of Transportation U.S. Department ADVISORY Federal Aviation Administration CIRCULAR Subject: Turbojet, Turboprop, Turboshaft, Date: 10/22/14 AC No: 20-147A and Turbofan Engine Induction System Initiated by: ANE-110 Icing and Ice Ingestion This advisory circular (AC) provides guidance for demonstrating compliance with the engine induction system icing and engine installation ice requirements of Title 14 Code of Federal Regulations (14 CFR) parts 23, 25, 27, 29, and 33. This AC discusses the icing environments depicted in appendices C and O of part 25, appendix C of part 29, and appendix D of part 33.
This AC discusses turboshaft engine installations, but not the rotary wing aircraft they are installed on.
Colleen M D'Alessandue 'Kim Smith, Acting Manager, Engine & Propeller Directorate Aircraft Certification Service
Contents Paragraph Page
1. Purpose4
2. Applicability4
3. Cancellation4
4. Related Regulations5
5. References and Related Reading Material6
6. Background7
7. Definitions8
8. Discussion11
9. Induction System Icing and Operation Test13
a. Critical Point Analysis13
b. Test Versus Analysis14
c. Test Facilities14
d. Elements of CPA15
e. Test Point(s) Selection17
f. Table 1 of § 33.68 Icing Conditions…17
g. Section 33.68(d) – Ground Operation20
h. Ground Idle Demonstrations21
i. Ice Crystal Icing22
j. Test Setup Considerations22
k. Direct Connect Inlet Engine Test Facilities23
l. Testing in Mixed Phase or Glaciated Atmospheric Conditions23
m. Test Results and Compliance Issues23
n. Mechanical Damage24
o. Engine Systems25
p. Auto-Recovery Systems26
q. Operation Instructions27
r. Special Considerations for Mixed Phase and Glaciated Atmospheric Conditions27
s. Comparative Analysis Guidance for Ice Crystal Icing28
t. Compliance Considerations30
u. Ice Crystal Accretion Explanation30
v. Ice Crystal Icing Assessment30
w. Appendix O of Part 25, Water Impingement31
10. Ice Slab Ingestion35
a. Why the Ice Slab Ingestion Test is Conducted35
b. Compliance Considerations35
c. Validated Analysis with Equivalent Soft Body Tolerance Testing36
d. Elements of a Validated Analysis36
e. Ice Slab Break Up38
f. Test Results39
g. Communication of Test Results40
11.. Induction System Icing Protection40
a. Acceptable Means of Compliance40
b. Inlet Design Point Selection41
c. Addressing Specific Operational Conditions42
d. Two-minute Delayed Selection of Inlet Anti-Ice Accretion Analysis43
e. Extended Operations Performance Standards43
f. Potential Airframe Ice Sources, Including Inlets43
g. Wing Sourced Ice for Aft Mounted Engines44
h. Aft Mounted Engine Installations45
i. Compliance with Mixed Phase and Glaciated Atmospheric Conditions under
§ 25.1093(b)(1)45
j. Compliance with Supercooled Large Drop Conditions under § 25.1093(b)(1)46
k. Natural Icing Flight Tests46
l. Identification of Ice Source47
m. Icing Test Point Monitoring47
n. Compliance47
o. Falling and Blowing Snow48
p. Test Results48
q. Compliance for Auxiliary Power Unit Installations49
12. Conclusion51
Appendix AA-1
Appendix B Advisory CircularB-1
1. Purpose.
a. This advisory circular (AC) provides guidance for demonstrating compliance with the engine induction system icing and engine installation ice requirements of Title 14 Code of Federal Regulations (14 CFR) parts 23, 25, 27, 29, and 33. This AC discusses turboshaft engine installations, but not the rotary wing aircraft they are installed on. See ACs 20-73A, AC 27-1B, and AC 29-2C for additional guidance on engine installation icing issues for rotary wing applications.
