Subject: TURBOJET, TURBOPROP, AND 

TURBOFAN ENGINE INDUCTION SYSTEM 

ICING AND ICE INGESTION 

Date: 2/02/04 

Initiated By: AIR-100 

AC No: 20-147 

Change: 

1. PURPOSE. This advisory circular (AC) provides guidance and acceptable methods, but not 

the only methods, for demonstrating compliance with the applicable engine induction system

icing and engine ice ingestion requirements. These requirements are applicable to the Federal 

Aviation Regulations, parts 23, 25, and 33 of Title 14 of the Code of Federal Regulations (14 

CFR parts 23, 25, and 33). The primary purpose of this AC is to reduce inconsistencies and 

eventual surprises to both engine manufacturers and engine installers, when installing a part 33 

certified engine in a part 23 or 25 aircraft. The guidance in this AC is not intended to address 

turboshaft engine installations, or the rotary wing aircraft they are installed on. Due to the 

complexity that those aircraft and installations pose for icing, AC 20-73, Aircraft Icing 

Protection,is considered the primary AC for those installations. Further, this AC is not intended 

to address mixed phase icing conditions (meaning, mixed water and ice precipitation), although 

there is a discussion on the subject. While these guidelines are not mandatory, they are 

historically based and are derived from extensive Federal Aviation Administration (FAA) and 

industry experience in determining compliance with the relevant regulations. This AC does take 

precedence over the engine and engine installation icing guidance in AC 20-73. It is important 

to note that AC 20-73 does contain useful information on the understanding and characterization 

of the icing environment. Additionally, AC 23-16, Powerplant Guide for Certification of Part 23 

Airplanes, does take precedence over the engine installation guidance provided in this AC, as 

one method of compliance to part 23 regulations. 

2. APPLICABILITY.

 a. The guidance provided in this document is directed to engine manufacturers, modifiers, 

foreign regulatory authorities, FAA engine type certification engineers and their designees. 

 b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for demonstrating compliance 

with the applicable regulations. The FAA will consider other methods of demonstrating 
AC 20-147 2/02/04 

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compliance that an applicant may elect to present. Terms such as “should,” “shall,” “may,” and 

“must” are used only in the sense of ensuring applicability of this particular method of 

compliance when the acceptable method of compliance in this document is used. While these 

guidelines are not mandatory, they are derived from extensive FAA and industry experience in 

determining compliance with the relevant regulations. Alternatively, if the FAA becomes aware 

of circumstances that convince us 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. 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, §§ 25.1093 and 25.1419. 

 c. Part 33, Airworthiness Standards: Aircraft Engines, §§ 33.68, 33.77(c), 33.77(e), 

33.89(b), and § 33.78, Rain and Hail Ingestion. 

4. RELATED READING MATERIAL (Latest Revisions). 

 a. AC 20-73, Aircraft Ice Protection. 

 b. AC 33-2B, Aircraft Engine Type Certification Handbook. 

 c. FAA Report No. FAA-RD-77-78, Engineering Summary of Powerplant Icing Technical 

Data, July 1977. 

 d. AC 23-16, Powerplant Guide for Certification of Part 23 Airplanes. 

5. BACKGROUND. 

The induction system icing requirements of parts 33, 23, and 25 are intended to provide 

protection for anticipated flight into icing conditions with no adverse effect on engine operation 

or serious loss of power or thrust. Propulsion systems certified under these requirements and 

operated in accordance with the airplane flight manual, have generally demonstrated safe 

operation when exposed to natural icing environments. This AC will supersede the engine and 

induction system icing guidance contained in AC 20-73 and AC 33-2 with regard to turbojet, 

turboprop and turbofan engine icing and engine installation icing approvals. Bear in mind AC 

20-73 does contain additional guidance on turboshaft engines and installations. The suggested 

test conditions called out in this AC are intended to be standardized engine icing certification test 

conditions. These standard conditions, in conjunction with any design-specific 
2/02/04 AC 20-147 

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critical test points, should be used together with any additional conditions that the Administrator 

determines to be critical. These standard test conditions have been determined, through more 

than 30 years of certification experience, to provide an adequate and consistent basis for engine 

icing certification with good service experience. The successful demonstration of the test 

conditions outlined in this AC is intended to address many potential engine power conditions, 

aircraft flight conditions, and environmental conditions that could otherwise prove to be costly 

and difficult to realistically test. Service experience, now in the hundreds of millions of hours, 

has also shown a long success record when using these test points to cover unknown 

environmental or operational factors. Finally, one should be aware that in Appendix C of part 

25, the environmental threat is considered probable, and therefore likely to occur. In 

comparison, this occurrence rate is far more probable than the remote threat posed by the rain 

and hail environmental threat in Appendix B of part 33. Therefore, a direct comparison of 

guidance between the acceptable test outcomes of the icing certification requirements and the 

rain and hail certification requirements is not appropriate. Again, it should also be recognized 

that although Appendix C is the certification standard, it is still possible to have in-service icing 

conditions that are more severe than the Appendix C conditions. Often, experience has shown 

that the actual icing environment in nature can be a combination of conditions, such as a 

continuous maximum cloud followed by a intermittent maximum cloud followed by a continuous 

maximum cloud, and so on. 

6. DEFINITIONS. The following are defined for the purpose of this AC: 

 a. Auto-recovery systems. Auto-recovery systems typically include auto-relight systems, 

stall recovery systems, or any 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 fraction of impinging water that freezes on impact. 

 c. Ice formations. Ice formations resulting from the impact of supercooled water droplets on 

propulsion system surfaces 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. 
AC 20-147 2/02/04 

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 (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 content, 

and droplet sizes between those that produce rime and glaze ice. 

 d. Ice shed cycles. The time period required to buildup and shed ice on a propulsion system 

surface for a given power and icing condition. A shed cycle can be identified through visual 

means (for example, high-speed camera which should view fan and booster or low pressure 

compressor (LPC) inlet guide vane components), and engine instrumentation (such as, vibration 

pickups, temperature probes, speed pickups, and so on). 

 e. Icing condition. A meteorological condition defined by the following parameters: 

 (1) Liquid Water Content (LWC). Concentration of liquid water in air, typically expressed 

in grams of water per cubic meter of air. 

 (2) Mean effective droplet diameter (MED or MEDD). A characteristic of a given icing 

cloud where the volume of water associated with droplets larger than the MED is equal to the 

volume of water associated with droplets smaller than the MED. 

 (3) Temperature. The total temperature associated with the icing cloud environment. 

Appendix C temperatures are static ambient temperatures. When a critical test point analysis is 

conducted, the local total temperatures at the engine inlet should be considered, based on 

applying the Appendix C static temperatures and assumed flight mach number. 

 f. Power loss instabilities. Engine operating anomalies such as non-recoverable or repeating 

surge, stall, rollback, or flameout, can result in engine power or thrust cycling. 

 g. Scoop factor (concentration factor). The ratio of nacelle inlet highlight area (AH) to the 

area of the captured air stream tube (AC) [scoop factor = AH/AC]. The highlight area is defined 

as the area bounded by the leading edge of the nacelle inlet. Scoop factor potentially 

concentrates liquid water available for ice formation in the inlet and additionally in the lowpressure compressor or engine core as a function of aircraft forward airspeed and engine power 

condition. The scoop factor affect depends on the droplet diameter, the simulated airspeed and 

the engine power level as well as the geometry and size of the engine. For small thrust engines 

(30 inch diameter and less), MEDD over 15 microns can influence the water concentration while 

typically, MEDD must be over about 40 microns to become an influence on water concentration 

for large engines with diameters in excess of 100 inches. 

 h. Serious loss of power or thrust. Engine operating anomalies such as non-recoverable or 

repeating surge, stall, rollback or flameout, which can result in noticeable engine power or thrust 

loss. The FAA (that is, the Engine and Propeller Directorate, the Transport Airplane Directorate, 

and the Small Airplane Directorate) expects there will not be a noticeable power or thrust loss. 

