U.S. Department

of Transportation

Federal Aviation

Administration

ADVISORY 

CIRCULAR

Subject: Turbojet, Turboprop, Turboshaft, 

and Turbofan Engine Induction System 

Icing and Ice Ingestion

Date: 10/22/14

Initiated by: ANE-110

AC No: 20-147A

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. 

If you have suggestions for improving this AC, you may use the Advisory Circular Feedback

form at the end of this AC. 
Contents

Paragraph Page 

1. Purpose.............................................................................................................................. 1

2. Applicability..................................................................................................................... 1

3. Cancellation...................................................................................................................... 2

4. Related Regulations.......................................................................................................... 2

5. References and Related Reading Material........................................................................ 2

6. Background....................................................................................................................... 4

7. Definitions......................................................................................................................... 5

8. Discussion......................................................................................................................... 8

9. Induction System Icing and Operation Test..................................................................... 11

a. Critical Point Analysis............................................................................................ 11

b. Test Versus Analysis.............................................................................................. 11

c. Test Facilities......................................................................................................... 12

d. Elements of CPA.................................................................................................... 12

e. Test Point(s) Selection........................................................................................... 14

f. Table 1 of § 33.68 Icing Conditions...................................................................... 15

g. Section 33.68(d) – Ground Operation................................................................... 17

h. Ground Idle Demonstrations................................................................................. 18

i. Ice Crystal Icing.................................................................................................... 19

j. Test Setup Considerations..................................................................................... 20

k. Direct Connect Inlet Engine Test Facilities.......................................................... 20

l. Testing in Mixed Phase or Glaciated Atmospheric Conditions............................ 20

m. Test Results and Compliance Issues...................................................................... 21

n. Mechanical Damage.............................................................................................. 21

o. Engine Systems...................................................................................................... 23

p. Auto-Recovery Systems......................................................................................... 23

q. Operation Instructions............................................................................................ 24

r. Special Considerations for Mixed Phase and Glaciated

Atmospheric Conditions............................................................................... 24 

s. Comparative Analysis Guidance for Ice Crystal Icing........................................... 25

t. Compliance Considerations.................................................................................... 28

u. Ice Crystal Accretion Explanation.......................................................................... 28

v. Ice Crystal Icing Assessment.................................................................................. 28

w. Appendix O of Part 25, Water Impingement.......................................................... 29

10. Ice Slab Ingestion.............................................................................................................. 33

a. Why the Ice Slab Ingestion Test is Conducted....................................................... 33

b. Compliance Considerations..................................................................................... 33

c. Validated Analysis with Equivalent Soft Body Tolerance Testing........................ 34

d. Elements of a Validated Analysis........................................................................... 34

e. Ice Slab Break Up.................................................................................................... 36

f. Test Results............................................................................................................. 37

g. Communication of Test Results.............................................................................. 38
11. Induction System Icing Protection................................................................................... 38

a. Acceptable Means of Compliance...........................................................................38

b. Inlet Design Point Selection.................................................................................... 39

c. Addressing Specific Operational Conditions.......................................................... 40

d. Two-minute Delayed Selection of Inlet Anti-Ice Accretion Analysis.................... 41

e. Extended Operations Performance Standards......................................................... 41

f. Potential Airframe Ice Sources, Including Inlets.................................................... 41

g. Wing Sourced Ice for Aft Mounted Engines...........................................................42

h. Aft Mounted Engine Installations........................................................................... 43

i. Compliance with Mixed Phase and Glaciated Atmospheric Conditions under

§ 25.1093(b)(1).............................................................................................. 43

j. Compliance with Supercooled Large Drop Conditions under § 25.1093(b)(1)...... 44

k. Natural Icing Flight Tests........................................................................................44

l. Identification of Ice Source..................................................................................... 45

m. Icing Test Point Monitoring.................................................................................... 45

n. Compliance..............................................................................................................45

o. Falling and Blowing Snow...................................................................................... 46

p. Test Results............................................................................................................. 47

q. Compliance for Auxiliary Power Unit Installations................................................48

12. Conclusion........................................................................................................................ 49

Appendix A...............................................................................................................................A1-1
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. FAARD-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. FAARD-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 
systems when demonstrating compliance to §§ 23.1093, 25.1093, 27.1093, and 29.1093, see 

paragraph 11.p.(3)(a) of this AC.

 d. Application of Cloud Extent Factors for § 33.68 Compliance Tests. Cloud extent factors 

are depicted in appendices C and O of part 25, appendix C of part 29, and appendix D of part 33. 

The horizontal cloud extent factor is a dimensionless number, which relates the length of a cloud 

to an average LWC across the cloud. It is used in the CPA to assess the probable icing content

during various aircraft mission and performance analyses.

 (1) Typically, the average LWC of a cloud is less for longer clouds. The horizontal cloud 

extent factor is not intended to be used to limit the severity of exposure to icing conditions, 

assuming the aircraft will be required to operate in that condition. For example, when in a

holding pattern which may require repeated passes through a severe icing environment, or 

continuously remain in that severe environment, the horizontal cloud extent factor should not be 

applied.

 (a) Engines and their inlet systems should demonstrate continuous operation in icing 

conditions without regard to time. Typically, an applicant assumes multiple clouds in their CPA, 

with a cloud extent factor equal to 1.0. This is because actual cloud extent factors are not a 

consideration for most engine operations, particularly in an aircraft hold pattern. This approach 

assures proper engine and induction system operation within the atmospheric conditions depicted

in appendices C and O of part 25, appendix C of part 29, appendix D of part 33, and as 

experience indicates, in actual icing environments. The following are some additional factors to 

consider:

 1. These cloud extent factors are also applicable to airframe flight profiles. Airplane 

applicants use the cloud extent factors depicted in the appendices during their evaluation of the

straight-line flight portion of their compliance demonstration. However, engines and induction 

systems have historically not been limited to or evaluated against a specific aircraft flight profile 

when considering icing environments. Instead, they are evaluated for unlimited operation in 

icing.

 2. While appendix C of parts 25 and 29 define supercooled clouds, appendix O of

part 25 includes freezing precipitation (that is, rain and drizzle).

 3. Appendix D of part 33 depicts ice crystals and mixed conditions (meaning, a 

mixture of supercooled water droplets and ice crystals). The cloud extent factor depicted in 

appendix D of part 33 relies primarily on limited data derived from the reference in paragraph

5.k. of this AC. The data covers several geographical regions including eastern Asia where ice 

crystal-related service events are prevalent.

9. Induction System Icing and Operation Test (§§ 33.68 and 33.89(b)).

 a. Critical Point Analysis (CPA). A CPA is an analytical method utilizing engine test data to 

show an engine meets the certification requirements of part 33. This method derives critical test 

points from the collection and analysis of data on icing effects on engine performance. 
Compliance with § 33.68 includes identifying, through analysis, the critical operating test points 

for icing within the declared operating envelope of the engine. A CPA should include 

consideration of the icing conditions depicted in appendix C of parts 25 and 29, and as we gain 

experience, in appendix D of part 33. A CPA should relate these appendices with the aircraft 

speed range and engine powers defined by the engine manufacturer. It should also include a 

prolonged flight operation in icing (for example, in-flight hold pattern), or repeat icing 

encounters. These combined elements within the CPA should identify the most critical 

operational icing conditions.

 (1) The applicant (you) should ensure that analysis is supported by test data. Your CPA 

should also include environmental and engine operational effects on accumulation, accretion 

locations, and the most critical engine operating conditions for ice shed and ingestion. You may 

propose conditions outside the requirements of the appendices (for example, recommending 

conditions that are more severe based on actual service experience). You may also supplement a

CPA with development test data (for example, wet and dry testing with thermocouple 

components).

 (2) Your CPA should include ice accretion calculations that account for freezing fraction 

and aerodynamic effects of the ice as it moves into the air inlet. For example, water ingestion 

into the fan module and core inlets, water impingement rates for critical surfaces, forward 

aircraft airspeed effects, engine configuration effects such as inter-compressor bleed, and altitude 

effects (such as bypass ratio effects). The CPA should also include an energy balance of critical 

engine surfaces (for example, latent heat and heat of fusion effects, metal-to-ice heat transfer 

effects, and ice insulating effects).

 (3) For anti-iced parts, the CPA should identify a critical test point determined from 

energy balance calculations of required heat loads, and encompassing the range of possible 

combinations of icing condition and engine power. In glaze ice conditions, assessing the effects 

of non-aerodynamic ice formations and their shedding is more complex. FAA Report number

FAA-RD-77-78, titled “Engineering Summary of Powerplant Icing Technical Data”, provides

additional guidance on performing a critical test point icing analysis.

 b. Test Versus Analysis. The CPA is not meant to replace testing. Rather, it provides a

means to predict critical test points. The CPA test points can replace the standard table points 

defined in Tables 1 and 2 of § 33.68, when applicants can show they are equivalent. Otherwise 

they supplement the standard certification test points. Today’s analysis tools are greatly 

improved over their predecessors; however, experience has shown the CPA is best used to 

predict the critical icing conditions for a given design, then use them in conjunction with the 

standard certification test points for certification compliance test purposes. Please note, even if 

the FAA concurs with a type certificate holder’s generic CPA method, this does not constitute 

our acceptance of the resulting critical icing test points for future certification projects. The 

content of an icing certification program for any given certification project will be evaluated on a 

case-by-case basis.

 c. Test Facilities. Facility limitations or weather can delay § 33.68 compliance 

demonstrations. These delays may often be avoided by using national and international icing 
test facilities. National and international icing test facilities for accomplishing engine icing 

compliance testing include the United States Air Force (USAF) McKinley Climatic Lab in 

Florida, the Arnold Engineering Development Center (AEDC) in Tennessee, and the Canada’s

National Research Council in Ottawa, the Global Aerospace Center for Icing and Environment 

(GLACIER), and GE Aviation Engine Testing, Research and Development Centre (TRDC) in 

Winnipeg. In Europe, there is the Delegation Generale Pour L’Armement (DGA) Aero Engine 

test facility in France. Other test facilities are likely to become available in the future as test 

capabilities improve. 

 (1) When considering any icing facility, the capability to provide simulated cloud

conditions that meet the requirements of § 33.68 should be thoroughly considered. Critical items 

that that you should also consider are:

 (a) A description of the icing facility, including the spray system arrangement, spray 

nozzle(s), water flow system, airflow system, and test facility operation.

 (b) Instrumentation should include facility operation monitoring, cloud simulation, and 

cloud property determination.

 (c) Air/water flow operating map.

 (d) Psychrometrics.

 (e) Pre-test icing rig functional checks.

 (f) Facility test procedures.

 (g) Test data measurement accuracy and capability description.

d. Elements of CPA. Your CPA should address, at minimum, the following icing issues:

 (1) Ice Shed Damage. Shed ice can cause engine damage if it impacts an engine surface 

with sufficient mass and velocity. The following types of damage are common, and you should

include them in your CPA with an assessment of each:

 (a) First Stage of Compressor (example, fan). Various rotating and non-rotating parts 

of the fan module, or first stage compressor section for non-fan engines, are susceptible to ice 

shed damage. For example, acoustic panels, fan rub strips, and fan blade tips are susceptible to 

ice shed from the inlet sensor(s), spinner, and the fan blade root.