b. This AC, 20-147A, replaces AC 20-147, dated February 2, 2004. Applicants should use this AC as their primary guidance concerning icing issues for engines and engine installations for parts 25 and 33, other guidance on the subject notwithstanding. For part 23 airplanes, AC 23- 16A takes precedence on engine installation icing compliance guidance. For parts 27 and 29 rotorcraft, ACs 27-1B, and 29-2C, respectively, take precedence on engine installation guidance (that is, §§ 27.1093 and 29.1093). Additionally, AC 20-73A contains useful information on the understanding and characterization of the icing environment, but it should not be viewed as guidance on engine icing methods of compliance discussed in this AC. It should be noted that neither supercooled large droplet (SLD) (refer to appendix O of part 25) nor Ice Crystals (refer to appendix D of part 33) icing environments are currently required for turboshaft engines and their aircraft installations. It should also be noted that the SLD (refer to appendix O of part 25) icing environment is currently not required for turbine engine induction systems (refer to § 25.1093) on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds, or for part 23 airplanes.
2. Applicability.
a. The guidance provided in this document is directed to engine and airframe manufacturers, modifiers, and foreign regulatory authorities.
b. This material is neither mandatory nor regulatory in nature, and it does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA (we) will consider other methods of demonstrating compliance that an applicant (you) may elect to present. Terms such as “should,” “shall,” “may,” and “must” are used only to ensure the applicability of this particular method of compliance when the method in this document is used. This guidance is derived from extensive FAA and industry experience in determining compliance with the relevant regulations. If we determine that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation as the basis for finding compliance.
c. This material does not change, create any additional, authorize changes in, or permit deviations from existing regulatory requirements.
3. Cancellation. AC 20-147, Turbojet, Turboprop, and Turbofan Engine Induction System Icing and Ice Ingestion, dated 2/2/04, is canceled.
4. Related Regulations.
a. Part 23, Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes, §§ 23.901(d)(2), 23.1093, and 23.1419.
b. Part 25, Airworthiness Standards: Transport Category Airplanes, §§ 25.1091, 25.1093,
25.1419 and 25.1420.
c. Part 27, Airworthiness Standards: Normal Category Rotorcraft, §§ 27.1093.
d. Part 29, Airworthiness Standards: Transport Category Rotorcraft, §§ 29.1093.
e. Part 33, Airworthiness Standards: Aircraft Engines, §§ 33.68, 33.77(c), 33.77(e), 33.89(b), and 33.78.
5. References and Related Reading Material.
a. AC 20-73A, “Aircraft Ice Protection”, dated 8/16/2006.
b. AC 23-16A, “Powerplant Guide for Certification of Part 23 Airplanes and Airships”, dated February 23, 2004.
c. AC 23.1419-2D, “Certification of Part 23 Airplanes for Flight in Icing Conditions”, dated April 19, 2007.
d. AC 27-1B, “Certification of Normal Category Rotorcraft”, dated September 30, 2008.
e. AC 29-2C, “Certification of Transport Category Rotorcraft”, dated September 30, 2008.
f. AC 33-2B, “Aircraft Engine Type Certification Handbook”, dated June 30, 1993.
g. U.S. Department of Transportation. Federal Aviation Administration. Report No. FAA- RD-77-76, “Engineering Summary of Powerplant Icing Technical Data”, by G. D. Pfeiffer and G. P. Maier, dated July 1977.
h. U.S. Department of Transportation. Federal Aviation Administration. Report No. FAA- RD-77-78, “Engineering Summary of Powerplant Icing Technical Data”, dated July 1977.
i. U.S. Department of Transportation. Federal Aviation Administration. Report No.
DOT/FAA/AR-97/66, August, 1998. “Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up To 9 km (30,000 ft)”, by R. K. Jeck.
j. U.S. Department of Transportation. Federal Aviation Administration. Report No.
DOT/FAA/AR-09/13, “Technical Compendium from Meetings of the Engine Harmonization Working Group”, by R. Mazzawy, dated April 2009.
k. U.S. Department of Transportation. Federal Aviation Administration. Report No.
