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AIA/EIWG Subcommittee on Engine Probe Icing:

A Process for Evaluating the Performance of Temperature

Probes, Combined Temperature and Pressure Probes, and

Static Pressure Probes in Icing Conditions

6 October 2017

AIA/EIWG Subcommittee on Engine Probe Icing Page 2 of 45

Introduction

In response to tasking by the FAA as defined in a letter dated July 3, 2013, titled, “Request Formation of Advisory Group to Address Specific Engine and Installation Icing Issues,” the Engine Icing Working Group (EIWG) has studied the issue of engine probe icing and its effect on engines and airframe systems. Guidance to evaluate the performance of Pitot (total pressure) probes and Pitot static (combined total and static pressure) probes in icing conditions exists in the form of SAE Aerospace Standard AS5562 and FAA TSO C16. In order to address temperature probes, combined temperature and pressure probes, or static pressure probes, the EIWG has developed a process which can be used to evaluate probe behavior in icing conditions and the resulting impact on engines and airframe systems. The original FAA letter addressed only engine inlet probes, however the process that has been developed can be used on any temperature probe, combined pressure/temperature probe, or static pressure probe regardless of its installation location. This report will be provided to the FAA in response to their tasking request for use in developing policy and guidance for engine probe icing. The FAA has participated on this AIA committee; however, conclusions stated within this report do not necessarily represent the views of the FAA. Once this report is submitted to the FAA, the FAA has stated that they will review the final conclusions, respond to the recommendations and make a decision as to how to proceed.

Executive Summary

The EIWG has developed a structured methodology for showing compliance with the requirements related to the performance of temperature probes, combined temperature and pressure probes, or static pressure probes in icing conditions. The process details areas for applicants to target in their analysis and describes how applicants can develop appropriate test points tailored to their application to show compliance with the engine and airframe requirements related to the performance of the probes in icing conditions. The EIWG recommends the FAA incorporate the process into policy and guidance on showing compliance to the regulations related to probe icing.

Background

The industry has experienced multiple cases of ice crystals blocking turbine engine air data (temperature and pressure) probes on small to medium-sized turbine powered airplanes in the recent past. These ice blockages have resulted in engine power rollbacks, lack of throttle response, and flight crew warnings for out-of-range probe data. Some airplane manufacturers have eliminated certain airframe air data probes and, instead use the engine probe data for airframe systems. Eliminating dedicated airframe probes and using engine inlet probe data for key airframe systems may result in the need for greater emphasis on icing compliance and hazard safety analysis to support overall aircraft safety. Eliminating the airframe probes also removes the ability of the engine control system to use the airframe signal as the classical referee, that is, the engine uses the airframe signal to validate data from the engine probes. Without the use of an airframe probe, the engine sensor is substantially more critical in measuring air data throughout the operating envelope of the aircraft.


Some engine manufacturers use heated inlet probes to prevent ice from accreting on or in the probe, while others use unheated probes. Heated probes can be effective in preventing ice from accreting on the probe while in supercooled droplet icing conditions that are typically found at low to mid-altitudes. Heated probes, however, can be susceptible to ice accretion from ice crystal icing conditions. These conditions are typically found at high altitude but can exist at lower altitudes. Applicants have redesigned certain heated temperature probes to reduce potential for ice crystal blockage but have not been able to completely eliminate it. Unheated probes are less susceptible to ice crystal icing conditions, but are more susceptible to lower and mid-altitude supercooled droplet icing conditions. Therefore, whether manufacturers use heated or unheated probes, problems with ice accretion can still occur. Engine manufacturers have incorporated Full Authority Digital Engine Control (FADEC) logic to detect and annunciate probe blockage and compensate for the probe error to maintain acceptable engine operation, but this logic has not been able to completely eliminate all issues.

Discussion

No specific rule targets the turbine engine air data probes, however they are indirectly addressed within 14 CFR sections 33.28, 33.68, 33.89 and 33.91 when part of the engine type design and within 14 CFR sections 23.1093(b), 25.1093(b), 27.1093(b) and 29.1093(b) when part of the aircraft installation. Historically, engine air data probes were tested during the engine’s § 33.68 induction system icing compliance testing for supercooled water droplets, as defined by Appendix C of part 25. They have also been subjected to DO-160 standards and test procedures during § 33.91 testing, but they have not addressed the effects of ice crystals on inlet probes. Note that testing completed at the probe level does not relieve the engine manufacturer from meeting the requirements of § 33.68 at the engine level. Amendment 33-34 to 14 CFR part 33 and Amendment 25-140 to 14 CFR part 25 introduced requirements for the engine and airframe systems to operate in mixed phase and ice crystal icing conditions (defined by Appendix D to part 33) and supercooled large drop (SLD) icing, (defined by Appendix O to part 25). Similar regulations are included in EASA CS-E and CS-25, including identical icing envelopes for ice crystal icing (defined in CS-25 Appendix P) and SLD icing (defined in CS-25 Appendix O). The FAA provides guidance in advisory circular (AC) 20-147A for demonstrating compliance with the engine induction system icing and engine installation icing requirements of 14 CFR parts 23, 25, 27, 29, and 33. While AC 20-147A discusses where probes could be susceptible to icing and the need to evaluate them, it does not provide specific guidance on how to do so. SAE Aerospace Standard AS5562 and FAA TSO C16b provide guidance on means to evaluate electrically heated Pitot and Pitot-static probes for in-flight icing conditions, however there is no guidance available on evaluating temperature probes. EASA regulation CS-E 780 includes similar requirements to the icing requirements of 14 CFR part 33. The related guidance included in AMC E 780 directs the applicant to determine the critical probe icing conditions using the guidance of AMC 25.1324 of CS-25. EASA regulation CS 25.1324 requires each flight instrument external probes system, including, but not limited to temperature probes, to be heated or have an equivalent means of preventing malfunction in the heavy rain conditions of table 1 of CS 25.1324 and in icing conditions. AMC 25.1324 provides some guidance related to the test setup and the conditions to be tested to show compliance with the regulation.


The EIWG has worked to develop a structured methodology for showing compliance with the referenced FAA requirements. The process details areas for an applicant to target in their analysis and describes how an applicant can develop appropriate test points tailored to their application to show compliance with the requirements related to probe icing. The process is applicable to any temperature or pressure probe that provides data to the electronic engine control system or other airframe systems, whether the probe is installed in the engine or on the airframe. The process is shown graphically in Figure 1. A detailed explanation of each step in the process follows.

Identify Candidate Test Points

Based on local conditions at probe· High IWC/short duration· Lower IWC/longer duration· Include AC 20-147A Table

Points for completeness· Etc.

StartCompare the Engine & Aircraft Operating Envelopes with the

Icing Envelopes

Consider1) the Atmospheric condition and extent aircraft will operate in; and 2) the possible failure modes of probes, including the loss of heater function.

Determine Ambient Conditions

Calculate water flux as a function of total temperature

Determine Local Conditions at Probe Based on Ambient Conditions

Scoop Factor Calculations for inlet mounted probe, concentrations effect, local velocities, etc. for airframe mounted probe.

Conduct System Level Analysis

Probe/Engine/Airframe OEMs develop pass/fail criteria

Determine How to Address Candidate Test Points

Determine:1) which points can be covered by other test conditions;

2) which points can actually be tested (and modify conditions to allow testing if facility limitation is why test can’t be completed); and

3) which points can be covered by analysis/test

Complete Testing/Analysis

Compile Test/Analysis Results

Quantify effect of test conditions on probe:1. signal error (temperature, pressure, etc.) including transient response rate including any build/shed cycle effects and when icing conditions are introduced or removed

2. accretion/shedding – size and shape of ice build up

Provide Results to Engine OEM

OEM determine acceptability of probe response on engine control 1) corruption due to icing2) any other failure due to icing (including Turbo machinery damage from probe icing build/shed cycle)

Include in safety assessment for §33.28:1) effect of loss of heater function

Engine effect acceptable?

Provide Engine Response to Probe Icing to Airframe OEM

1. Change in thrust2. Changes in control schedules3. Failure indications/Fault accommodation4. Changes in displayed parameters5. probe signal error

Determine Effect on Aircraft

1.Effect of signal error on airframe systems2. Effect of engine response (change in thrust /fault accommodation/etc.)

Aircraft Effect Acceptable?

Completion

Engine OEM incorporate engine response into final installation manual

Airframe OEM document aircraft response and compliance with Installation Manual requirements as required

No

No

Yes

Yes

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Figure 1 Process Flow Chart


Step 1: Compare the Engine & Aircraft Operating Envelopes with the Icing Envelopes

The initial step requires defining the engine and aircraft operating envelopes (altitude, airspeed,

temperature). The part 25 Appendix C and part 33 Appendix D icing envelopes need to be

compared to the aircraft operating envelope to determine applicability of those envelopes. The

part 25 Appendix O icing envelope should also be evaluated as required to support engine

and/or aircraft certification. Due to the tendency of probes to react in much shorter time frames

than ETOPS conditions, no unique evaluation is required for ETOPS airplanes.

In support of engine and airframe safety analyses, possible failure modes of the proposed probe

should be considered at this stage as well. For example, if the probe is a heated probe, failure

of the heater element should be considered. These failure modes do not necessarily need to be

evaluated as part of the test program, but they do need to be addressed so the installer of the

probe understands the impacts of failure modes of the probe.

The integrity of the data provided from the probe should also be evaluated so the airframe and

engine manufacturers can determine what systems the probe is adequate for use in (i.e.

systems with catastrophic safety effects will require a more robust probe or multiple data

sources, whereas systems with only minor safety effects can use a less robust probe).

Step 2: Determine Ambient Conditions

The envelopes of part 25 Appendices C and O and part 33 Appendix D define ambient or free

stream concentrations of liquid water and ice crystals. Based on these concentrations and the

aircraft operating speeds, water flux as a function of total temperature can be calculated. For

glaciated testing the highest water flux cases tend to be the most severe. This is not

necessarily the case for super-cooled liquid water or mixed phase. Other parameters to

consider in selecting critical points include highest total cooling load, minimum predicted surface

temperature, maximum water to air mass flux ratio, etc. Refer to Appendix 1 for an example of

the process of defining ambient conditions to be evaluated.

