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. The 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 purpo of this AC is to reduce inconsistencies and
eventual surpris 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 tho aircraft and installations po for icing, AC 20-73, Aircraft Icing
Protection,is considered the primary AC for tho installations. Further, this AC is not intended
to address mixed pha icing conditions (meaning, mixed water and ice precipitation), although
there is a discussion on the subject. While the guidelines are not mandatory, they are
historically bad 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 uful 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 compliance that an applicant may elect to prent. Terms such as “should,” “shall,” “may,” and “must” are ud only in the n of ensuring applicability of this particular method of compliance when the acceptable method of compliance in this document is ud. While the 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 adver effect on engine operation or rious loss of power or thrust. Propulsion systems certified under the requirements and operated in accordance with the airplane flight manual, have generally demonstrated safe operation when expod to natural icing environments. This AC will superde 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 A渴望的意思
C are intended to be standardized engine icing certification test conditions. The standard conditions, in conjunction with any design-specific
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2/02/04 AC 20-147
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critical test points, should be ud together with any additional conditions that the Administrator
determines to be critical. The 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 rvice 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 otherwi 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 the 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 pod 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-rvice icing
conditions that are more vere than the Appendix C conditions. Often, experience has shown
公司乔迁祝福语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 purpo 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 the.
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 clo 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 increa with lower temperature up to a test point
where no additional adhesion is gained with additional lower temperature.
AC 20-147 2/02/04
(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 tho 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 expresd 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, bad 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 cyclin
g.
g. Scoop factor (concentration factor). The ratio of nacelle inlet highlight area (A H) to the area of the captured air stream tube (A C) [scoop factor = A H/A C]. 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 low-pressure 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 ab裴国华
out 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 ud above, refers to flight crews tactile feel during the event, or the u of typical engine test instrumentation, or flight deck instrumentation (such as, N1, N2, vibes, exhaust gas temp).
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2/02/04 AC 20-147
Par 6 Page 5 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 veral shed cycles) is stable and not trending upwards or downwards.丝带玫瑰花
j. Sustained power loss. A permanent reduction in power or thrust at the engine’s primary
power t parameter (for example, fan rotor speed, engine pressure ratio). A sustained
measurable power loss is considered a “vere power loss” in the context of the icing
懒云requirement. Power or thrust loss 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 respon during shedding may be
上床睡觉的英语
from the thermodynamic engine respon 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 adver effect on
engine operation or sustained loss of power. An icing encounter, including a prolonged
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encounter, should not be of conquence to the crew, and should not invalidate the engine's滑板板面
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. The instructions will be ud as a recommendation for in-rvice 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
respons during the ice accumulation process (for example, isochronous control respon to
accreting ice) are considered acceptable as long as there are no power loss. At the conclusion
of the test point, during the acceleration to takeoff power, the measured parameters should
AC 20-147 2/02/04 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 respon to shed ice ingestion is acceptable). Clo coordination is necessary by all parties to ensure that test plans are in reasonable bounds for the anticipated u
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 ctions corresponding to the applicable parts (§§ 33.68 and 33.89(b); 33.77; and 23.1093 and 25.1093).
a. Mixed Pha or Glaciated Icing Conditions. Mixed pha 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 pha or glaciated icing conditions, with the possible exception of two known potentially vulnerable engine design features. The two design features are (1) pronounced inlet bends (such as particle-parator 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. The two design features should either be avoided or carefully scrutinized by analysis and testing to assure their non-susceptibility to mixed pha or glaciated icing conditions. Additionally, there have been cas of icing encounters in mixed pha icing conditions where ice detectors have not detected ice formation. Ice detection systems should be evaluated for the conditions.
b. Auto-recovery systems. The u of auto-recovery systems is acceptable for certain engine certification testing. The FAA supports the u of auto-recovery systems, or other protective engine systems or devices, while in rvice, and allows the u 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 u after certain ice ingestion or accretion induced flameouts (§ 33.77), the auto-relight 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 u of an auto recovery systems may be acceptable would be rare ice ingestion events resulting from vere (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 u of auto-recovery systems when demonstrating compliance to §§ 33.68 and 33.77.
c. U 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 m
easured 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 Page 6 Par 7