This is especially important when considering that icing encounters are considered a frequent 

event, and multiple encounters for each flight is a reasonable assumption. The word 

“noticeable”, as used above, refers to flight crews tactile feel during the event, or the use of 

typical engine test instrumentation, or flight deck instrumentation (such as, N1, N2, vibes, 

exhaust gas temp).
2/02/04 AC 20-147 

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 i. Steady Operation. During icing testing, the engine should demonstrate steady, reliable, and 

smooth operation while sitting on test point (during multiple build or shed cycles, if ice is 

accreting), as well as during throttle transients. The term “steadily” is intended to address both 

stabilized ice accretions and stabilized engine operation. Ice accretions are considered stabilized 

when either ice is not forming on any engine parts, or the accreting ice has demonstrated a 

regular shed cycle when viewed by a video camera or instrumentation indication. Engine 

operation is considered stabilized when the measured engine parameters are not changing, or a 

regular, repeatable shed cycle has been demonstrated through the recording of measured engine 

parameters. The applicant should determine what parameters need to be monitored to determine 

steady operation of the engine during the icing test. 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. 

 j. Sustained power loss. A permanent reduction in power or thrust at the engine’s primary 

power set parameter (for example, fan rotor speed, engine pressure ratio). A sustained 

measurable power loss is considered a “severe power loss” in the context of the icing 

requirement. Power or thrust losses that are not sustained are temporary in nature and may be 

related to the effects of ingesting super-cooled water or ice particles, or possibly the effects of 

ice accumulation or ice shedding. The engine’s momentary response during shedding may be 

from the thermodynamic engine response to the ice ingestion. 

 k. Water impingement rate. The rate (gm/Sq. m/min) at which a portion of the surface area 

of a solid object is impacted by the water droplets in a moving air stream. 

7. DISCUSSION. The induction system icing requirements of §§ 23.1093, 25.1093, and 33.68 

are intended to provide protection for flight into icing conditions with no adverse effect on 

engine operation or sustained loss of power. An icing encounter, including a prolonged 

encounter, should not be of consequence to the crew, and should not invalidate the engine's 

compliance with other part 33 requirements (for example, §§ 33.15, 33.19, 33.63, 33.65, 33.75, 

33.77, 33.83, 33.89(b), and the like). The engine should have sufficient durability to operate 

through prolonged or repeated environmental encounters, such as icing, without special 

operational or maintenance interventions. Operational procedures to assist ice shedding, such as 

throttle manipulation, should not be relied on, or be required to comply with parts 23, 25, and 33 

in-flight icing requirements. It is acceptable to provide engine throttle manipulation (for 

example, power run-ups to shed ice) instructions to shed accumulated ice during ground 

operations. These instructions will be used as a recommendation for in-service ground 

operation, although they would be mandatory if they were utilized during the ground icing 

compliance demonstration of §§ 33.68(b), 23.1093(b)(2), and 25.1093(b)(2). The applicant 

should provide instrumentation and video or photographic coverage to supplement test results 

under §§ 33.68, 23.1093, and 25.1093. The applicant should determine the parameters, which 

may include both visual and instrumented indications that need to be monitored. The 

demonstration should include stable build or shed cycles (that is, steady operation) with either no 

ice buildup or no additional ice buildup on the engine or inlet. Normal engine control system 

responses during the ice accumulation process (for example, isochronous control response to 

accreting ice) are considered acceptable as long as there are no power losses. At the conclusion 

of the test point, during the acceleration to takeoff power, the measured parameters should 
AC 20-147 2/02/04 

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demonstrate a smooth and steady acceleration characteristic, unless the applicant can provide an 

acceptable justification for a performance change while on test point (for example, 

thermodynamic engine response to shed ice ingestion is acceptable). Close coordination is 

necessary by all parties to ensure that test plans are in reasonable bounds for the anticipated use 

of the airplane. (You will find background information on for guidance on low engine speed 

compliance testing if you refer to attachment A of this AC.) The body of this AC is arranged in 

three sections corresponding to the applicable parts (§§ 33.68 and 33.89(b); 33.77; and 23.1093 

and 25.1093). 

 a. Mixed Phase or Glaciated Icing Conditions. Mixed phase icing conditions occur when 

supercooled liquid water droplets and ice particles coexist in a cloud, often around the outskirts 

of a thunderhead cloud formation. Service experience generally indicates that turbine engines 

are not susceptible to mixed phase or glaciated icing conditions, with the possible exception of 

two known potentially vulnerable engine design features. These two design features are (1) 

pronounced inlet bends (such as particle-separator inlets), or inlet flow reversals, where inlet 

flow can stagnate and accumulate ice, and (2) high solidity dual row front stage compressor 

stators that can be susceptible to non-aerodynamic ice buildup on the stator air foils resulting in 

core airflow blockage. These two design features should either be avoided or carefully 

scrutinized by analysis and testing to assure their non-susceptibility to mixed phase or glaciated 

icing conditions. Additionally, there have been cases of icing encounters in mixed phase icing 

conditions where ice detectors have not detected ice formation. Ice detection systems should be 

evaluated for these conditions. 

 b. Auto-recovery systems. The use of auto-recovery systems is acceptable for certain engine 

certification testing. The FAA supports the use of auto-recovery systems, or other protective 

engine systems or devices, while in service, and allows the use of auto-recovery systems during 

ice slab ingestion certification testing as defined in § 33.77. Generally, compliance with §§ 

33.68 and 33.77 requires a demonstration that no flameout, sustained power loss, surge or stall, 

or rundown is evident. Although ignition systems have generally been found to be reliable for 

auto-relight use after certain ice ingestion or accretion induced flameouts (§ 33.77), the autorelight system should not be relied on during typical icing encounters (§ 33.68). Auto-recovery 

systems are regarded as only back-up devices, and should not be routinely needed. An example 

where the use of an auto recovery systems may be acceptable would be rare ice ingestion events 

resulting from severe (that is, significantly outside Appendix C) icing conditions. In addition, 

auto recovery systems are not considered the primary protection for continued safe engine 

operation during normal ice sheds or accumulations while operating in typical icing conditions. 