DOT/FAA/AR-03/48, MAY 2003. “Assessment of Effects of Mixed-Phase Icing Conditions on Thermal Ice Protection Systems”, by Kamel Al-KHalil.
l. Royal Aircraft Establishment (Farnborough) Technical Note No: Mech. Eng. 283. “The Analysis of Measurements of Free ice and Ice/Water Concentrations in the Atmosphere of Equatorial Zone”, by Ian I. McNaughton, B.Sc., Dip. R.T.C., dated 1959.
m. “The Icing of an Unheated Non-rotating Cylinder in Liquid Water Droplet-Ice Crystal Clouds”, by E. P. Lozowski, J. R. Stallabrass & P. P. Hearty, National Research Council Canada Report LTR-LT-96, dated February 1979.
n. “Further Icing Experiments on an Unheated Non-rotating Cylinder”, LTR-LT-105, by J.R.
Stallabrass and P. F. Hearty, dated November 1979.
o. “Snow Concentration Measurements and Correlation with Visibility”, by J. R. Stallabrass, AGARD Conference Proceedings No. 236 Icing Testing for Aircraft Engines, dated April 1978.
p. “The Estimation of Snowfall Rate Using Visibility”, by Rasmussen, R.M., J.
Vivekanandan, J. Cole, B. Myers, and C. Masters, 1999: Journal of Applied Meteorology, 38, 1542-1563.
q. “Calibration and Acceptance of Icing Wind Tunnels, Aerospace Recommended Practice (ARP) 5905”, issued 2003-09: Society of Automotive Engineers.
r. “Appendix D – An Interim Icing Envelope”, by R. S. Mazzawy and J. W. Strapp, SAE 2007-01-3311, dated September, 2007.
6. Background.
a. The induction system icing requirements of §§ 23.1093, 25.1093, 27.1093, 29.1093 and of § 33.68 are to provide protection for flight into icing with no adverse effect on engine operation or power. Propulsion systems that meet these requirements when operated in accordance with the aircraft flight manual, have generally demonstrated safe operation upon exposure to natural icing environments.
b. The successful demonstration of the test conditions outlined in the regulation is intended to address many potential engine power conditions. These conditions include aircraft flight conditions, and environmental conditions that could otherwise prove to be costly and difficult to realistically test.
c. A direct comparison between the severity of compliance requirements for icing versus compliance requirements for rain and hail is inappropriate. In § 33.68, the regulation references appendices C and O of part 25, and appendix D of part 33. The environmental threats depicted in all of these appendices have occurred. However, operators have also encountered icing conditions that are more severe. Indeed, the probable occurrence rate of encountering the icing conditions depicted in appendix C of part 25 or 29 is far greater than the remote threat posed by the rain and hail environmental conditions depicted in appendix B of part 33. As a result of these differences in rates of occurrence, as well as fundamental differences of environment and corresponding assumed in-flight profiles, a comparison of compliance requirements across different threat types will be flawed, incongruous, and ultimately inappropriate.
d. The icing environments described in this AC are considered probable to encounter.
Appendices C and O of part 25 and appendix C of part 29, address 99% of supercooled droplet icing conditions; they are based on atmospheric icing data. Additionally, the probability of exceeding both liquid water content (LWC) and median volume diameter (MVD) while operating at or below the temperature specified in both appendices C, of parts 25 and 29 is estimated at approximately 10-3 encounters for each flight; thus a probable encounter. Appendix D of part 33 is derived from the available flight data (see reference in paragraph 5.j. of this AC) and represents a 99% ice crystal condition based on the current data.
e. Finally, the icing environments depicted in each of the referenced appendices define single cloud lengths in terms of 17.4 nautical miles, while nature doesn’t provide icing clouds so discretely. Experience has shown that actual icing environment can be a combination of conditions. For example, as depicted in appendix C of parts 25 and 29, a continuous maximum cloud followed by an intermittent maximum cloud, followed by a continuous maximum cloud, and so on, is probably what a natural environment demonstrates. To account for this difference between natural icing environments and the environments depicted in the appendices, this guidance effectively provides an equivalent icing condition that is representative of multiple clouds in sequence.
f. The body of this AC is arranged in three areas corresponding to the applicable parts (that is, §§ 33.68, 33.89(b), 33.77, 23.1093, 25.1093, 27.1093, and 29.1093 ). The first area addresses part 33 induction system icing compliance guidance (§ 33.68 compliance), in paragraph 9 of this
AC. The second area addresses engine ice slab ingestion compliance guidance (§ 33.77 compliance), in paragraph 10 of this AC. The third area addresses installation icing requirements under parts 23, 25, 27, and 29, in paragraph 11 of this AC.
g. The icing conditions depicted in appendix O of part 25, and appendix D of part 33 are not applicable to turboshaft engines or their installations. Turboshaft engines need only comply with the icing conditions depicted in appendix C of part 29.
h. The icing conditions depicted in appendix O of part 25 are not applicable to turbine engine induction systems on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds, or on part 23 airplanes. Turbine engine induction systems on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds need only comply with the icing conditions depicted in appendix C of part 25, appendix D of part 33, and in falling and blowing snow within the limitations established for the airplane for such operation.