 (b) Since engine icing compliance testing is performed in test cells and not actual flight 

conditions, the effects of ice density, hardness, and adhesion strength as it sheds should be 

assessed to realistic flight conditions. For example, in realistic flight conditions the ice shed 

cycle for rotating surfaces such as fan blades, or first stage compressor blades for non-fan 

engines, is strongly influenced by the rotor speed and the adhesive strength of the ice to the 

surface. The adhesive strength of ice generally increases with decreasing surface temperature. 
The ice thickness and the rotor speed at the time of the shed, defines the impact threat. Available 

data typically shows that shedding from unheated rotating and static parts can be quite variable 

and difficult to predict. This is due to uncertain factors such as adhesion strength properties, 

local thermodynamic properties, and ice structure (such as ice bridging). In some cases,

applicants have utilized consistently demonstrated shedding trends in their CPA, as opposed to

absolute values, since there is often uncertainty in precise shed predictions.

 (c) When determining the critical conditions for fan module damage, the surface 

temperature, exposure time, and rotor speed are important considerations, as well as atmospheric

icing conditions and scoop factor. In particular, extended operation in a holding condition power 

level during very cold and continuous maximum icing conditions will maximize the adhesion of 

ice on rotating first stage compressor or fan components. This can result in large ice accretions 

and resulting sheds which can damage the engine or cause power loss.

 (2) Compressor Damage. When ice formations on static components shed, they often 

result in damage. This type of damage generally occurs on the first blade set in the high-pressure 

compressor (that is, intermediate pressure compressor for three spool engines, or first stage of 

compression for conventional turboshaft engines). Establishing the critical conditions for these 

glaze ice accretions requires careful consideration, since they occur at specific limited conditions 

of low freezing fractions, over a range of local mach numbers and air densities. The critical 

conditions may not occur during any of the power settings discussed in this AC (for example, 

flight-idle, 50% and 75% of maximum continuous or 100% maximum continuous);

consequently, you should evaluate the power setting at the critical condition. Finally, since icing 

is a common environmental condition, you should evaluate any engine compressor damage that 

results from ice testing against the possibility of multiple occurrences.

 (3) Engine Operability and Compressor Rematch. Ice shed from upstream components 

may enter the core compressor. The presence of ice, or water from melted ice, in the gas path 

may cause engine component cycle changes. The engine should be capable of accelerating from 

minimum flight-idle to takeoff power, and should demonstrate takeoff power set procedures on 

the ground at any icing condition, without unacceptable power loss or power loss instabilities. 

Ice sheds should not result in flameout, rollback, or surge. Any anomalous engine behavior 

should be raised to the FAA Engine or Aircraft Certification Office (ECO or ACO) for 

evaluation, and if found acceptable, it should be documented in the engine’s installation manual. 

You should consider as part of the CPA, the engine accelerations and decelerations relative to 

operability challenges (for example, surge and stall). Assume that the minimum engine bleed 

schedule allowed for the condition being tested minimizes the operability margin. CPA testing 

should demonstrate those conditions where the minimum operability margin is expected.

 (4) Core and Booster Ice Blockage. Ice accretion on internal engine vanes from glaze ice 

accretions may affect airflow capacity and rematch of the engine cycle. This should be 

considered in your CPA. For engine powers that can sustain flight, you should reconcile ice 

accretion through a demonstration of several ice build shed cycles. Show that there are no 

adverse operating effects from ice builds or sheds.
 (5) Sensor Fouling. Ice accretion and blockage of control sensors can result in erroneous 

engine pressure and temperature measurements. Critical sensors include inlet total pressure and 

temperature probes, and inter-compressor temperature probes. A power loss or power loss 

instability could result if erroneous measurements are used by the engine control system’s 

control law to establish power or thrust ratings, or if used to schedule other systems required to 

operate the engine (for example, variable stator vanes). Critical sensors should be designed to

operate when exposed to the conditions defined in 33.68 and the associated appendices, with 

minimal ice accretion and no erroneous measurements that would result in an unacceptable 

operating characteristic, such as a power loss. In accordance with 33.68 icing requirements, 

turboshaft engines need only be evaluated against the environmental icing conditions depicted in

appendix C of part 29. The effects of installation on the local icing conditions at the probe 

should be accounted for in all types of applicable installations. Additionally, ice accretion on 

upstream sensors can shed and cause engine damage to downstream rotating hardware. You 

should evaluate engine inlet probes for supercooled droplet icing susceptibility, as well as ice 

crystal icing susceptibility.

 e. Test Point(s) Selection. Test points selected for a supercooled droplet environment should

address the applicable icing envelopes depicted in appendices C and O of part 25, and appendix 

C of part 29. Test points for an ice crystal icing environment should be representative of the 

meteorological conditions depicted in appendix D of part 33. Typically, the supercooled droplet 

test points include those defined in the regulation (either § 33.68 or § 25.1093) and any 

additional test points identified as part of your CPA. You should consider pertinent service 

experience as well as the anticipated use of the aircraft when selecting critical icing test points. 

Do consider the following when constructing an icing test matrix:

 (1) Section 33.68, Acceptable Means of Compliance. The engine should be capable of 

operating acceptably under the meteorological conditions depicted in appendices C and O of part 

25, and appendix C of part 29, covering the engine operating envelope defined in 

§ 33.68(b)(2)(ii)(B), and under the conditions of ground icing defined in § 33.68(d). Section

33.68(c) permits eliminating specific standard certification test points so long as you choose to 

run a similar CPA test condition that is more severe in terms of ice accretion mass at critical 

engine locations or to produce an equivalent level of safety to the standard test points.

 (2) Rotorcraft Turboshaft Engines. Engines installed on rotorcraft present their own 

unique conditions. Holding phase test conditions are not applicable to turboshaft applications. 

Therefore, turboshaft engine testing should include test conditions that address prolonged hover 

or continued operation in a relatively small area of operation. For example, prolonged 

operations in the local vicinity of the airport or possibly local operations around oil rigs at sea. 

This may require additional test, analysis, or both, to show installation configuration dilution 

benefits in prolonged operations, based on the aircraft induction system design.

 f. Table 1 of § 33.68 Icing Conditions.

 (1) The supercooled LWC shown in Table 1 of § 33.68 represents ambient icing conditions 

for an open inlet ground test facility or equivalent, within the inlet duct of a direct connect test 
facility. An analysis of local enrichment for flight effects, as would normally be done for a CPA 

LWC, is not necessary for the table points.

 (2) For Table 1 test conditions 1 and 2, applicants should run the engine under both rime 

and glaze icing conditions. You should test under both conditions for at least 10 minutes each at

100%, 75%, and 50% maximum continuous power, and for 10 minutes at a flight-idle. Follow 

each power setting with a snap acceleration to takeoff power. If ice is building at the end of 10

minutes at the three higher power settings, continue running the test point until the engine 

demonstrates stabilized building and shedding, or until the engine no longer operates

satisfactorily.

 (3) Table 1, conditions 1 and 2 are intended to partially represent a broad test matrix of 

environmental and engine operating conditions to be used when showing compliance with 

§ 33.68. This test matrix includes power settings from idle to 100% maximum continuous. They 

are representative of exposure to conditions typical of both high altitude where rime ice 

formations occur, as well as conditions typical of low altitude where glaze ice formations often 

occur.

 (a) Icing conditions are normally run for a minimum of 10 minutes for all engine 

powers that can sustain level flight, or longer if the natural engine ice shed cycle is not 

established. Special consideration and tests should be conducted to adequately demonstrate

engine inlet screens and inlet air passages that might accumulate snow or ice due to restrictions 

or contours. At low power, such as idle descent power, the required time period is limited to 10

minutes. The rime icing condition should be run at an engine speed associated with top of 

descent operation for fixed wing aircraft, as depicted in appendix C of part 25. The glaze icing 

condition should be run at the minimum engine speed associated with lower altitudes at the end 

of the descent phase of flight for fixed wing applications. Applicants for rotorcraft applications

should propose an appropriate flight profile to address this low power test period duration.

 (4) Conditions 3 and 4 (holding phase). The engine should continue to meet part 33

requirements while demonstrating it can operate indefinitely in a flight hold pattern under icing 

conditions. Guidance on test durations and procedures to achieve this are outlined throughout 

this AC. The test program for turbofan and turboprop applications should include test point

conditions (for example, icing conditions and power settings) to address the effects of prolonged 

exposure in icing conditions that are typical of in-flight holding patterns.

 (a) Condition 4 in Table 1 represents a rime icing condition that is typically 

encountered on transport category airplanes. Condition 3 in Table 1 represents a mixed rime and 

glaze icing condition that was originally derived from the European Union’s JAR-E ice 

requirements of LWC, temperature, and droplet size. The engine and inlet should be capable of 

prolonged exposure to the conditions specified in Table 1. For fixed wing aircraft, a 45-minute 

test exposure will typically demonstrate several ice shed cycles. You should follow each 

accretion point for condition 3 and 4 with a snap acceleration to takeoff power. To demonstrate 

unlimited icing operation, the engine should exhibit stabilized operation at the conclusion of the 

45-minutes test condition, prior to the snap acceleration.
 (5) For All Conditions. Engine operation in all icing conditions should be uneventful, 

uninterrupted, and without any significant adverse effects. It should also include the ability to 

continue in operation and accelerate and decelerate normally without adverse operational effects. 

Some power reduction is acceptable at idle power settings in icing conditions, due to the cycle 

effects of pumping ice and water; however, all other operation should be unaffected.

 (a) You should determine what parameters need to be monitored to determine the 

stabilized operation of the engine during the icing test. Engine operation is considered stabilized

either when measured engine parameters are not changing, or when a regular, repeatable shed 

cycle has been demonstrated through the recording of measured engine parameters. Variations 

in measured parameters are acceptable during the performance of the ice test, as long as the 

long-term trend (typically the duration of several shed cycles) is stable and not trending upwards 

or downwards.

 (b) Engines with manually activated icing protection systems (including systems for 

probes), should be tested. You may test these systems by stabilizing the engine for at least 2

minutes in the icing atmosphere (1-minute for turbo shaft engines) with the protection systems 

off (that is, prior to turning the icing protection system on). Conduct this test at flight-idle and 

above. Doing so simulates the delay expected for the pilot to recognize the icing condition.

 (c) Fully automatic systems may use an appropriate delay. Systems that are automatic 

and controlled by the full authority digital electronic engine control (FADEC) do not require the 

2-minute delay in the ice protection system demonstration. Where the engine’s anti-icing system 

relies on an ice detector to indicate the presence of icing conditions, a delay in the anti-ice 

selection is likely. Therefore, delayed selection testing should still be demonstrated.

 (d) Stable engine operation should occur under the tested icing conditions. A stable 

engine operation is intended to address both stabilized ice accretion and shed cycles during 

steady-state engine operation. Ice accretions are considered stabilized when either ice is not 

forming on any engine part or the accreting ice has demonstrated a regular shed cycle when 

viewed by a video camera, or as indicated by instrumentation. Engine operation is considered 

stabilized when the measured engine parameters are not changing, or a regular and repeatable 

shed cycle has been demonstrated through the recording of measured engine parameters. You 

may provide a justification to the FAA for a performance change while operating a steady-state 

test point. Momentary performance changes such as a thermodynamic engine response to shed

ice ingestion may be acceptable. These momentary performance changes are due to an ice shed 

which can momentarily affect the thermodynamic cycle.

 (6) Turboprop and Turboshaft Engines Equipped with an Inlet Screen. You may perform 

engine icing tests utilizing the aircraft inlet system or alternately with a bellmouth. Some 

airframe installations utilize an inlet screen, which in some cases is included as part of the testing 

for the part 33 compliance demonstration. The inlet screen compliance demonstration for icing 

may be addressed separately from the § 33.68 engine ice compliance testing. If addressed 

separately, then you should note the requirement for an engine inlet screen in the engine 

installation manual.
 (a) If the inlet screen is not addressed under § 33.68, then compliance could be 

demonstrated under the installation requirements of §§ 23.1093, 25.1093, for turboprop, and 

§§ 27.1093, or 29.1093, for turboshaft installations. Icing testing that utilizes inlet screens 

should demonstrate acceptable operation, including multiple shed cycles representative of longterm or unlimited operation in icing conditions. Protruding or unusual inlet features may require 

specialized tests or analysis to assure they don’t adversely affect operations in icing conditions.