Within the ice crystal envelope defined by part 33 Appendix D, total water content (TWC) in g/m3

has been determined based upon the adiabatic lapse defined by the convective rise of 90%

relative humidity air from sea level to higher altitudes and scaled by a factor of 0.65 to a

standard cloud length of 17.4 nautical miles. In-service occurrences show that several air data

probe (both temperature and pressure probe) icing events in glaciated conditions occurred

outside of the Appendix D envelope in terms of altitude and outside air temperature. In that

context, the environment to be considered should be Appendix D enlarged to encompass ISA

+30°C conditions and should be extended to a minimum temperature of -70°C. In addition, the

Appendix D envelope should be expanded to cover ISA -5°C conditions above 25,000 ft. This

expanded Appendix D envelope and a sampling of conditions where air data probes have

experienced in-service ice crystal icing events is shown in Figure 2.


Figure 2 Expanded Convective Cloud Ice Crystal Envelope

In addition, based on data available from multiple sources, EASA AMC 25.1324 suggests that

the standard cloud of 17.4 NM and the associated average TWC concentration values provided

by part 33 Appendix D may not provide the most conservative conditions for air data probe

testing. The ‘max’ or ‘peak’ TWC concentration values should be considered instead of the

‘17.4 NM’ values provided by part 33 Appendix D. These ‘max’ or ‘peak’ values are available in

FAA document DOT/FAA/AR-09/13 and correspond to the ‘17.4 NM’ values multiplied by a

factor of 1.538 (1/0.65).

Step 3: Determine Local Conditions at the Probe Based on Ambient Conditions

Based on the ambient conditions determined in Step 2, the local conditions at the probe should

be determined. Probes are typically mounted a sufficient distance from the mounting surface

(e.g. the fuselage skin or engine inlet) to accurately sense the freestream parameter of interest

(total temperature, total pressure, etc.). However, when flying through particles such as

supercooled water droplets, ice crystals or rain, there can be a concentration effect near the

mounting surface. This concentration effect is primarily due to inertia and drag effects, but can

also be affected by large particles which splash and/or break up yet remain in proximity to the

boundary layer. This effect is highly installation dependent and it can vary significantly

depending on probe location and probe design.

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)

Appendix D Envelope

Appendix D Expansion

Industry Reported Probe Icing Events

ISA+30

ISA+20

ISA -5


The determination of local conditions involves reviewing the installed position of the probe (e.g.

in the engine inlet or on the airframe) and determining local flow velocities and concentration

effects for airframe mounted probes or scoop factor effects for inlet mounted probes.

AC 20-73A presents some of the methods that have been used to calculate drop impingement

and water catch at the location of interest for supercooled liquid water cases. For ice crystal

icing conditions, there are currently no accepted methods for calculating ice crystal trajectories

or the effects of ice crystal bouncing and break up on the local concentration levels at the point

of interest; however, preliminary research indicates the ice crystal concentration can be

significantly higher than ambient conditions (reference SAE papers 2011-38-0050 and 2015-01-

2146), so conservative assumptions should be made in regard to local ice water content for ice

crystal icing conditions in accordance with the state of the art analytical tools. The assumptions

made regarding concentration factors should be documented.

Similar analysis would need to be completed to determine local conditions for probes installed in

other places, such as behind the fan on a turbofan engine or in the core of the engine. The

local conditions may be significantly different than the free stream or ambient conditions,

including local probe angle of attack, Mach number, pressure, temperature, etc.

Step 4: Identify Candidate Test Points

Based on the local conditions at the probe, the candidate test points can be determined. An

example of one method to determine the candidate test points for ice crystal icing conditions is

included in Appendix 1.

A range of temperatures representative of the icing envelopes should be tested for the probes.

The probe design should be reviewed to determine whether the probe is more susceptible to

higher water concentrations for short durations, to lower water concentrations for longer

durations, or to cyclic conditions, such as those defined in Test Point 3 of § 33.68 Table 1

(Amendment 33-34). As noted in Step 2, the maximum ambient water concentrations should

account for peak TWC conditions corresponding to the values of part 33 Appendix D multiplied

by 1.538. Testing at the peak TWC values is consistent with the guidelines of EASA AMC

25.1324.

In addition to the peak TWC conditions, the maximum ambient water concentrations can be

reduced and the duration extended to address conditions where lower water concentrations for

longer durations are critical. Testing at different water concentrations also provides evidence

that the installed probe will work throughout the operating envelope and not just at the maximum

level. The peak concentration can be scaled to a standard (17.4 nm) cloud by dividing the peak

TWC values by 1.538. It is also recommended that a concentration of one half of the peak

value be evaluated. From Appendix D, Figure D3 the scale factor for the standard cloud is 1,

and the scale factor for one half of the peak value is ½*1.538 = 0.769. Based on Appendix D

Figure D3, this scale factor corresponds to a cloud extent of approximately 215 nm. Review of

in-service engine ice crystal icing events (reference SAE paper 2015-01-2086) shows that the

99th percentile cloud length for events where engine damage occurred is 354 nm (657 km), with

the large majority of engine events occurring in clouds of less than 215 nm (400 km) in length.


Experience has shown that in general, probes do not take as long to respond to ice crystal icing

as engines do, and therefore it is reasonable to reduce the maximum cloud length for a probe to

215 nm and use the corresponding TWC of one half the peak value.

The duration of each test point also needs to be considered. AMC 25.1324 and AS5562 both

require test conditions in ice crystal icing conditions be run for a minimum of 2 minutes. In

general, this is a sufficient length of time to evaluate the probe’s behavior. However, for cases

where the probe builds ice, the duration should be extended as required to quantify the size of

ice which may accrete and to ensure any build/shed cycles are completely evaluated. For the

reduced concentration points, the tests should be run for a sufficient length of time to traverse a

17.4 nm or 215 nm cloud as appropriate to the test condition. For typical cruise airspeeds,

traversing a 17.4 nm cloud takes on the order of 2 to 3 minutes. For these cases, the conditions

completed with the peak TWC for 2 minutes is a more severe test than 2 to 3 minutes at a

reduced TWC, and the lower TWC case can therefore be eliminated from consideration. For

typical cruise airspeeds, traversing a 215 nm cloud takes approximately 25 minutes. To ensure

the test adequately addresses the concerns, the one half of peak TWC conditions should be run

for 30 minutes (17.4 nm / 2 minutes = 8.7 nm/minute, therefore 215/8.7 = 24.7 minutes rounded

up to 30 minutes).

In regards to liquid water icing testing, for engine mounted components test/analysis points

should be identified per the conditions stated in Table 1 of § 33.68. This requires 10-minute

duration glaze ice and rime ice tests at various power settings (airflows) as well as 45-minute

glaze and rime ice holding conditions. For airframe mounted temperature probes, test/analysis

points should be identified per the conditions stated in Tables 1 and 2 of EASA AMC 25.1324 of

CS-25. This requires 15 minute durations per continuous maximum cloud concentrations, 5

minute durations per intermittent maximum cloud concentrations and either 30 minute or 10

minute duration cyclical liquid water concentrations. For both engine mounted and airframe

mounted probes, in addition to the above requirements a critical point analysis (CPA) should be

performed based on the Appendix C requirements to determine if there are additional critical

points within the operating envelope that should be considered for test/analysis. The critical

points which the engine is tested against to show compliance with § 33.68 may or may not be

critical points for the probe. For example, high airflow conditions like maximum continuous

power may be more critical for an inlet mounted probe than for the engine as a whole. It is

therefore important for the applicant to determine the critical conditions for the probe as part of

this step and not rely solely on the § 33.68 points for demonstrating proper operation of the

probe. The points defined in § 33.68 and AMC 25.1324 represent conditions at the engine inlet

for the installation. For a component (probe) test, the appropriate concentration factor needs to

be determined so that the local conditions at the probe match what it will see as installed.

For unheated probes in liquid water environments, the applicant should evaluate the impact of

ice accumulation and shedding in regards to altitude. Analysis and/or testing at altitude should

be considered. For heated probes, it is accepted to test liquid water conditions in a non-

pressure controlled wind tunnel since this is considered more conservative than testing at higher

altitudes.


In regards to mixed liquid and ice crystal conditions, test/analysis points should be selected with

the guidance stated in section 12.2 of EASA AMC 25.1324 of CS-25. This specifies the total

water content (TWC) should be based on a 2.6 nautical mile cloud. One clarification needed is

that the AMC does not specify how to designate the amount of liquid water vs ice crystal

contents. Appendix D defines the liquid water portion of mixed phase conditions to be ≤1.0 g/m3

for clouds of less than 50 nautical mile extents for temperatures above -20°C and zero for

temperatures below -20°C. SAE Aerospace Standard AS5562 assumes the liquid water content

is per the Appendix C intermittent maximum cloud and the remainder of the TWC is ice crystals.

The EIWG’s recommendation is to align with SAE Aerospace Standard AS5562 and assume a

liquid water content (LWC) per the Appendix C intermittent maximum cloud and the balance of

the TWC to be ice crystals for all mixed phase conditions. This recommendation results in

consistency with AS5562 and conservatively results in mixed phase conditions in colder

conditions with more liquid water present than the Appendix D requirements. An example of

one method to determine the candidate test points for mixed phase conditions is included in

Appendix 1.

The duration for mixed phase icing conditions also needs to be considered. For the maximum

TWC in mixed phase conditions, it is appropriate to run test conditions for 2 minutes as this is

sufficient time for an air data probe to reach a steady state and stabilized condition. In addition,

flight testing in well developed, large diameter mesoscale convective systems completed as part

of the HAIC/HIWC flight test campaigns showed a low frequency of mixed phase regions.