Details will be provided later in this AC about the use of auto-recovery systems when 

demonstrating compliance to §§ 33.68 and 33.77. 

 c. Use of Cloud Extent Factors for § 33.68. In Appendix C, a cloud extent is the distance 

vertically (vertical extent) or horizontally (horizontal extent) that a cloud extends. Vertical 

extent is normally measured in feet while horizontal extent is measured in nautical miles. The 

cloud extent factor is a dimensionless number, which relates the length of a cloud to an average 
2/02/04 AC 20-147 

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LWC across the cloud. These relationships described within Appendix C should be used in the 

critical point analysis (CPA) for assessing the probability of occurrence of icing conditions 

during various aircraft mission and performance analyses. These factors are applicable to 

airframe flight profiles where the straight-line flight portion of the evaluations may use the cloud 

extent factor included in Appendix C, in Figures 3 and 6 as applicable. However, engines and 

induction systems being evaluated under § 33.68 have historically not been limited to or 

evaluated against a specific aircraft flight profile when considering icing environments. Instead, 

under § 33.68, they are evaluated for unlimited operation in icing. It is emphasized that the 

criteria in Appendix C represent those icing conditions that could result from encounters with 

supercooled clouds. Other conditions that may contribute to aircraft engine icing conditions not 

currently included in Appendix C, and not currently required to be addressed under § 33.68 are, 

for example, freezing precipitation (that is, rain, drizzle, sleet, hail, and snow), ice crystals, and 

mixed conditions (meaning, mixture of supercooled water droplets and ice crystals). 

Rulemaking is currently being considered for these conditions. To account for the differences 

described above, multiple clouds should be assumed, with an extent factor equal to 1.0, as actual 

cloud extent is not a consideration for engine operations, particularly in an aircraft hold pattern. 

This approach will assure unlimited engine and induction system operation within the 

atmospheric conditions defined by Appendix C, and as experience indicates, in actual icing 

environments that include Appendix C in its entirety. The cloud horizontal extent factor was not 

intended to be used to limit the severity of exposure to icing conditions, where it is reasonable to 

assume the aircraft will be required to operate in that condition (for example, the holding pattern 

which may require repeated passes through a severe icing environment, or continuously remain 

in that severe environment). As a general rule, engines and induction systems should be shown 

to operate continuously in icing without regard to time in icing conditions. The only exception 

to this would be for low engine power conditions where sustained level flight is not possible. 

Even then, a conservative approach must be used where multiple horizontal and vertical cloud 

extents in series are assumed. 

 d. Low Engine Speed Compliance Testing in Continuous Maximum Clouds. During § 33.68 

compliance test demonstrations, when demonstrating engine speeds (N1) below minimum for 

sustaining level flight (meaning, below “light-hold” N1), and in continuous maximum icing 

conditions, the test duration should be no less than 10 minutes and do not need to continue 

beyond 30 minutes. It may be terminated in-between 10 and 30 minutes, if and when stabilized 

operation in icing is shown. These low N1 test duration’s are based on multiple appendix C 

clouds in series while in descent. At engine powers that can sustain flight, all compliance test 

conditions must be run indefinitely, until stabilized operation in icing is demonstrated. 

Stabilized operation is the same as “steady operation”, and is defined in this AC. These test 

duration criteria should be applied to all icing demonstration test points that are within, or close 

to the continuous maximum conditions of Appendix C. At the end of each test point, the engine 

should be accelerated to maximum power to shed any residual ice, and then it can be shutdown. 

(You will find background information for low engine speed compliance testing if you refer to 

attachment A of this AC.) 

 e. Low Engine Speed Compliance Testing in Intermittent Maximum Clouds. For § 33.68 

compliance test demonstrations defined in Appendix C, intermittent maximum icing conditions, 

the test duration should be 10 minutes. Indefinite operation is not required when operating at 

these low N1's, so the test can be terminated and the engine accelerated to takeoff power after the 
AC 20-147 2/02/04 

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10-minute demonstration period. This 10-minute test duration can be applied to all engine 

power conditions up to, but not including light hold power. These low N1 test durations are 

based on multiple Appendix C clouds in series while in descent. At engine powers that can 

sustain level flight, all compliance test conditions should be run until stabilized operation is 

demonstrated, which may be well beyond 10 minutes. Stabilized operation is the same as 

“steady operation” and is defined in this AC. (You will find background information on low 

engine speed compliance testing if you refer to attachment A of this AC.) These criteria should 

be applied to all icing demonstration test points that are within or close to the intermittent 

maximum conditions of Appendix C. At the end of each test point, the engine should be 

accelerated to maximum power to shed any residual ice and then it can be shut down. 

SECTION 1. INDUCTION SYSTEM ICING (§§ 33.68 and 33.89(b))

8. CRITICAL POINT ANALYSIS (CPA). Compliance with the requirements of § 33.68 

includes identifying, through analysis, the critical operating test points for icing within the 

declared operating envelope of the engine. The CPA should include a range of possible 

combinations of icing conditions. This range should relate to Appendix C of part 25, aircraft 

speed range, and engine powers as defined by the engine manufacturer, and prolonged operation 

in icing (for example, in-flight hold pattern), or repeat icing encounters. The CPA should be 

validated by empirical test data. This analysis should consider both critical ice accumulation 

conditions (that is, rime ice and glaze ice), environmental and engine operational effects on 

accumulation, accretion locations, as well as the most critical engine operating conditions for ice 

shed and ingestion. Some manufacturers have included within their CPA, a best practice of 

including conditions that may be outside of Appendix C, that have been identified through 

service difficulty of other engine models. Often the CPA is supplemented with development test 

data (for example, wet and dry testing with thermocouple components). The methodology used 

to calculate ice accretions should account for freezing fraction and pertinent aerodynamic 

effects. For example, water ingestion into fan inlet and core inlet, water impingement rates for 

critical surfaces, forward aircraft air speed effects, engine configuration effects such as intercompressor bleed, and altitude effects such as bypass ratio effects. This should be in conjunction 

with 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. For anti-iced parts, the 

critical test point should be determined from energy balance calculations of required heat loads 

encompassing the range of possible combinations of icing condition and engine power. In 

instances of low freezing fraction in glaze ice conditions, additional complexities arise from 

assessing the effects of non-aerodynamic ice formations and their shedding. Federal Aviation 

Administration (FAA) Report No. FAA-RD-77-78, Engineering Summary of Powerplant Icing 

Technical Data, provides additional guidance on performing a critical test point icing analysis. 

 a. Test versus Analysis. To approach the problem of reducing a matrix of potentially 

hundreds of test points to a few appropriately chosen test points, a certification process was 

developed over 30-years ago where a number of standard AC table test points (shown in tables 1 

and 2 of this AC) have been demonstrated through full engine test which is then augmented by a 

CPA. The CPA was not meant to replace testing, or eliminate or replace the standard table test 

points, but instead to provide a means to predict other critical test points and test them in 
2/02/04 AC 20-147 

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addition to the standard AC table test points. The FAA maintains this view of the CPA 

complementing the standard AC table test points provides an adequate methodology to cover all 

icing conditions. 

The FAA recognizes an improvement in the fidelity of analysis tools that are available today. 