 (b) At the conclusion of each steady-state icing test point, the engine should be 

accelerated to takeoff power (throttle or power lever movement of one second or less) or 

shedding should be induced by other methods, depending on the critical shed methodology. The 

throttle or power lever motion should be the most critical when considering the ice shed effects 

on engine operability. In some cases, a quick deceleration before accelerating to takeoff power 

may be more critical to the ice shed effects on engine operation. Assess this effect and account 

for your assessment in your test proposal.

 g. Section 33.68(d) – Ground Operation. Section 33.68(d) provides two icing test points that 

represent freezing fog conditions; a falling or blowing snow icing encounter during ground 

operation; and a supercooled large droplet (SLD) test point that represents freezing rain or 

drizzle. The SLD condition is similar to freezing fog in terms of ambient temperature range and

the LWC levels. However, the larger droplets can penetrate farther back on the surface of the 

engine spinner with the potential for ice shedding into the fan blades.

 (1) The ground-fog icing, SLD, and falling or blowing snow demonstrations of § 33.68(d) 

should continue for at least 30 minutes, or until acceptable operation is demonstrated. If you 

cannot achieve acceptable operation then you should demonstrate the periodic engine speed runups. These run-ups will become mandatory in the engine operating manual and aircraft flight 

manual for operating in these icing conditions, since they were required to comply with the icing 

testing requirements. Turboshaft engines do not need to address SLD conditions.

 (2) Falling and Blowing Snow. Service experience has shown that compressor damage 

occurs as a result of exposure to prolonged periods of falling snow ingestion during ground 

operation. Based on our review of service events we have found that airports have continued to 

operate with falling snow concentrations that result in 0.25 mile or less visibility (about 0.9 

gm/m3

of snow). While visibility in snow can be a poor indicator of precipitation rates (see 

reference in paragraph 5.o. of this AC), the following calculation is based on equivalent rainfall 

rate and gives a similar result as the 0.25 mile visibility criteria:

 (a) Visibility and Snow Fall Rates. The maximum precipitation rate for moderate snow 

is equivalent to 2.5 millimeters per hour (mm/hr) of rainfall over approximately 30 minutes. 

Using a typical fall speed for snow of 0.8 meters per second (m/s), this translates into a snow/ice

concentration of 0.9 gm/m3

. From a Transport Canada data set of 338,000 minutes of snowfall 

data, the 95% and 99% values were 2 and 4 mm/hr respectively, showing that a 2.5 mm/hr 

threshold provides an extreme value of snowfall rates. Holdover time tables for de-icing fluids 

are only good for the extreme values of moderate snow, defined as 2.5 mm/hr (or a visibility of 

1⁄4 of a statute mile). This is equivalent to the engine certification levels described here.
 (b) Engine core icing service events in snow conditions confirm the critical snow 

accretion temperature range is 250 to 320 Fahrenheit (-40 to 00 Celsius). The engine service 

events have demonstrated that a snow environment is conducive to ice accretion behind the fan 

in front compressor stages of the engine, at low engine power. The engine test for snow 

conditions was developed to represent the engine conditions where snow can form accretions aft 

of the fan on the core inlet and first stages. You should choose a temperature within the range 

provided above to achieve icing behind the fan.

 h. Ground Idle Demonstrations. Section 33.68 requires operation at ground idle setting for at 

least 30-minutes under the icing conditions defined in § 33.68(d), followed by acceleration to 

takeoff power or thrust. Since a broad temperature range is provided within the regulation, you

should identify the most critical temperature, as determined by the CPA, and target that range. 

The CPA analysis can be used to demonstrate that colder conditions are less critical to the 

engine’s operation than the CPA condition that is demonstrated during ground icing tests.

Turboprop and turboshaft engine inlet screen icing compliance demonstrations may be addressed 

separately from the § 33.68 engine ice compliance testing. If so, you should note the 

requirement for an engine inlet screen in the engine installation manual. We recommend you

give special attention to these inlet features:

 (1) Unless data is provided indicating otherwise, the minimum ground idle engine speed 

will be considered the most critical engine operating condition. Unless a more critical ground 

operating speed is identified, you should show that the engine continues to perform acceptably at 

its minimum ground idle speed attainable within all icing conditions. This test demonstration 

generally establishes the maximum allowable ground icing operation time for the engine. In 

addition, this test usually establishes the maximum allowable time in the engine operation 

instructions between engine run-ups to shed ice. This time may be prescribed as a single 

duration or, if supported by test conduct and results, the maximum time interval allowed between 

performing a repetitive, intentional, ice shedding engine run-up procedure. You should 

demonstrate stabilized ice accretion, either naturally or with run-ups, to achieve a ground icing 

taxi time which is appropriate for long taxi times in icing conditions.

 (2) Engine ice shed characteristics (for example, period, extent, location(s)) during the 

duration of the § 33.68(d) test demonstration, should be examined (that is, by means of visual 

and/or suitable engine flowpath instrumentation) to determine the engine’s natural (unassisted by 

operator input) tendency to shed compressor system rotating and static surface ice accumulations 

as well as the effectiveness of any intentional (that is, prescribed, operator initiated) ice shedding 

engine run-up procedure(s). Understanding these natural and, if applicable, assisted ice shedding

characteristics will aid in determining the necessary ground operational limitations in icing 

requirements (for example, total and periodic exposure durations, prescribed procedures) for the 

engine.

 (3) The resulting substantiated ground operating procedure in icing must be included in 

the engine’s operating instructions. If required, the applicable aircraft flight manual should 

specify the maximum total time allowed for ground icing operation and, if appropriate, the 

maximum time period between run-ups consistent with the compliance test demonstration. 
Alternate run-up procedures are also acceptable if you can show these will result in acceptable 

engine acceleration to takeoff power.

 i. Ice Crystal Icing (part 33 turbofan and turboprop, only).

 (1) Ice crystal icing conditions that are hazardous to turbine engines are found in and 

around cloud formations containing deep convection, in scales ranging from isolated and 

organized convective cells and complexes, to tropical storms (for example, monsoons, 

hurricanes, typhoons). Information on the mixed phase icing condition is located in the 

reference shown in paragraph 5.i. of this AC. Total water content (TWC) up to 5 gm/m3 can 

exist at these high altitudes, composed mostly of ice crystals. The maximum size of ice particles 

within convective weather clouds can be up to 10 mm or larger, but mass is typically 

concentrated at a much smaller median mass diameter of approximately 200 microns.

 (2) Appendix D of part 33 defines compliance requirements in terms of ice crystal and 

mixed phase icing environments. This mixed phase environment extends beyond the icing 

envelopes depicted in appendices C and O of part 25 or appendix C of part 29. TWC 

concentration levels in appendix D of part 33 are based on predicted “adiabatic” condensation 

rates in convective storm updrafts. This has been shown to be a reasonable upper estimate based 

on available flight test data, mostly from the 1950s. However, the accuracy of these flight test 

data is no longer adequately traceable, and the flight patterns adopted were not optimum for 

deriving TWC extent (cloud length) statistics. Therefore, the TWC extent probability of 99%

put forth by appendix D is likely increasingly conservative with increasing cloud length. Until a 

more comprehensive mixed phase and glaciated atmospheric envelope is defined from flight test 

data using modern cloud instrumentation to determine TWC and ice crystal size measurement, 

appendix D of part 33 should be treated as the critical ice crystal icing condition when 

demonstrating compliance for § 33.68. Likewise, applicants should evaluate engine inlet probes

for the conditions depicted in appendix D of part 33.

 j. Test Setup Considerations. The LWC levels depicted in appendices C and O of part 25 and 

appendix C of part 29 are intended as supercooled droplet ambient icing conditions. Tests may 

be conducted with a simulated cloud which is produced outside of the inlet and ingested into the 

engine. Under such a test environment, the LWC within the inlet ducting should replicate the 

engine operating conditions of an airplane in icing conditions at actual airspeeds. The inlet icing 

concentration or dilution effect is dependent on droplet size, engine fan speed (for turbofan 

engines), and simulated forward airspeed. For example, engine operation at idle descent power 

with simulated forward airspeed that is less than flight speed, due to facility limitations, might 

require a compensating increase in the test level of LWC concentration above what is depicted in 

appendices C and O of part 25. This increase would be greater for larger supercooled droplet 

diameters. The engine size is also a variable that affects the difference in the LWC inlet 

concentration between flight conditions and the engine test environment, with small engines 

potentially needing the greatest compensation. Rotorcraft engine applications may have 

different criteria and applicants should propose a test that addresses those specific turboshaft 

installation issues.
 k. Direct Connect Inlet Engine Test Facilities. Icing tests that provide a simulated icing 

cloud by direct connection of facility piping to the front flange of the engine, where no inlet air 

spillage is allowed, may cause test facility effects to alter the test parameters (for example, LWC 

and MVD). You must provide the FAA with data that demonstrates the simulated test conditions

are representative of an installed engine operating in the icing environments as depicted in

appendices C and O of part 25 and appendix C of part 29, as applicable. Suitable data may be 

direct measurement data concerning LWC within the inlet, or an acceptable validated analysis of 

water droplet trajectories for the test setup. In some cases, you may need to adjust the LWC to

address any effects of the test setup (for example, non-uniformity across the engine face).

 l. Testing in Mixed Phase or Glaciated Atmospheric Conditions (part 33 turboprop and 

turbofan engines only).

 (1) Flight testing is a method of demonstrating engine operation in icing conditions, 

particularly mixed phase or glaciated atmospheric conditions. Under these conditions, two 

important flight test considerations are: the measurement of ambient meteorological data, and the 

ability to correlate the measured engine performance to a more severe icing point. To address 

the correlation, it is necessary that you have a fully instrumented engine with temperature 

sensors strategically located in the core flow passage. This instrumentation will collect data 

during the icing environment encounter. Since the environment encountered may not be as 

severe as the levels needed, scaling of the measured data to show satisfactory engine operation at 

the more severe point should be attempted to the extent possible. Any proposals for scaling 

should be supported by data from experiments, and the scaling approach should be conservative.

 (a) An accurate measurement of the ambient meteorological condition is essential. Past 

engine flight tests in this environment demonstrated that a combination of LWC probe, TWC

probe, particle sizing and imaging probe, ice accretion detector, and TAT probe are necessary to 

fully characterize the ambient environment.

 (b) An onboard real-time meteorological data display with GPS positioning capability 

is also helpful to find high ice crystal concentration areas. This allows the pilot to position the 

aircraft for the test. Since testing around a thunderstorm elevates the risk level of a flight test, an 

on-site rapid data reduction capability would give a timely indication if the test objective is met,

and minimizes the number of flights into this hazardous environment.

 (2) Simulation of the Critical Mixed Phase and Glaciated Atmospheric Condition in a 

Ground Icing Facility. As indicated in the reference shown in paragraph 5.i. of this AC, facility 

simulation of ice crystal icing conditions is difficult and not currently done routinely. It is not 

known how well any facility simulation method actually replicates the natural environment. 