At -10°C, mixed phase regions amount to less than about 5% of the total in-cloud distance

traversed and maximum average distances across mixed phase zones were about 8 nautical

miles. Frequency of mixed phase zones decreased with decreasing temperature. At -30°C, the

spatial fraction of mixed phase zones was less than 0.2%, and maximum average distance

across mixed phase zones was about 1.9 nautical miles. It is believed that these well-

developed storm cells produce large amounts of falling and recirculating ice that would tend to

glaciate any new updraft, thus resulting in lower liquid water contents, and smaller regions of

mixed phase conditions. For smaller or still developing cells with less glaciation and less

circulation, regions of mixed phase conditions with higher liquid water content could exist for

longer times. These areas of mixed phase conditions are still likely to exist as separate regions

within the storm, resulting in alternating between mixed phase and fully glaciated conditions as

the storm is traversed. A conservative approach to represent these smaller or fresh convective

cells is to perform a cyclic test alternating between mixed phase and fully glaciated conditions.

Therefore, in addition to the test conditions for maximum TWC conditions for two minutes, tests

should be completed cycling between mixed phase and fully glaciated conditions. For these

cyclic conditions, the TWC should be set to one half of the 2.6 nautical mile scaled TWC. This

lower TWC is justifiable as the HAIC/HIWC flight test results indicate that extended regions of

liquid water seem less likely in high IWC conditions. Therefore, lower IWC, and resulting lower

TWC, values are required for mixed phase conditions to be sustained without transitioning to

fully glaciated conditions. The LWC for the mixed phase conditions in a developing storm cell is

not yet well understood. In order to define a conservative test, the LWC should be assumed to

be the Appendix C Intermittent Maximum value for the conditions to be tested. As discussed in

Steps 10 and 13, other engine or airframe level mitigation may be necessary to ensure


acceptable aircraft operation depending on the test results in these conservative conditions.

Each cycle should alternate between 2 minutes in mixed phase conditions and 2 minutes in fully

glaciated conditions. To simulate flying through a developing storm system or holding in such

conditions, the cycles should be continued until repetitive, stabilized operation has been shown,

or for a maximum of 30 minutes.

For the case of a heated probe, test conditions with the heater turned off should be considered,

whether to address an inadvertent type encounter where the heater isn’t turned on immediately

or to address a transient power interruption. The timing should be coordinated with the engine

and airframe manufacturers to ensure appropriate conditions are defined.

Step 5: Conduct a System Level Analysis

A system level analysis should be jointly completed by the probe, engine and airframe

manufacturers to define the pass/fail criteria for the probe when exposed to the candidate test

points determined in Step 4. The probe remaining free of ice is not necessarily the pass criteria.

The acceptance criteria need to consider the criticality of the probe and the air data it provides

to the engine and the airframe. A simplified acceptance criterion is for the engine and airframe

systems that utilize the data from the probe to continue to operate within some acceptable

range in icing conditions.

The probe manufacturer can typically only quantify what the effect is at the probe level (e.g. the

probe output/accuracy is ±x° when exposed to a specific condition). The engine manufacturer

needs to translate that probe effect into the resulting system impact at the engine level (e.g. ±x°

of measured temperature error equates to ±y thrust or to some effect on engine operability) The

airframe manufacturer needs to know the probe level effect (e.g. ±x° measured temperature

error) and the installation level effect on airframe systems that use data from the probe along

with the engine effect and the impact the engine effect has at the aircraft level.

Shedding of any ice that accretes on the probe should also be evaluated as part of the pass/fail

criteria to ensure no damage to downstream components occurs.

The acceptance criteria need to consider the types of conditions and the possible failure modes of probes.

· “Classic” failure mode of a temperature probe in ice crystal icing conditions is to clog with partially melted crystals driving the probe reading to 0°C (the temperature of the slush).

· Other failure modes exist, such as smaller errors for some short period of time or fluctuations in the probe output with build and shed cycles.

· Damage to downstream components as a result of ice shedding.

Step 6: Determine How to Address Candidate Test Points

An analysis should be completed of the candidate points identified in Step 4. The points should

be evaluated to determine which points are covered by other conditions, or can be covered by

analysis supported by testing at other conditions.


The points should also be evaluated to determine if they can be tested at the proposed test

facility. When a particular test condition cannot be achieved due to a test facility limitation, the

scaling methods of AS5562 sections 3.3.2 through 3.3.4 may be used to vary temperature,

airspeed and water flux to define test conditions which can be achieved. If a significant number

of test points need to be modified in this manner, an alternate test facility capable of testing

more of the proposed envelope, should be considered so as to minimize the number of

candidate points which are not directly tested. As noted in AS5562 section 3.3.1 there is no

acceptable method to scale the altitude for ice crystal icing conditions at this time. Therefore,

ice crystal icing conditions must be tested at the altitude determined in Step 4.

It may be possible to change the probe design such that any points that cannot be tested are no

longer critical. Alternatively, it may be possible to impose limitations on the probe’s operating

envelope. However, this may require engine or aircraft level design changes or limitations. Any

limitations on the probe installation must be clearly identified and communicated to the engine

and aircraft manufacturers so those limitations can be adhered to as installed.

As stated in Step 1 above, the system level effects of the loss of heater function should also be

considered. This may be addressed either in a test matrix or by analysis. This activity is

intended to document the effect on the probe with the heater inoperative in the worst case icing

conditions being evaluated.

Step 7: Complete Testing/Analysis

Testing of all conditions identified as needing to be tested in Step 6 should be completed in a

facility capable of meeting the test conditions. The configuration of the test article should match

the intended installation including orientation as closely as possible, and the probe itself should

match the type design configuration, except as necessary to install instrumentation or other test

equipment.

The following requirements should be met for each test condition. Where noted and

emphasized in italics, the requirements were copied from SAE AS5562, the requirements are

reprinted with permission SAE AS5562 Copyright © 2017 SAE International. Further distribution

of this material is not permitted without prior permission from SAE.

1. Probe Mounting Location (AS5562 §4.2)

The location of the probe within the calibrated test section shall be selected such that the

LWC and/or IWC at the surfaces of interest meet or exceed the required test condition.

2. Probe Mount Heating Requirements (AS5562 §4.3)

If it is necessary to heat the probe mounting arrangement in the icing wind tunnel, the

mount heating system shall be designed so as not to invalidate the test results.

3. Installation Heat Sink Effects (AS5562 §4.4)

The probe shall be mounted to an aluminum heat sink of 0.10 inches (2.5 mm) in

thickness and 100 square inches (645 cm2), or any required configuration of equivalent

thermal capacity, which will be exposed to the ambient environment of the icing wind

tunnel.


Note: if specific installation data is available, the as-tested heat sink effects should

reflect the conditions that would exist in the installation.

4. Probe Power for Electrically Heated Probes (AS5562 §4.5)

A probe heater voltage shall be specified for use in all test conditions. The value of the

probe voltage utilized shall be recorded in the test results and form part of the test

record.

NOTE: It is commonly accepted to test the probe at 10% below the nominal rated

voltage to cover voltage variation at the probe location on aircraft during normal aircraft

operation.

NOTE: The probe test voltage value shall be provided to the installer to support

installation approval.

5. Non-Electrically Heated Probes

For probes which are heated using some method other than an electrical heater, the

heat source should be set to the minimum allowable value expected for the installation.

For example, a probe heated using bleed air should be tested with the bleed air supply

at the minimum expected regulation pressure and temperature. The specific conditions

tested regarding the air supply shall be provided to the installer to support installation

approval. It is important to fully characterize this air supply including: in-line pressure

drop, piping clearances, local heat transfer characteristics and any control orifices in the

supply line and/or sensor.

6. Tunnel Blockage (AS5562 §4.6)

The percent blockage, using the projected frontal area of the UUT [Unit Under Test], of

the icing tunnel test section shall be assessed to verify it does not invalidate the test

results.

7. Angle of Attack

The effect of angle of attack (AOA) in the intended installation should be considered. If it

is determined that the angle of attack will have no significant impact, all test conditions

can be run at a single angle of attack. If it is determined the angle of attack could have

an impact, or if it cannot be determined if it would, the probe shall be tested at angles of

attack of -15°, 0° and +15° for each icing test condition, or if nominal angle of attack is

known, nominal - 15°, nominal, and nominal + 15° for each icing condition.

8. Data Collection Sample Rate (AS5562 §4.8)

The data collection sample rate for all tests shall be a minimum of 20 Hz.

9. Electrically Heated Probe Test Unit Selection (AS5562 §4.9)

Probe qualification tests shall be performed on a unit fitted with a heater circuit having

the lowest electrical performance acceptable on a production article as defined by the

acceptance test procedure. The test voltage may be adjusted to simulate the lowest

performing probe.

10. Non- Electrically Heated Probe Test Unit Selection


Probe qualification tests shall be performed on a unit having the lowest performance

acceptable on a production article as defined by the acceptance test procedure. The

inputs may be adjusted to simulate the lowest performing probe (e.g. a bleed air heated

probe may have the air pressure and/or air temperature adjusted to simulate the lowest

performing probe heater).

11. Test Duration

Ice crystal conditions tests completed at the peak TWC values shall be run for 2

minutes. Tests completed at one half of the peak TWC value shall be run for 30

minutes.

For mixed phase condition tests, tests completed at the maximumTWC values shall be

run for 2 minutes, and tests completed at the reduced TWC values shall be run as cyclic

tests. Each cycle should alternate between 2 minutes in mixed phase conditions and 2

minutes in fully glaciated conditions. The cycles should be continued until repetitive,

stabilized operation has been shown, or for a maximum of 30 minutes

For liquid water icing conditions, test/analysis points should be identified per the

conditions stated in Tables 1 and 2 of EASA AMC 25.1324 of CS-25. This requires 15

minute durations per continuous maximum cloud concentrations, 5 minute durations per

intermittent maximum cloud concentrations and either 30 minute or 10 minute duration

cyclical liquid water concentrations as defined in the AMC.

12. Test Particle Size Distribution

The particle size MMD (Median Mass Dimension) for the test conditions shall match that

defined by 14 CFR part 33 Appendix D, unless it can be justified that a different size will

not have a significant effect on the test.

Step 8: Compile Test/Analysis Results

The test and analysis results should quantify the effect of the conditions on the probe including:

1. Signal error (temperature, pressure, etc.) including transient response rate and including

any build/shed cycle effects and when icing conditions are introduced or removed.

2. Measurement accuracy and any changes when icing conditions are introduced or

removed, and any changes when the heater is turned on.