However, based on experience with the various applicants, the FAA believes that the CPA is 

best utilized as a method to predict the critical icing conditions for a given design, and then use 

these conditions in conjunction with the standardized AC table test points for certification 

compliance test purposes. It should be noted that FAA concurrence with a type certificate 

holders generic CPA method does not automatically constitute FAA 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. Some 

engine manufacturers have had difficulty in consistently achieving the agreed on test points that 

are necessary for compliance demonstration, due to facility or weather limitations. This 

difficulty is based on test facility limitations that can be expensive and often impractical to 

overcome. To directly address this repeated shortfall, the FAA has identified several national 

and international icing resources for accomplishing § 33.68 compliance testing. These resources 

include the Air Force’s McKinley Climatic Lab (Florida), the Air Force’s AEDC (Tennessee), 

and the Canadian government’s NRC (Toronto), to name a few. There are additional test 

facilities that are suited for component or model testing. 

 b. Elements of CPA. The CPA should address, at minimum, the following icing issues: 

 (1) Ice shed damage. Ice accretion on engine surfaces (for example, blades, vanes, 

sensors) may eventually shed. The shed ice can subsequently cause engine damage if it impacts 

an engine surface with sufficient mass and velocity. 

 (2) Fan module. Acoustic panels, fan rub strips, and fan blade tips are susceptible to ice 

shed from inlet sensor(s), spinner, and fan blade root. The effects of ice density, hardness, and 

adhesion strength should be assessed to realistic flight conditions. The ice shed cycle for 

rotating surfaces, such as fan blades, is strongly influenced by 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 rotor speed at the time of the shed defines 

the impact threat. In determining the critical conditions for fan module damage, surface 

temperature, exposure time, and rotor speed are important considerations in addition to more 

typical parameters, such as icing condition and scoop factor In particular, extended operation in a 

holding condition in very cold continuous maximum icing conditions will maximize the 

adhesion of ice on rotating fan components. 

 (3) Compressor damage. A common damage scenario in turbofan engines, is the accretion 

of glaze (non-aerodynamic) ice formations on static components (for example, sensors, vanes, 

and bleed ducts upstream of the compressor) which when it sheds, results in damage. This type 

of damage generally occurs on the first blade set in the high-pressure compressor (intermediate 

pressure compressor for three spool engines). Establishing the critical conditions for these glaze 

ice accretions requires careful consideration as 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 recommended by this AC (that is, flight-idle, 50-
AC 20-147 2/02/04 

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percent and 75-percent of maximum continuous takeoff or 100-percent maximum continuous, 

whichever is applicable). Any engine damage that results from ice testing should be evaluated 

against the possibility of multiple occurrences, since icing is a common environmental condition. 

 (4) 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 the engine to assume new operating conditions (that is, engine component cycle rematch). 

The engine should be capable of accelerating from minimum flight-idle and ground idle to 

takeoff power, at any icing condition, without power loss, or instability (surge or stall). Ice 

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

be raised to the cognizant Aircraft Certification Office (ACO) for evaluation and if found 

acceptable, should be documented in the engine’s installation manual. The applicant should 

consider as part of the CPA both engine accelerations and decelerations relative to operability 

challenges. Critical test point testing should demonstrate those conditions where minimum 

operability margin is expected. 

 (5) Core and Booster ice blockage. Ice accretion on internal engine vanes due to the 

presence of glaze ice accretions may affect flow capacity and rematch of the engine cycle and 

should be considered in the CPA. At engine powers that can sustain flight, ice accretion should 

be reconciled through a demonstration of several ice build shed cycles to demonstrate no adverse 

operating effects of either the ice builds or sheds. 

 (6) Sensor fouling. Ice accretion and blockage of control sensors can result in erroneous 

engine pressure and temperature measurements. A power loss or power loss instability could 

result if these measurements are used by the engine control systems control law to establish 

power or thrust ratings, or to schedule other systems required to operate the engine (for example, 

variable stator vanes). Critical sensors should be designed to operate without accreting ice 

sufficient to cause an erroneous measurement that would result in an unacceptable operating 

characteristic. Additionally, ice accretion on upstream sensors can shed and cause engine 

damage to downstream rotating hardware. 

9. TEST POINT(S) SELECTION. The icing test points selected must address the Appendix C 

icing envelope. Typically, the test points include those described in this AC (see table 1 and 

table 2 test points of this AC), and any additional test points identified as part of the CPA. The 

applicant should consider pertinent service experience as well as the anticipated use of the 

aircraft when selecting critical icing test points. The following should be considered when 

constructing an icing test matrix: 

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

operating acceptably under the meteorological conditions of Appendix C over the engineoperating envelope, as described in § 33.68(a), and under conditions of ground fog (§ 33.68(b)). 

Experience has indicated that testing to the conditions specified in the following tables have 

been a successful means of showing compliance, if used in conjunction with the critical 

conditions determined in the CPA.
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Table 1 

Icing Conditions for Engine Certification Testing 

Icing Condition 1 

(glaze ice) 

2 

(rime ice) 

3 (§ 33.68(b)) 

(ground fog icing) 

Liquid Water Content, 

gr/meter3

2 

(minimum) 

1 

(minimum) 

0.3 

(minimum) 

Inlet Temperature, °F 

(Total Temperature) 

23 

(+/- 2 F) 

-4 

(+/- 4 F) 

15 - 30 

Mean effective water droplet 

diameter, microns 

22 

(+/- 3) 

15 

(+/- 3) 

20 

(minimum) 

Specifically for the icing conditions defined in the table 1.

• Conditions 1 and 2. Operate the engine steadily (see definitions for “steady operation”) 

under icing conditions 1 and 2 for at least 5 minutes at takeoff setting, and at least 10 minutes 

each at 75-percent of maximum continuous power (M.C.), 50-percent of M.C., and at a flightidle setting, then accelerate to takeoff. If ice is still building up at the end of 10 minutes, 

continue running until the engine demonstrates stabilized operation (meaning, stabilized building 

and shedding is demonstrated or the engine will no longer operate satisfactorily). For test point 

number 1, the mean effective droplet diameter has been reduced by 3 microns in order to 

position the test point within the Appendix C envelope, at 23° Fahrenheit and LWC=2. This was 

done to avert the past discussions on this test point being out of the appendix C envelope. 

• Test points 1 and 2 above, are intended to partially cover a widely bounded test matrix of 

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

§33.68(a). This test matrix includes power settings from idle to takeoff during exposure to 

conditions typical of high altitude where rime ice formations occur, as well as conditions typical 

of low altitude where glaze ice formations often occur. Icing conditions 1 and 2 are normally 

run for a minimum of 5 minutes when at takeoff power. All other power settings below takeoff 

should be run for a minimum of 10 minutes, or longer if the natural ice shed cycle is not 

established. Also, engines that employ an inlet de-ice system that may have some unheated inlet 

surfaces may need to be run longer. 

• Condition 3. Operate steadily at ground idle setting for at least 30 minutes under icing 

condition 3 followed by acceleration to takeoff setting. Since a broad temperature range is 

provided, the applicant should identify the most critical temperature, as determined by the CPA, 

and target that range. If ice is still building at 30 minutes, continue running until the engine 

demonstrates stabilized operation (that is, stabilized building and shedding is demonstrated or 

the engine will no longer operate satisfactorily). For large bypass fan engines, shedding at this 

low speed from static surfaces is not likely at 30 minutes and beyond. For these engines, this test 

generally establishes the maximum allowable time between engine run-up to shed ice. 