Consequently, no testing standard for this icing condition currently exists. Therefore, ground 

simulation to demonstrate compliance to the ice crystal icing certification requirements will be 

evaluated on a case-by-case basis until uniform industry standards are developed.

 m. Test Results and Compliance Issues. During all icing tests (that is, supercooled droplet, 

or ice crystal icing), the engine should operate without accumulating ice that would adversely 

affect engine operation. Accumulating ice can adversely affect engine operation (for example, 
flameout, surge, stall, run-down, high vibrations, slow acceleration, or lack of throttle or power 

lever response), or can cause a sustained loss of power or thrust. Additionally, the applicant 

should accurately monitor icing test point conditions. Video surveillance or other 

instrumentation may provide the means to identify any source of ice damage, especially in those 

instances where test apparatus may also shed ice (for example, icing nozzles, special test 

instrumentation, or icing tunnel walls).

 (1) Sustained Loss of Power or Thrust and Power Loss Instabilities. The engine should 

not experience a sustained power loss while operating at approved ratings in icing conditions.

 (2) Temporary Power Loss. Temporary power losses below engine power and thrust 

ratings selected under § 33.8 are acceptable if you show that the engine has a sufficient margin to

avert any power loss instability, such as rollback, surge, stall, high vibrations or flameout.

 (3) Momentary Power Loss. Momentary power loss caused by pumping or processing of 

ice debris through the fan module and compressor during the ice shed ingestion process is 

usually acceptable. Any accepted temporary or momentary power loss or temporary high 

vibrations must be documented in the engine installation manual.

 n. Mechanical Damage. The engine should not exhibit more than limited engine damage 

from any cause because of § 33.68 icing testing. Some limited damage can be acceptable if the 

resultant power loss is minor. Additionally you must fully account for cumulative damage from 

repeat encounters. Limited engine damage will be considered acceptable provided you satisfy

the following criteria:

 (1) Continued In-Service Use. You should evaluate any resultant damage and demonstrate 

that it does not affect the engine performance for continued in-service use. This includes both 

continued safe operation, with no imminent failures expected, and no significant power loss.

 (2) Sustained Power Losses. The engine should not experience any sustained power loss 

beyond 1.5% (that is, the nominal accepted level considered to be within measurement 

capability).

 (3) Temporary or Momentary Power Loss. Temporary or momentary power loss should 

be reviewed by both the airframe and engine manufacturer to assess installed operational impact. 

Any resultant temporary or momentary power loss, or high vibrations found acceptable by the 

FAA during the compliance demonstration, should be recorded in the installation manual by the 

engine manufacturer.

 (4) Validation Basis. Analytical tools used to substantiate the criteria for determining 

acceptable damage should demonstrate an acceptable validation basis. For example, validation 

could utilize engine tests or rig tests to substantiate the accuracy of results. An acceptable 

analytical tool validation basis includes test data which yield conservative results.

 (5) Engine Damage. Damage to the engine or engine components as a result of icing 

compliance testing should not exceed the maintenance manual limits. Cumulative damage for 
repeated encounters should be considered part of this assessment. Any damage findings should 

be brought to the attention of the FAA for approval.

 (6) High Vibrations. You must ensure that high vibrations are assessed for the specific 

aircraft installation. The acceptability of high vibrations is evaluated by the airframer. Bring 

high vibrations to the attention of the ACO or ECO approving the engine; it will contact the 

ACO approving the engine installation before the final approval, and inform them of this 

condition. Any high vibrations should be included in the installation manual by the engine 

manufacturer.

 (7) Communication of Results. The installation and operating manuals required by § 33.5 

should provide information describing all engine conditions observed during engine certification 

icing tests. Prior to these tests, the engine manufacturer should provide the FAA with a process 

to evaluate the acceptability of any potential damage that could occur during icing tests.

 (a) Natural icing flight tests are conducted to demonstrate compliance with §§ 23.1093,

25.1093, 27.1093, or 29.1093; they require an FAA preapproved process for evaluating damage 

resulting from icing tests. Also, if periodic engine power run-ups are used to minimize icing 

damage during the ground icing operation demonstration defined in § 33.68(d), then the 

applicant must document the run-up procedure. Complete documentation must contain a 

description of the run-up requirements and the required run-up intervals, and it must be 

contained in the operation manual as mandatory within icing conditions. Prior to demonstration, 

the engine manufacturer should coordinate the proposed run-up shedding procedures with the 

airplane manufacturer, so as to assure that procedures are practical and appropriate for the 

intended operations.

 (b) Any acceptable power loss anomalies from for example, ice accretion, ice shed, 

water runback and refreeze and shed, and the effects on performance and operation, should be 

documented in the installation manual. Both the engine certifying ACO and the installing ACO 

should carefully evaluate any high vibrations induced from ice accretions during ice testing.

These vibrations should be documented by the engine manufacturer in the engine’s installation 

manual.

 o. Engine Systems. Applicants may use an automatic engine control system to initiate an ice 

protection system to meet the requirements defined in § 33.68, provided that system operation 

does not result in potential crew action. Examples of engine characteristics that may be 

noticeable to the flight crew are exhaust gas temp fluctuations, large speed fluctuations, or 

audible surging. Further, any engine control system required for engine ice protection 

certification under part 33 should not react adversely with other aircraft systems, aircraft 

handling qualities and performance, or human factor considerations.

 (1) Any unacceptable adverse interactions may result in the engine being deemed uninstallable under parts 23 or 25 airplanes or parts 27 or 29 rotorcraft. Critical issues which you

must address include: crew interface, uncommanded thrust or power changes, thrust or power 

setting, asymmetric engine behavior, pilot workload and appropriate flight deck indication and 

procedures, the effect on aircraft handling, pilot ability, and human factors. Additionally, any 
engine system required to show compliance under § 33.68 should meet the following 

requirements:

 (a) System Reliability. Demonstrate the capability of the system for reliably sensing 

the conditions, which enables the engine system function, throughout the approved operating 

envelope.

 (b) Dispatch. The engine system function should be available for all dispatch 

configurations. The system should be configured in its most critical dispatch state for 

certification icing tests.

 (c) Electronic Faults. Applicants should demonstrate that engine system functions will

not be lost due to any probable electronic fault(s).

 (d) Other Environmental Testing. Engine system functions should not be affected 

when the system and any associated electronic systems are exposed to the required operating 

environments, including high intensity radiated fields (HIRF) and lightning.

 (e) Power Requirements. For those systems powered solely with a dedicated engine

alternator (either directly or using another engine system such as a FADEC), applicants should 

demonstrate the system sensing and performance function at minimum certified rotor speeds. 

Minimum certified engine speed is the minimum idle rotor speed achievable anywhere in the 

icing envelope.

 p. Auto-Recovery Systems. Auto-recovery systems should not be needed under § 33.68 

testing since these icing conditions are considered to be within the engine’s certified operational 

envelope and a probable encounter. The intent of § 33.68 is to certify engines to perform and 

operate reliably in the icing conditions depicted in appendices C and O of part 25, appendix C of 

part 29, and appendix D of part 33, as applicable.

 (1) Auto-recovery systems are considered back-up devices that are only needed following 

rare ice ingestion events resulting from icing conditions outside appendices C and O of part 25,

appendix C of part 29, and appendix D of part 33. If auto-recovery systems activate during the 

icing test, then the applicant should alert the installing ACO of the activation. Auto-recovery 

systems are not the primary protection for continued safe engine operation during normal ice 

sheds or accretion while operating in icing conditions depicted in appendices C and O of part 25,

appendix C of part 29, and appendix D of part 33.

 (2) Therefore, applicants may perform the § 33.68 compliance testing with auto-recovery 

systems enabled, but the systems should not activate during the § 33.68 test sequence. 

Additionally, continuous ignition should not be selected during the § 33.68 compliance testing. 

To assure non-activation of an enabled auto-recovery system, the applicant should have 

displayed an instrumented signal that monitors the auto-recovery system activation. If activation 

monitoring cannot be accomplished, then disabling the auto-recovery system will be necessary.
 q. Operation Instructions. Any operating procedure (for example, ground run-up procedures) 

required to ensure continued operational compliance with ground icing conditions or falling and 

blowing snow evaluated under § 33.68(d), and the installation requirements of 

§§ 23.1093(b)(1)(ii), 23.1093(b)(2), 25.1093, 27.1093(b)(1)(ii) or 29.1093(b)(1)(ii), should be 

communicated to the installer in the operating instructions as a requirement. The requirement

for each applicable operating procedure should further be included in the limitation section of the 

airplane flight manual. It may be necessary to coordinate with the installer on these procedures 

to ensure that they can be effectively implemented in-service. The installer may translate the 

operating instructions into a procedure tailored at the airplane level if it results in an operation

that is equivalent to the demonstrated operating procedure. An equivalent operation should be 

conservatively representative of the demonstrated operating procedure.

 r. Special Considerations for Mixed Phase and Glaciated Atmospheric Conditions (part 33

turbofan and turboprop only). Part 33 requirements for the type certification approval of engines 

operating in an ice crystal environment are located in § 33.68, and appendix D of part 33. These 

requirements have been developed in response to service events. The root cause of these events 

can be traced to ice buildup within the core flow path of the affected engines. Engine inlet probe 

blockage and the resulting signal corruption should also be analyzed. Adverse effects created by

this type of icing on the engine include un-commanded rollback of power or flameout, as well as 

compressor stall and core hardware damage.

 (1) Ice crystals have only been recognized as a threat to turbine engines in recent years. In 

response, the FAA has worked with industry to develop standards that address this threat. 

During this process, we found that although capabilities are developing, the present ice crystal 

tools and test techniques have not been fully developed and validated sufficiently. Therefore, we 

developed a phased-in approach to address the ice crystal threat during an engine type 

certification program. The following paragraphs describe this phased-in approach.

 (2) New engines must address the known in-service experience in ice crystal environments 

(for example, core damage and engine flameout events). Until ice crystal tools and test 

techniques have been developed and validated, the engine manufacturer should use a 

comparative analysis to specific field events. This analysis approach should show that new 

engine cycle or design features, or both, will result in acceptable engine operation.

 (3) Acceptable operation includes the absence of rollback, rundown, stall, flameout, and 

unacceptable compressor blade damage as described earlier in paragraph 8.a of this AC. 

Additionally, we recommend the engine manufacturer incorporate developed and validated

technology as it becomes available, and work toward a full engine test substantiation of 

operation in ice crystal environments.

 (4) Long-term, we anticipate an acceptable demonstration will eventually include a CPA

of an ice crystal environment as depicted in appendix D of part 33. All engine power levels,

including in-flight idle operation, will be evaluated in these conditions. The critical conditions 

will be demonstrated to the FAA through a combination of testing and validated analysis using 

the latest tools and technology when proposing the compliance methodology. Computational 
tools to be used in this analysis would be calibrated by either rig calibration test data or engine 

test measurements.

 s. Comparative Analysis Guidance for Ice Crystal Icing (part 33 turbofan and turboprop 

only).

As stated earlier, until ice crystal tools and test techniques have been developed and validated, 

the engine manufacturer should use a comparative analysis to specific field events. This analysis 

should show the new engine cycle or design feature, or both, would result in an acceptable 

engine operation when subjected to the ice crystal environment depicted in appendix D of part 

33. This comparative analysis should consider both suspected susceptible design features, as 

well as mitigating design features.

 (1) Susceptible Design Features. These features could include:

 (a) Stagnation points which could provide an increased accretion potential.

 (b) Exposed core entrance (as opposed to hidden core).

 (c) High turning rates in the inlet, in the booster and core flowpath (particularly 

compound turning elements).

 (d) Protrusions into the core flowpath (for example, bleed door edges and measurement 

probes).

 (e) Unheated surfaces on booster and front core stages.

 (f) Narrow vane-to-vane circumferential stator spacing leading to a small stator passage 

hydraulic diameter.