7. Definitions. The following are defined for the purpose of this AC:
a. Auto-Recovery Systems. Engine systems that ensure that engines operate just before or immediately after an upset (that is, power loss or stall) without operator intervention. Autorecovery systems include auto-relight systems, stall recovery systems, and other engine system intended to recover the operability of an engine following a flameout, surge, stall, or a combination of these.
b. Freezing Fraction. The ratio or percentage of water that impacts a surface and freezes.
The freezing fraction is defined as a number between 0 and 1, and will determine the type of ice formation.
c. Cloud Extent Factor. The distance that a cloud extends vertically (vertical extent) or horizontally (horizontal extent). Vertical extent is normally measured in feet while horizontal extent is measured in nautical miles. The cloud extent factor is a dimensionless number, which uses the length of a cloud to determine an average liquid water content across the cloud.
d. Highlight Area. The area bounded by the leading edge of the nacelle inlet. This may be different for turboshaft installations where complex inlet schemes are utilized.
e. Ice Formations. These result from the impact of supercooled water droplets on propulsion system surfaces; formations are classified as follows:
(1) Glaze Ice. A clear, hard ice, which forms at temperatures close to (but below) freezing, in air with high liquid water content and large droplet sizes. Droplets impacting the surface do not freeze immediately, but run back along the surface until freezing occurs. Glaze ice typically has a non-aerodynamic shape and is more susceptible to aerodynamic forces that result in shedding. Glaze ice typically has both a lower freezing fraction and lower adhesive properties than rime ice. Glaze ice is often a concern for static hardware while rime ice is often a concern for rotating hardware.
(2) Rime Ice. A milky white ice which forms at low temperatures, in air with low liquid water content and small droplet sizes. Rime ice typically forms in an aerodynamic shape, on both rotating and static engine hardware. The freezing fraction is high for rime ice, typically approaching a value of 1.0. Rime ice typically has greater adhesion properties than glaze ice but often a lower density. Adhesion properties increase with lower temperature up to a test point where no additional adhesion is gained with additional lower temperature.
(3) Mixed or Intermediate Ice. A combination of glaze and rime ice which forms with rime patches slightly aft of the glaze ice portions. This ice forms at temperatures, liquid water contents, and droplet sizes between those that produce rime and glaze ice.
f. Ice Shed Cycles. The time period required to build up and then shed ice on a propulsion system surface for a given power and icing condition. A shed cycle can be identified visually (for example, with high-speed cameras, and engine instrumentation such as vibration pickups, temperature probes, pressure probes, speed pickups, etc.). For rotating surfaces, such as fan blades, the ice shed cycle is strongly influenced by rotor speed and the adhesive strength of ice to the surface. In general, ice adhesive strength increases as surface temperature decreases.
g. Icing Conditions. These meteorological conditions are defined by the following parameters:
(1) Liquid Water Content (LWC). The concentration of liquid water in air, typically expressed in grams of water for each cubic meter of air.
(2) Median Volume Diameter (MVD). The drop diameter which divides the total water volume present in a droplet distribution in half (that is, half the water volume is in larger drops and half the volume in smaller drops). Note the MVD used in appendix O of part 25, and appendix D of part 33, is equivalent to the MED in appendix C of parts 25 and 29 for an assumed Langmuir type droplet distribution.
(3) Mean Effective Diameter (MED). Similar to MVD. See definition of MVD.
(4) Median Mass Dimension (MMD). The particle size (sphere of equivalent mass) which divides the total ice mass present in an ice particle distribution in half (that is, half the ice mass is in larger particles and half the ice mass is in smaller particles).
