2. Flammable fluid leaks;
3. If applicable, the exterior condition of the turbocharger and wastegate installation;
4. If applicable, tail pipe, multi-segmented V-band clamps, and firewalls;
5. Electrical wiring;
6. Fuel and oil lines and fittings; and, otherwise, make a cursory inspection of the engine installation; and
7. Other items as defined by the engine, propeller, or airplane manufacturer.
For more information refer to § 23.611, Accessibility provisions.
Paragraph 23.901(d)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What special considerations are there for turbine engine installations?
Paragraph 23.901(d) requires that carcass vibration and rain, hail, ice, and bird ingestion be considered for all turbine engine installations. This requirement ensures the installation does not degrade the certificated capability of the engine.
What considerations and methods of compliance are typical for the vibration requirements?
Compliance with rules concerning vibration usually involves instrumentation and testing.
These tests may consist of a combination of bench vibration tests and full-scale airplane tests. Analysis can be used to support the placement of instrumentation and to identify the critical test conditions. Coordination with the engine manufacturer may be necessary to understand the engine vibration limits and to design the tests to show the installation vibration characteristics are within the engine certification limits throughout the airplane flight envelope.
What considerations and methods of compliance are typical for the ingestion requirements?
Compliance with the rain ingestion requirements of § 23.901(d)(2) is typically shown with design analysis that identifies areas of concern and tests for those areas of concern.
Part 33 engine certification testing may be used for part 23 compliance if the engine certification testing addressed the areas of concern identified by the installation design analysis (for example, use of an installation representative test inlet system). Part 33 5
engine certification testing may also be used for part 23 compliance if specific conditions addressed in the rule were addressed during engine certification testing. For airplanes with a certification basis before Amendment 23-53, a 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 resulting 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 that can allow water pooling. This consideration is usually not a concern with simple pitot style inlets typical of part 23 turbofan/jet engine installations. Part 23 turbine engine installations typically require testing since most of these are turbopropeller installations. However, if design analysis shows the installation will not affect the water ingestion characteristics and engine certification addressed the specific conditions addressed in the part 23 rule, this analysis combined with engine certification testing may be enough to demonstrate rain ingestion compliance. Because of the large diversity of turbine engine installations in part 23 airplanes, all installations should be evaluated to determine whether areas of concern exist. 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 whether the engine has been subjected to part 33 rain ingestion testing. If the engine does not have Amendment 33-6 or a later amendment as part of the certification basis, the engine must have a safe service history of rain ingestion in similar installations, under § 23.903(a)(2)(iv).
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 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.
What are some other examples of issues not addressed by engine certification that need to be addressed for installation compliance with § 23.901(d)?
Amendment 23-53 added requirements for ice, hail, and birds similar to those in part 33.
Examples of installation issues normally not addressed by engine certification, but that should be addressed for installation compliance, include the following:
1. 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 6
2. Consideration of items such as inlet splitters, acoustic liners, and so forth, that may be damaged by impact with ice, hail, and birds.
Paragraph 23.901(e)
ADC Cross-reference: 5.1(d)
What instructions are typically provided under the engine and propeller type certificates?
Sections 33.5 and 35.3 list the requirements for engine and propeller installation instruction manuals. Examples of subjects typically provided under the engine and propeller type certificates may include, but are not limited to, the following:
1. Engine and propeller operating limitations;
2. Engine and propeller mounting provisions;
3. Allowable fuel and oil specifications;
4. Allowable fuel and oil temperatures;
5. Fuel inlet pressures;
6. Allowable accessory temperature limits;
7. Vibration limits;
8. Accessibility requirements;
9. Turbine engine inlet distortion limits; and
10. Abnormal engine failure load conditions such as turbine engine fan blade failure.
It is important to identify and accommodate all engine and propeller certification dependencies with the aircraft installation to ensure compatibility of the hardware and operational limitations.
Is it possible to certificate an airplane to use a fuel other than one certificated for use on the engine?
No, the fuel used in the airplane must be certificated for use in the engine as well as the airplane to comply with § 23.901(e). Other fuels such as automobile gasoline have been certificated for use in an airplane with a separate certification for using the fuel in the 7
engine. All applicable sections of 14 CFR part 33 and part 23 must be considered when certificating a fuel for use in an engine and in an airplane, respectively.
