01-28000. Issuance of Policy Statement, Compliance with Induction System Icing Protection for Part 23 Airplanes

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Notice of policy statement.

SUMMARY:

This document announces an FAA general statement of policy applicable to turbine powered, normal, utility, acrobatic, and commuter category airplanes. This document advises the public, in particular small airplane owners and modifiers, of information related to compliance with the engine induction system icing protection requirements applicable to turbine powered, part 23, normal, utility, acrobatic, and commuter category airplanes. This notice is necessary to tell the public of FAA policy.

FOR FURTHER INFORMATION CONTACT:

Paul Pellicano, Federal Aviation Administration, Small Airplane Directorate, Regulations and Policy Branch, ACE-111, 901 Locust, Room 301, Kansas City, Missouri 64106; telephone [phone]; fax [phone].

SUPPLEMENTARY INFORMATION:

Background

This notice announces the following policy statement, ACE-01-23.1093(b). The purpose of this statement is to address compliance with the engine induction system icing protection requirements applicable to turbine powered, part 23, normal, utility, acrobatic, and commuter category airplanes.

What is the general effect of this policy?

The FAA is presenting this information as a set of guidelines suitable for use. However, we do not intend that this policy set up a binding norm; it does not form a new regulation, and the FAA would not apply or rely on it as a regulation.

The FAA Aircraft Certification Offices (ACO's) and Flight Standards District Offices (FSDO's) that certify changes in type design and approve alterations in normal, utility, acrobatic, and commuter category airplanes should try to follow this policy when appropriate. Applicants should expect the certificating officials would consider this information when making findings of compliance relevant to compliance with the engine induction system icing protection requirements applicable to turbine powered, part 23, normal, utility, acrobatic, and commuter category airplanes.

As with all advisory material, this statement of policy identifies one way, but not the only way, of compliance.

General Discussion of Comments

Has FAA taken any action to this point?

We issued a notice of policy statement, request for comments. This proposed policy appeared in the Federal Register on August 1, 2001 (66 FR 39815) and the public comment period closed August 31, 2001.

Was the public invited to comment?

The FAA encouraged interested people to join in making this proposed policy. We received comments from two airplane manufacturers.

The comments were related to similarity of part 25 guidance, the severity and subjectivity of the falling and blowing snow criteria, applicability of auxiliary power units (APU), policy differences between inlet styles, an ice shedding example in the policy which contradicts operational regulations, and making the policy into an Advisory Circular. The comments on the subjectivity of the falling and blowing snow criteria and the ice shedding example resulted in revisions to the policy.

The proposed policy was coordinated with the appropriate technical specialists at the Transport Airplane Directorate and the Engine and Propeller Directorate and it does not contradict any part 25 policy.

The 1/4 mile visibility criteria for falling and blowing snow comes from the definition of heavy snow in the FAA Aeronautical Information Manual (AIM) and agrees with transport and rotorcraft directorate policy. The policy states that other criteria may be applicable, such as that provided in Advisory Circular 29-2C. Another resource is FAA Report DOT/FAA/AR-97/66, “Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.)” and this is added to the policy.

The proposed policy states that all turbine installations, regardless of inlet type, should have a design analysis performed and if no accumulation sites of concern exist, then the analysis may be sufficient. A typical part 23 turbopropeller installation has accumulation sites of concern that may not exist for a typical turbojet or turbofan installation with a pitot style inlet. Also, the policy does not distinguish between inlet styles in the applicability of service history.

The policy will be incorporated into the next revision to Advisory Circular 23-16 and in the interim will be posted on the Internet to provide a wide circulation.

The Policy

The purpose of this policy statement is to provide compliance guidance for the engine induction system ice protection requirements contained in 14 CFR, part 23, § 23.1093(b), which is applicable to part 23 turbine powered airplanes. Except for the information contained in Advisory Circulars (AC's) 20-73 and 23.1419-2A, this guidance cancels and supersedes previous guidance on § 23.1093(b) compliance for part 23 normal, utility, acrobatic, and commuter category airplanes.

The guidance contained in AC 20-73 and AC 23.1419-2A, relevant to § 23.1093(b) compliance, is still applicable.

Applicants and FAA Aircraft Certification Offices (ACO's) involved with certification of small airplanes should generally follow this policy. Applicants should expect that the ACO would consider this information when making findings of compliance. However, in determining compliance with certification standards, each ACO has the discretion to deviate from these guidelines when the applicant demonstrates a suitable need. To ensure standardization, the ACO should coordinate deviation from this policy with the Small Airplane Directorate.