3. Accretion/shedding - size and shape of ice buildup.

4. Frequency of build/shed cycles and impact on signal error and the transient probe

response to shedding

Step 9: Provide Results to Engine OEM

The results of the testing and analysis should be provided to the engine manufacturer so that

the acceptability of the probe response can be determined. This evaluation should be based on


corruption of the probe output signal due to icing and any other failure mode seen during the

testing.

Step 10: Determine Acceptability of Engine Effects

Based on the results of the testing and analysis, the engine manufacturer should determine

whether the effect of the probe response to the icing conditions is acceptable or not. If the

response is acceptable, proceed to Step 11, if not, the pass/fail criteria should be reviewed to

ensure an acceptable engine response is defined and the process should be repeated from

Step 1. The revised system analysis may require review of the candidate test points and/or

design changes to the probe or installation. It is important to note that the pass/fail criteria

should not simply be changed to match the results, rather the system analysis step should be

repeated to ensure correctness and the design reviewed or changed as necessary.

Uncertainty regarding test data validity: For an engine inlet probe, static testing outside of the

engine inlet system may not exhibit the same build and shed behavior due to variations in

airflow and vibration levels. Results from an integrated inlet/engine test are likely to be quite

different due to changes in vibration and local airflow, and likely to result in differences in

shedding behavior compared to an isolated probe in an icing tunnel.

Step 11: Provide Engine Response to Probe Icing to Airframe OEM

The engine manufacturer should document the response of the engine based on the probe

response to the icing conditions tested and provide that response to the airframe manufacturer.

The engine response evaluation should address:

1. Probe Signal error

2. Failure indications/Fault accommodation

3. Changes in engine operating characteristics (surge/stall, flameout, etc.) due to signal

error

4. Change in thrust or power setting

5. Change in displayed parameters

Step 12: Determine Effect on Aircraft

The airframer should review the data provided by the engine manufacturer in Step 11 to

determine the impact to the aircraft.

Step 13: Determine Acceptability of Aircraft Effects

Based on the results of the testing and analysis, the airframe manufacturer should determine

whether the effect of the probe response to the icing conditions is acceptable at an aircraft level

or not. If the response is acceptable, proceed to Step 14, if not, the pass/fail criteria should be

reviewed to ensure an acceptable aircraft response is defined and the process should be

repeated from Step 1. The revised system analysis may require review of the candidate test

points and/or design changes to the probe or installation, and/or re-evaluation of the aircraft

response.


Step 14: Document the Acceptability of Aircraft Effects

Once the aircraft effect has been determined to be acceptable, final documentation of the probe,

engine and aircraft response to the icing environments should be completed. This includes the

engine manufacturer incorporating information about the engine response into the engine

installation instructions provided under § 33.5. This information should include the data

provided to the airframe OEM in Step 11, a description of the icing environments evaluated and

any other pertinent data from the safety assessments required by 33.28(e) and 33.75(a)

describing the probe and engine response. The airframe manufacturer should document the

response and compliance with any requirements included in the engine installation instructions

as required. This can include information included in the Airplane Flight Manual, the system

safety documentation for the aircraft, compliance documents, etc.

Conclusions and Recommendations

The EIWG has developed a process to evaluate probe behavior in icing conditions and the

resulting impact on engine and airframe systems. The EIWG proposes that this process can be

used to show compliance with the engine and airplane-level requirements of temperature

probes, combined temperature and pressure probes or static pressure probes in the icing

conditions of supercooled water, mixed phase and ice crystal icing, and SLD icing, as defined in

the respective envelopes of 14 CFR part 25 Appendix C, 14 CFR part 33 Appendix D, and 14

CFR part 25 Appendix O. This process is applicable to any temperature probe, combined

temperature and pressure probe, or static pressure probe that provides data to the engine

control system or any airframe system, whether the probe is installed in the engine or on the

airframe. The 14 steps of this process are, in summary:

1. Compare the Engine & Aircraft Operating Envelopes with the Icing Envelopes

2. Determine Ambient Conditions

3. Determine Local Conditions at the Probe

4. Identify Candidate Test Points

5. Conduct a System Level Analysis

6. Determine How to Address Candidate Test Points

7. Complete Testing/Analysis

8. Compile Test/Analysis Results

9. Provide Results to Engine OEM

10. Determine Acceptability of Engine Effects

11. Provide Engine Response to Airframe OEM

12. Determine Effect on Aircraft

13. Determine Acceptability of Aircraft Effects

14. Document the Acceptability of Aircraft Effects

The EIWG proposes that the FAA review this process for incorporation into policy and guidance

on showing compliance to the regulations applicable to temperature probes, combined

temperature and pressure probes, or static pressure probes when operating in all types of icing


conditions. The EIWG further proposes that the FAA review this process with EASA to achieve

harmonization with EASA policy and guidance.

In addition, the EIWG proposes expanding the Appendix D envelope to ISA +30°C and ISA -5°C

above 25,000 ft with both the ISA +30°C and ISA -5°C extended to a minimum temperature

of -70°C as shown in Figure 2. Any changes made to the Appendix D envelope regarding

altitude/temperature envelope or total water content levels need to be incorporated into this

document and any FAA guidance developed from this document.

It is the EIWG’s opinion that the mixed phase test conditions defined in Step 4 are conservative

in terms of water concentrations and durations of the conditions, however no data exists to

justify using any lower concentration or duration. The EIWG recommends additional research

be conducted to understand the prevalence of mixed phase conditions, and the total and liquid

water concentrations in mixed phase conditions to ensure these conditions are not overly

conservative.

Additionally, there are currently no accepted methods for calculating ice crystal trajectories or

the effects of ice crystal bouncing and break up on the local concentration levels at the point of

interest in ice crystal icing conditions. The EIWG recommends additional research be

conducted to provide additional data in order to determine acceptable methodologies and/or

assumptions with regards to local ice water content for ice crystal icing conditions.

References and Related Material

1. FAA Letter to Aerospace Industries Association, “Request Formation of Advisory Group

to Address Specific Engine and Installation Icing Issues” dated 3 July 2013.

2. SAE AS5562, Ice and Rain Minimum Qualification Standards for Pitot and Pitot-static

Probes, issued 08/2015

3. FAA TSO C16b, ELECTRICALLY HEATED PITOT AND PITOT-STATIC TUBES, dated

January 27, 2017

4. FAA Advisory Circular AC 20-147A “Turbojet, Turboprop, Turboshaft and Turbofan

Engine Induction System Icing and Ice Ingestion, dated October 22, 2014

5. EASA CS-E Amendment 4, AMC E 780

6. EASA AMC 25.1324, Flight instrument external probes, Amendment No: 25/16

7. DOT/FAA/AR-09/13 “Technical Compendium from Meetings of the Engine

Harmonization Working Group”, March 2009

8. FAA Advisory Circular AC 20-73A, “Aircraft Ice Protection”, dated August 16, 2006

9. SAE International 2011-38-0050 “An Analysis of Turbofan Inlet Water and Ice

Concentration Effects in Icing Conditions”, S. Liao, X. Liu and M. Feulner

10. SAE International 2015-01-2146 “Ice Crystal Ingestion In a Turbofan Engine”, M.

Feulner, S. Liao, B. Rose and X. Liu

11. SAE International 2015-01-2086 “Studies of Cloud Characteristics Related to Jet Engine

Ice Crystal Icing Utilizing Infrared Satellite Imagery”, T. Tritz, J. Mason, M. Bravin, and A.

Sharpsten

AIA/EIWG Subcommittee on Engine Probe Icing Sample Candidate Test Point Identification Using Water Mass Flux as the Critical Parameter

Page 17 of 45

Appendix 1: Sample Candidate Test Point Identification Using Water Mass Flux as the Critical

Parameter

The following is an example application to illustrate the process of a critical point

analysis, assuming that water mass flux is the parameter of criticality. A CPA using

water mass flux as the critical parameter will naturally select the highest airspeeds

within the flight envelope. However, these types of conditions may not be the most

critical for all probe designs and/or types of icing conditions. Therefore, it is

recommended that other parameters besides water mass flux be considered. These

may include, for example, maximum total cooling load, minimum dynamic pressure,

maximum water-to-air mass flux ratio and minimum anti-icing heater power available.

Consideration of other parameters of criticality will tend to result in a wide range of

airspeeds considered.

In addition, the CPA may need to consider a range of additional influences that are

not detailed herein, for example:

- Inlet scoop factor

- Engine power setting

- Droplet size

In order to determine the water flux a probe will be exposed to in service, the

ambient conditions to be evaluated need to be defined. The following process

defines a method of determining the ambient conditions for ice crystal and mixed

phased icing based on the aircraft operating conditions.

First, the ice crystal conditions are identified and then the process is repeated for

mixed phase conditions.

The ice crystal icing envelope is defined as a function of altitude and temperature in

Appendix D of 14 CFR part 33 as shown in Figure 3.


Page 18 of 45

Figure 3 Part 33 Appendix D Convective Cloud Ice Crystal Envelope

Within the ice crystal envelope, total water content (TWC) in g/m3 has been

determined based upon the adiabatic lapse defined by the convective rise of 90%

relative humidity air from sea level to higher altitudes and scaled by a factor of 0.65

to a standard cloud length of 17.4 nautical miles. To determine the peak TWC, the

values from part 33 Appendix D are multiplied by 1.538 (1/0.65). Figure 4 displays

peak TWC values over a range of ambient temperature within the boundaries of the

ice crystal envelope. Note that Figure 4 also includes an isothermal line of total

water content for -70°C which is not included in part 33 Appendix D. The -70°C

curve was obtained by extrapolating the other curves to a -70°C condition. The TWC

data of Figure 4 is also presented in tabular form in Table 1.