Typically, the AFM specifies about 1⁄2 this time (15 minutes) between run-ups. The engine 

should be shown to operate continuously (including run-ups, if necessary) at the ground icing 
AC 20-147 2/02/04 

Page 12 Par 9 

condition, as might well occur at an airport where aircraft can be held on the ground several 

hours in bad weather awaiting takeoff. Periodic run-ups are acceptable as part of this 

demonstration. 

For all conditions. While at flight-idle engine speed and above, for engines with icing protection 

systems, stabilize the engine for at least 2 minutes in the icing atmosphere with these protection 

systems off, prior to turning on the icing protection system. Systems that are automatic and 

controlled by the full authority digital electronic/engine control (FADEC) do not require the 2-

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

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

likely following failure of the detector, and so delayed selection testing should still be 

demonstrated. 

• For all conditions. Engine operation in these icing conditions should be reliable, 

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

continue in operation and accelerate and decelerate. Some power reduction is acceptable at idle 

power settings due to the cycle effects of pumping ice and water, but all other operation should 

be unaffected. 

• The engine must operate “steadily” under the tested icing conditions. The term 

“steadily” is defined in the definitions section and is intended to address both stabilized ice 

accretions and stabilized engine operation. Ice accretions are considered stabilized when either 

ice is not forming on any engine parts, or the accreting ice has demonstrated a regular shed cycle 

when viewed by a video camera or instrumentation indication, or both. Engine operation is 

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

repeatable shed cycle has been demonstrated through the recording of measured engine 

parameters. 

• At the conclusion of each steady state ice test point, the engine should be accelerated to 

takeoff power. The throttle 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. The applicant should 

assess this effect and account for their assessment in their test proposal. 

 b. Section 33.68(b). Section 33.68(b) provides an icing test point, which represents a typical 

freezing fog icing encounter during ground operation. The guidance contained in AC 33.2B, 

with respect to snow ingestion testing, is considered outdated (see paragraph 17 of this AC for 

additional guidance). The ground-fog icing demonstration should continue for at least 30 

minutes or until stabilized operation (see “steady operation” in the definitions section of this AC) 

is demonstrated. If this stabilized operation cannot be demonstrated then periodic engine speed 

run-ups should be demonstrated. These run-ups would then become mandatory in icing 

conditions since they were required to comply with the icing requirements. The effect of 

ingesting snow during ground-operations can and should be evaluated, but not concurrently with 

ground fog. Service experience has demonstrated compressor damage (see paragraph 8b(3) of 

this AC) as a result of exposure to prolonged periods of falling snow during ground operation. 

Based on review of service events, airports have continued to operate with falling-snow 
2/02/04 AC 20-147 

Page 13 Par 9 

concentrations that result in 0.25 mile or less visibility. The enhancement of freezing fraction in 

the presence of wet, sticky snow suggests that icing tests conducted with only liquid water 

require approximately twice that concentration shown in table 1 for condition 3. Utilization of 

liquid water in-lieu of snow is not recommended, depending on the hardware configuration being 

tested, ice crystals may be more appropriate. Sections 23.1093(b)(ii) and 25.1093(b)(ii) require 

that engines must operate satisfactorily in falling and blowing snow throughout the engine power 

range, within the limitations established for the airplane for such operation. 

 c. Holding phase. This test, when performed as part of a part 33 demonstration, is intended 

to be applicable to part 33 engine components. Both the engine (§ 33.68) and aircraft inlet 

induction system (§§ 23.1093 and 25.1093) should operate safely in an in-flight holding phase 

without a time limit. The test program for turbofan and turboprop applications should include 

test points (for example, icing condition and power setting) to address the effects of prolonged 

exposure in icing conditions typical of in-flight holding patterns. Test point 1 in table 2, below, 

represents a rime icing condition that is typically encountered on transport category airplanes. 

Test point 2 in table 2 of this AC represents a mixed rime and glaze icing condition that was 

originally derived from the 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 2. 

A 45-minute minimum test exposure followed by acceleration to takeoff power will typically 

demonstrate several ice shed cycles and should normally be sufficient to assess compliance for 

the engine. 

Table 2 - Holding Conditions 

Total Air Temperature

(degrees / Fahrenheit)

Liquid Water Content

(g/m3) 

Mean Effective 

Droplet Diameter

Test 

Point 

Turbofan Turboprop (minimum) (microns)

 1. -4 

(+/- 4°F) 

6 

(+/- 2°F) 

0.25 15-20 

 2. 14 

(+/- 2°F) 

6 

(+/- 2°F) 

 0.30 6 minutes* 

 1.70 1-minute* 

15-20 

 20 +/-3 

NOTE: * alternate between these two conditions for at least 45 minutes 

10. TEST SETUP CONSIDERATIONS. The Liquid Water Content (LWC) levels defined in 

appendix C are intended as ambient icing conditions. Tests may be conducted with a simulated 

cloud outside of the inlet that is ingested into the engine. Under such a test environment, the 

LWC within the inlet ducting may be more or less than the engine inlet LWC concentration if the 

engine were actually installed in an airplane and flying through those icing conditions at actual 

airspeeds. This inlet concentration effect is dependent on droplet size, engine fan speed, and 

simulated forward airspeed. 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 
AC 20-147 2/02/04 

Page 14 Par 10 

cause test facility effects that could alter the test parameters (for example, LWC and MED). The 

applicant must provide the FAA with substantiation that the required simulated test conditions 

adequately simulate an installed engine flying through the Appendix C conditions. This 

substantiation can be in the form of direct measurement of LWC within the inlet or by acceptable 

validated analysis of water droplet trajectories for the test setup. In some cases, additional LWC 

may be needed to address any shortfall in concentration effects due to the test setup (for 

example, non-uniformity across the engine face). 

11. TEST RESULTS AND COMPLIANCE ISSUES. During all icing tests, the engine 

should operate without the accumulation of ice, which would adversely affect engine operation 

(for example, flameout, surge, stall, run-down, high vibrations, slow acceleration or lack of 

throttle response) or cause a sustained loss of power or thrust. Additionally, the applicant should 

accurately monitor icing test point conditions either through video surveillance or 

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

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

instrumentation). 

 a. Sustained loss of power or thrust and power loss instabilities. There should be no 

sustained power loss while operating at approved ratings in icing conditions. Temporary steady 

state power losses below the engine power and thrust ratings selected in accordance with § 33.8 

can be accepted if it is proven that there is sufficient margin against any power loss instability, 

such as rollback, surge, stall, high vibes or flameout. Although usually not acceptable, 

temporary power anomalies that may be found acceptable could include those temporary 

anomalies caused by pumping or processing of ice debris through the fan and compressor during 

the ice shed ingestion process. If temporary power loss or temporary high vibrations are deemed 

acceptable to the Administrator, they should be documented in the engine installation manual. 

 b. Mechanical Damage. There should be no engine damage as a result of § 33.68 icing 

testing. In some circumstances, some limited damage may be accepted. The acceptance of any 

damage must fully account for the cumulative damage from repeat encounters, provided the 

applicant satisfies the following criteria: 

 (1) Continued in-service use. Any resultant damage should be shown to be acceptable for 

continued in-service use. 

 (2) Sustained power losses. There should be no resultant sustained power loss; 

 (3) Temporary power loss. Although not generally acceptable, any resultant temporary 

steady state power loss, surge or high vibrations, if found acceptable by the Administrator, 

should be recorded in the installation manual. 