 (2) Mitigation Features. These susceptible design features can be significantly mitigated 

by one or more of the following design features. Mitigating design features could include:

 (a) Heated surfaces in the fan, booster, and forward core compressor stages.

 (b) Elevated rotor speeds.

 (c) Hidden core entrance.

 (d) Low frontal cross-sectional area on flowpath probes.

 (e) Inter-compressor bleed scheduling to remove both the ice crystal media and any upstream shedding, from the flowpath.

 (f) Circumferential spacing of stators set to enhance tolerance to ice blockage

(generally denoted by the hydraulic diameter of the stator passage).
 (g) Increasing core compressor airfoil tolerance to soft-body damage. Soft-body 

damage is typically airfoil bending, whereas hard-body damage results in tearing and fracture.

 (3) Similarity to Engines Proven Safe to Operate in Mixed Phase or Glaciated 

Atmospheric Conditions (part 33). Although ice crystal icing conditions are hazardous to turbine

engine operation, severe incidents involving this type of meteorological condition are not 

common. Many currently certified engine designs have been proven by their field service 

experience to be safe to operate in these conditions. New engine designs that are similar to those 

of proven engine designs are allowed to show compliance by comparative analysis.

 (a) Several steps are required to demonstrate compliance by similarity analysis. First,

you should identify the baseline (certified) engine, and supply evidence that this engine is safe to 

operate in mixed phase or glaciated icing conditions. This evidence can be field service 

experience and/or a certification report.

 (b) Second, identify the icing-pertinent engine features that may influence mixed phase 

or glaciated ice accretion within the target (certifying) engine; then compare them to the baseline 

engine. This comparison should establish that the new engine model is less than or equally 

susceptible to icing as the baseline model.

 (4) Comparative Analysis Versus CPA. A comparative severity analysis should be 

performed to show the operational envelope of the target engine does not make it more 

susceptible to ice crystal icing than the baseline engine. As the icing community’s knowledge

improves through research, in the long term applicants should perform a CPA to fully assess ice 

crystal icing. This severity analysis should consider both environmental and engine operational 

effects on accumulation, accretion locations, as well as ice shedding.

 (5) Innovative or Novel Designs. If the new engine cycle or design feature, or both,

contains innovative design ideas such that a comparative analysis with current engines and 

specific engine events is not possible, then you should demonstrate that the innovative feature(s)

will not be susceptible to any adverse effects of operating in an ice crystal environment. Such a 

demonstration entails a two part process. In part one you should document the physical basis 

regarding why this design would result in acceptable operation in ice crystals. In part two you 

should generate physical evidence to substantiate the claims of part one. For purposes of 

substantiation, water particles may be used instead of ice crystals. However, data collected with 

water particles should be corrected to account for ice crystal characteristics, including liquid to 

solid phase thermodynamic effects. The use of water to simulate crystals will be re-evaluated by 

the FAA as ice crystal test techniques improve through research and development.
Table A. Example Steps That May Be Used

Part 1 Part 2

Discuss consequences of ice accretion. Identify test article (engine or component).

Identify design features that promote tolerance 

to critical ice accretion.

Establish test conditions promoting ice 

accretion (with allowance for use of liquid 

water if testing with ice crystals is not 

feasible).

Discuss consequences of ice crystals on 

promotion of ice accretion.

Evaluate tolerance of new design features over 

a range of test points.

 t. Compliance Considerations (part 33). Engine icing events in an ice crystal environment

appear to be associated with accretion of ice deeper in the compressor, as compared to

supercooled droplet environments where accretion occurs farther forward.

 u. Ice Crystal Accretion Explanation. The accretion process is believed to be a result of ice 

crystals passing through the engine inlet and into the front compressor stages. This process 

continues until the local conditions within the engine are conducive to forming a liquid layer on 

vane and blade surfaces. Once the local conditions are conducive to the formation of a liquid 

layer, the vane and blade surface condition allows impinging crystals to melt and stick to the 

liquid layer and accrete on the surface. This is a combination of conductive and convective heat 

transfer from ice melting, evaporation, and surface contact.

 (1) The combination of both the liquid water and solid ice crystal draw significant heat 

away from the impinged surface. An impingement surface that was initially as high as 1200

Fahrenheit (490 Celsius) can be cooled down to 320 Fahrenheit (00 Celsius) by this dual phase ice

and water media on the surface.

 v. Ice Crystal Icing Assessment (part 33, turbofan and turboprop only). You should assess 

engine operation across the whole icing envelope. Cruise, hold, and descent power settings 

should be evaluated in mixed phase and glaciated atmospheric conditions as depicted in 

appendix D of part 33. Note that no established standardized CPA for ice crystal icing currently 

exists. However, to aid you, we are recommending some possible CPA point selections 

described in Table B below. We also recommend applicants use the associated general criteria 

when selecting proposed conditions to be evaluated.
Table B. General Criteria for Selecting Conditions to be Evaluated

Service History This suggests that a selection of conditions at high ambient 

temperatures depicted in appendix D of part 33 icing envelope,

are consistent with higher TWC levels. However, lower 

temperatures might push the accretion aft, where there may be 

a more critical ice accretion site with lower TWC. Therefore,

both high and low temperatures within the envelope should be 

evaluated.

Power Levels To promote ice crystal melting, power levels with internal 

total air temperature within the core flowpath should be

between 320 Fahrenheit (00 Celsius) and approximately 1200

Fahrenheit (490 Celsius). The power level adjusts the 

accretion site forward or aft within the engine.

High Altitude High altitude (low air density) allows greater ice accretion 

mass before shedding from static surfaces.

 (1) The operating conditions for this evaluation should be chosen from appendix D of part 

33, covering the flight phases to include climb, cruise, idle-descent, and holding. The water 

contents depicted in appendix D represent the level for a standard exposure distance of 17.4 

nautical miles. To adjust the water content level as required, so that it reflects the expected icing 

exposure period, use the distance scale factor depicted in appendix D. Service experience 

suggests that straight line exposure distances of 20 to 80 nautical miles may be encountered, as 

well as holding in cloud. As our understanding of water content in relation to distance flown 

becomes more precise, this encounter distance may be adjusted. Currently we hold that

applicants who test to this greater distance will assure their engines are robust and meet the 

minimum standards of the regulations.

 (2) As noted earlier, ice crystals promote icing at engine sites rearward and at higher local 

air temperatures than would exist with only supercooled liquid. These rearward internal 

flowpath accretion sites will also have higher air loads, which can limit ice accretion due to 

shedding. Therefore, the CPA should consider not only ice accretion, but also the likelihood of 

ice shedding.

 w. Appendix O of Part 25, Water Impingement.

 (1) The water impingement region may become greater when encountering the large water 

droplets depicted in appendix O of part 25. This occurs because larger droplets have greater 

inertia and follow a more ballistic trajectory less prone to influence by local airflow streamlines

that bend to deflect air around an object. A way to illustrate this droplet inertia effect is through 

the use of the modified inertia parameter variable, Ko , as shown in Figure 1 of this AC (see 

reference g, in section 5 of this AC. This parameter is a relationship based on the momentum of 

the droplet, air viscosity, and size scale of the impingement region (with a correction for 
Reynolds number). It is used to correlate the streamline effect of the airflow in deflecting a 

droplet and preventing it from impinging on an object. The equation for Ko is provided below 

for a droplet with diameter (d) with an upstream velocity (Vo) traveling in an airflow with a 

viscosity (μ).

 (2) The impingement efficiency Em is simply a decimal fraction of the water droplets that 

are aligned with an object far upstream that actually do impinge on the surface. For very large 

droplets, impingement efficiency approaches unity and is progressively less for smaller droplets. 

Published correlations exist for various object shapes of interest for ice impingement. Such a 

correlation between Ko and impingement efficiency Em, is shown below in Figure 1 for several 

object shapes.
Figure 1. Droplet Impingement Efficiency for Different Shapes

Figure 1a. Definition of Inertia Parameter, Equation 3.10 from Reference 5.g of this AC 

Inertia Parameter K = 2(d/2)2 ρH20 V0 (Dimensionless) EQU 3.10 9 LC μ

Where Characteristic length LC of object is:

Airfoils LC = Chord Cylinders LC = Radius

Spheres LC = Radius Cones LC = Base Radius
 (3) Lower impingement efficiencies (less than a value of one) are indicative of droplet 

deflection from the object. When droplets are deflected, they don’t accrete on the object. This 

deflection is caused by streamlines moving around the object. For most shapes, a modified 

inertial parameter of 10 or greater implies a ballistic path for the droplets. A ballistic path means 

the droplet does not follow stream lines and can impinge the vane farther back than a nonballistic droplet. This results in ice accretion farther back on the vane.

 (4) Impingement of a supercooled water droplet on a surface is dependent on the droplet 

size, and the size of the geometric feature. The smaller the drop size and the bigger the feature, 

the more likely the drop will follow the aerodynamic streamlines around the body and fail to 

impinge. Larger drops, with more inertia are less likely to follow streamlines, and therefore they 

will impinge on larger features. In the engine, such features include the inlet and spinner. The 

applicant must account for the potential impingement of larger drops on engine geometric 

features of this scale. Figure 2 of this AC illustrates the consequences of encountering the larger 

water droplets depicted in appendix O of part 25. Figure 2 also shows what happens with target 

objects of typical size ranges for larger transport engines. The shaded area in Figure 2 covers

appendix C of part 25 and appendix C of part 29, as applicable. For a given modified inertia 

parameter value (Ko) of 10, the droplet size range is seen to behave ballistically for small 

“target” objects, such as engine airfoils, airfoil cascades, and probes. Consequently, large 

droplets as depicted in appendix O of part 25 will not significantly affect surface water 

impingement and resulting ice accretions on small airfoils within engines. For larger targets, 

however, applicants should anticipate greater impingement on surfaces, for example: the nacelle 

and engine spinner. Other surfaces unprotected by surface heating systems are also at risk for 

water impingement by the sized droplets depicted in appendix C of parts 25 and 29, as 

applicable.

Figure 2. Ballistic Boundary as a Function of Target and Droplet Size

10 100 1000 10000

Droplet Diameter - microns

Airfoils

Probes

Nacelle 

Lip

Spinner

Inlet 

Diameter

Cascades

.01

.1

1

10

“Target” Dimension - ft

Ballistic (Minimal 

Deflection by 

Streamlines)

Locus of Modified Inertia Parameter = 10

Droplet 

Size 

Range 

of 

Appdx C

Ko > 10 (example)

Ko = 10

Ko < 10 (example)

10 100 1000 10000

Droplet Diameter - microns

Airfoils

Probes

Nacelle 

Lip

Spinner

Inlet 

Diameter

Cascades

.01

.1

1

10

“Target” Dimension - ft

Ballistic (Minimal 

Deflection by 

Streamlines)

Locus of Modified Inertia Parameter = 10

Droplet 

Size 

Range 

of 

Appdx C

Ko > 10 (example)

Ko = 10

Ko < 10 (example)
10. Section 33.77, Ice Slab Ingestion. 

 a. Why the Ice Slab Ingestion Test is Conducted. The intent of the ice slab ingestion test is 

to demonstrate tolerance to ice ingestion due to shedding from inlet (nacelle) surfaces in the 

front of the engine. This test also establishes limits for ice released from other aircraft or 

rotorcraft surfaces during parts 23, 25, 27, and 29 certifications. The dimensions of the test slab 

are related to engine size (defined by inlet highlight area), based on service experience. 

Manufacturers of turboshaft engines with complex inlet designs should propose an equivalent 

inlet highlight area once they have defined the ice slab size. Engine manufacturers should also 

include in their compliance plan, an analysis of the potential installation effects of the engine 

induction system.