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)


Page 19 of 45

Figure 4 Total Water Content – Peak Values

Table 1 Total Water Content – Peak Values

Ambient Temperature

Altitude (ft) 0°C -10°C -20°C -30°C -40°C -50°C -60°C -70°C

2000 0.26 - - - - - - -

4000 1.40 0.85 - - - - - -

6000 2.46 1.82 0.85 - - - - -

8000 3.44 2.72 1.74 0.26 - - - -

10000 4.34 3.56 2.58 1.11 - - - -

12000 5.16 4.33 3.35 1.90 0.55 - - -

14000 5.90 5.04 4.07 2.63 1.29 - - -

16000 6.56 5.67 4.72 3.32 1.98 0.59 - -

18000 7.14 6.25 5.31 3.94 2.62 1.29 - -

20000 7.64 6.76 5.84 4.51 3.22 1.94 0.49 -

22000 8.06 7.20 6.31 5.03 3.76 2.54 1.14 -

24000 8.40 7.57 6.71 5.49 4.25 3.09 1.75 0.41

26000 8.66 7.89 7.06 5.90 4.70 3.59 2.31 1.02

28000 8.84 8.13 7.34 6.25 5.09 4.04 2.82 1.59

30000 8.93 8.31 7.57 6.54 5.44 4.44 3.28 2.11

32000 8.95 8.42 7.73 6.78 5.74 4.79 3.69 2.58

34000 - 8.47 7.83 6.97 5.98 5.09 4.05 3.01

36000 - - 7.87 7.10 6.18 5.35 4.36 3.38

38000 - - - 7.17 6.33 5.55 4.63 3.71

40000 - - - 7.19 6.43 5.70 4.84 3.99

0

1

2

3

4

5

6

7

8

9

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

TWC

(g

/m3

)

Altitude (ft)

Peak TWC Levels: Adiabatic Lapse from Sea Level @ 90% Relative Humidity

0°C

-10°C

-20°C

-30°C

-40°C

-50°C

-60°C

-70°C


Page 20 of 45

Ambient Temperature

Altitude (ft) 0°C -10°C -20°C -30°C -40°C -50°C -60°C -70°C

42000 - - - 7.15 6.48 5.80 5.01 4.23

44000 - - - 7.06 6.48 5.85 5.13 4.41

46000 - - - 6.92 6.43 5.85 5.20 4.55

48000 - - - 6.72 6.33 5.80 5.22 4.64

50000 - - - - 6.18 5.70 5.19 4.69

52000 - - - - - - 5.11 4.68

54000 - - - - - - 4.99 4.63

56000 - - - - - - - 4.53

58000 - - - - - - - 4.38

60000 - - - - - - - 4.18

In service events show that several probe icing events (both temperature and

pressure probes) in glaciated conditions have occurred outside of the 14 CFR part

33 Appendix D domain in terms of altitude and outside air temperature.

Furthermore, a reported event occurred at a temperature of -70°C. In that context,

the convective cloud ice crystal envelope should be enlarged to encompass ISA

+30°C conditions and to extend to -70°C. In addition, the Appendix D envelope

should be expanded to cover ISA -5°C conditions above 25,000 ft. This expanded

envelope is in Figure 5.

Figure 5 Expanded Convective Cloud Ice Crystal Envelope

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)

Appendix D Envelope



Page 21 of 45

Figure 5 can be overlaid on Figure 4 to show the relationship of maximum water

content as a function of altitude and temperature, including the expansion of the

Appendix D envelope, as shown in Figure 6

Figure 6 Peak Total Water Content Levels as a Function of Altitude and Temperature

The peak values of each TWC curve can also be plotted to show the peak TWC as a

function of temperature, as shown in Figure 7.

0

1

2

3

4

5

6

7

8

9

10

0 10000 20000 30000 40000 50000 60000

TWC

(g

/m3

)

Altitude (ft)

Peak TWC Levels As a Function of Altitude and Temperature

0°C-10°C-20°C-30°C-40°C-50°C-60°C-70°CAppendix D Temperature/Altitude EnvelopeAppendix D Expansion


Page 22 of 45

Figure 7 Maximum Total Water Content as a Function of Temperature

Points of interest for the analysis can be plotted on the altitude/temperature

envelope. The points of interest should include the most critical conditions where

icing is likely to occur. For this example, it is assumed that those critical conditions

occur at the corners of the envelope within the design operating envelope of the

aircraft. In addition to the corner points of the envelope, several points in the middle

of the envelope are also selected for completeness. The points selected for this

example are plotted in Figure 8. Note that the highlighted points at higher altitude

conditions have been selected to correspond to the peak total water content for the

selected ambient temperature rather than following the edge of the

temperature/altitude envelope. When selecting these points, the analysis may need

to address other considerations in addition to the peak total water content, for

example, system failure modes that impact engine core airflow. The completed

analysis should provide an evaluation of the total threat relative to the target

operating envelope and determine the most threatening conditions based on that

evaluation.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

-70 -60 -50 -40 -30 -20 -10 0

TWC

(g

/m3 )

Ambient Temperature (°C)

Appendix D Peak TWC

Appendix D Peak TWC Extrapolated to -70°C


Page 23 of 45

Figure 8 Selected Ice Crystal Points for Analysis

The points can then be plotted on the total water content versus altitude envelope of

Figure 6 to determine the total water content at each point; this is shown in Figure 9.

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)

Appendix D Envelope


Points for Analysis

Points correspond to peak

TWC for given ambient

temperature


Page 24 of 45

Figure 9 Total Water Content for Selected Points

The points represented on Figure 9 give the altitude (ambient pressure), temperature

and total water content for the points of interest. These parameters are summarized

in Table 2.

Table 2 Selected Ice Crystal Points for Analysis (Altitude, Temperature, TWC)

Altitude Ts Peak TWC

Point # Ft °C g/m3

1 4000 -3 1.20

2 12055 -3 4.85

3 24228 -3 8.13

4 4000 -10 0.85

5 15450 -10 5.51

6 27761 -10 8.10

7 9000 -20 2.17

8 20300 -20 5.91

9 32808 -20 7.78

0

1

2

3

4

5

6

7

8

9

10

0 10000 20000 30000 40000 50000 60000

TWC

(g

/m3

)

Altitude (ft)

0°C-10°C-20°C-30°C-40°C-50°C-60°C-70°CAppendix D Temperature/Altitude EnvelopeAppendix D ExpansionSelected Points for Analysis


Page 25 of 45

Altitude Ts Peak TWC

Point # Ft °C g/m3

10 14000 -30 2.64

11 25150 -30 5.73

12 38400 -30 7.18

13 19000 -40 2.93

14 30000 -40 5.44

15 41500 -40 6.47

16 34850 -50 5.21

17 40000 -50 5.70

18 45000 -50 5.85

19 39700 -60 4.82

20 45000 -70 4.49

21 45000 -60 5.17

Using these conditions, along with an assumed airspeed, the water flux can then be

calculated. To cover all conditions in the operating envelope, both a minimum and a

maximum airspeed should be analyzed. For this example, a minimum airspeed of

100 KCAS is assumed, while the maximum airspeed is assumed to be 325 KCAS up

to 29,000 ft and Mach 0.85 above 29,000 ft. For completeness, a hold airspeed

between the minimum and maximum speeds should also be evaluated, for this

example a holding airspeed of 180 KCAS is assumed. These airspeeds are

representative of a typical business jet aircraft. Actual airspeeds used for analysis

should match the operating envelope of the intended installation.

The airspeed can be used to calculate a Mach number which is used in order to

calculate the total temperature. The airspeed and ambient temperature can also be

used to calculate the free stream velocity (true airspeed) of the flow.

The total water flux is then simply the total water content times the velocity of the

flow. This calculated water flux is the ambient water flux. The total water flux at the

probe could be significantly higher as noted in Step 3 due to concentration effects. It

is possible that the concentration effect is greater at lower airspeeds such that low

speed conditions result in a larger water flux at the probe location than high speed

conditions. It is therefore important to evaluate the installation of the probe and

ensure that the critical conditions are evaluated.

Performing those calculations for each of the above points at both the high and low

speed conditions results in a table of 63 points as shown in Table 3. Note for this

example, the total water content has been assumed to be the same as the ice water

content (IWC), that is, the assumption is that the cloud is fully glaciated. For mixed

phase conditions the split between ice and liquid water would need to be addressed,

as discussed later in this appendix.


Page 26 of 45

Table 3 Selected Ice Crystal Points for Analysis

Point

# Altitude Ts TT Ps Mach Free Stream TWC Free Stream

TWC flux

Ft °C °C psia m/s KCAS g/m3 g/(cm2 ·

min)

Low

Sp

eed

1L 4000 -3 -1.6 12.69 0.163 54 100 1.4 0.43

2L 12055 -3 -1.1 9.33 0.19 63 100 4.9 1.83

3L 24228 -3 0.2 5.64 0.243 80 100 8.2 3.97

4L 4000 -10 -8.6 12.69 0.163 53 100 1.1 0.34

5L 15450 -10 -7.8 8.14 0.203 66 100 5.5 2.16

6L 27761 -10 -6.4 4.83 0.263 85 100 8.2 4.2

7L 9000 -20 -18.4 10.5 0.179 57 100 2.1 0.73

8L 20300 -20 -17.5 6.67 0.224 71 100 5.8 2.5

9L 32808 -20 -15.6 3.83 0.294 94 100 7.8 4.38

10L 14000 -30 -28.1 8.63 0.197 62 100 2.7 0.99

11L 25150 -30 -27 5.42 0.248 78 100 5.7 2.68

12L 38400 -30 -24.5 2.94 0.335 105 100 7.2 4.51

13L 19000 -40 -37.8 7.04 0.218 67 100 2.9 1.18

14L 30000 -40 -36.4 4.36 0.276 84 100 5.5 2.81

15L 41500 -40 -33.9 2.53 0.36 110 100 6.5 4.3

16L 34850 -50 -45.8 3.48 0.308 92 100 5.2 2.89

17L 40000 -50 -44.6 2.72 0.348 104 100 5.7 3.57

18L 45000 -50 -43.2 2.14 0.39 117 100 5.9 4.11

19L 39700 -60 -54.9 2.76 0.345 101 100 4.8 2.93

20L 45000 -70 -63.8 2.14 0.39 112 100 4.5 3.02

21L 45000 -60 -53.5 2.14 0.39 114 100 5.2 3.55

Hig

h S

pee

d

1H 4000 -3 5.9 12.69 0.406 134 250 1.4 1.08

2H 12055 -3 17 9.33 0.608 200 325 4.9 5.86

3H 24228 -3 28.4 5.64 0.762 251 325 8.2 12.42

4H 4000 -10 -1.3 12.69 0.406 132 250 1.1 0.84

5H 15450 -10 12.1 8.14 0.647 210 325 5.5 6.9

6H 27761 -10 25.1 4.83 0.815 265 325 8.2 13.03

7H 9000 -20 -3.2 10.5 0.575 184 325 2.1 2.36

8H 20300 -20 5.4 6.67 0.707 226 325 5.8 7.88

9H 32808 -20 16.7 3.83 0.85 271 305 7.8 12.68

10H 14000 -30 -10.7 8.63 0.63 197 325 2.7 3.16

11H 25150 -30 -0.7 5.42 0.775 242 325 5.7 8.36

12H 38400 -30 4.3 2.94 0.838 262 265 7.2 11.3

13H 19000 -40 -17.7 7.04 0.69 211 325 2.9 3.73

14H 30000 -40 -6.2 4.36 0.85 260 325 5.5 8.65


Page 27 of 45

Point

# Altitude Ts TT Ps Mach Free Stream TWC Free Stream

TWC flux

Ft °C °C psia m/s KCAS g/m3 g/(cm2 ·

min)

