(1) Acceptable Damage. The extent of damage is equivalent to or less than what was incurred and accepted during the engine certification testing.
(2) All Systems Operating Normally. The extent of damage is equivalent or less than what was incurred and accepted during the § 33.68 tests.
(3) Delayed Activation of Induction System Anti-Icing. If ice ingestion tests under § 33.77 do not adequately represent the particular airframe installation, then the delayed antiicing system activation test should be considered (caution should be used and all safety precautions exercised). For this condition, the acceptance criteria described in paragraph 10 of this AC should be used. The airframe manufacturer still must consider all potential ice shedding sites (for example, inboard wing and radome).
(a) Similar to the § 33.77 ice slab ingestion test compliance, the use of engine autoignition and recovery systems are allowed when showing compliance with the delayed activation tests of parts 23, 25, 27, or 29, as long as these automatic systems cannot be easily turned off by the flight crew.
(b) The difference in anti-iced inlets versus de-iced inlets is significant. Anti-iced inlets are designed so that no ice accretes on the inlet while operating in icing conditions. De-iced inlets allow some ice accretion, but are designed for a cyclic shedding of ice from the engine inlet into the engine. De-iced inlets typically incorporate, as part of their design, an inlet particle-separator that stops the ingestion of ice into the core of the engine. Engine autorecovery systems should not be a compensating design feature utilized to minimize the negative effects of an inadequate particle-separating inlet that is not in full compliance with §§ 23.1093,
25.1093, 27.1093, or 29.1093.
(4) Damage From Testing in Non-Representative Icing Conditions. When damage results from icing test conditions that fall significantly outside the icing envelopes depicted in appendices C and O of part 25, appendix C of part 29, or appendix D of part 33, or when the flight test is conducted in an abnormal manner and results in excessive ice shed damage, this may result in a test failure relative to the pretest pass or fail criteria. Any abnormal conditions should be discussed with the FAA to determine if the test can be deemed “passed”. An example of an abnormal operation is flying with one engine at idle while the aircraft is operated in level flight.
(5) Unacceptable Damage. The icing test conditions are representative of in-service encounters, but the resultant airframe or engine ice sheds caused damage that exceeded the acceptance criteria described in paragraph 8.a.(1) of this AC.
q. Compliance for Auxiliary Power Unit (APU) Installations. Compliance for an auxiliary power unit (APU) installation is demonstrated under §§ 23.901(f), 25.901(d) or 29.901(d), but
APUs must also be addressed and shown as acceptable for installation under §§ 23.1093(b),
25.1093(b) or 29.1093(b) (see section 11 of this AC). An essential APU is used to provide air and/or power necessary to maintain safe aircraft operation. A non-essential APU is used to provide air and/or power as a matter of convenience and may be shut down without jeopardizing safe aircraft operation. The icing conditions depicted in appendix O of part 25 are not applicable to APU induction systems on airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds.
(1) Non-Essential APU. An applicant for a transport category airplane with a nonessential APU installation that is restricted from use in icing conditions is not required to show compliance with § 25.1093(b). This includes the icing conditions depicted in appendices C and O of part 25, appendix D of part 33, and in falling and blowing snow conditions. An applicant for a transport category airplane with a non-essential APU installation that is not restricted from use in icing conditions or falling and blowing snow must show compliance with § 25.1093(b).
To do this, you must demonstrate that the APU’s operation will not affect the safe operation of the airplane when subject to those conditions, both on the ground and in-flight.
(2) Essential APU. An applicant for a transport category airplane with an essential APU installation is required to demonstrate that it can safely operate in natural icing conditions, both on the ground and in-flight. Thus, if you are applying for a transport category airplane icing approval with an essential APU installation, you must show compliance with § 25.1093(b). This includes the icing conditions depicted in appendices C and O of part 25, and appendix D of part 33, and in falling and blowing snow within the limitations established for the airplane for such operation.