Related Material
AC 23.1521-1B, Type Certification of Automobile Gasoline in Part 23 Airplanes with Reciprocating Engines
AC 23.1521-2 with Change 1, Type Certification of Oxygenates and Oxygenated Gasoline Fuels in Part 23 Airplanes with Reciprocating Engines
AC 20-24B, Qualification of Fuels, Lubricants and Additives for Aircraft Engines
AC 20-122A, Anti-misfueling Devices: Their Availability and Use
Paragraph 23.901(f)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What should be considered when determining the applicable parts of these regulations as applied to Auxiliary Power Units (APU)?
The typical Auxiliary Power Unit (APU) is a combustion cycle powerplant operated primarily to generate electrical, pneumatic, and hydraulic power for ground and limited flight operations and not thrust for flight. Each of the rules applicable to powerplant installations must be considered carefully relative to the design features and operating requirements for a given APU. Below are three examples.
1. APUs typically do not have propellers connected to them and, in such a case, it is obvious, the portions of this part that apply to propellers should not be considered for such an installation.
2. APUs generally have a much smaller operating envelope than do thrust producing powerplants and, as such, due consideration should be given to the requirements in areas such as induction system icing (§ 23.1093).
3. An APU that is used to provide emergency power to meet the requirements of § 23.1351 would be considered differently than an APU intended only for ground use.
What regulations should be considered for Auxiliary Power Units using technologies other than the typical combustion cycles?
In the future, designs using exotic technologies to produce auxiliary electrical, pneumatic, and hydraulic power, such as fuel cells, may be proposed. These designs will 8
require careful selection of the applicable powerplant rules and may require special conditions depending on how similar they are to the typical combustion cycle designs.
Related Material
AC 20-128A, Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure 9
23.903 Engines (Amendment 23-54)
CAR 3 Cross-reference: 3.415
ADC Cross-reference: 5.2
What guidance is available for replacing a reciprocating engine with a turbine engine?
Guidance for turbine engine airplane conversions is provided in AC 23-14, Type Certification Basis for Conversion from Reciprocating Engine to Turbine Engine- Powered Part 23 Airplanes. This AC highlights the differences between the two engine installations that must be accounted for including, but not limited to:
1. Fuel system compatibility with turbine fuel;
2. Fuel Gelling and icing;
3. Part 34 fuel and venting requirements for turbine engines; and
4. Inlet icing and falling and blowing snow requirements.
What guidance is available for certification of the installation of unique types of engines?
Typical airplane engines fall into two categories: spark ignition reciprocating and turbine engines. These types account for most airplanes in service today, but other types such as compression ignition reciprocating (diesel cycle) and sinusoidal cam spark ignition engines have been proposed. When certifying an airplane with engines that are different from the typical engines, the unique features of the new engines should be carefully considered relative to the rules of 14 CFR part 23. Special conditions may be necessary to account for these unique features. Suitable guidance material will be written as necessary to capture any certification experience and other considerations about these installations. For example, Policy Memo PS-ACE100-2002-004, Diesel Engine Installation, was released in May 2003 in response to current engine and airplane certification projects involving diesel engines.
Related Material
AC 23-14, Type Certification Basis for Conversion from Reciprocating Engine to Turbine Engine-Powered Part 23 Airplanes
PS-ACE100-2002-004, Diesel Engine Installation
Paragraph 23.903(a)
ADC Cross-reference: 5.2(a)
Does an engine(s) require a type certificate if the airplane is to be certified under 14 CFR part 23?
Yes, the engine(s) must have an approved type certificate (according to 14 CFR part 33 or CAR Part 13) for normal, utility, acrobatic, and commuter category airplanes certificated under part 23. The airplane, with the engine(s) installed, must also comply with 14 CFR part 34, § 34.11.
Is it possible to certificate an airplane under 14 CFR part 23 with an engine(s) that does not have a type certificate?
No, it is not possible to certificate an airplane under 14 CFR part 23 with an engine(s) that does not have a type certificate under 14 CFR part 33. There are rules other than part 23 that do allow engines that are not certificated under part 33. Engines may be approved as part of the airplane for airplanes originally certificated in the Primary Category under 14 CFR part 21, § 21.17(b), or airplanes certificated under Joint Aviation Requirements―Very Light Airplane (JAR-VLA), or Sport Plane (Transport Canada Design Standard TP10141E).