15H 41500 -40 -6.2 2.53 0.85 260 250 6.5 10.15

16H 34850 -50 -17.7 3.48 0.85 255 292 5.2 7.98

17H 40000 -50 -17.7 2.72 0.85 255 259 5.7 8.74

18H 45000 -50 -17.7 2.14 0.85 255 231 5.9 8.94

19H 39700 -60 -29.1 2.76 0.85 249 261 4.8 7.23

20H 45000 -70 -40.6 2.14 0.85 243 231 4.5 6.58

21H 45000 -60 -29.1 2.14 0.85 249 231 5.2 7.74

Ho

ld S

pee

d

1D 4000 -3 2.1 12.69 0.31 96 180 1.20 0.69

2D 12055 -3 3.6 9.33 0.35 112 180 4.85 3.25

3D 24228 -3 6.9 5.64 0.43 143 180 8.13 6.95

4D 4000 -10 -5.0 12.69 0.31 95 180 0.85 0.49

5D 15450 -10 -2.8 8.15 0.37 118 180 5.51 3.90

6D 27761 -10 0.9 4.83 0.46 152 180 8.10 7.37

7D 9000 -20 -14.4 10.50 0.33 102 180 2.17 1.33

8D 20300 -20 -11.9 6.67 0.40 127 180 5.91 4.52

9D 32808 -20 -7.4 3.83 0.50 166 180 7.78 7.73

10D 14000 -30 -23.7 8.63 0.36 110 180 2.63 1.74

11D 25150 -30 -20.8 5.42 0.44 138 180 5.73 4.74

12D 38400 -30 -14.9 2.94 0.56 184 180 7.18 7.92

13D 19000 -40 -32.8 7.04 0.39 119 180 2.93 2.09

14D 30000 -40 -29.5 4.36 0.47 150 180 5.44 4.88

15D 41500 -40 -23.3 2.53 0.60 193 180 6.47 7.48

16D 34850 -50 -38.0 3.48 0.52 163 180 5.21 5.09

17D 40000 -50 -35.1 2.72 0.58 182 180 5.70 6.24

18D 45000 -50 -31.4 2.14 0.64 203 180 5.85 7.15

19D 39700 -60 -45.9 2.76 0.57 177 180 4.82 5.12

20D 45000 -70 -53.1 2.14 0.64 194 180 4.49 5.23

21D 45000 -60 -42.3 2.14 0.64 199 180 5.17 6.17

The total water flux versus temperature can now be plotted for the points in Table 3.

Note that these water flux values represent the free stream values. Local values at

the probe need to be determined based on concentration factors expected to be

seen as installed.

From the plot of water flux versus temperature (Figure 10), the maximum expected

value can be determined, and points can be identified for testing or analysis. As

discussed in Step 4 of the main body of this report, it is recommended that points be

selected across the temperature range of expected operation and at the upper limit

of the water flux value and at values of 0.65 (1/1.538) and 0.5 times the maximum to


Page 28 of 45

address the effect of lower water contents for longer durations. The temperatures

selected are roughly equally spaced across the temperature range. For reference,

the ice crystal test point conditions from AS5562 (Table 5, Class 4 aircraft) are also

plotted on Figure 10.

The points identified for testing are included in Figure 10.

Figure 10 Water Flux versus Total Temperature

The recommended test conditions from Figure 10 are tabulated in Table 4. These

are the minimum set of conditions that should be tested for the probe for this

example. Note that these recommended test conditions define the ambient

conditions and they must be adjusted by the appropriate concentration factor to

define the local conditions at the probe.

0

2

4

6

8

10

12

14

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40

Tota

l W

ate

r Fl

ux

(g/c

m2

·min

)

Total Temperature (°C)

Water Flux versus Total Temperature for Analyzed Points

High Speed Points

Low Speed Points

Hold Speed Points

Recommended Ice Crystal Test Points

AS5562 Points

Upper Extent of Analyzed Points


Page 29 of 45

Table 4 Recommended Ice Crystal Test Conditions

Recommended Ice Crystal Test Point # Total Temperature Water Flux

(°C) (g/cm2·sec)

1 15 12.53

2 15 8.15

3 15 6.27

4 0 10.93

5 0 7.10

6 0 5.46

7 -15 9.32

8 -15 6.06

9 -15 4.66

10 -30 7.72

11 -30 5.02

12 -30 3.86

13 -40 6.65

14 -40 4.32

15 -40 3.32

From these recommended test points, corresponding ambient conditions which cover

the operating envelope need to be identified. To determine the ambient conditions,

the analyzed points can be compared to the recommended test points, as shown in

Figure 11. The analyzed points which match closest to the recommended peak

TWC points are selected as the ambient conditions to test at the three different TWC

levels. In this example that would be points 9H, 15H, 18H, 20H and 21H as noted on

Figure 11.


Page 30 of 45

Figure 11 Ice Crystal Test Point Identification

The resulting points are summarized below in Table 5 and plotted against the

Appendix D envelope in Figure 12 and on the total water content versus altitude

envelope in Figure 13 and on the maximum total water content versus temperature

envelope in Figure 14. It is also important to note that these test points define the

ambient conditions to be tested. The local conditions at the probe still need to be

determined, as described in Step 3 of the main body of this document.

The duration of the test points is also tabulated in Table 5 based on the length of

time it would take to traverse clouds of 17.4 nm and 215 nm.

0

2

4

6

8

10

12

14

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40

Tota

l W

ate

r Fl

ux

(g/c

m2

·min

)


Water Flux versus Total Temperature for Analyzed Points

High Speed Points

Low Speed Points

Hold Speed Points


AS5562 Points


Pt#20H, -40.6°,flux 6.58

Pt#21H, -29.1°,flux 7.74

Pt#15H, -6.2°,flux 10.15

Pt#18H, -17.7°,flux 8.94

Pt#9H, +16.7°,flux 12.68


Page 31 of 45

Table 5 Recommended Ice Crystal Test Points

Point # Altitude Ts TT Ps Mach

Free Stream Velocity

Free Stream Water Flux TWC

Time to Traverse Cloud

Ft °C °C psia m/s KTAS g/(cm2 · min) g/m3

17.4 nm

(min)

215 nm

(min)

1 32808 -20 16.6 3.83 0.85 271 528 12.68 7.8 2.0 24.5

2 32808 -20 16.6 3.83 0.85 271 528 8.24 5.1 2.0 24.4

3 32808 -20 16.6 3.83 0.85 271 528 6.34 3.9 2.0 24.4

4 41500 -40 -6.3 2.53 0.85 260 506 10.15 6.5 2.1 25.5

5 41500 -40 -6.3 2.53 0.85 260 506 6.60 4.2 2.1 25.5

6 41500 -40 -6.3 2.53 0.85 260 506 5.075 3.2 2.1 25.5

7 45000 -50 -17.8 2.14 0.85 254 495 8.94 5.9 2.1 26.1

8 45000 -50 -17.8 2.14 0.85 254 495 5.81 3.8 2.1 26.1

9 45000 -50 -17.8 2.14 0.85 254 495 4.47 2.9 2.1 26.1

10 45000 -60 -29.2 2.14 0.85 245 477 7.74 5.2 2.2 27.0

11 45000 -60 -29.2 2.14 0.85 245 477 5.03 3.4 2.2 27.0

12 45000 -60 -29.2 2.14 0.85 245 477 3.87 2.6 2.2 27.0

13 45000 -70 -40.6 2.14 0.85 249 484 6.58 4.5 2.2 26.7

14 45000 -70 -40.6 2.14 0.85 249 484 4.28 2.9 2.2 26.7

15 45000 -70 -40.6 2.14 0.85 249 484 3.29 2.2 2.2 26.7


Page 32 of 45

Figure 12 Recommended Ice Crystal Test Points Relative to Appendix D Envelope

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)

Appendix D Envelope


Points for Analysis



Page 33 of 45

Figure 13 Recommended Ice Crystal Test Points Relative to Appendix D Total Water

Content

0

1

2

3

4

5

6

7

8

9

10

0 10000 20000 30000 40000 50000 60000

TWC

(g

/m3

)

Altitude (ft)

0°C -10°C

-20°C -30°C

-40°C -50°C

-60°C -70°C

Appendix D Temperature/Altitude Envelope Appendix D Expansion

Selected Points for Analysis Recommended Ice Crystal Test Points


Page 34 of 45

Figure 14 Recommended Ice Crystal Test Points Relative to Appendix D Maximum

Total Water Content

As noted in the main body of this report, the test conditions for a standard (17.4 nm)

cloud are not critical due to the short length of time required to traverse the cloud.