 (1) The engine manufacturer and the installer should closely coordinate the ice slab sizing

and density. This coordination ensures that ice accumulation sites on the airframe which can

potentially be ingested by the engine are addressed under § 33.77. Airframe locations that 

should be considered include for example, the inboard section of the wing for an aft fuselage 

mounted engine, the radome, and antenna. Ice slab ingestion is demonstrated under § 33.77, but 

must be addressed and shown as acceptable for the installation under §§ 23.901(d)(2), 23.1093, 

25.1091(e), 25.1093, 27.1093, or 29.1093 (see paragraph 11 of this AC).

 (2) The airframer responsible for the induction system should assess the inlet cowl ice 

accumulations in accordance with §§ 23.901(d)(2), 23.1093, 25.1093, 27.1093, or 29.1093. 

They should also provide pertinent test variables to the engine manufacturer for incorporation 

into a § 33.77 test demonstration. If an application or product inlet has not been selected at the 

time of engine certification, the engine manufacturer should include in the engine installation 

manual all pertinent inlet icing assumptions, test data, and results. See paragraph 11 of this AC 

for installer assessment guidance.

 b. Compliance Considerations. Compliance may be demonstrated by the standard engine ice 

slab test or by using a validated analysis procedure that uses equivalent soft body testing. The 

test demonstration should use ice slab trajectories aimed at critical engine locations. Applicants 

should pick locations based on the ice accretion and shed characteristics of the induction system 

likely to be installed on the engine. The most critical impact location should be tested.

 (1) Slab Dimension Considerations. If the engine manufacturer type certificate applicant 

lacks specific knowledge of the icing characteristics of an aircraft inlet system, it may select test 

conditions typical of an installation already in-service, in combination with conservative 

assumptions on the future installation. The ice slab size, thickness, and density defined in the

§ 33.77 compliance demonstration should be evaluated against parts 23, 25, 27, and 29 engine 

installation and inlet system icing requirements. Applicants should use a minimum ice slab 

density equivalent to a 0.9 specific gravity, unless a different value is considered more 

appropriate (see AC 20-73A, AC 23.1419-2D, AC 25.1419-1, AC 27-1B, or AC 29-2C (or latest 

revisions) for more guidance on ice shedding).

 (2) Engine Test Considerations. Applicants should determine if the ice slab size, 

thickness, and density are appropriate for the specific engine installation. If appropriate, the test 
results of § 33.77 might be used by the airframe manufacturer to comply with the natural icing 

flight test requirements of §§ 23.1093, 25.1093, 27.1093, and 29.1093. Execution of the ice slab 

ingestion test typically involves targeting the slab to enter the air stream ahead of the fan, intact,

at the outer diameter of the inlet duct. This is intended to mimic the ice release from the inlet 

and results in impact on the outer diameter of the fan.

 c. Validated Analysis with Equivalent Soft Body Tolerance Testing. Compliance may also 

be shown by a validated analysis procedure that uses appropriate soft body damage testing. If 

the applicant elects this alternate compliance approach, the engine will be certified for the 

minimum standard ice slab consistent with the engine inlet area as defined in § 33.77. We

recognize that alternate soft body damage testing may, in some circumstances, include objects 

that are larger than the standard ice slab based on inlet area. However, certification of an ice 

slab larger than the standard size by this validated analysis method is not currently allowed.

 d. Elements of a Validated Analysis. This analytical model may be used alone or in 

conjunction with the results of a certification medium bird or other soft body ingestion test. A 

validated analysis must contain sufficient elements to show compliance. These elements may 

include:

• Full fan (fan engines) or first stage compressor (non-fan engines) blade modeling using 

the latest techniques such as finite element analysis;

• Blade material properties for yield or failure, or both, as appropriate;

• Dynamic and time variant capability;

• Thrust or power variance prediction if required to account for blade or other damage; 

and

• Appropriate engine or component testing, or both, with impact at the outer 1/3 of the 

first stage blade span location. The fan is the first stage blade row for turbofan engines.

 (1) The analysis of the ice slab impact on the fan must properly account for critical 

controlling parameters:

• Relative kinetic energy normal to the leading edge chord;

• Incidence angle – relative slab speed and blade speed;

• Slab dimensions; and

• Slab orientation.

 (2) Any predicted power loss or blade damage (distortion, cracking, tearing) must be 

assessed against the criteria of this AC. Figures 3 and 4 of this AC help describe the 

contribution of these parameters when establishing the threat to the blade.
Figure 3. Normal Component of Kinetic Energy of Ice Slab

Figure 4. Ice Slab Orientation Effects

Dependent on orientation at fan face the “bite” can be 

increased substantially

Load shared by 6-7 blades

Load shared by 3 blades

rotation

slab

Relative KE 

component 

normal to 

leading edge is 

critical
 (3) The relative kinetic energy of the ice slab should be determined from an assessment of 

flight conditions that control engine rotor speed versus ice slab velocity. Engine test results from 

previous ice slab testing may be used to support the predicted ice slab velocity. The applicant’s 

analysis should assume the most critical orientation, unless they can show that an alternate ice 

slab orientation is more conservative for ice slab testing purposes.

 e. Ice Slab Break Up. Typically, the ice slab breaks into smaller pieces during ingestion. 

The applicant’s analysis should use the largest slab size consistent with a conservative 

assessment of slab “break up” that can occur within the air stream ahead of the fan. Data derived 

from a number of tests are included in Figure 5 below. The largest ice piece is typically 1/3 to 

1/2 the original size. For compliance by analysis, the applicant should assume 1/2 of the original 

slab length unless evidence suggests that this is not conservative relative to ice slab testing.

Figure 5. Ice Slab Break Up Experience

1⁄2 slab dimension – minimum allowed
 f. Test Results. Section 33.77(c) requires that under the conditions in § 33.77(e), ice 

ingestion must not cause a sustained power or thrust loss, or require the engine to be shutdown. 

The following paragraphs cover these points in greater detail:

 (1) Sustained Power Loss. Applicants should evaluate the impact of any first stage blade 

bending or damage on potential sustained engine power loss. Power loss associated with blade

damage from the slab should be less than 1.5%. As soft body blade damage is common from 

medium bird ingestion, applicants may use the medium bird ingestion test results to show 

compliance with this requirement. If the medium bird ingestion test results is less than 1.5% 

permanent power loss, and no cracks, tears, or blade piece breakout occurs due to a bird 

introduced at the outer 1/3 of the fan blade span, then the § 33.77 requirement is met.

 (a) If power loss exceeds 1.5% when utilizing the bird test, the manufacturer must 

provide a validated analysis that shows consistency with the bird test results. The manufacturer 

must also demonstrate the standard ice slab would produce less than the 1.5% power loss.

 (b) Applicants must also demonstrate by test that any cracks, tears, or blade piece 

breakout will not result in “unacceptable sustained power or thrust loss” within 100 flight cycles.

This is considered sufficient to allow continued engine use until the next scheduled “A” check

for fixed wing applications, or similar inspection periods for rotorcraft applications. Note that

any damage resulting from this test must be documented in the engine installation manual.

 (2) Engine Operability. Engine damage should not cause surge, flameout, or prevent 

transient operation.

 (3) In-Service Capability. Engine damage should not result in a failure or a performance 

loss that would prevent continued safe operation during a conservative flight operations scenario. 

For example, an engine should be able to operate within the time period for an “A” check or 

greater, if appropriate testing validates a continued period of in-service capability. The period of 

in-service capability to be demonstrated may vary with installation if the damage is not readily 

evident to the crew or visible on preflight inspection (for example, tail mounted engine location).

 (4) Other Anomalies. Engine damage should not result in any other anomaly (for 

example, vibration) that may cause the engine to exceed operating or structural limitations.

 (5) Auto-Recovery Systems. If during the ice slab ingestion testing under § 33.77, an 

engine incurs a momentary flameout and auto-relight, then the acceptance of that test is

predicated on including the auto-relight system as a required part of the engine’s type design. 

Furthermore, additional criteria would be required where the ignition system is fully operable 

before each dispatch. The reason for the additional dispatch criteria is to ensure the ignition 

system’s critical relight function is reliably available during the next flight. We allow use of the

auto-recovery system during § 33.77 certification testing to account for ice accretion and 

shedding that results from an inadvertent delay in actuating the anti-icing system. We consider

this delay to be an abnormal operational result where operability effects, like momentary 

flameout and relight, may be accepted.
 g. Communication of Test Results.

 (1) The installation and operation instructions required by § 33.5 should provide 

information on the size, thickness, and density of the ice slab ingested; any anomalous behavior 

such as high vibrations, and any affect on the engine’s ability to operate at the commanded 

power setting or rating. By including the information in the installation manual, you provide 

these test results to the installer.

 (2) The icing certification report addressing § 33.77 compliance should include 

information regarding ice slab orientation and trajectories, slab breakup, impact locations, 

descriptions of any resultant damage, as well as all other pertinent data defining the engine's 

capability or response to the ice ingestion event. Additionally, if the auto-recovery system is 

required to comply with § 33.77, then the functional state of the recovery system (for example, 

one igniter inoperative) becomes a limitation that needs to be communicated to the installer via

the engine installation manual to ensure compliance with the delayed activation requirements of 

§§ 23.1093, 25.1093, 27.1093, or 29.1093.

11. Induction System Icing Protection (§§ 23.1093, 25.1093, 27.1093, and 29.1093). 

Applicants usually use the results of § 33.77 for compliance with §§ 23.1093, 25.1093, 27.1093, 

and 29.1093, instead of a test demonstration of the 2-minute delayed activation of inlet anti-ice

(1-minute for turbo shaft engines). The § 33.77 results have also been used to show compliance 

with §§ 23.1093, 25.1093, 27.1093, and 29.1093 for other airframe ice sources. The engine 

manufacturer and the airframer should coordinate closely to ensure the § 33.77 test covers all 

potential ice sources. Similarly the icing compliance demonstration of § 33.68 should be 

coordinated with the installer to verify the engine icing compliance demonstration is applicable 

to the installation icing compliance requirements. The icing conditions depicted in appendix O 

of part 25 are only applicable to turbine engine induction systems on part 25 airplanes with a 

maximum takeoff weight less than 60,000 pounds. 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.

 a. Acceptable Means of Compliance (§§ 23.1093(b)(1), 25.1093(b)(1), 27.1093(b), and 

29.1093(b)). As a general rule, engine induction systems should operate continuously in icing 

conditions without regard to time, as in a hold condition. For fixed wing aircraft, a 45-minute 

test duration in a standard continuous maximum cloud is an acceptable demonstration of 

continuous in-flight operation, so long as the test demonstration result isn’t indicating an 

imminent test criteria failure. An exception would be for low engine power conditions where a 

sustained level flight is not possible. Even then, a conservative approach must be used when a 

series of multiple horizontal and vertical cloud extent factors are assumed. The horizontal cloud

extent factor is not intended to be used to limit the severity of exposure to those icing conditions 

where it is reasonable to assume the aircraft will be required to operate.

 (1) Applicants must adequately analyze the engine inlet anti-ice system performance and 

address any potential ingestion hazards to the engine from predicted ice buildup on the engine 

inlet. This includes runback and lip ice. If an applicant can show that both the inlet anti-ice 

system performance and the first stage compressor blade (for example, fan blade for turbofan 
engines) capability are equivalent to previous certification experience, then certification may be 

shown through similarity to previous designs.