(a) Acceptable operation of an APU in the icing conditions depicted in appendix C to part 25 should be demonstrated using the same compliance methods for turbine engine installations described in this AC.
(b) Acceptable operation in the icing conditions depicted in appendix O to part 25 and appendix D to part 33, should be demonstrated using the same compliance methods for turbine engine installations described in this AC. However, many transport category airplane APU installations have an inlet located in the empennage section of the airplane fuselage that can generate a shadowing effect, and thereby prevent large icing droplets (SLD) and ice crystals from entering the APU inlet. For this type of installation, the § 25.1093 compliance for the icing environment depicted in appendix O to part 25 and appendix D to part 33, may be demonstrated by an acceptable validated analysis. To be acceptable, a validated analysis should show that icing conditions depicted in appendix O to part 25 and appendix D to part 33, are adequately addressed by the compliance demonstration for the conditions depicted in appendix C to part 25.
(c) APU compliance with the falling and blowing snow conditions, within the limitations established for the airplane for such operation, should be demonstrated using the same compliance methods for turbine engine installations described in this AC.
12. Conclusion. Although applicants may conduct representative tests under §§ 33.68, 33.77,
23.1093, 25.1093, 27.1093, and 29.1093, flight test events during aircraft certification may still occur which appear inconsistent. In all likelihood, these results will not be inconsistent when evaluated based on the scope, intent, and limitations of the certification testing. Only through appropriate test criteria, appropriate critical point selection, reliable instrumentation, and adequate documentation, can the impact of icing on turbine engines be fully addressed. Because of the relative frequency with which operators experience icing encounters, and because of the potential adverse impact on safety, we recommend all applicants adopt a conservative approach when establishing icing testing plans and demonstrations of compliance.
Appendix A
APPENDIX A Reference: Low engine speed part 33 compliance testing (part 33 turbofan and turboprop engines only)
1. The Federal Aviation Administration (FAA; us), Engine and Propeller Directorate, Standards Staff, ANE-110, has been requested to provide a position relative to test time duration during § 33.68 icing tests at non-flight sustainable engine speeds. The following paragraphs provide a technical rationale for the standardized compliance test times for Appendix C conditions. The FAA’s position has traditionally been, and continues to be, that engines are held to a higher standard than aircraft for icing certification and the engines must demonstrate unrestricted and unlimited operation in icing conditions. Therefore, during the § 33.68 compliance test demonstration, engines must remain on-point until stable operation is demonstrated.
Some applicants have described this requirement as being burdensome when operating the engine at non-flight sustainable rotor speeds (meaning, below lightweight hold engine operation). In response to this observation, the FAA has developed criteria that allow for a reasonable test period and termination time for icing testing at non-flight sustainable engine speeds. It is emphasized that this criteria is only applicable to icing testing when the engine is operating at non-flight sustainable engine speeds.
2. The maximum intermittent maximum liquid water content (LWC) severity depicted in appendix C of part 25 is over three times more severe than the continuous maximum LWC. Due to the significant differences in severity between intermittent maximum and continuous maximum icing conditions depicted in appendix C of part 25, two separate criteria have been considered. They are: (1) short test sequences for the more severe intermittent maximum conditions, and (2) longer test sequences for continuous maximum conditions.
3. Appendix C of part 25, paragraph (a) - Continuous Maximum Icing Conditions. In an effort to preserve flight safety in icing conditions, the FAA strives to retain a margin of safety through suitable demonstrations of compliance. We have established the criteria for part 33 icing testing through conservative assessments, the better to ensure that compliance demonstrations are reflective of real world conditions. Our rationale is detailed below.
a. Conservative test perspective. Engine designs should be capable of operations exceeding the single cloud definitions depicted in appendix C of part 25. A robust design is necessary, since an encounter of LWC at a stated inlet temperature, at any mean effective diameter (MED) is a probable event for the cloud length specified. This expected occurrence rate is too frequent to be addressed by tests durations based on a single cloud extent. Thus for demonstration of compliance with § 33.68, we use a test duration representative of multiple cloud extents.
b. Conservative test period: icing test demonstrations at non-flight sustainable speeds. The FAA has developed a conservative methodology for assessing the worst case condition to be expected in service (that is, long descent loiter). The following analysis is intended to show the rationale behind the test duration we have selected for low engine speed testing under § 33.68.