The resulting duration is only slightly longer than the 2 minutes duration defined for

the peak TWC conditions, therefore those points are deemed to be not critical and

they can be removed from the final listing of test points. The final ice crystal test

points, including test durations, are tabulated in Table 6.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

-70 -60 -50 -40 -30 -20 -10 0

TWC

(g

/m3 )

Ambient Temperature (°C)

Appendix D Peak TWC

Appendix D Peak TWC Extrapolated to -70°C

Recommended Test Points


Page 35 of 45

Table 6 Final Ice Crystal Test Points

Point # Altitude Ts TT Mach Free Stream Velocity Total

Water Flux TWC Duration

Ft °C °C m/s KTAS g/(cm2 ·

min) g/m3 (min)

1 32808 -20 16.6 0.85 271 528 12.68 7.8 2

3 32808 -20 16.6 0.85 271 528 6.34 3.9 30

4 41500 -40 -6.3 0.85 260 506 10.15 6.5 2

6 41500 -40 -6.3 0.85 260 506 5.075 3.2 30

7 45000 -50 -17.8 0.85 254 495 8.94 5.9 2

9 45000 -50 -17.8 0.85 254 495 4.47 2.9 30

10 45000 -60 -29.2 0.85 245 477 7.74 5.2 2

12 45000 -60 -29.2 0.85 245 477 3.87 2.6 30

13 45000 -70 -40.6 0.85 249 484 6.58 4.5 2

15 45000 -70 -40.6 0.85 249 484 3.29 2.2 30

Once the ice crystal test points have been identified, the same basic process is

repeated for mixed phase conditions. The significant differences between the

process for ice crystals and mixed phase conditions are a lower temperature limit

of -40°C for mixed phase conditions compared to -70°C for ice crystals and a lower

total water content for the mixed phase conditions.

It is commonly recognized that below -40°C no liquid conditions exist anymore.

Therefore, testing in mixed phase conditions does not need to consider temperatures

at or below -40°C. The EIWG recommends a lower static temperature limit of -35°C

for mixed phase test conditions.

Based on several sources of information, EASA guidance published in AMC 25.1324

recommends the use of TWC’s scaled for a 2.6 nautical mile cloud rather than the

standard 17.4 nm cloud for mixed phase conditions. This guidance matches the total

water contents called out for mixed phase test conditions defined in AS5562 for Pitot

probes. For consistency, these same water contents should be used for evaluating

temperature probes, combined temperature and pressure probes, and static

pressure probes. The resulting total water contents are presented in Figure 15. The

TWC data of Figure 15 is also presented in tabular form in Table 7. Both Figure 15

and Table 7 include data for -35°C, this -35°C data was obtained from interpolating

between the -30°C and -40°C curves.


Page 36 of 45

Figure 15 Total Water Content for Mixed Phase Conditions (Appendix D Peak Values

Scaled for 2.6 nm)

0

1

2

3

4

5

6

7

8

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

TWC

(g

/m3

)

Altitude (ft)

0°C

-10°C

-20°C

-30°C

-35°C

-40°C


Page 37 of 45

Table 7 Total Water Content for Mixed Phase Conditions (Appendix D Peak Values Scaled for 2.6 nm)

Altitude 0°C -10°C -20°C -30°C -35°C -40°C

0 - - - - - -

2000 0.20 - - - - -

4000 1.07 0.65 - - -

6000 1.88 1.39 0.65 - - -

8000 2.63 2.08 1.33 0.20 - -

10000 3.32 2.72 1.97 0.84 0.33 -

12000 3.95 3.31 2.56 1.45 0.93 0.42

14000 4.51 3.85 3.11 2.01 1.50 0.98

16000 5.02 4.34 3.61 2.53 2.02 1.51

18000 5.46 4.78 4.06 3.01 2.51 2.00

20000 5.84 5.16 4.46 3.45 2.95 2.46

22000 6.16 5.50 4.82 3.84 3.36 2.87

24000 6.42 5.79 5.13 4.20 3.72 3.25

26000 6.62 6.03 5.40 4.51 4.05 3.59

28000 6.75 6.21 5.61 4.77 4.33 3.89

30000 6.83 6.35 5.78 5.00 4.58 4.16

32000 6.84 6.44 5.91 5.18 4.78 4.38

34000 - 6.47 5.99 5.33 4.95 4.57

36000 - 6.46 6.02 5.42 5.07 4.72

38000 - - 6.00 5.48 5.16 4.84

40000 - - 5.94 5.50 5.20 4.91

42000 - - - - 5.21 4.95

44000 - - - - 5.18 4.95

46000 - - - - - 4.91

Figure 15 and Table 7 present the total water content. For mixed phase conditions,

the liquid portion of the total water content needs to be identified. Appendix D

defines the liquid water portion of mixed phase conditions to be ≤1.0 g/m3 for clouds

of less than 50 nautical mile extents for temperatures above -20°C and zero for

temperatures below -20°C. SAE Aerospace Standard AS5562 assumes the liquid

water content is per the Appendix C intermittent maximum cloud and the remainder

of the TWC is ice crystals. The EIWG’s recommendation is to align with SAE

Aerospace Standard AS5562 and assume a liquid water content (LWC) per the

Appendix C intermittent maximum cloud and the balance of the TWC to be ice

crystals for all mixed phase conditions.

For unheated probes, pure liquid (Appendix C) conditions are likely to be most

severe. The presence of any ice crystals will tend to erode any forming ice and thus

reduce ice thickness. However, for heated probes it is unknown whether mixed

phase or fully glaciated conditions represent a more severe condition and what

LWC/TWC combination might be most severe if it does. Therefore, a range of


Page 38 of 45

LWC/TWC ratios may need to be considered. For this example, only the Appendix C

liquid water contents will be analyzed, but it is possible lower LWC’s will need to be

considered to define the worst case condition.

For the mixed phase condition analysis, the same points of interest used for the ice

crystal analysis shown in Figure 8 can be used, with the exception that points at or

below -40°C ambient temperature are replaced with points at -35°C since there will

be no liquid water present in conditions at or below -40°C. The resulting subset of

points is shown on the altitude vs. temperature envelope in Figure 16, and the Total

Water Content versus Altitude envelope in Figure 17. The resulting TWC’s are

summarized in Table 8.

Figure 16 Selected Mixed Phase Points for Analysis

-70

-60

-50

-40

-30

-20

-10

0

10

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Am

bie

nt

Te

mp

era

ture

(°C

)

Altitude (ft)

Appendix D Envelope


Mixed Phase Points for Analysis


Page 39 of 45

Figure 17 Mixed Phase Condition Total Water Content Levels as a Function of

Altitude and Temperature

Table 8 Selected Mixed Phase Points for Analysis (Altitude, Temperature, TWC)

Altitude Ts 2.6 nm Scaled

TWC

Point # Ft °C g/m3

1 4000 -3.0 0.92

2 12055 -3.0 3.71

3 24228 -3.0 6.21

4 4000 -10.0 0.65

5 15450 -10.0 4.21

6 27761 -10.0 6.20

7 9000 -20.0 1.66

8 20300 -20.0 4.52

9 32808 -20.0 5.95

10 14000 -30.0 2.02

11 25150 -30.0 4.38

12 38400 -30.0 5.49

13 16500 -35.0 2.15

14 27500 -35.0 4.27

15 41000 -35.2 5.21

0

1

2

3

4

5

6

7

0 10000 20000 30000 40000 50000 60000

TWC

(g

/m3

)

Altitude (ft)

0°C

-10°C

-20°C

-30°C

-40°C

Appendix D Temperature/Altitude Envelope


Selected Mixed Phase Points for Analysis


Page 40 of 45

The liquid water portion of the total water content is determined based on the

intermittent maximum LWC from Appendix C. To maximize the LWC, the mean

effective drop diameter is assumed to be 15 microns. This is consistent with the

method used for Pitot probes in AS5562. As noted previously, maximizing the liquid

water content conservatively results in mixed phase conditions in colder conditions

with more liquid water present than the Appendix D requirements. After the liquid

water portion is determined from Appendix C, the remaining water content is

assumed to be completely glaciated. For some of the low altitude conditions, the

Appendix C intermittent maximum liquid water contents are higher than the Appendix

D total water contents. For those points where the Appendix C intermittent maximum

LWC’s were higher than the Appendix D TWC’s, the Appendix C continuous

maximum LWC’s (scaled for a 17.4 nm cloud) were used instead of the intermittent

maximum values. The resulting liquid and ice water concentrations are summarized

in Table 9.

Table 9 Selected Mixed Phase Points for Analysis (Altitude, Temperature, LWC & IWC)

Altitude Ts

2.6 nm Scaled TWC

LWC IWC

Point # Ft °C g/m3

g/m3 g/m3

1* 4000 -3.0 0.92 0.73 0.19

2 12055 -3.0 3.71 2.80 0.91

3 24228 -3.0 6.21 2.80 3.41

4* 4000 -10.0 0.65 0.55 0.10

5 15450 -10.0 4.21 2.48 1.73

6 27761 -10.0 6.20 2.48 3.72

7* 9000 -20.0 1.66 0.33 1.33

8 20300 -20.0 4.52 1.88 2.64

9 32808 -20.0 5.95 1.88 4.07

10 14000 -30.0 2.02 1.13 0.89

11 25150 -30.0 4.38 1.13 3.26

12 38400 -30.0 5.49 1.12 4.36

13 16500 -35.0 2.15 0.68 2.03

14 27500 -35.0 4.27 0.69 3.95

15 41000 -35.2 5.21 0.67 4.74

* Appendix C intermittent maximum liquid water content is higher than the

Appendix D total water content. For these points, the LWC is assumed to be

the Appendix C continuous maximum value

Using the points identified in Table 9, the same process used for the ice crystal

conditions can be repeated for the mixed phase conditions to determine the total

water flux. The resulting calculations are summarized in Table 10.