(5) Total Temperature. The ambient temperature plus the ram rise. For icing testing in test cells, the total engine inlet temperature includes static temperature of the cloud as depicted in appendices C and O of part 25, appendix C of part 29, and appendix D of part 33, plus the assumed flight airspeed.
(6) Static Temperature. The ambient temperature calculated from the local measured total temperature on the aircraft, minus the temperature rise from velocity effects. Appendices C and O of part 25, appendix C of part 29, and appendix D of part 33 temperatures are assumed to be static ambient temperatures.
h. Power Loss Instabilities. These are caused by engine operating anomalies. These types of anomalies could include non-recoverable or repeating surge, stall, rollback, or flameout, which can result in engine power or thrust cycling. A rotating compressor stall is one kind of power loss instability that might be acceptable, upon review with the FAA: if it is not noticeable to the flight crew, and if it does not result in unacceptable stresses or operability effects either at steady-state conditions, or during acceleration.
i. Scoop Factor (concentration factor). The ratio of the nacelle inlet highlight area (AH) to the area of the captured air stream tube (AC) [scoop factor = AH/AC]. Scoop factor compares the liquid water available for ice formation in the engine inlet, to that available in the low-pressure compressor or engine core, as a function of aircraft forward airspeed and engine power condition. The scoop factor effect depends on the droplet diameter, the simulated airspeed, and the engine power level, as well as the geometry and size of the engine. This may be different for turboshaft installations where complex inlet schemes are utilized.
j. Serious Loss of Power or Thrust. A non-recoverable or repeating engine surge, stall, rollback, or flameout occurrence, which can result in noticeable engine power or thrust loss.
k. Stabilized Operation. This is when the engine demonstrates steady, reliable, and smooth operation while operating at the test condition (that is, during multiple ice build or shed cycles, if ice is accreting), and during throttle or power transients. Engine operation is considered stabilized when measured engine parameters are not changing, or when a regular repeatable shed cycle has been demonstrated through the recording of measured engine parameters with stabilized ice accretions.
l. Stabilized Ice Accretion. A condition when ice accretion is not increasing on any engine part, or when the accreting ice has demonstrated a regular shed cycle when viewed by a video camera or instrumentation indication.
m. Sustained Power Loss. A permanent loss in engine power or thrust. Typically, sustained power loss is calculated at rated takeoff power. See definitions below on “Temporary Power Loss” and “Momentary Power Loss” for power or thrust losses that are not sustained.
n. Descent Idle Engine Speed. An altitude-dependent, minimum flight-idle engine speed that is generally in effect from the top of descent (TOD) to where the approach phase of flight begins.
This is typically when flaps are deployed and idle speed increases for an approach. Since this is different for turboshaft installations, the applicant can propose appropriate criteria.
o. Momentary Power Loss. A short-duration reduction in engine power or thrust associated with a transient event (for example, ice shedding).
p. Noticeable. A tactile feeling during an event, or the use of typical engine test instrumentation or flight deck instrumentation (such as, N1, N2, vibrations, exhaust gas temp) to indicate an event.
q. Temporary Power Loss. A reduction or loss in engine power or thrust occurring during an icing encounter; this may be related to the effects of ingesting supercooled water or ice particles,
or possibly the effects of ice accumulation within the flowpath. The amount of temporary power loss should be communicated to the installer from the engine manufacturer.
r. Power rollback. An uncommanded reduction in engine rotor speeds, and subsequent loss of power and power control.
8. Discussion.
a. An icing encounter, including a prolonged encounter, should neither cause a significant increase in workload, nor elicit concern from the flight crew. Additionally, an icing encounter should not result in damage to the engine, its systems or subsystems, loss of performance or an operability deficiency, or any other compromise of compliance with the engine certification basis. Relevant changes in bill of material hardware or software made during the compliance program should be re-evaluated with respect to icing.
(1) The engine should have sufficient durability to operate through prolonged or repeated environmental icing encounters without special operational or maintenance interventions. For example, operational procedures to assist ice shedding, such as throttle manipulation or power change, should not be used to comply with the in-flight icing requirements of parts 23, 25, 27, 29 and 33.