References

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 23.1419-2A, Certification of Part 23 Airplanes for Flight in Icing Conditions.

AC 20-73, Aircraft Ice Protection.

AC 29-2C, Certification of Transport Category Rotorcraft.

Considerations Regarding Approval for Flight in Known Icing

It is important to know that compliance with § 23.1093(b) for induction system icing protection, the initial requirement being incorporated by Amendment 23-7, is independent of approval for flight into icing conditions (§ 23.1419 compliance). Propulsion system items that were intended to be certificated for approval for flight into icing conditions are addressed under § 23.929, initially adopted by Amendment 23-14. Service experience has shown that airplanes encounter icing conditions even if the airplane is not approved for flight into icing conditions. This is particularly true with turbine powered airplanes, which typically have an expanded operating flight envelope as compared to reciprocating engine powered airplanes. To provide a minimum level of ice protection for all for part 23 normal, utility, acrobatic, and commuter category airplanes, compliance with all the requirements contained in § 23.1093 must be demonstrated even if the aircraft is not approved for flight into icing conditions. Therefore, for turbine powered airplanes, compliance with § 23.1093(b) is required even if approval for flight into icing conditions is not sought.

Use of Similarity and Service Experience

The use of similarity and service experience is appropriate to lessen the design risk associated with an installation. Once an applicant has developed data on an installation, then the applicant may use this data, when suitable, for substantiation on later projects with similar installations. It is common and proper for an applicant to base analytical methods and test point definitions on experience and testing of previous, similar certification programs performed by the applicant. However, since certification data helps define the type design of an airplane, for one applicant to use data from another applicant's certification program as substantiation, access to the specific design and test considerations used by the second applicant would be required. Therefore, the proper use of similarity data by an applicant to support analytical methods and testing requirements would be difficult if the data was not based on the applicant's past projects or if the project is not being performed in cooperation with another applicant.

Even if previous experience and data are used, each inlet/engine installation and the associated operating characteristics can be different and should be considered individually. Therefore, it is not appropriate to use similarity or service experience by itself for the purpose of demonstrating compliance to the § 23.1093(b) requirement. Rather, such means as 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.

Use of Tunnel Test Data

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(b). 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 are:

(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.

Also, 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.

14 CFR Part 33 Engine Certification

An applicant seeking airplane certification should coordinate the installation of an engine with the engine manufacturer. The engine manufacturer should be able to identify critical points, conditions, and operational requirements that may need to be addressed when showing compliance with the installation requirements. However, it is inappropriate to assume that any part 33 engine certification program would fully address all the part 23 engine installation requirements.

It should be emphasized that it is the responsibility of the airplane applicant and not the engine manufacturer to show compliance with the part 23 induction system ice protection requirements. Items such as use of an inlet system recommended by the engine manufacturer would still require installation substantiation to show compliance with part 23 requirements.

It is appropriate to use engine type certification data as the basis for reducing design risk, analysis, testing, and so forth; however, when showing compliance with § 23.1093(b) it is still the responsibility of the installer to evaluate this data and demonstrate how the data is applicable to the particular application. Therefore, close coordination of the engine and airplane applicant can ease certification burdens and enhance the safety of a particular engine installation.

Falling and Blowing Snow Requirement

The requirement contained in § 23.1093(b)(1)(ii), incorporated initially by Amendment 23-15, is to evaluate the installed powerplant system to ensure no hazardous effects are encountered when operating in falling and blowing snow. A hazardous effect could be in the form of 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. The requirement was incorporated separately from icing and water ingestion requirements due to the unique characteristics of snow. Therefore, it is inappropriate to assume that compliance with engine induction system icing requirements means that compliance with snow requirements have been met.

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. Therefore, the effect of ingesting snow during ground operations and critical in-flight operations should be evaluated. The snow environment that has been seen to be critical 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), review of the installation should be performed to identify potential inlet, engine, and airframe sites where snow accumulation and shedding is possible. Also, review of the airplane operation should be performed to determine critical conditions that should be addressed.

Although all turbine engine installations should be evaluated, turbopropeller installations generally have different areas of concern than turbofan/jet installations. 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. 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 been generally been found to be more critical than snow tests. Therefore, a turbofan/jet installation may be found acceptable by inlet design and airplane operation analysis, while turbopropeller installations will normally require testing in operationally representative conditions.