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

acceptable damage should be shown to have a sufficient validation basis (for example, engine 

tests, rig tests, service experience) to substantiate the accuracy of results or be shown to yield 

conservative results.
2/02/04 AC 20-147 

Par 11 Page 15 

 (5) Disposition of Damage. Disposition of damage to any engine or engine component 

may not be obtainable solely by comparing the damage against the maintenance manual limits. 

The cumulative damage for repeated encounters should be evaluated. 

 (6) Communication of results. The Installation and Operating Manuals required by § 33.5 

should provide information describing any resultant engine condition observed during engine 

certification icing tests. The engine manufacturer should provide a process to permit disposition 

of any potential damage that could occur during natural icing flight tests conducted to 

demonstrate compliance with §§ 23.1093 or 25.1093, if the installing FAA Aircraft Certification 

Office finds this acceptable. Also, if periodic engine power run-ups are necessary to minimize 

damage from icing during the ground icing operation demonstration of § 33.68(b), then this runup must be documented. Documentation must contain a description of the run-up requirements 

and the required run-up intervals and it must be contained in the operating manual and 

airworthiness limitations section of the instructions for continued airworthiness (ICA). Any 

power loss anomalies due to accumulation, shed, runback and the like, and their effects on 

performance and operation should be documented in the Installation Manual. Both the engine 

certifying ACO and the installing ACO should carefully consider any high vibrations induced 

from ice accretions during ice testing. This too should be documented as described above. 

 c. Engine systems. It is permissible to use engine systems (that is, automatic, engine initiated 

ice protection systems) to fulfill § 33.68 requirements provided that its operation is not expected 

to result in crew action. Examples of engine characteristics that may not be transparent to the 

flight crew are exhaust gas temp fluctuations, or audible surging. Additionally, any engine 

system required to show compliance with § 33.68 should meet the following requirements: 

 (1) System reliability. Demonstrate the capability of the system for reliably sensing the 

conditions, which enables the function, throughout the operating envelope; 

 (2) Dispatch. The function should be available for all dispatchable configurations. The 

system should be configured in its most critical dispatch state for certification icing tests; 

 (3) Electronic faults. If the system uses electronics, substantiate that the function is not 

lost due to any single or probable multiple electronic faults; 

 (4) Other environmental testing. The function should not be affected when the system and 

any associated electronic systems are exposed to required operating environments, including 

high intensity radiated fields (HIRF) and lightning; and 

 (5) Power requirements. For those systems that are powered solely with a dedicated 

engine alternator (either directly or using another engine system such as full authority digital 

electronic/engine control (FADEC)), it should be demonstrated that over the operating envelope, 

that the function (that is, sensing and performance) is provided at the minimum certified rotor 

speeds. Minimum certified engine speed is the minimum idle rotor speed achievable anywhere 

in the flight envelope.
AC 20-147 2/02/04 

Page 16 Par 11 

 d. Auto-recovery systems. Auto-recovery systems should not be needed during § 33.68 

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

envelope. The intent of § 33.68 is to certify engines that will be able to perform and operate 

reliably in the icing conditions described Appendix C. Auto-recovery systems are considered to 

be back-up devices that are only needed following rare ice ingestion events that result from icing 

conditions significantly outside Appendix C, and should be communicated to the installing ACO 

if activation is expected or experienced in these rare occasions. Auto-recovery systems are not 

the primary protection for continued safe engine operation during normal ice sheds, or accretion 

while operating in icing conditions described in Appendix C. Therefore, it is acceptable to 

perform § 33.68 compliance testing with auto-recovery systems enabled, but they should not 

activate throughout the § 33.68 test sequence. Additionally, continuous ignition should not be 

selected during § 33.68 compliance testing. To assure non-activation of an enabled autorecovery system, it may be necessary to display an instrumented signal that monitors autorecovery system activation. If activation monitoring cannot be accomplished, then disabling of 

the auto-recovery system may be necessary. 

 e. Operating instructions. Any operating procedure (for example, ground run-up procedures) 

required to ensure continued operational compliance with ground icing conditions evaluated 

under § 33.68(b), or falling and blowing snow conditions which can be evaluated under § 

33.89(b) as a precursor to §§ 23.1093(b)(ii), or 25.1093(b)(ii), should be communicated to the 

installer in the Operating Instructions as a requirement, and should 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. 

SECTION 2. ICE SLAB INGESTION (§ 33.77)

12. INTENT OF ICE SLAB INGESTION TEST. The intent of the ice slab ingestion test is 

to demonstrate that the ingestion of an ice slab, which may form after a delayed activation of 

induction system anti-icing, will not adversely affect engine operation. It is intended that the 

engine manufacturer will conservatively consider the potential installation effects of the engine 

induction system. Also, there should be close coordination with the installer to ensure that 

potential airframe ice accumulation sites that can result in ice ingestion into the engine (for 

example, inboard section of wing for an aft fuselage mounted engine) are either demonstrated 

under § 33.77 or addressed under §§ 23.901(d)(2), 23.1093, or 25.1093 (see paragraph 18a(2) of 

this AC). The induction system manufacturer or installer should assess these accumulations in 

accordance with §§ 23.901(d)(2), 23.1093, or 25.1093 and provide pertinent test variables to the 

engine manufacturer for incorporation into a test demonstration in accordance with § 33.77. In 

the case where an application or product inlet has not been selected at the time of engine 

certification, the engine manufacturer should provide all pertinent inlet assumptions and test data 

and results in the engine installation manual for use by the future installer. 

13. TEST CONSIDERATIONS. The test demonstration should consider ice slab sizes and 

trajectories aimed at critical engine locations that are based on the ice accretion and shed 

characteristics of the induction system that is likely to be installed on the engine. Lacking such 
2/02/04 AC 20-147 

Par 13 Page 17 

specific knowledge, the applicant may select test conditions, which are typical of a condition for 

a representative installation in-service. The ice slab size, thickness, and density used in § 33.77 

compliance demonstration should be assessed for appropriateness against parts 23 and 25 

compliance requirements. The reason for this is that part 33 specifically addresses continuous 

maximum icing conditions, and parts 23 and 25 address all part 25 appendix C conditions and all 

areas of the airframe that could present an ice slab into the engine inlet. See AC 20-73, AC 

23.1419-2B, and AC 25.1419-1 for more guidance on ice shedding. If it is determined that the 

ice slab size, thickness, and density are appropriate for the engine installation, then the part 33 

test results can often be used by the airframe manufacturer to comply with the natural icing flight

test requirements which are related to delayed activation of the induction anti-icing system. 

Historically, slab thickness used in compliance testing for § 33.77 has typically been identified 

as approximately 0.5” thick, with a few installations identifying approximately 0.33” thick slabs. 

These thicknesses have typically been sufficient to cover any potential airframe ice slab threats 

for compliance to parts 25 and 23 compliance. Experience has shown that the method of 

delivering the ice slab to the face of the engine and the orientation of the slab (for example, the 

edge of the slab versus the face of the slab) can have a significant effect on the outcome of the 

test. 

Therefore, it is incumbent upon the applicant to determine and test for the most realistic release 

method. This release method should account for the most critical possible orientation at impact. 

