in-flight ram effects in any critical point analysis or test demonstration of these flight conditions.

(b) Operate throughout its flight power range, including minimum descent idle rotor speeds achievable in flight, in the icing conditions defined for turbojet, turbofan, and turboprop engines in Appendices C and O of [part 25 of this chapter](/on/2024-09-19/title-14/part-25), and for turboshaft engines in Appendix C of [part 29 of this chapter](/on/2024-09-19/title-14/part-29). In addition:

(1) It must be shown through Critical Point Analysis (CPA) that the complete ice envelope has been analyzed, and that the most critical points must be demonstrated by engine test, analysis, or a combination of the two to operate acceptably. Extended flight in critical flight conditions such as hold, descent, approach, climb, and cruise, must be addressed, for the ice conditions defined in these appendices.

(2) It must be shown by engine test, analysis, or a combination of the two that the engine can operate acceptably for the following durations:

(i) At engine powers that can sustain level flight: A duration that achieves repetitive, stabilized operation for turbojet, turbofan, and turboprop engines in the icing conditions defined in Appendices C and O of [part 25 of this chapter](/on/2024-09-19/title-14/part-25), and for turboshaft engines in the icing conditions defined in Appendix C of [part 29 of this chapter](/on/2024-09-19/title-14/part-29).

(ii) At engine power below that which can sustain level flight:

(A) Demonstration in altitude flight simulation test facility: A duration of 10 minutes consistent with a simulated flight descent of 10,000 ft (3 km) in altitude while operating in Continuous Maximum icing conditions defined in Appendix C of [part 25 of this chapter](/on/2024-09-19/title-14/part-25) for turbojet, turbofan, and turboprop engines, and for turboshaft engines in the icing conditions defined in Appendix C of [part 29 of this chapter](/on/2024-09-19/title-14/part-29), plus 40 percent liquid water content margin, at the critical level of airspeed and air temperature; or

(B) Demonstration in ground test facility: A duration of 3 cycles of alternating icing exposure corresponding to the liquid water content levels and standard cloud lengths starting in Intermittent Maximum and then in Continuous Maximum icing conditions defined in Appendix C of [part 25 of this chapter](/on/2024-09-19/title-14/part-25) for turbojet, turbofan, and turboprop engines, and for turboshaft engines in the icing conditions defined in Appendix C of [part 29 of this chapter](/on/2024-09-19/title-14/part-29), at the critical level of air temperature.

(c) In addition to complying with [paragraph (b)](/on/2024-09-19/title-14/section-33.68#p-33.68(b)) of this section, the following conditions shown in Table 1 of this section unless replaced by similar CPA test conditions that are more critical or produce an equivalent level of severity, must be demonstrated by an engine test:

Table 1—Conditions That Must Be Demonstrated by an Engine Test

| Condition | Total air temperature | Supercooled water concentrations (minimum) | Median volume drop diameter | Duration |
| --- | --- | --- | --- | --- |
| 1. Glaze ice conditions | 21 to 25 °F (−6 to −4 °C) | 2 g/m3 | 25 to 35 microns | (a) 10-minutes for power below sustainable level flight (idle descent). |
| | | | | (b) Must show repetitive, stabilized operation for higher powers (50%, 75%, 100%MC). |
| 2. Rime ice conditions | −10 to 0 °F (−23 to −18 °C) | 1 g/m3 | 15 to 25 microns | (a) 10-minutes for power below sustainable level flight (idle descent). |
| | | | | (b) Must show repetitive, stabilized operation for higher powers (50%, 75%, 100%MC). |
| 3. Glaze ice holding conditions (Turbojet, turbofan, and turboprop only) | Turbojet and Turbofan, only: 10 to 18 °F (−12 to −8 °C) | Alternating cycle: First 1.7 g/m3 (1 minute), Then 0.3 g/m3 (6 minute) | 20 to 30 microns | Must show repetitive, stabilized operation (or 45 minutes max). |
| | Turboprop, only: 2 to 10 °F (−17 to −12 °C) | | | |
| 4. Rime ice holding conditions (Turbojet, turbofan, and turboprop only) | Turbojet and Turbofan, only: −10 to 0 °F (−23 to −18 °C) | 0.25 g/m3 | 20 to 30 microns | Must show repetitive, stabilized operation (or 45 minutes max). |
| | Turboprop, only: 2 to 10 °F (−17 to −12 °C) | | | |

(d) Operate at ground idle speed for a minimum of 30 minutes at each of the following icing conditions shown in Table 2 of this section with the available air bleed for icing protection at its critical condition, without adverse effect, followed by acceleration to takeoff power or thrust. During the idle operation, the engine may be run up periodically to a moderate power or thrust setting in a manner acceptable to the Administrator. Analysis may be used to show ambient temperatures below the tested temperature are less critical. The applicant must document any demonstrated run ups and minimum ambient temperature capability in the engine operating manual as mandatory in icing conditions. The applicant must demonstrate, with consideration of expected airport elevations, the following:

Table 2—Demonstration Methods for Specific Icing Conditions

| Condition | Total air temperature | Supercooled water concentrations (minimum) | Mean effective particle diameter | Demonstration |
| --- | --- | --- | --- | --- |
| 1. Rime ice condition | 0 to 15 °F (−18 to −9 °C) | Liquid—0.3 g/m3 | 15-25 microns | By engine test. |
| 2. Glaze ice condition | 20 to 30 °F (−7 to −1 °C) | Liquid—0.3 g/m3 | 15-25 microns | By engine test. |
| 3. Snow ice condition | 26 to 32 °F (−3 to 0 °C) | Ice—0.9 g/m3 | 100 microns (minimum) | By test, analysis or combination of the two. |
| 4. Large drop glaze ice condition (Turbojet, turbofan, and turboprop only) | 15 to 30 °F (-9 to -1 °C) | Liquid—0.3 g/m3 | 100 microns (minimum) | By test, analysis or combination of the two. |

(e) Demonstrate by test, analysis, or combination of the two, acceptable operation for turbojet, turbofan, and turboprop engines in mixed phase and ice crystal icing conditions throughout [Appendix D of this part](/on/2024-09-19/title-14/part-33/appendix-Appendix%20D%20to%20Part%2033), icing envelope throughout its flight power range, including minimum descent idling speeds.

[Amdt. 33-34, [79 FR 66536](https://www.federalregister.gov/citation/79-FR-66536), Nov. 4, 2014]