However, it needs to be reemphasized that the installation should be evaluated to decide on the required level of substantiation. For example, aft mounted turbofan/jet installations may have concerns with accumulation and in-flight snow shed from wing surfaces after take-off. Also, there are turbofan installations with S-type inlet ducts that would have many of the same concerns as turbopropeller installations. Additionally, part 33 engine certification does not address snow ingestion and some turbofan/jet engines, in addition to turbopropeller engines, may have internal accumulation sites that may allow snow to melt, refreeze, and shed causing internal engine damage. Therefore, all turbine engines should be evaluated with close coordination with the engine manufacturers.

When evaluating the conditions for showing compliance, the following airplane operations should be considered:

1. Static operation with the engine at idle for 30 minutes, with the ability to attain take-off power. 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 take-off roll.

If found acceptable, the engine may be able to be run up at higher power settings during the 30 minute period for the purposes of ice/snow shed. If run-ups are performed during compliance demonstration, these procedures should be incorporated as limitations in the Flight Manual.

Before run-ups are accepted, the practicality of the procedures should be evaluated. For example, if an engine must be run at a high power setting that may allow the airplane to slide or create hazards to other airplanes, then the procedures may not be acceptable.

2. Higher power settings, which could result in increased snow ingestion, associated with taxi/hold ground operations.

3. For airplanes with identified sites of possible hazardous snow accumulation and all inlets with bypass ducts (for example, typical turbopropeller inlets), a take-off run to take-off speed. This condition is considered critical since

(a) accumulated snow may liberate at this dynamic condition; and

(b) the static, idle point will not provide the ram effects that create bypass flow for bypass ducts.

4. For airplanes with identified sites of possible hazardous snow accumulation, take-off climb. This condition is considered critical since accumulated snow may liberate at this dynamic condition.

5. Extended in-flight operations such as hold patterns.

6. Operation when engine rotor speeds are low, such as during descent from high altitudes. An engine is highly susceptible to snow/ice accretion during this condition.

It should be noted that the preceding conditions are operational considerations and not meant to require flight test at all the conditions. As mentioned earlier, each installation may have different critical operational considerations and only the critical conditions may need further substantiation than just analysis.

Also, 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 that incorporates an inlet screen that precludes the ingestion of hazardous quantities of materials, the critical concern to be addressed 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.

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. Per 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 desired 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. However, 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 impingement. Therefore, the applicant should address this consideration when choosing the appropriate snow environment.

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. The applicant is responsible for defining the critical ambient temperatures for snow tests.

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 bypassed, with snow crystals sticking to partially heated interior inlet surfaces, leading to melting and refreeze. In all cases, the applicant must identify and evaluate the critical temperature for the configuration proposed. Company developmental tests or experience with similar induction systems may be used to determine critical conditions.

It should be emphasized that the purpose of the requirement is to evaluate the engine's induction system ice protection capability in snow environments that can be expected during the operational life of the airplane. Addressing the snow environment, detailed in resource materials such as the AIM, at critical operational conditions for a particular airplane, provides a good gauge to evaluate the system's capability. Most configurations will not require flight test in all operational conditions.

Snow concentration corresponding to the visibility prescribed is often extremely difficult to locate naturally. Furthermore, it is often difficult to maintain the desired concentrations for the duration of testing. Because of these testing realities, it is very likely that exact target test conditions will not be achieved for all possible test conditions. Therefore, those involved in certification must exercise reasonable engineering judgement in accepting critical test conditions and alternate approaches, with early coordination between the applicant and the FAA addressing these realities.

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 snow fall 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 very 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.

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.” Therefore, 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 desired 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.

An additional area of emphasis for § 23.1093(b)(1)(ii) compliance is the words in the regulation “. . .within the limitations established for the airplane for such operation.” 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. The only plausible Flight Manual limitation that may be acceptable would be prohibitions for ground operations such as taxi, take-off, engine runs, and so forth. However, the case of flying into snow after deployment must be considered.

Ice Fog

The basic requirement contained in § 23.1093(b)(2), also incorporated by Amendment 23-15, addresses the condition of idling the engine on the ground to ensure no adverse ice build-up (for example, no surges, adverse power loss, and so forth), commonly referred to as “ice fog.” A way to view the § 23.1093(b)(2) requirement is as an extension upon 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 run-ups are accepted, the practicality of the procedures should be evaluated.

Issued in Kansas City, Missouri, on October 23, 2001.

Michael Gallagher,

Manager, Small Airplane Directorate, Aircraft Certification Service.