Related Material
AC 21.17-3, Type Certification of Very Light Airplanes under § 21.17(b)
AC 21-37, Primary Category Aircraft
AC 23-11A, 14 CFR Part 23 Type Certification of an Airplane Originally Certificated to Joint Aviation Regulations – Very Light Airplane (JAR-VLA) Standards
Is it possible to affect the type certificate of the engine by its installation in an airplane?
This section requires compliance with the engine manufacturer’s installation drawing, installation manual, and engine specific operating instructions. Non-compliance with the engine manufacturer’s installation requirements can invalidate the type certification status of the engine, making the engine non-compliant with § 23.903(a)(1). This, in turn, would prevent certification of the airplane under part 23. Examples of engine requirements that could be affected by the airplane installation may include, but are not limited to, the following:
1. Fuel delivery flow and pressure requirements;
2. Allowable turbine engine inlet aerodynamic distortion; and
3. Compliance with 14 CFR part 34, Fuel Venting and Exhaust Requirements for Turbine Engine Powered Airplanes.
Paragraph 23.903(b)
ADC Cross-reference: 5.2(b)
What is the intent of this rule?
Airplanes with turbine engines should be designed to minimize the hazard to the airplane if a maximum energy rotor burst (typically the energy level associated with a one-third sector of the engine rotor at the critical rotating speed) occurs. Airplanes with turbine engines should also be designed to minimize the hazard to the airplane if any other uncontained turbine engine failure (i.e., fan blade), including engine case burn through, occurs. It is impossible to protect any design in an absolute sense, but good engineering judgment must be used to minimize the effects of these failures. This requires a design with features that protect structure, systems, and components from the adverse effects of heat, pressure, and loads associated with these failures. Minimizing the hazard has been interpreted through the years as allowing continued safe operation, with emergency procedures that allow for landing the airplane.
Airplanes using new technology materials or novel designs should be carefully evaluated for their ability to minimize the effects of engine rotor burst and case burn through. A Small Airplane Directorate policy letter dated April 7, 1989, defined the word “minimize” as used in § 23.903(b)(1) as “to reduce to a minimum, decrease to the least practical amount.” The “least practical amount” has to consider what is attainable with current technology and materials. In these cases, the standard has become the existing certificated airplanes. These airplanes used discrete source damage analysis to select the most effective and suitable design precautions and to substantiate that these precautions minimized the rotorburst hazard to the least practical amount. A typical design case minimizing the effects consists of a crown skin cut with a limit gust load that then is used as the ultimate load when analyzing the remaining structure. Advisory Circular 20-128A, in the Related Materials below, contains information on the discrete source damage analysis.
Information on engine case burn through is contained in the related material.
What compliance methods are generally acceptable?
At the most basic, a review of drawings, schematics, system descriptions, hazard assessments, and a rotor burst analysis with inspections of the installation are acceptable means of compliance. Design similarity and service experience can also be considered with the means above to comply with this rule provided a clear link to the similar airplane can be established. Advisory Circular 20-128A provides guidance on methods to mitigate the hazard to critical systems such as engine and airplane control cables and 12
wiring, flammable fluid tanks and lines, pneumatic ducts, and hydraulic lines if a rotor burst occurs.
Related Material
AC 20-128A, Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure
AC 20-135, Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards, and Criteria
Paragraph 23.903(c)
ADC Cross-reference: 5.2(c)
What do the words “…in at least one configuration…” mean?
The intent of this rule is to ensure that, during any one flight, any initial failure condition that could prevent the continued safe operation of one engine will not prevent the continued safe operation of the other engine(s). Compliance with this rule requires there should be no immediate crew intervention necessary for continued safe flight.
Related Material
AC 23.1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions
Paragraph 23.903(d)
ADC Cross-reference: 5.2(d)
What concerns are there with starting and stopping a piston engine?
The intent of the rule relative to starting is to ensure the engine(s) can be started without creating a hazardous condition. Procedures and limitations must be established and documented in the Airplane Flight Manual and on proper airplane placards to cover all allowed conditions for starting engine. These conditions must include ground starts for all airplanes and in-flight restarts for multiengine airplanes.
The intent of the rule relative to stopping an engine is to ensure that any engine on a multiengine airplane can be stopped in flight if necessary to prevent a hazardous condition without creating an airplane hazard. According to § 23.901(d)(2), commuter category airplanes also require the stopping system components to be fire resistant.