 (2) You must show by analysis, and verify by test, that the engine inlet anti-ice system 

provides adequate protection under all flight operations. Your analysis should include the 

conditions shown in Table C of this AC. Note that additional critical points may be needed 

depending on the specifics of the airplane and engine design. Bleed crossover points may also 

need to be analyzed, if applicable.

 b. Inlet Design Point Selection. If the inlet is evaporative under the critical points in 

continuous maximum icing conditions, and if it is running wet under intermittent maximum icing 

conditions (refer to appendix C of parts 25 and 29), then the design is satisfactory. Turbofan and 

turboprop engine inlet anti-ice systems have historically demonstrated good service experience 

with inlet anti-ice systems that run wet, when runback ice has been evaluated at the conditions 

described in Table C of this AC. For turbofan and turboprop inlets that are certified for 

unlimited operation in icing, runback ice formation is allowed during hold, descent, and 

diversions under the conditions shown in Table C of this AC. Turbofan and turboprop installers

should determine the critical accretion conditions for their inlets in each of the scenarios in Table 

C below. They should then compare each result to the amount of ice the engine was

satisfactorily demonstrated to ingest during engine certification (refer to § 33.77).

Table C. Inlet Lip and Runback Ice (part 33 turbofan and turboprop engines only)

Flight 

Condition

Descent Hold Straight-Line Flight

Icing Design 

Condition

6500 foot 

descent through 

a continuous 

maximum 

cloud, as

depicted in

appendix C of

part 25, with a 

cloud extent 

factor of 1, 

followed by an

intermittent 

maximum cloud 

exposure, as 

depicted in 

appendix C of 

part 25.

45-minute hold in a

continuous 

maximum cloud, as 

depicted in 

appendix C of part 

25, with a cloud

extent factor of 1.

45-minute exposure in a 

continuous maximum cloud, as 

depicted in appendix C of part 25, 

with a cloud extent factor as 

depicted in appendix C of part 25, 

followed by an intermittent 

maximum cloud exposure, as 

depicted in appendix C of part 25.
 c. Addressing Specific Operational Conditions.

 (1) Descent. If the engine inlet anti-ice system is not fully evaporative during descent 

through a continuous maximum cloud as depicted in appendix C of part 25, then the total amount 

of ice accretion (including inlet lip and runback ice) must be calculated for both the continuous 

maximum and intermittent maximum icing conditions depicted in appendix C, and for the

operating conditions shown in Table C above. Airspeed and scoop factor (if applicable for the 

inlet design) should be part of this assessment. Ingestion of these calculated quantities of ice 

buildup for a given condition must not result in more ice ingestion damage (that is, based on slab 

size or kinetic energy) than the amount of ice the engine was satisfactorily demonstrated to 

ingest during the engine certification (refer to § 33.77).

 (2) Holding. The engine inlet anti-icing system must be capable of safe operation in the 

cloud conditions depicted in appendix C of parts 25 or 29 for extended operations. The extended 

holding condition is for 45 minutes (except, 30 minutes is used for parts 27 and 29) at the critical 

accretion ice conditions for the continuous maximum icing cloud, using a horizontal cloud extent 

factor of one.

 (a) If the inlet is evaporative under continuous maximum icing conditions, and it is 

running wet under intermittent maximum icing conditions, then the design is satisfactory. This 

is because the descent condition provides less power to the anti-ice system and will always 

become the critical accretion condition in intermittent maximum icing conditions.

 (b) If the inlet is running wet in a maximum continuous atmospheric condition, then the 

applicant should calculate the amount of ice that would accumulate during the holding 

conditions as described in Table C above. The total amount of calculated ice should include 

runback and lip ice for the condition being analyzed. Ingestion of this ice must not result in 

more damage than the amount of ice the engine was satisfactorily demonstrated to ingest during 

engine certification tests (refer to § 33.77). This damage assessment should be based on the

kinetic energy at the leading edge of the first stage compressor blade (for example, the fan for 

turbofan engines), and should include considerations of ice slab size, velocity, density, and 

compressor (or fan) rotor speed.

 (3) Straight-Line Flight. For straight-line flight (for example, cruise, and diversion to 

alternate airport) the engine inlet anti-icing system should be capable of operating sufficiently to 

assure safe operation for extended time in icing conditions. The straight-line flight evaluation 

must be evaluated and may include the use of the cloud extent factor depicted in appendix C of

parts 25 or 29. If the inlet is evaporative under continuous maximum icing conditions, and if it is 

running wet under intermittent maximum icing conditions, then the design is satisfactory. If the 

inlet is running wet in a maximum continuous atmospheric condition, then the applicant should 

calculate the amount of ice that would accumulate during the straight-line flight conditions 

identified in Table C of this AC (that is, continuous maximum exposure followed by an 

intermittent maximum exposure). Ingestion of these calculated quantities of ice (that is, the total 

amount of ice including runback and lip ice for the condition) should not result in more ice 
ingestion damage than the amount of ice the engine was satisfactorily demonstrated to ingest 

during engine certification tests (refer to § 33.77).

 (4) Running Wet Inlet Anti-ice Systems. For running wet systems, an alternative method 

of compliance would be to demonstrate the normal (no crew action required) ice build and shed 

cycles during exposures representative of descent, holding, or straight-line flights (turboshaft 

engines can propose alternate representative exposures). This may be accomplished through 

tanker or ground testing, with conservative corrections for water contents consistent with 

exposures discussed in paragraphs (1) through (3) above. Ingestion of this ice must not result in 

more damage than the amount of ice the engine has satisfactorily demonstrated to ingest during 

engine certification tests (§ 33.77). The period of in-service capability to be demonstrated may 

vary with installation if the damage is not readily evident to the crew or visible on preflight 

inspection (for example, tail mounted positions). Damage should be reviewed with the FAA and 

documented in the certification report.

 d. Two-minute Delayed Selection of Inlet Anti-Ice Accretion Analysis (part 33 turbofan and 

turboprop engines only). The § 33.77 testing criteria were developed to account for historical 

means of compliance regarding the 2 minute delayed selection of inlet anti-ice. For traditional 

pitot-style engine inlet designs, a part 23 or 25 applicant does not need to consider this scenario 

if the applicant shows compliance with § 33.77 testing criteria that utilizes a standard slab size.

 (1) For inlet designs other than the traditional pitot-style inlets, a 2-minute delay 

calculation may be required, given that inlet lip ice can form during the delayed activation of the 

engine inlet anti-ice system. Applicants should calculate the amount of inlet lip ice that forms 

using a continuous maximum condition depicted in appendix C of part 25, with a cloud extent 

factor of one. Of the total lip ice, only the ice on the inner barrel side of the stagnation point 

would be ingested into the engine.

 (2) For the 2-minute delay approach, applicants may assume that 1/3 of the ice on the inlet 

perimeter is ingested as one piece. This assumption is consistent with the historical approach 

taken by the engine manufacturers. The maximum damage to the fan blade occurs at a high fan 

speed; the critical condition occurs with the maximum inlet nacelle heat. Therefore, 2 minutes is 

a reasonable time to include the pilot reaction to select the anti-ice system and, for the engine 

thereafter, to shed the ice from the inlet perimeter.

 e. Extended Operations Performance Standards (ETOPS) (part 25 installations only). If 

ETOPS certification is desired, the applicant should consider the maximum ETOPS diversion 

scenarios. For example, if a lightweight two-engine diversion occurs at a 10,000 foot altitude

followed by a cabin depressurization, then this puts the airplane into icing conditions that will 

promote ice buildup. Therefore, the inlet icing should be evaluated. Ingestion of the ice must 

not result in more ice ingestion damage than has been satisfactorily demonstrated during engine 

certification ice ingestion (refer to § 33.77).

 f. Potential Airframe Ice Sources, Including Inlets. The engine manufacturer should consider 

the potential installation effects of the engine induction system and work with the installer to 

ensure that potential ice accretions formed at airframe ice accumulation sites are adequately 
tested during the engine ice ingestion testing of § 33.77. Fuselage sources of ice accretion 

include the inboard section of the wing for an aft fuselage mounted engine, and a radome for any 

installation configuration, to name two. The induction system manufacturer or installer should 

assess ice accumulations in accordance with parts 23, 25, 27, or 29 (that is, §§ 23.901(d)(2), 

23.1093, 25.1093, 27.1093, or 29.1093). They should also provide pertinent test variables to the 

engine manufacturer for incorporation into the § 33.77 test demonstration. If an application or 

product inlet has not been selected by the time of engine certification, the engine manufacturer 

should list all pertinent inlet assumptions, test data, and results in the engine installation manual 

for use by the future installer.

 (1) It is normally sufficient to show that ice accretions from these sites are smaller in size, 

and therefore have less or equal kinetic energy potential than the ice slab used in § 33.77 testing. 

As described in paragraph 10.e of this AC, the slab typically breaks into pieces when entering 

the inlet. For the comparison of airframe ice to the results of the § 33.77 test, it may be assumed 

that airframe ice sources break in half when entering the inlet.

 (2) The applicant may elect to compare the ice based on the kinetic energy of the ice slab. 

Kinetic energy may be used as an acceptable method for comparing the airframe ice source to 

the results of the § 33.77 test. Any kinetic energy method must be agreed to by the applicant’s 

ACO.

 (3) Demonstration of ice shedding with natural or icing tanker testing can be an acceptable 

means of compliance for airframe ice that accumulates in front of the engine, while in the 

conditions depicted in appendix C of part 25 or 29. However, the applicant must demonstrate 

conservatism relative to long durations in these icing conditions (such as for several successive 

cloud extent exposures). Ingestion of shed ice during natural or icing tanker testing must not 

result in more damage than the amount of ice the engine was satisfactorily demonstrated to 

ingest during engine certification tests (refer to § 33.77). The period of in-service capability to 

be demonstrated may vary with the installation if the damage is not readily evident to the crew or 

visible on preflight inspection (for example, tail mounted positions).

 g. Wing Sourced Ice for Aft Mounted Engines (part 33 turbofan and turboprop engines, 

only). Clear ice may occur on the upper surfaces of the wing when cold-soaked fuel (due to 

aircraft prolonged operation at high altitude) remains in contact with the fuel tanks’ upper 

surfaces after landing, and during time on the ground when the airplane is exposed to conditions 

of atmospheric moisture (for example, fog, precipitation, and condensation of humid air) at 

ambient temperatures above freezing. Atmospheric moisture, when in contact with cold wing 

surfaces, may freeze. If undetected and still present during takeoff, the ice is most likely to shed 

when the wings flex at takeoff rotation. Simultaneous ice shedding from both wings of an 

airplane with aft mounted engines has resulted in ice ingestion damage and power loss in both 

engines during takeoff.

 (1) When fuel management procedures allow wing fuel tanks to remain mostly full after

landing, cold fuel is in contact with the wing upper skin. As a result, clear ice may form on cold 

wing surfaces during the time on the ground; this holds true even if conditions remain above 
freezing and are not expected to be, or recognized as, icing conditions. Fuel temperature may 

need to be analyzed, including the effect of fuel schedules and refueling procedures.

 h. Aft Mounted Engine Installations. For aircraft with aft mounted engines, applicants

should demonstrate that any undetected ice in front of engine inlets is not greater than what was

shown during engine and aircraft certification. Applicants should also demonstrate that shedding 

the ice accumulated due to cold-soaked fuel will not result in hazardous engine operation.