(2) The applicant may provide procedures for engine throttle manipulation or power change (for example, power run-ups) to shed ice accumulated during ground operations. These procedures will be used as in-service ground operation recommendations, although they would be mandatory if used during the compliance demonstration of §§ 33.68, 23.1093, 25.1093,
27.1093, and 29.1093.
(3) Applicants should ensure they collect sufficient data to demonstrate regulatory compliance. Video or photographic coverage may be useful for supplementing test results. In addition, applicants should determine the most critical measured parameters which indicate ice accretion and shedding within the engine. These parameters provide critical data for showing regulatory compliance during the compliance plan. In addition they help validate the critical point analysis (CPA) assumptions and demonstration intent. The parameters may include both visual and instrumented indications that need to be monitored during the icing test to show acceptable operation in icing conditions.
(a) This demonstration should include stabilized ice accretion (that is, stabilized operation) with either no ice buildup or no additional ice buildup on the engine or nacelle inlet.
Normal engine control system responses during the ice accumulation process (for example, isochronous control responses to accreting ice) are acceptable if no sustained power losses are encountered. Any significant temporary power loss during operation in icing conditions should be reported to the installer by the engine manufacturer.
b. Mixed Phase or Glaciated Atmospheric Conditions. The FAA and the aviation community have recently become aware of the potential effects of mixed phase icing conditions. They occur when mixtures of supercooled liquid water droplets and ice particles (mixed phase) or ice
crystals alone (glaciated) exist in a cloud, often around the outskirts of a deep convective cloud formation. Ice crystal icing conditions exist when all the liquid water particles in the cloud have frozen into ice particles. Mixed phase and ice crystal icing has caused more than 100 turbine engine power losses. Compliance methodologies for these conditions are being introduced in this AC for the first time for turbojet and turbofan engines. These conditions do not apply to turboshaft engines.
(1) Traditionally, industry has considered the susceptibility of turbine engines to mixed phase or ice crystal conditions as minimally consequential, with two exceptions: (1) pronounced inlet bends (such as particle-separator inlets) or inlet flow reversals, and (2) high solidity, high turning front stage compressor stators. Industry has recognized the inlet bends are a concern since that is where inlet flow can stagnate and accumulate ice. Similarly, industry has recognized that high solidity, high turning front stage compressor stators are a concern because stator airfoils are susceptible to non-aerodynamic ice buildup. These flowpath blockages and significant turns can result in accretion zones and core airflow blockage from ice accretion.
(2) Recently, operators have reported service experience on fixed wing applications with icing within the engine core stream due to mixed phase and glaciated atmospheric conditions.
These reported service events included ice crystal accretion within the engine’s flowpath and within engine inlet probes, including temperature and pressure probes. This ice accretion has produced significant adverse impacts, including uncommanded engine power rollback or flameout, with occasional core hardware damage.
(3) Operators have also reported total air temperature (TAT) probe malfunctions during many of these engine icing events. A heated TAT probe malfunction is a known indicator of ice crystals. Ice accumulation within the TAT probe produces a false TAT of air temperature near 320 Fahrenheit (00 degrees Celsius). Ice detection systems should be evaluated for these conditions.
(4) Events attributed to icing within the engine core stream due to mixed phase or ice crystal conditions, and TAT probe malfunctions, suggests that ice crystal icing is of greater concern than originally thought. The root cause of these events can be traced to ice buildup within the core flowpath of the affected engine. In general, these events occur near convective clouds at ambient temperatures warmer than the international standard atmosphere, and outside the icing envelopes depicted in appendices C and O of part 25, and appendix C of part 29.
(5) We have established appendix D of part 33 to define the mixed phase or glaciated atmospheric envelope.
c. Auto-Recovery Systems. Compliance with §§ 33.68 and 33.77 is intended to demonstrate no flameout, sustained power loss, surge or stall, or rundown will occur when operating in icing conditions. The auto-recovery provides protection in-service against multiple forms of power loss. However, auto-recovery protections should not be relied on during the compliance demonstration because auto-recovery systems are back-up devices. For additional information about the use of auto-recovery systems when demonstrating compliance to §§ 33.68 and 33.77, see paragraphs 9.p., and 10.f.(5) of this AC. For additional information on auto-recovery