While the rule does not explicitly state any requirements for stopping an engine in normal conditions, such as ground shutdown at the end of operations, there should be no hazards associated with these conditions. Conditions that may require stopping an engine in flight include:
1. Engine compartment fires;
2. Turbocharger or exhaust system failures;
3. Engine or accessory failures resulting in excessive vibration; and
4. Propeller malfunction.
The design of the system should consider and minimize hazards during both starting and stopping, such as the following:
1. The collection of flammable liquids and vapors;
2. Any adverse affects on engine accessories from reduced rotation, starting and stopping;
3. Vibrations that adversely affect the structural integrity of the engine, its components, or aircraft structures; and
4. Any conditions that would prevent safely restarting the engine.
Paragraph 23.903(e)
ADC Cross-reference: 5.2(e)
What concerns are there with starting and stopping a turbine engine?
The intent of the rule relative to starting is to ensure the engine(s) can be started without creating a hazardous condition. Procedures and limitations must be established and documented in the Airplane Flight Manual and proper airplane placards to cover all allowed conditions for starting the engine. These conditions include ground starts as well as in-flight restarts. Provisions and procedures for in-flight restarts must be established and documented in the Airplane Flight Manual.
The intent of the rule relative to stopping an engine is to ensure that any engine on an airplane can be stopped in flight if necessary to prevent a hazardous condition without creating an airplane hazard. Conditions that may require stopping an engine in flight include:
1. Engine compartment fires;
2. Engine rotorburst; and
3. Engine or accessory failures resulting in excessive vibration.
The design of the system should consider and minimize hazards during both starting and stopping, such as the following:
1. The collection of flammable liquids and vapors;
2. Any adverse affects on engine accessories from windmilling;
3. Vibrations that adversely affect the structural integrity of the engine, its components, or aircraft structures; and
4. Any conditions that would prevent safely restarting the engine.
Related Material
AC 23-2, Flammability Tests
AC 33.74/92-1A, Turbine Engine Continued Rotation and Rotor Locking
Paragraph 23.903(f)
ADC Cross-reference: 5.2(f)
What differences are there between reciprocating and turbine engines when showing compliance with this rule?
The requirement to define the in-flight restart envelope applies to all normal, utility, acrobatic, and commuter category airplanes. A full complement of tests to define the inflight restart envelope may not be necessary. For example, normally aspirated reciprocating engine in-flight restart procedures typically only specify the airspeed to restart the engine. In this case, the “restart envelope” is the entire flight envelope for the selected airspeed. On the other hand, defining a turbine engine restart envelope typically requires data from a comprehensive altitude/airspeed envelope test program. Review the engine installation manual to understand the engine limitations relative to the starting envelope. We recommend you coordinate with the engine manufacturer to determine the start testing performed during the certification of the engine.
1. Airplanes powered by reciprocating engines.
The amount of in-flight restart envelope testing is dependent on the particular design features of the engine installation. Comprehensive in-flight restart envelope testing is not usually necessary for single-engine airplanes powered by conventional reciprocating engines. The service experience of these airplanes with typical reciprocating engine installations is that a reliable in-flight engine restart can be made, provided the airplane is equipped with an adequate engine starting system.
Thus, in-flight restart envelope testing has not been necessary provided a mechanically sound engine can be restarted under typical power-off flight conditions.
However, if the engine installation contains new or non-conventional design features, more extensive in-flight restart envelope testing may be necessary. This is particularly the case when the flight conditions impact the ability to restart the engine in flight. For example, consider the effects of new or unusual engine designs or operating cycles, propeller feathering systems, engine control systems, fuels or fuel system arrangements, engine starting systems, etc. Any in-flight restart envelope limitations and/or starting procedures should be defined in the proper areas of the Airplane Flight Manual (AFM). This information is required for compliance with § 23.1585.
As with conventional designs, extensive in-flight restart envelope testing may not be required provided the applicant can substantiate this position on the new or nonconventional design features. Flight tests are generally the most appropriate means to develop this substantiating data.
2. Airplanes powered by turbine engines.
Turbine engine starting is sensitive to small differences in flight conditions, primarily pressure altitude, speed, temperature, and attitude. Therefore, it is necessary to test the engine restart envelope to ensure that turboprop or turbofan engines can be reliably restarted in flight. These flight conditions and the associated starting procedures (windmill, starter assisted, or otherwise) should be defined in the proper areas of the Airplane Flight Manual (AFM). This information is required for compliance with § 23.1585.
Paragraph 23.903(g)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
Related Material
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes 16