 (1) For fixed wing applications, a wing mounted ice detection system may allow minimal 

accretion prior to alerting the flight crew as long as the accumulated ice thickness is limited to a 

defined value. This defined value should demonstrate a sufficient margin relative to both engine 

ingestion (refer to § 33.77) and performance and handling qualities as demonstrated under 

§ 25.1419. The largest clear ice slab area that could be ingested into an engine is the same area 

as the engine’s inlet highlight area. In cases where this large slab is very thin, it may not be 

possible to reach the engine in one piece. Therefore, consistent with a conservative assessment 

of slab “break up” that can occur within the air stream ahead of the fan, the applicant may 

consider the ice slab as multiple pieces. You may evaluate any potential resulting damage to

individual engine components, such as fan blades, by comparison to other ice ingestion tests 

such as the ice slab discussed in paragraph 10 of this AC. Additionally, you may compare the 

ingestion results to other soft body ingestion test results, such as the 2-inch hail ingestion tests, 

as long as the mass and kinetic energy effects are accounted for. Finally it may be appropriate to 

address the potential effect on the engine fan shaft, or other structure, if the combined impact of 

a large sheet of ice (or multiple pieces) striking many fan blades simultaneously could generate 

unacceptable loads on the engine components. This might in some instances be addressed by 

analysis.

 i. Compliance with Mixed Phase and Glaciated Atmospheric Conditions Under 

§ 25.1093(b)(1) (part 33 turbofan and turboprop engines only). The results of previous airfoil 

testing in a mixed phase icing environment indicates these icing conditions do not appreciably 

accrete on unheated aircraft wings. Furthermore the testing showed mixed phase environment 

results in the same or less ice accretion than an equivalent amount of supercooled liquid water. 

The overall power required by the running-wet ice protection system was essentially unchanged 

between all-liquid and mixed phase conditions.

 (1) However, in the running-wet mode, the local power density was much higher around 

the stagnation area in the mixed phase conditions, compared to the purely liquid conditions. 

This is due to the power required to offset the thermodynamic heat-of-fusion necessary to melt 

the impacting ice particles that either fully or partially stick to the surface.

 (2) This may also explain why pitot-style inlets have not proved to be susceptible to mixed 

phase ice accretion within the inlet, and why the compliance methods to § 25.1093 for the icing 

conditions depicted in appendix C of part 25, have historically adequately addressed those inlets. 

Engines designed with reverse flow intakes, or with intakes involving considerable changes in 

airflow direction should demonstrate acceptable operation in the icing conditions depicted in

appendix D of part 33. Compliance for pitot-style inlets, without considerable changes in 
airflow direction, may be shown through qualitative analysis of the design, and supported by 

similarity to previous designs that have shown successful service histories.

 j. Compliance with the Supercooled Large Drop (SLD) Conditions Under § 25.1093(b)(1)

(part 33 turbofan and turboprop engines only). The icing conditions depicted in appendix O of 

part 25 are not applicable to turbine engine induction systems on airplanes with a maximum 

takeoff weight equal to or greater than 60,000 pounds.

 (1) In-Flight Exposure to Appendix O of Part 25 Conditions.

 (a) For airframe ice sources, the installer may use either quantitative or qualitative 

analysis of the design. The qualitative analysis must be supported by similarity to previous 

designs that have shown a successful service history in order to have confidence that the 

historical methodology for certification as represented by § 33.77, Table 1 – “Minimum Ice Slab 

Dimensions Based on Engine Inlet Size”, is appropriate.

 1. For engine inlets, compliance may be shown through quantitative or qualitative 

analysis of the design. Qualitative analysis must be supported by similarity to previous designs

that have shown a successful service history. Compliance to the “Large Droplet Condition” of 

Table 1 of § 25.1093(b)(2), titled “Table 1 - Icing Conditions for Ground Tests”, would still be 

required for ground idle taxi conditions. If similarity is not shown, then an assessment of inlet 

impingement limits, differing catch efficiency, distribution effects, and water contents depicted

in appendix O of part 25 should be accomplished.

 2. To demonstrate compliance with § 25.1093, the exposures should be consistent 

with, but conservative to the aircraft operational approvals, as well as the applicable definitions 

in Part II of appendix O of part 25, for both the ice protection system and the potential airframe 

accumulations with respect to shedding.

 (2) Ground Taxi Exposure to Appendix O of Part 25 Conditions. The service experience 

considered for this advisory material indicates that engine fan damage events exist which 

resulted from exposure to SLD during ground taxi operations. For this reason, an additional 

compliance condition was added to § 25.1093(b)(2) for a 30-minute, idle power exposure to SLD 

on the ground. Applicants should include the terminal falling velocity of SLD (for example, 

freezing rain, freezing drizzle) in their trajectory assessment, relative to the protected sections of 

the inlet. The 100 micron minimum mean effective diameter (MED) was selected as a 

reasonable achievable test condition, given current technology. We recommend however, that 

applicants who choose to certify by analysis, should evaluate engine operation in the icing 

conditions depicted in appendix O of part 25. This evaluation should include various drop sizes, 

with the appropriate drop size distribution, to find a critical condition.

 k. Natural Icing Flight Tests. Natural icing flight tests are intended to demonstrate that each 

turbine engine is capable of operating throughout the flight power range of the engine (including 

idling), without an adverse effect. This includes the accumulation of ice on the engine, inlet 

system components, or airframe components that would have an adverse effect on the engine 

operation or cause a serious loss of power or thrust. The icing conditions depicted in appendix O 
of part 25 are only applicable to turbine engine induction systems on part 25 airplanes with a 

maximum takeoff weight less than 60,000 pounds.

 (1) Based on multiple natural ice engine damage and operability events experienced on 

natural icing flight tests and in-service airplanes, natural ice encounters must be used to show 

compliance with §§ 23.1093(b)(1) or 25.1093(b)(1), and in some cases for §§ 27.1093(b)(1) and 

29.1093(b)(1). For airplanes that are not intended to be certificated for flight in icing conditions, 

other methods may be used to replace the natural icing flight tests. When replacing the natural 

icing flight tests on these airplanes, you can show compliance to § 23.1093(b)(1) by analysis

such as ground testing, dry air flight testing, and similarity.

 (2) In addition to showing your engine inlet icing analysis model is accurate, several other 

key issues exist that the natural ice encounter addresses. These include:

• The adequacy of flight crew procedures when operation in icing conditions;

• The acceptability of control indications to the flight crew as the airplane responds 

to engine fan blade ice shedding during various conditions;

• The performance of the engine vibration indication system, as well as other 

engine indication systems; and 

• The confirmation that the powerplant installation performs satisfactorily while in 

icing conditions. This powerplant installation is comprised of the engine, inlet, 

and the anti-ice system.

 l. Identification of Ice Source. Applicants should provide a means to identify the source of 

any ice that may be ingested by the engine during the natural icing certification testing. Special 

attention should be given to non-representative ice accretions on flight test instrumentation 

probes or other surfaces forward of the engine during prolonged operation in icing conditions.

 m. Icing Test Point Monitoring. The applicant should provide acceptable monitoring of the 

icing test point condition (that is, LWC, droplet diameter, temperature) for sufficient time 

exposure to ensure the icing encounter is representative of the environmental icing conditions 

depicted in appendices C and O of part 25 certification icing conditions, appendix C of part 29,

or appendix D of part 33 conditions, as applicable. 

 n. Compliance. Compliance with §§ 23.1093 or 25.1093 is required even if flight into icing 

conditions is not intended or approved (§§ 23.1419 or 25.1419 compliance). Applicants must 

therefore, propose an acceptable icing engine-installation test. The proposed test should include 

the natural ice encounter criteria. The icing conditions depicted in appendix O of part 25 are 

only applicable to turbine engine induction systems on part 25 airplanes with a maximum takeoff 

weight less than 60,000 pounds.

 (1) Typically, an adequate test sequence includes three natural fan ice shed cycles at each 

of the following conditions (inlet anti-ice turned "on"): descent (flight-idle, or anti-ice idle), 

holding (power set to maintain level flight through a range of anticipated airplane gross weight), 

and maximum climb, unless a more critical engine power setting exists. These fan shed cycles 
should result from natural ice accumulation and shedding, not induced or forced shedding by 

throttle or power lever excursions or manipulations during each condition.

 (2) This test should be conducted at a steady-state engine thrust level. The test may also 

involve flying through the same icing cloud multiple times (lapping) for the fan to accumulate 

enough ice for a shed cycle to occur. The airplane may exit the icing conditions between each 

fan shed cycle as needed to clear any other unprotected airplane surfaces from ice. Applicants 

should ensure the test engine ignition system is off during the icing conditions. This will avoid 

masking any adverse engine operating conditions during the natural icing encounter. This may 

require pulling several airplane circuit breakers to disable the test engine’s auto-ignition or 

recovery system, or both. We recommend that applicants establish engine damage criteria and 

obtain FAA approval of these criteria before conducting the natural ice encounter test. Our 

experiences have shown that pass or fail criteria, such as allowable limits of engine damage, are

best agreed on before certification testing.

 o. Falling and Blowing Snow. Sections 23.1093(b)(1)(ii), 25.1093(b)(1), 27.1093(b)(ii), and 

29.1093(b)(ii) require engines to operate satisfactorily in falling and blowing snow throughout 

the flight power range, and ground idle. Ground idle would include aircraft operations on or 

around the airport environment, both before and after flight. Accordingly, the effect of ingesting 

snow during ground operations should be evaluated.

 (1) Consistent with service experience, airports continue operations with falling snow 

concentrations that result in a 0.25 mile or less visibility (about 0.9 gm/m3, see paragraph 9.g.(2) 

of this AC). For turbine engines operating in these conditions, in-flight service experience 

shows that snow sheds from engine or aircraft accumulation sites can cause severe operability 

affects when ingested by these engines. Typical turbofan or turbojet installations, using simple 

pitot (straight duct) inlets, have minimal areas for snow accumulation. For these inlets, in-flight 

icing tests have generally been found to be more critical than snow tests. The applicant should 

evaluate engine inlets that incorporate plenum chambers, screens, particle-separators, variable 

geometry, inlet barrier filters (rotorcraft), or any other feature, such as an oil-cooler, struts or 

fairings, which may provide a potential accumulation site for snow. Further, any airframe 

accumulation sites upstream of the engine inlet should also be considered by the applicant.

 (2) A temperature range of 250 to 320 Fahrenheit (-40 Celsius to +00 Celsius) is common in 

a heavy snow environment, and can result in “wet sticky snow” which may accumulate on 

unheated surfaces (airframe and engine) that are subject to impingement. Engine service events 

have demonstrated that a snow environment is conducive to ice accretion behind the fan in front 

compressor stages of the engine, at low engine power. Service experience has demonstrated 

compressor damage as a result of exposure to prolonged periods of falling snow during ground 

operation. The new engine test for snow conditions is developed to represent the engine 

conditions where snow can form accretions aft of the first stage compressor (for example, the fan 

on turbofan engines) on the core inlet and first stages of the core compressor. The applicant 

should choose a temperature within the range provided above to achieve icing behind the first 

stage compressor (for example, the fan for turbofan engines).
 p. Test Results. The applicant should carefully consider all evidence of ingestion and 

damage to the engines, and their potential sources. If damage is incurred, the possible test 

outcomes include:

 (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. Deiced 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.
10/22/14 AC 20-147A

This document does not represent final agency action on this matter and should not be viewed as a guarantee 

that any final action will follow in this or any other form.

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.

A1-1
10/22/14 AC 20-147A

 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.

 (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.

A1-2
10/22/14 AC 20-147A

6. 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.

A1-3
10/22/14 AC 20-147A

APPENDIX B

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:

Paragraph ____________________ 

Page ______

Type of error (check all that apply): Editorial:-----Procedural-----

Conceptual____ 

 Description/Comments:______________________________________________

_________________________________________________________________

_________________________________________________________________

Recommend paragraph ______ on page ______ be changed as follows:

 (attach separate sheets if necessary)

_________________________________________________________________

In a future change to this advisory circular, please include coverage on the following subject: 

(briefly describe what you want added attaching separate sheets if necessary)

________________________________________________________________

Name: __________________________ 

A2-1