This analysis is not based on expected or representative flight profiles or typical icing conditions.
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(1) A typical descent gradient for pilots is 3 miles horizontal transit for every 1000-foot vertical descent (15.8:1 gradient). Approximately, this works out to a 60-mile descent initiation test point for a 20,000-foot descent, with a nominal descent slope of about 3.6 degrees. If a conservative 2-degree descent slope is considered, then the descent initiation would start at 120 miles out for a 22,000-foot vertical descent. A total descent altitude of 22,000 feet above ground level (agl) was used because Appendix C goes from sea level to 22,000 feet, although it is recognized that icing occurs at altitudes greater than 22,000.
(2) In order to convert a descent gradient into a meaningful descent rate in feet each minute, a horizontal airspeed must be assumed. For this conservative analysis, an average horizontal air speed of 220 knots (250 miles per hour) is assumed. This average airspeed is for the complete descent through all 22,000 feet, and assumes periodic head winds and profile variations for all types of fixed wing aircraft. This makes the assumption more conservative for transport category aircraft and less conservative for small lightweight propeller aircraft. This is a conservative approximation above 10,000 feet agl, where speed is not limited.
(3) At a 2-degree descent gradient and 220 knots (250 mph) horizontal air speed, the result of these inputs is about a 750 feet descent each minute from 22,000 feet, for a total time of 30 minutes. Consequently, when stabilized operation has not been demonstrated, it is reasonable that engine manufacturers should test engines at flight-idle and up to engine speeds that result in non-sustainable flight (that is, less than light hold power) for a total of not more than 30-minutes of continuous maximum icing operation in the conditions depicted in appendix C of part 25.
4. Appendix C of part 25, part 1, paragraph (b) - Intermittent Maximum Icing Conditions. The intermittent maximum icing conditions depicted in appendix C of part 25, are more severe than the continuous maximum conditions. Just as the previous continuous maximum discussion provides for multiple cloud extents in a series, the same philosophy will be applied here for intermittent maximum conditions, to assure robust design demonstrations. The increased severity of an intermittent cloud depicted in appendix C, in conjunction with the reduced horizontal and vertical extents of these clouds in appendix C, necessitates a proportionately reduced maximum compliance test demonstration period.
The FAA has a very long and successful historical experience base for compliance test demonstrations in intermittent maximum icing conditions. This historical basis has shown that a 10-minute test period has been sufficient. Additionally, the 10-minute period would represent about five to ten sequential intermittent maximum clouds on a horizontal extent basis, depending on aircraft speed, as depicted in appendix C of part 25.
5. Conclusion: Appendix A of this AC provides the rationale for the standard test time durations described in this AC and the icing rule (14 CFR 33.68). In summary, shorter compliance test demonstration periods have been rationalized here for low power descent conditions and for intermittent maximum conditions where there are shorter cloud extents.
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Appendix B
APPENDIX B. Advisory Circular Feedback Information
Advisory Circular Feedback Information
If you have comments or recommendations for improving this advisory circular (AC), or suggestions for new items or subjects to be added, or if you find an error, you may let us know about by using this page as a template and 1) emailing it to 9-AWA-AVS-AIR500- Coord@faa.gov or 2) faxing it to the attention of the AIR Directives Management Officer at 202- 267-3983.
Subject (insert AC number and title) Date: (insert date)
Comment/Recommendation/Error: (Please fill out all that apply)
An error has been noted:
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Recommend paragraph ______ on page ______ be changed as follows:
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In a future change to this advisory circular, please include coverage on the following subject:
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