14. TEST RESULTS. Section 33.77(c) requires that the ingestion of ice, under the conditions 

stipulated in § 33.77(e), may not cause a sustained power or thrust loss, or require the engine to 

be shutdown. The intention of this ice slab test is to account for inadvertent delayed activation 

of the induction anti-icing system. A nominal 2-minute delay is assumed. The following criteria 

should be met: 

 a. Sustained power losses. There should be no resultant sustained power loss. Any fan blade 

bending or damage should be assessed against potential sustained power loss and the resulting 

damage should not result in a power loss greater than the ability to assess power level. That is, it 

should not be outside the capability to measure and assess engine power level. 

 b. Engine operability. Damage should not adversely affect engine operability (that is, should 

not cause surge, flameout, nor prevent transient operation). 

 c. In-service capability. Damage should not result in a failure or a performance loss that 

would prevent continued safe operation for a conservative flight cycle scenario (for example, 

within fly back limits or greater if appropriate testing is done to validate 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 positions). 

 d. Other anomalies. Damage should not result in any other anomaly (for example, vibration) 

that may cause the engine to exceed operating or structural limitations.
AC 20-147 2/02/04 

Page 18 Par 14 

 e. Auto-recovery systems. If during § 33.77 ice slab ingestion testing, an engine does incur a 

momentary flameout and auto-relight, then normally the acceptance of that test would be 

predicated on the inclusion of the auto-relight system as being a required part of the engines type 

design, and an additional dispatch criteria would be required, where the ignition system must be 

functional (that is, fully operable) prior to each dispatch. The reason for the additional dispatch 

criteria is to ensure the ignition system’s critical relight function is reliably available during the 

subsequent flight. The reason for the allowance of auto-recovery systems during § 33.77 

certification testing is to account for ice accretion and shedding, as a result of an inadvertent 

2-minute delay in actuating the anti-icing system, which is considered to be an abnormal 

operational result where mild operability effects (for example, momentary flameout and relight) 

may be accepted. 

15. COMMUNICATION OF TEST RESULTS. The installation and operating 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 engines ability to 

operate at the commanded power setting or rating. The icing certification report should include 

information regarding ice slab orientation and trajectories, slab breakup, impact locations, 

description of any resultant damage, and any 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 to ensure 

compliance with the delayed activation requirements of §§ 23.1093 or 25.1093. 

SECTION 3. INDUCTION SYSTEM ICING PROTECTION (§§ 23.1093 and 25.1093)

16. 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 the accumulation of ice on the engine, inlet system 

components, or airframe components that would have an adverse affect on engine operation or 

cause a serious loss of power or thrust. Based on multiple engine natural ice damage and 

operability events experienced on natural icing flight test and in-service airplanes, the FAA 

requires natural ice encounters for showing compliance with §§ 23.1093(b)(1) or 25.1093(b)(1). 

However, for airplanes that are not intended to be certificated for flight in icing conditions, the 

FAA will accept other methods in lieu of natural icing flight tests, such as analysis, ground 

testing, dry air flight testing, and similarity, to show compliance to § 23.1093(b)(1). Aside from 

the benefit of validating the engine inlet icing analysis model, there are several other key issues 

that the natural ice encounter addresses. These evaluations include: (1) The adequacy of the 

flight crew procedures for operation in icing conditions, (2) acceptability of tactile inputs to the 

flight crew as the airplane responds to engine fan blade ice shedding during a variety of airplane 

operating conditions, (3) performance of the engine vibration indication system as well as other 

engine indication systems and, (4) confirmation that the powerplant installation as a whole (for 

example, engine, inlet, anti-ice system) performs satisfactorily while in icing conditions. 
2/02/04 AC 20-147

Par 16 Page 19 

 a. Identification of ice source. A means should be provided to aid in identifying 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. 

b. Icing test point monitoring. The applicant should provide sufficient monitoring of the 

icing test point condition (that is, LWC, droplet diameter, temperature) against the time to ensure 

that the icing encounter is representative of the appendix C conditions under part 25. 

 c. Compliance. Compliance with §§ 23.1093 or 25.1093 is required even if flight into icing 

approval (§§ 23.1419 or 25.1419 compliance) is not obtained. Compliance with the natural ice 

encounter criteria should be proposed by the applicant and agreed to by the FAA prior to the test. 

However, typically an adequate test sequence includes three natural fan ice shed cycles at each 

of the following conditions (with inlet anti-ice turned "on"); descent (flight-idle), holding (power 

necessary to maintain level flight for a range of anticipated airplane gross weight conditions), 

and maximum climb, unless a more critical engine power setting exists. These encounters 

should be conducted at a steady state engine thrust level and although not preferred, sometimes 

have involved flying through the same icing cloud multiple times (lapping) in order for the fan to 

accumulate enough ice for a shed cycle to occur. (Caution is emphasized when flying in natural 

ice conditions.) These fan shed cycles should be due to natural ice accumulation and not 

induced or forced by throttle excursions or manipulations, or both, during each condition. It has 

also been allowed for the airplane to exit the icing conditions between each fan shed cycle for 

the purpose of clearing any other unprotected airplane surfaces from ice. To avoid masking any 

adverse engine operating conditions during the natural icing encounter, the test engine’s ignition 

system should be selected off during the icing conditions. This may require pulling several 

airplane circuit breakers to disable the test engine’s auto-ignition or recovery system, or both, 

and caution should be used and all safety precautions exercised. Lastly, based on past 

experience, it is advisable that the applicants establish and gain concurrence with the FAA for 

engine damage criteria prior to conducting the natural ice encounter test. 

17. FALLING AND BLOWING SNOW. Sections 23.1093(b)(ii) and 25.1093(b)(ii) require 

that engines must operate satisfactorily in falling and blowing snow throughout the flight power 

range. The effect of ingesting snow during ground operations can and should be evaluated. 

Service experience has demonstrated compressor damage (see paragraph 8b(3) of this AC) as a 

result of exposure to prolonged periods of falling snow during ground operation. Based on 

review of service events, airports have continued to operate with falling-snow concentrations 

that result in a 0.25 mile or less visibility. In-flight service experience has also shown that snow 

can shed from engine or aircraft accumulation sites and cause severe operability affects on 

turbine engines. Therefore, all engine inlets, including those with plenum chambers, screens, 

particle-separators, variable geometry, or any other feature, such as an oil cooler, struts or 

farings, which may provide a potential accumulation site for snow should be evaluated. Also, 

any airframe accumulation sites upstream of the engine inlet should also be considered. 
AC 20-147 2/02/04

Page 20 Par 18 

18. 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: 

 a. Acceptable damage. The extent of damage is equivalent to or less than that incurred and 

accepted during engine certification testing. 

 (1) All systems operating normally. The extent of damage is equivalent or less than that 

incurred and accepted during the § 33.68 tests. 

