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
23.904 Automatic power reserve system (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.905 Propellers (Amendment 23-43)
CAR 3 Cross-reference: 3.416
ADC Cross-reference: 5.3
Related Material:
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
PS-ACE100-2002-008, Propeller Testing Vd Versus VNE
Paragraph 23.905(a)
ADC Cross-reference: 5.3(a)(1)
What regulations cover propeller certification?
Propeller certification is covered under 14 CFR part 35. These rules set the minimum requirements for propellers in a manner similar to that for engines. Installation information from the propeller manufacturer is used to comply with § 23.901(e)(1).
Do the rules in 14 CFR part 35 cover propeller blades that are constructed of composite material?
The rules in 14 CFR part 35 were written to cover wooden and metal propeller blades, and do not, by themselves, address all issues unique to blades constructed of composite materials. The following airworthiness issues are typically addressed with special conditions, according to § 11.19, as part of the propeller certification under part 35.
1. Safety assessment
2. Centrifugal load tests
3. Fatigue limits and evaluation
4. Bird impact
5. Lightning strike
Paragraph 23.905(b)
ADC Cross-reference: 5.3(a)(2)
What should be considered when determining compliance with this rule?
Evaluate the maximum propeller power and shaft rotational speed for each airplane installation throughout its normal operational envelope. The installed limits must be at or 18
below the certificated propeller and engine power and rotational speed limits (listed on the propeller and engine type certificate data sheets) that are typically based on static ground testing performed before flight test.
Engine power and propeller shaft rotational speed limits must not be exceeded during transient events.
14 CFR part 23, § 23.33, contains additional requirements pertaining to the installed propeller speed and pitch limits.
Paragraph 23.905(c)
ADC Cross-reference: 5.3(a)(3)
No specific policy is available as of December 31, 2002.
Paragraph 23.905(d)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
How do I know if a propeller blade pitch control system has been approved for use with my propeller?
The propeller Type Certificate Data Sheet (TCDS) identifies operationally compatible components of the propeller blade pitch control system that may or may not be included in the propeller type design. If the propeller blade pitch control system components are included in the propeller type design (verify this with the propeller manufacturer or the FAA office controlling the propeller type design), the components have demonstrated compliance with 14 CFR part 35, § 35.42, Blade pitch control system component test.
How do I get approval for a propeller blade pitch control system if it has not been approved as part of the propeller type certificate?
The propeller manufacturer may not have tested components that are not included in the propeller type design according to § 35.42. For components that have not been previously tested, demonstrate compliance with § 35.42 either by testing or by similarity to prior approved components.
Paragraph 23.905(e)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Is an airplane required to comply with § 23.905(e) if it is not being certificated to fly in icing conditions?
Yes, compliance with § 23.905(e) for pusher propeller airframe icing protection is required for all normal, utility, acrobatic, and commuter category airplanes certificated under part 23 even if the airplane is not being certificated for flight in icing conditions (§ 23.1419 compliance). Service experience has shown that airplanes can inadvertently encounter icing conditions even if the airplane is not approved for flight into icing conditions.
What type of icing encounter should be considered when showing compliance with § 23.905(e) if the airplane is not being certified to fly in icing conditions?
In the case where the airplane is not being certificated to fly in icing conditions, compliance with § 23.905(e) should be shown for an inadvertent encounter with icing conditions. Ice accumulations and shapes should be based on a typical inadvertent encounter of five minutes duration. The five minutes in the icing condition is broken down as follows:
1. 1 minute for detection of the icing condition.
2. 1 minute for the pilot to take actions necessary to exit the icing condition.
3. 1 minute to execute a 180-degree turn.
4. 2 minutes to exit the icing condition.
Does the type certification of a propeller account for any foreign object damage requirements?
Propeller type certification does not include any type of foreign object damage requirements, unless accomplished with special conditions or special tests.
What foreign object damage concerns are unique for pusher propeller installations?
Most propeller installations are subject to ice being shed into them. The source of the ice for tractor propeller installations is the ice accumulated and shed from the propeller spinner. Typically, for tractor propellers, the amount of ice results in no significant propeller damage. Airplanes with pusher propellers that are very close to the fuselage and well back from the front of the airplane's nose can be subject to ice shed from the forward fuselage and from the wings. The ice from these areas may cause significant propeller damage.
Related Material:
AC 23.1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions