similarity or service experience should be supplemented with either analysis, even if only basic design analysis to substantiate similarity, or testing, or a combination of both.
Can compliance with § 23.1093 be accomplished with analysis alone?
No, analysis alone is not sufficient to show compliance. Testing is normally required to demonstrate compliance as analytical methods have been difficult to validate. It is possible, for airplanes not being certified to § 23.1419, that flight testing will not be required given analysis that is backed by similarity to and service experience of other certified installations.
How can ground testing be used to show compliance?
Artificial icing conditions can be generated in icing tunnel facilities. Artificial icing for compliance to ground ice fog requirements can also be generated with an icing spray rig set up in front of the airplane. The icing conditions are very dependent on the calibration of the nozzles. SAE ARP 5905 contains calibration guidelines for icing tunnels. The personnel operating icing spray rigs should be familiar with the calibration of icing nozzles.
Artificial snow can be generated in some ground facilities to supplement natural snow testing. Considerations are provided in the discussion under § 23.1093(b).
How can tunnel test data be used for compliance?
An area where there has been much discussion has been the use of tunnel test data instead of full-scale, airplane flight test data for showing compliance with § 23.1093. The use of tunnel test data is a common, appropriate, and often efficient means to reduce the amount of testing required by the applicant for showing compliance. However, the extent that this data can be used for compliance is dependent upon how representative the test article and test conditions are to the installation and airplane operating conditions.
It is not uncommon for tunnel testing to be performed on a prototype or test inlet that often has design differences from the production inlet used by an installer. When using tunnel test data, or any test data for that matter, as a basis for testing or certification, the applicant must address the differences and the impact of the differences. Three areas of difference usually addressed include the following:
1. Heated versus non-heated inlets;
2. Inlets with movable or variable internal devices (for example, movable vanes used to select bypass modes on a number of turbopropeller inlets) versus fixed inlets; and
3. Differences in geometry even if the inlet type (fixed versus variable) is the same.
As an example, if tunnel testing is performed with a heated inlet and an applicant incorporates a non-heated inlet, ice runback/refreeze may be reduced, but items such as ice accretion characteristics will be different.
It must be ensured that the tunnel tests were performed at the critical points. Advisory Circular 20-73, Aircraft Ice Protection, provides guidance on critical points determination.
How can testing with airborne icing tankers be used for compliance?
Tanker icing tests are not used as a sole means of compliance, but may be used:
1. To verify analyses or scaled icing tunnel tests
2. To obtain data at the limits of the Appendix C icing envelopes
3. To evaluate ice-shedding characteristics
SAE ARP 5904 provides useful information on icing tanker testing.
One limitation of icing tankers in the past has been their inability to generate droplet sizes at the low end of the Appendix C envelope, which may be critical for inlets with curves and bends, such as some turboprop inertial separation inlets.
How can flight test in natural icing conditions be used for compliance?
Airplanes being certified for flight into icing conditions according to § 23.1419 will require flight test into natural icing conditions. The results of this testing can be used to show compliance with § 23.1093.
Airplanes not being certified for flight into icing conditions according to § 23.1419 do not necessarily require flight test into natural icing conditions to show compliance with § 23.1093. It is acceptable to use analysis backed with sufficient similarity and service experience to show compliance.
Related Material
AC 20-73, Aircraft Ice Protection
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
AC 23.1521-1B, Type Certification of Automobile Gasoline in Part 23 Airplanes with Reciprocating Engines
SAE ARP 5904, Airborne Icing Tankers
SAE ARP 5905, Calibration and Acceptance of Icing Wind Tunnels
Paragraph 23.1093(a)
ADC Cross-reference: 5.45(a)
What does the term preheat mean and how is it different from heat rise?
Current interpretation of the term preheat as written in § 23.1093(a)(4) and § 23.1093(a)(6) is that it is a change in temperature above the ambient requirements stated in the regulation. Thus, it is interpreted as interchangeable with the term heat rise.
The regulation is written to provide rules of compliance that can be reasonably demonstrated without testing over the whole range of the icing and engine power envelopes. According to Appendix C of part 25, the icing temperature envelope extends down to -22 °F, a condition that is difficult to attain during a normal flight test program.
The interpretation of § 23.1093(a)(6) is that a “preheat of 60 °F” requires a change of 60 °F above the 30 °F ambient baseline temperature. This interpretation ensures that the system will always provide sufficiently heated air to prevent icing even if the airplane encounters the worse case scenario with –22 °F temperatures and the engines at idle as with a descent to landing.
Related Material
AC 23.1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions
Paragraph 23.1093(b)
ADC Cross-reference: 5.45(b)
Why are the falling and blowing snow requirements separate from the icing and water ingestion requirements?
The falling and blowing snow requirement was incorporated separately from the icing and water ingestion requirements due to the unique characteristics of snow. The requirement contained in § 23.1093(b)(1)(ii), incorporated initially by Amendment 23- 15, is to evaluate the installed powerplant system to ensure that no hazardous effects are encountered when operating in falling and blowing snow. A hazardous effect could be unacceptable engine operating characteristics (for example, adverse power loss, surges, and so forth) due to inlet blockage or engine damage resulting from conditions such as snow, which may accumulate, melt, refreeze, shed, and then be ingested by the engine.
Therefore, compliance with engine induction system icing requirements does not show compliance with the snow requirements in § 23.1093(b)(1)(ii).
What should be considered relative to complying with the falling and blowing snow requirements?
The effect of ingesting snow during ground operations and critical in-flight operations should be evaluated. Service experience has demonstrated that engine damage can occur as a result of prolonged ground operations in falling and blowing snow. Also, in-flight service experience has shown that snow, which has melted and refrozen, can shed from engine, inlet, or airplane accumulation sites, resulting in adverse engine operability or engine damage. The critical snow environment is a ”wet, sticky snow,” which accumulates on unheated exterior and interior surfaces subject to impingement.
When showing compliance with § 23.1093(b)(1)(ii), the installation should be reviewed to identify potential inlet, engine, and airframe sites where snow accumulation and shedding is possible. The airplane ground and in-flight operational envelope should be reviewed to determine critical conditions that should be addressed. The information from these reviews should be used to identify the critical conditions to evaluate compliance.
The following differences between turbopropeller and turbofan/jet installations should be considered:
1. Typical turbopropeller installations have inlets that incorporate complex geometry with features such as particle separators, plenum chambers, screens, oil coolers, and so forth, where hazardous snow accumulations may occur. Turbopropeller installations normally require testing in operationally representative conditions.
2. Typical turbofan/jet installations, using simple pitot (straight duct) inlets, have minimal, if any, areas for snow accumulation. For these inlets, in-flight icing tests have generally been found to be more critical than snow tests. Turbofan/jet installation may be found acceptable by inlet design and airplane operation analysis.
Care should be taken to identify potential airplane accumulation sites that could shed into the engine(s), such as in the case of aft-mounted engines. Atypical installations with S-type inlet geometries should be treated like the turbopropeller installations when finding compliance with § 23.1093(b)(1)(ii).
What operationally representative condition should be considered?
When evaluating the conditions for showing compliance, the following airplane operations should be considered:
1. Takeoff power after static operation with the engine at idle for 30 minutes.
This condition is considered critical due to the operational consideration of idling an engine on the ground with minimal ability for de-ice/anti-ice. The primary concern is the loss of power at takeoff roll.
It is acceptable to incorporate engine run-up to higher power settings during the 30 minute period for the purposes of ice/snow shed, provided the procedure does not 128
create any hazards. A potential hazard could be the airplane sliding toward other airplanes due to the higher power settings required. If run-ups are performed during compliance demonstration, these procedures should be incorporated as limitations in the flight manual.
2. Higher power settings
Higher power settings, which could result in increased snow ingestion, associated with taxi/hold ground operations.
3. Takeoff run to takeoff speed
Airplanes with identified sites of possible hazardous snow accumulation and all inlets with bypass ducts (for example, typical turbopropeller inlets), should consider a takeoff run to takeoff speed. This condition is considered critical since—
a. Accumulated snow may come loose and the engine can ingest it in this condition;
and
b. The static, idle point will not provide the ram effects that create bypass flow for bypass ducts.