Page 41 of 45

Table 10 Selected Mixed Phase Points for Analysis

Point # Altitude Ts TT Ps Mach Free Stream TWC Free Stream

TWC flux

Ft °C °C psia m/s KCAS g/m3 g/(cm2 · min)

Low

Sp

eed

1L-M 4000 -3 -1.6 12.69 0.163 54 100 0.92 0.30

2L-M 12055 -3 -1.1 9.33 0.19 63 100 3.71 1.40

3L-M 24228 -3 0.2 5.64 0.243 80 100 6.21 2.98

4L-M 4000 -10 -8.6 12.69 0.163 53 100 0.65 0.21

5L-M 15450 -10 -7.8 8.14 0.203 66 100 4.21 1.67

6L-M 27761 -10 -6.4 4.83 0.263 85 100 6.20 3.16

7L-M 9000 -20 -18.4 10.5 0.179 57 100 1.66 0.57

8L-M 20300 -20 -17.5 6.67 0.224 71 100 4.52 1.93

9L-M 32808 -20 -15.6 3.83 0.294 94 100 5.95 3.35

10L-M 14000 -30 -28.1 8.63 0.197 62 100 2.02 0.75

11L-M 25150 -30 -27 5.42 0.248 78 100 4.38 2.05

12L-M 38400 -30 -24.5 2.94 0.335 105 100 5.49 3.46

13L-M 16500 -35 -33.0 7.80 0.207 64 100 2.15 0.83

14L-M 27500 -35 -31.8 4.88 0.261 81 100 4.27 2.08

15L-M 41000 -35 -29.0 2.59 0.355 110 100 5.21 3.44

Hig

h S

pee

d

1H-M 4000 -3 5.9 12.69 0.406 134 250 0.92 0.74

2H-M 12055 -3 17 9.33 0.608 200 325 3.71 4.45

3H-M 24228 -3 28.4 5.64 0.762 251 325 6.21 9.36

4H-M 4000 -10 -1.3 12.69 0.406 132 250 0.65 0.52

5H-M 15450 -10 12.1 8.14 0.647 210 325 4.21 5.30

6H-M 27761 -10 25.1 4.83 0.815 265 325 6.20 9.85

7H-M 9000 -20 -3.2 10.5 0.575 184 325 1.66 1.83

8H-M 20300 -20 5.4 6.67 0.707 226 325 4.52 6.13

9H-M 32808 -20 16.7 3.83 0.850 271 305 5.95 9.67

10H-M 14000 -30 -10.7 8.63 0.630 197 325 2.02 2.39

11H-M 25150 -30 -0.7 5.42 0.775 242 325 4.38 6.37

12H-M 38400 -30 4.3 2.94 0.838 262 265 5.49 8.63

13H-M 16500 -35 -14.4 7.80 0.658 204 325 2.15 2.63

14H-M 27500 -35 -3.7 4.88 0.810 251 325 4.27 6.43

15H-M 41000 -35 -0.6 2.59 0.850 268 253 5.21 8.38

Ho

ld S

pee

d

1D-M 4000 -3 1.6 12.69 0.292 109 180 0.92 0.60

2D-M 12055 -3 3.2 9.33 0.340 112 180 3.71 2.49

3D-M 24228 -3 7.1 5.64 0.433 143 180 6.21 5.33

4D-M 4000 -10 -5.5 12.69 0.292 95 180 0.65 0.37

5D-M 15450 -10 -3.1 8.14 0.363 118 180 4.21 2.98

6D-M 27761 -10 1.4 4.83 0.466 151 180 6.20 5.61


Page 42 of 45

Point # Altitude Ts TT Ps Mach Free Stream TWC Free Stream

TWC flux

Ft °C °C psia m/s KCAS g/m3 g/(cm2 · min)

7D-M 9000 -20 -14.8 10.5 0.32 102 180 1.66 1.01

8D-M 20300 -20 -11.9 6.67 0.399 127 180 4.52 3.45

9D-M 32808 -20 -6.4 3.83 0.519 166 180 5.95 5.92

10D-M 14000 -30 -23.9 8.63 0.353 110 180 2.02 1.33

11D-M 25150 -30 -20.5 5.42 0.441 126 180 4.38 3.31

12D-M 38400 -30 -13.2 2.94 0.588 184 180 5.49 6.06

13D-M 16500 -35 -28.5 7.80 0.370 115 180 2.15 1.48

14D-M 27500 -35 -24.8 4.88 0.463 143 180 4.27 3.66

15D-M 41000 -35 -16.5 2.59 0.623 191 180 5.21 5.97

The total water flux versus temperature can now be plotted for the points in Table 10.

Note that these water flux values represent the free stream values. Local values at

the probe need to be determined based on concentration factors expected to be

seen as installed.

From the plot of water flux versus temperature (Figure 18), the maximum expected

value can be determined, and points can be identified for testing or analysis. For the

ice crystal example, points were selected at the upper limit of the water flux value

and at values of 0.65 (1/1.538) and 0.5 times the maximum to address the effect of

lower water contents for longer durations. The points at 0.65 times the maximum

water flux were determined to be not critical due to the duration of the test points;

that same rationale applies in the case of mixed phase conditions, so only the

maximum and ½ of the maximum TWC flux values are recommended for testing or

analysis. For reference, the mixed phase test point conditions from AS5562 (Table

4, Class 3 & 4 aircraft) are also plotted on Figure 18.

The points identified for testing are included in Figure 18 and tabulated in Table 11.


Page 43 of 45

Figure 18 Water Flux versus Total Temperature for Mixed Phase Conditions

Table 11 Recommended Mixed Phase Test Conditions

Total Temperature Water Flux

Point (°C) (g/cm2·sec)

1 20 10.00

2 20 5.00

3 0 8.60

4 0 4.30

5 -20 5.40

6 -20 2.70

7 -35 5.79

8 -35 2.90

From these recommended test points, corresponding ambient conditions which cover

the operating envelope need to be identified, similar to the process that was followed

for determining the ice crystal test conditions. To determine the ambient conditions,

the analyzed points can be compared to the recommended test points, as shown in

Figure 18. The analyzed points which match closest to the recommended peak

TWC points, are selected as the ambient conditions to test at the different TWC

0

2

4

6

8

10

12

14

-50 -40 -30 -20 -10 0 10 20 30 40

Tota

l W

ate

r Fl

ux

(g/c

m2

·min

)


Water Flux versus Total Temperature for Analyzed Mixed Phase Points

High Speed Points

Hold Speed Points

Low Speed Points

Recommended Mixed Phase Test Points


AS5562 Mixed Phase Points

Pt #9H-M, Flux 9.67Pt #15H-M, Flux 8.38

Pt #15D-M, Flux 5.97

Pt #15L-M, Flux 3.44


Page 44 of 45

levels. In this example that would be points 9H-M, 15H-M, 15D-M and 15L-M as

noted on Figure 18.

Table 12 Final Mixed Phase Test Points

Point # Altitude Ts TT Ps Mach

Free Stream Velocity

Total Water

Flux TWC LWC IWC Test

Ft °C °C psia m/s KCAS g/(cm2 ·

min) g/m3 g/m3 g/m3

1 41000 -35 -0.6 2.59 0.85 268 233 8.38 5.21 0.69 4.52 2 minutes

2 41000 -35 -0.6 2.59 0.85 268 233 4.19 2.61 0.69 1.92 Cyclic

3 32808 -20 16.7 3.83 0.85 271 305 9.67 5.95 1.88 4.07 2 minutes

4 32808 -20 16.7 3.83 0.85 271 305 4.84 2.98 1.88 1.09 Cyclic

5 41000 -35 -16.5 2.59 0.623 191 180 5.97 5.21 0.69 4.52 2 minutes

6 41000 -35 -16.5 2.59 0.623 191 180 2.99 2.61 0.69 1.92 Cyclic

7 41000 -35 -29.0 2.59 0.355 110 100 3.44 5.21 0.69 4.52 2 minutes

8 41000 -35 -29.0 2.59 0.355 110 100 1.72 2.61 0.69 1.92 Cyclic

It is important to note that these test points define the ambient conditions to be

tested. The local conditions at the probe still need to be determined, as described in

Step 3 of the main body of this document.

The type of test for each point is also included in Table 12. The high TWC points

should be tested for the same 2 minute duration as the peak TWC ice crystal points

were tested. The lower concentration points should be tested as cyclic test

conditions as described in Step 4 in the main body of this document.

The list of conditions for ice crystal icing conditions (identified in Table 6), mixed

phase conditions (identified in Table 12) and the supercooled liquid water conditions

identified in Step 4 in the main body of this document should be considered the

candidate test points to be evaluated per Step 6 of the process.

As noted above, the example provided assumes that water mass flux is the critical

parameter. A CPA using water mass flux as the critical parameter will naturally

select the highest airspeeds within the flight envelope. However, these types of

conditions may not be the most critical for all probe designs and/or types of icing

conditions. Therefore, it is recommended that other parameters besides water mass

flux also be considered. These may include, for example, maximum total cooling

load, minimum dynamic pressure, maximum water-to-air mass flux ratio and

minimum anti-icing heater power available. Consideration of other parameters of

criticality ensure a thorough test of the probe’s capability to operate throughout the

defined icing envelopes.

AIA/EIWG Subcommittee on Engine Probe Icing

Page 45 of 45

Appendix 2: Task Group Participants

Name Representing email

Chairpersons

Tom Dwier Textron Aviation [email protected]

Phil Dang Honeywell [email protected]

Industry Participants

Alice Calmels Airbus [email protected]

Steve Glenn Boeing [email protected]

Shah Ghayem Boeing [email protected]

Melissa Bravin Boeing [email protected]

Sébastien Dijon Esterline [email protected]

Chuck Califf GE [email protected]

Jason Tan GE [email protected]

Dave Dischinger Honeywell [email protected]

Morris Anderson Honeywell [email protected]

David Orchard NRC [email protected]

Bill Fletcher Rolls Royce [email protected]

Vince LoPresto UTAS [email protected]

Bob Sable UTAS [email protected]

Chris Skwarek UTAS [email protected]

Brian Matheis UTAS [email protected]

Robin Rotondo UTAS [email protected]

Ken Smith Williams International [email protected]

Charlie Bonnen Williams International [email protected])

Aaron Binns Williams International [email protected]

Francois Larue Zodiac Aerospace [email protected]

Certification Authority Participants

Angus Abrams EASA [email protected]

John Fisher FAA [email protected]

Alan Strom FAA [email protected]

Gary Horan FAA [email protected]

Larry Field FAA [email protected]

Tom Bond FAA [email protected]

Doug Bryant FAA [email protected]

Bob Hettman FAA [email protected]

Chris Baczynski Transport Canada [email protected]

Michel Provencher Transport Canada [email protected]

David Johns Transport Canada [email protected]

Roop Dhaliwal Transport Canada [email protected]

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