01-347. Issuance of Policy Memorandum, Notice of Compliance with the Engine Ingestion Requirements Applicable to Turbine Powered, 14 CFR Part 23, Normal, Utility, Acrobatic, and Commuter Category 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 more information related to compliance with the engine ingestion 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:

Randy Griffith, Federal Aviation Administration, Small Airplane Directorate, Regulations and Policy Branch, ACE-111, 901 Locust, Room 301, Kansas City, Missouri 64106; telephone [phone]; fax [phone]; email: <[e-mail]>.

SUPPLEMENTARY INFORMATION:

Background

This notice announces the following policy statement, ACE-00-23.901(d)(2). The purpose of this statement is to address compliance with the engine ingestion 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, and acrobatic 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 ingestion 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 September 1, 2000 (65 FR 53338) and the public comment period closed October 2, 2000.

Was The Public Invited To Comment?

The FAA encouraged interested people to join in making this proposed policy. We received one comment. The commenter, while fully agreeing with the content, noted that the policy would be better if in an FAA Advisory Circular. We have noted the commenter's concerns. We will eventually provide the pertinent information in this policy in a revision to Advisory Circular 23-16, Powerplant Guide for Certification of Part 23 Airplanes. In the interim, the issuance of a policy statement is more timely and effective. Additionally, experience with a recent certification project resulted in further clarification of the draft policy. As a result, we have explained compliance considerations related to critical conditions for turbopropeller engine installations as compared to turbojet/fan engine installations. If these added compliance considerations cause concern, please send your comments to <[e-mail]>.

The Policy

Background

The current § 23.901(d)(2) requirement was incorporated by Amendment 23-53. However, the basic requirement, which has evolved into the current § 23.901(d)(2), was incorporated by Amendment 23-18.

Amendment 23-18 required that the engine installation provide continued engine operation without a sustained loss of power when operated at flight idle in rain for at least three minutes. The rate of rain ingestion was to be not less than 4 percent, by weight, of the engine induction airflow rate. The rule was incorporated due to reports of turbine engine power loss while operating in heavy rain. The intent of the rule was twofold: (1) to ensure that installation effects do not result in deterioration of the engine's rain ingestion tolerance determined by engine certification; and (2) to evaluate the engine's capability for rain ingestion for engines that were certificated before Amendment 33-6 since rain ingestion requirements were not added to 14 CFR part 33 until Amendment 33-6. Therefore, the rate of rain ingestion to be considered was based upon the part 33 engine certification requirement at the time.

Revisions of Standards

Amendment 23-29 revised the requirement to consider rated takeoff power/thrust. Also, the preamble to Amendment 23-29 further defined the intent of § 23.901(d)(2) by specifically stating that the rule is to ensure that installation effects do not result in any deterioration of the powerplant rain ingestion tolerance. Therefore, compliance with § 23.901(d)(2) required a separate determination for engine installation other than the requirements addressed by part 33 (for example, engine certification without further installation certification is inadequate to demonstrate compliance with the part 23 requirement).

Amendment 23-43 added a requirement that the installation be evaluated at the maximum installed power/thrust for takeoff. This new requirement was due to engine installations where rated takeoff power could be less than installed takeoff power; for example, de-rate thrust. The amendment also added a requirement that the engine be accelerated and decelerated safely under the rain conditions; however, Amendment 23-51 removed this consideration.

Amendment 23-53 added the current rule. The current amendment requires the installed engine to withstand ingestion of rain, hail, ice, and birds at a level not less than that established under engine certification. The significant changes with the new rule include operating concerns other than loss of power (for example, engine surges), the addition of hail, ice, and bird ingestion requirements, and replacement of specific rain quantification with the conditions used during engine certification. Under Amendment 23-53, the airplane applicant needs to evaluate the conditions used to address rain, hail, ice, and bird ingestion during engine certification and how the installation relates to these conditions.

Means of Compliance

When showing compliance with the rain ingestion requirements for all amendment levels of § 23.901(d)(2), compliance is typically accomplished with design analysis that identifies areas of concern and test when there are areas of concern. Part 33 engine certification testing may be used for compliance if the engine certification testing (1) addressed the areas of concern identified by the installation design analysis (for example, use of an installation representative test inlet system) and (2) specific conditions addressed in the rule were addressed during engine certification testing. For airplanes with a certification basis prior to Amendment 23-53, test is typically required if the specific operating considerations contained in the part 23 rule were not addressed during engine certification.

When evaluating areas of concern with the installation, consider areas where water pooling with subsequent ingestion or shed of localized “slugs” of water normally not addressed during engine certification might occur. Some examples are inlet system channels, indentations, and so forth. These are typical of turbopropeller or S-duct type inlets that have complex geometry to allow water pooling. This consideration is usually not a concern with simple pitot style inlets typical of most part 23 turbofan/jet engine installations. However, due to the large diversity of turbine engine installations in part 23 airplanes, all installations should be evaluated to determine if areas of concern exist. For example, there are turbofan installations that use S-style inlet ducts that may have areas of concern.

Therefore, part 23 turbine engine installations typically require testing since the vast majority of these are turbopropeller installations. However, if design analysis shows that the installation will not affect the water ingestion characteristics (for example, a simple and typical pitot style inlet installation) and engine certification addressed the specific conditions addressed in the part 23 rule, this analysis combined with engine certification testing may be adequate to demonstrate rain ingestion compliance.

Also, since the rain ingestion requirements in part 33 were not added until Amendment 33-6, the airplane applicant needs to evaluate the engine's certification basis to determine if the engine has been subjected to part 33 rain ingestion testing. If the engine does not have Amendment 33-6 or a subsequent amendment as part of the certification basis, in accordance with § 23.903(a)(2)(iii), the engine must have a safe service history of rain ingestion in similar installations.

If it is determined that testing for rain ingestion should be performed, flight test is not required. The intent of the part 23 rule is to ensure that the engine installation has not deteriorated the rain ingestion tolerance of the certificated engine. Since a ground static engine test normally demonstrates engine certification compliance, use of installation ground tests at the required power/thrust settings has been commonly accepted as a means of compliance.

The applicant can use design analysis to determine critical configurations and conditions of the installation. This might reduce required installation tests to the critical configurations and conditions instead of repeating the entire part 33 test conditions. Engine certification should address the results of the critical point analysis for the engine; therefore, it is important for the engine installer to research the conditions and requirements used for engine certification.

Other Considerations for Compliance

Amendment 23-53 also added requirements for ice, hail, and birds. Examples of installation issues normally not addressed by engine certification, but that should be addressed for installation compliance, include the following: ice build-up on areas where ice shed may be ingested by the engines (for example, ice shed from wings and airframe sources into aft mounted engines) and consideration of items such as inlet splitters, acoustic liners, and so forth, that may be damaged by impact with ice, hail, and birds.

Issued in Kansas City, Missouri on December 14, 2000.

Michael Gallagher,

Manager, Small Airplane Directorate, Aircraft Certification Service.