4. Takeoff climb
Airplanes with identified sites of possible hazardous snow accumulation should consider a takeoff climb. This condition is considered critical because accumulated snow may come loose and the engine can ingest it in this condition.
5. Extended in-flight operations
Conditions such as holding patterns should be considered.
6. Operation when engine rotor speeds are low
Operations with low engine rotor speeds, such as during descent from high altitudes, should be considered. An engine is highly susceptible to snow/ice accretion during this condition.
Is a flight test required for all these operational conditions?
The preceding conditions are operational considerations and not meant to require flight test at all the conditions. Each installation may have different critical operational considerations and only the critical conditions may need flight testing.
When appropriately substantiated by the applicant, some of the conditions can be, and have been, simulated and accepted by the FAA. For example, for a turbopropeller engine 129
that incorporates an inlet screen that precludes the ingestion of hazardous quantities of materials, the critical concern may only be the effect of snow accumulation and release from the inlet and screen. In this case, the inlet, bypass duct, inlet screen, and so forth, could be blocked to simulate snow accumulation on an identified area of concern. Since accumulation during dynamic operation would be simulated, the effects of snow ingestion could be determined through ground tests (for example, effects of operability on items such as reverse flow). Such methodologies need to be substantiated by means such as design analysis, operational review, tunnel tests, icing tests, and so forth, and coordinated early with the FAA.
How is the actual snow amount quantified for testing?
When testing in ”falling and blowing snow” the actual snow amount is often difficult to quantify. The FAA Aeronautical Information Manual (AIM), an official FAA guide to basic flight information and air traffic control procedures, may be used as guidance for what constitutes falling and blowing snow. According to the AIM, paragraph 7-1-18, heavy snow, which is representative of what may be expected in operation, is defined as visibility of 1/4 mile or less as limited by snow (not snow and fog). These conditions are usually indicative of the wet snow environment necessary for test. When using the 1/4 mile or less visibility for test, including flight tests, this value can be determined using ground conditions. Useful information on snow characterization can also be found in FAA Report DOT/FAA/AR-97/66, Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.). Advisory Circular 29-2C (Certification of Transport Category Rotorcraft), section AC 29.1093, paragraph c(4)(iv), also provides information on snow quantification, including desired snow concentration, which is acceptable for use on part 23 airplanes. Whatever method is used to characterize the snow, as mentioned earlier, the design consideration that has been found to be critical is snow that accumulates on surfaces subject to impingement. Therefore, the applicant should address this consideration when choosing the appropriate snow environment.
What temperatures are considered appropriate for testing?
The applicant is responsible for defining the critical ambient temperatures for snow tests.
The primary consideration is to demonstrate operability in a snow environment that is critical as far as snow accumulation on exterior and interior areas of impingement (for example, wet, sticky snow). However, for a snow environment indicative of a representative concentration expected for the airplane, temperature is also an important consideration.
Typically, in natural conditions, a temperature range between 25 and 34 degrees Fahrenheit has been found conducive to the heavy snow environment. However, colder temperatures may be critical to some configurations. For example, in some installations, colder exterior surfaces may be by-passed, with snow crystals sticking to partially heated interior inlet surfaces, leading to melting and refreeze. Company developmental tests or experience with similar induction systems may be used to determine critical conditions.
Is it difficult to locate and maintain the desired conditions for the test?
Snow concentration corresponding to the visibility prescribed is often extremely difficult to locate naturally and it is often difficult to maintain the desired concentrations for the duration of testing. Because of this, it is likely that exact target test conditions will not be achieved for all possible test conditions. Those involved in certification must exercise reasonable engineering judgment in accepting critical test conditions and alternate approaches, with early coordination between the applicant and the FAA addressing these realities.
Is it possible to use artificially produced snow for the test?
Artificially produced snow is an excellent developmental tool and has been used successfully to show potential problem areas and critical test points. When the desired natural snow concentration is not found, artificial means may be used to supplement the snow amount. However, when snow testing is required, the use of simulated snow is normally not used as the sole means of compliance. The desired heavy snow environment produces ”wet, sticky snow,” which accumulates on unheated exterior and interior surfaces subject to impingement. Most artificial means (for example snow blowers) produce snow pellets that are dissimilar to the snowflakes associated with ”wet, sticky snow.” Also, simulated snow produced indoors does not accumulate moisture from snowfall as seen in naturally created snow, with critical temperatures for simulated snow varying significantly from natural snow. Therefore, quantification of artificially produced snow for critical conditions can be difficult and subjective. If artificial means is proposed as a means of compliance, the applicant should provide data and substantiation on how the artificial means will effectively simulate the critical, desired operational consideration.