 (2) 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 anti-icing 

system activation test should be considered. (Caution should be used and all safety precautions 

exercised.) For this condition, the acceptance criteria defined in paragraphs 13 and 14 of this 

AC, should be used. The airframe manufacturer still must consider all potential ice shedding 

sites (for example, inboard wing and radome). Similar to the accepted compliance of § 33.77 ice 

slab ingestion tests (outlined in section 2 of this AC), the use of engine auto-ignition and 

recovery systems are allowed to show compliance with the delayed activation tests of parts 23 or 

25, as long as these automatic systems can not be easily turned off by the flight crew (that is, a 

flight crew that inadvertently forgets to turn on the engine anti-ice protection is also likely not to 

have selected any other engine protection features such as continuous ignition, prior to entering 

the inclement weather). It is important to note the difference in anti-iced inlets versus de-iced 

inlets. De-iced inlets produce a cyclic shedding of ice from the engine inlet into the engine and 

typically incorporate, as part of their design, an inlet particle-separator that precludes the 

ingestion of ice into the core of the engine. It should be recognized that an engine auto-recovery 

system 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 or 25.1093. 

 b. Damage from testing in non-representative icing conditions. Damage resulting from icing 

test conditions which fall significantly outside Appendix C icing envelopes, or when the airplane 

flight test is conducted in an abnormal manner and results in excessive ice shed damage, may be 

given additional considerations relative to compliance with the provisions of either §§ 23.1093 

or 25.1093, and in some cases may be disregarded. An example of abnormal operation could be 

flying with one engine at idle while the aircraft is operated in level flight. 

 c. Unacceptable damage. The icing test conditions was representative of in-service 

encounters and the resultant airframe or engine ice sheds caused damage that exceeds the criteria 

established in paragraph 11b of this AC. 
2/02/04 AC 20-147

Page 21 Par 19 

19. CONCLUDING REMARKS. Although applicants may conduct representative tests under 

§§ 33.68, 33.77, 23.1093, and 25.1093, flight test events may still occur which appear 

inconsistent. In all likelihood, those results would not be inconsistent when judged in light of 

the scope, intent, and limitations of the certification testing. Only through reliable 

instrumentation and photographic evidence can the icing test disparities be fully understood. 

Because of the relatively frequent encounters with icing conditions in conjunction with the 

potential impact on safety, the FAA takes a conservative approach when accepting icing 

compliance standards. 

David W. Hempe 

Manager, Aircraft Engineering Division 
AC 20-147 2/02/04

A1-1 

ATTACHMENT A 

Reference: Policy on low engine speed part 33 compliance testing 

The Federal Aviation Administration (FAA), Engine and Propeller Directorate's 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 FAA's position has traditionally 

been, and continues to remain, 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 § 33.68 compliance test demonstration, engines must remain onpoint until stable operation has been demonstrated. This has been pointed out as being onerous 

when operating the engine at non-flight-sustainable engine speeds (meaning, below lightweight 

hold engine operation). In response to this observation, the FAA has developed criteria that 

allows for a reasonable test period and termination time for icing testing of non-flightsustainable engine speeds. It is emphasized that this test time termination criteria is only 

applicable to icing testing when the engine is operating at non-flight-sustainable engine speeds. 

The maximum liquid water content (LWC) severity depicted in Appendix C of part 25 shows 

that intermittent maximum water content is over three times more severe than the continuous 

maximum water content. Due to the significant severity differences between intermittent 

maximum and continuous maximum icing conditions in Appendix C, two separate criteria have 

been considered. 

Appendix C, paragraph (a) - Continuous Maximum Icing Conditions. In an effort to preserve 

safety of flight in icing conditions, the FAA strives to retain sufficient margin to expected icing 

conditions when assessing compliance demonstrations. Conservative assessments are applied in 

several assumptions. The FAA must expect engine designs to exceed the single cloud 

definitions of appendix C since the encounter of LWC at a stated inlet temperature at any mean 

effective diameter (MED) is predicted to be a probable event. The expected occurrence rate is 

too frequent to consider a single cloud extent during §33.68 compliance test demonstrations. 

With this objective, the FAA has developed a conservative methodology for assessing the worst 

case (that is, long descent loiter) that can reasonably be expected. The following analysis is 

intended to approximate a conservative approach to bounding the maximum part 33 low engine 

speed test time and is not based on expected or representative flight profiles or typical icing 

conditions. 

A typical descent gradient and a rule of thumb for pilots is 3 miles horizontal for 1000-foot 

vertical descent gradient (15.8:1 gradient). This would work out to about 60-mile descent 

initiation test point for a 20,000-foot descent. This works out to be about a 3.6-degree nominal 

descent slope. If we take a conservative 2-degree descent slope, then the descent initiation 

would start at 120 miles out for a 22,000-foot vertical descent. A total descent altitude of 22,000 

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 feet. 
2/02/04 AC 20-147

A1-2 

In order to convert a 15.8:1 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 prop aircraft. It is 

understood that this is a conservative approximation over 10,000 feet agl where speed is not 

limited. 

The result of these inputs is, at 2-degree descent gradient and 220 knots (250 mph) horizontal 

air speed, you get about a 750-ft descent each minute, from 22,000 feet in 30 minutes. 

Consequently, it can be justly stated that the engine manufacturers should reasonably test 

engines at flight-idle up, and 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 appendix C continuous 

maximum icing conditions, when stabilized operation has not been demonstrated. 

Appendix C, paragraph (b) - Intermittent Maximum Icing Conditions. It is widely understood 

that under part 25, the intermittent maximum icing conditions of Appendix C are more severe 

than the continuous maximum conditions. Just as the above continuous maximum discussion 

provides for multiple cloud extents in series, the same philosophy will be applied here for 

intermittent maximum conditions, to assure robust design demonstrations. This increased 

severity of an intermittent cloud in conjunction with the reduced horizontal and vertical extents 

of these clouds 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 an Appendix C horizontal extent 

basis, depending on speed. This should provide the FAA with sufficient confidence in an engine 

design when exposed to icing conditions in-service.
AC 20-147 2/02/04

A1-3 

Conclusion 

In conclusion, during § 33.68 compliance test demonstrations, while demonstrating engine 

speeds below minimum for sustaining flight (that is, below light hold N1), while in continuous 

maximum icing conditions, the test duration should be no less than 10 minutes and need not 

continue beyond 30 minutes, and may be terminated in-between 10 and 30 minutes, if and when 

stabilized operation in icing is shown. At engine powers that can sustain flight, all compliance 

test conditions must be run indefinitely, until stabilized operation is demonstrated. These test 

duration criteria should be applied to all icing demonstration test points that are within or close 

to the continuous maximum conditions of Appendix C. At the end of each test point the engine 

should be accelerated to maximum power to shed any residual ice and then it can be shut down. 

 For § 33.68 compliance test demonstrations of Appendix C intermittent maximum icing 

conditions, the test duration should be no less than 10 minutes. For compliance demonstrations 

where engine speeds are below the minimum for sustaining flight (that is, below light hold N1), 

while in intermittent maximum icing conditions, the test duration should be no less than 10 

minutes and need not continue beyond 10 minutes and may be terminated thereafter. There is no 

requirement for indefinite operation when operating at these low N1's so the test can be 

terminated and the engine accelerated to takeoff power after the 10-minute demonstration period. 

This 10-minute test duration can be applied to all engine power conditions up to, but not 

including light hold power. At engine powers that can sustain flight, all compliance test 

conditions must be run indefinitely, until stabilized operation is demonstrated. These criteria 

should be applied to all icing demonstration test points that are within or close to the intermittent 

maximum conditions of Appendix C. At the end of each test point the engine should be 

accelerated to maximum power to shed any residual ice and then it can be shut down.