What is the difference between falling and blowing snow?
The concentration of snow entering the inlet in blowing snow will normally exceed the amount in falling snow; hence, the need to address ”blowing snow.” The location of the inlets should be considered to determine critical directions of blowing snow in relation to snow accumulation on impingement surfaces. Snow blowing in excess of 15 knots is the appropriate compliance condition. Means such as use of another airplane's propeller, taxiing the airplane in excess of 15 knots, and so forth, may be used to simulate blowing.
Is it possible to use flight manual limitations to comply with § 23.1093(b)(1)(ii)?
The only possible flight manual limitation that may be acceptable would be prohibitions for ground operations such as taxi, takeoff, engine runs, and so forth. As with all environmental considerations, such as rain, ice, hail, lightning, and so forth, operation in snow is considered an unavoidable, meteorological hazard that must be addressed including flying into snow after deployment.
What should be considered relative to complying with the § 23.1093(b)(2) requirements?
The requirement of § 23.1093(b)(2), incorporated by Amendment 23-15, addresses running the engine in an atmospheric condition commonly referred to as ”ice fog.” The engine is run at idle power in this condition to ensure no ice build-up that produces adverse engine operational characteristics (for example, no surges or adverse power losses). The § 23.1093(b)(2) requirement is as an extension of the 14 CFR part 25, Appendix C, icing envelope addressed in §§ 23.1093(b)(1)(i) and 23.1419. Therefore, the methodologies and analysis used for compliance with § 23.1093(b)(1)(i) can be extended for § 23.1093(b)(2) compliance.
It is often difficult to encounter all the ambient conditions required by § 23.1093(b)(2);
therefore, when testing, one or more of the conditions is typically simulated. For example, a common and acceptable method of compliance is using water spray devices to simulate the water conditions required while testing at the required ambient temperature conditions. Other manufacturers have used thermal analysis combined with dry air tests using ice shapes/simulated blockage to demonstrate compliance, which is also acceptable if properly substantiated.
The rule allows an engine run-up periodically to higher power settings to shed ice. As with snow testing, if run-ups are performed during compliance demonstration, then these procedures should be incorporated as limitations in the flight manual. Also, before runups are accepted, the practicality of the procedures should be evaluated.
Related Material
FAA Aeronautical Information Manual (AIM)
FAA Report DOT/FAA/AR-97/66, Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.)
AC 23.1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions
AC 20-73, Aircraft Ice Protection
AC 29-2C, Certification of Transport Category Rotorcraft
Paragraph 23.1093(c)
ADC Cross-reference: 5.45(c)
No specific policy is available as of December 31, 2002.
23.1095 Carburetor deicing fluid flow rate (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
Paragraph 23.1095(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1095(b)
No specific policy is available as of December 31, 2002.
23.1097 Carburetor deicing fluid system capacity (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
Paragraph 23.1097(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1097(b)
No specific policy is available as of December 31, 2002.
23.1099 Carburetor deicing fluid system detail design (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.1101 Induction air preheater design (Amendment 23-43)
CAR 3 Cross-reference: 3.610
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What is the purpose of the induction air preheater?
Some carburetor and fuel injection system designs do not experience icing. Designs that are susceptible to icing typically incorporate a means to preheat the air supply entering the system. The purpose of this system is to add sufficient heat to replace the heat lost due to vaporization of the fuel such that the mixing chamber temperature cannot drop to the freezing point of water. This is typically accomplished with a system consisting of an engine exhaust heated jacket surrounding the induction duct, which serves to preheat the inducted air. There is a control installed in the flight deck for adjusting the preheater valve such that the system may be operated only when required to prevent ice formation.
Paragraph 23.1101(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1101(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.1101(c)
No specific policy is available as of December 31, 2002.