Paragraph 23.905(f)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Why are pusher propellers required to have visible markings and tractor propellers are not?
Visibility of the propeller disk is of concern during ground operations. Because ground personnel and boarding and deplaning passengers are accustomed to tractor propellers, there is a significantly higher probability of inadvertent contact with a turning pusher propeller. Increased visibility of tractor propellers could also enhance ground operations safety. However, any additional visual distractions in the pilot's normal viewing area could adversely affect collision avoidance.
Paragraph 23.905(g)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Do the requirements for propeller certification cover engine exhaust impingement?
Propeller type certification does not include any type of engine exhaust requirements, unless accomplished with special conditions or special tests.
What are the concerns for engine exhaust impingement on pusher propeller installations?
In most pusher propeller installations, the engine exhaust pass through the propeller disc.
The affects of the exhaust temperatures and composition on the materials of the propeller are of major concern. Factors affecting the exhaust vary with the flight conditions and must be considered relative the propeller’s ability to withstand them.
Paragraph 23.905(h)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What other concerns are there for pusher propeller installations?
Similar to the concerns with ice in § 23.905(e), there is increased potential of foreign object damage because the propeller is aft of major portions of the airplane. Therefore, 21
the design of parts like those used to meet § 23.901(c) are required to ensure that they will not separate from the airplane such that they could contact the propeller.
23.907 Propeller vibration (Amendment 23-51)
CAR 3 Cross-reference: 3.417
ADC Cross-reference: 5.3
What should I consider when showing compliance with this rule?
For acceptable means of compliance, refer to AC 20-66A, Vibration and Fatigue Evaluation of Airplane Propellers.
The following applies to all propellers except conventional fixed-pitch wood propellers:
1. The most important part of approving a propeller installation is to demonstrate that the propeller steady and vibration stresses do not exceed the safe levels established by the propeller manufacturer. It is often beneficial to use approved propeller type design data (contact the propeller manufacturer) to approve a propeller for use on an aircraft.
2. Some propeller TC data sheets contain a section called Note 9. This note simplifies the vibration compliance for propellers under § 23.907 for normal category, single reciprocating engine airplanes in tractor configuration. The acceptable engine and propeller model combinations are listed in this note, and they can be used to determine compliance within the defined limitations. If the engine or airframe is modified such that the power of the stock engine model is increased during any phase of airplane operation, the potential exists to increase propeller stresses. Therefore, the Certificate Management Aircraft Certification Office (CMACO) should be contacted prior to using Note 9 as approval for § 23.907. Such modifications include, but are not limited to, the addition of a turbocharger, increased boost pressure, increased compression ratio, increased RPM, altered ignition timing, electronic ignition, full authority digital engine controls (FADEC), or tuned induction or exhaust.
3. Perform a flight-vibration stress survey. This test typically consists of placing instruments on the propeller hub, shanks, and blades with strain gauges and operating the airplane both on the ground and in flight to verify that the stress and vibration levels are within those certificated for the propeller.
Generally, reciprocating engines are the dominant contributor to any steady and vibration stress levels the propeller will experience in operation, but there are exceptions. For example, agricultural and acrobatic category airplane maneuvers may exceed the stress levels the engine produces during such operation. Additional testing may be needed to show compliance with § 23.907 for special use or unique 23
aircraft categories. Turbopropeller installations have stress levels that are generally driven by aerodynamic forces during ground and flight operations.
The stress survey should identify any operational and maintenance limitations to note in either the airplane Instructions for Continued Airworthiness or the Airplane Flight Manual (AFM), or both. These limitations may also need to be incorporated into the propeller airworthiness limitations section. According to § 23.1549, operational limitations are to be made available to the crew by the use of appropriate placards and powerplant instrument markings (i.e., red and yellow arcs).
4. The vibratory stress levels in pusher propeller installations can be much higher than on tractor aircraft configurations. Any small change in the airplane configuration can influence the propeller stress levels, especially if the change is upstream and in the propeller inflow area. Engine exhaust locations and nozzle shapes can impact propeller-operating temperatures. Engine exhaust flows that impinge on the propeller blades can introduce potential corrosion issues and affect propeller blade life. Small changes to the propeller spinner shape and size can change the inboard propeller blade and hub operating temperatures. For these reasons, perform the vibration survey under the same caustic and high temperature environmental conditions identified in § 23.905(g).
5. The Airplane Instructions for Continued Airworthiness (ICA) should require regular calibration of the tachometer to ensure that the limits are not exceeded.
Related Material:
AC 20-88A, Guidelines on the Marking of Aircraft Powerplant Instruments (Displays)
AC 20-66A, Vibration and Fatigue Evaluation of Airplane Propellers
AC 35.37-1A, Guidance Material for Fatigue Limit Tests and Composite Blade Fatigue Substantiation
Paragraph 23.907(a)
ADC Cross-reference: 5.3(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.907(b)
ADC Cross-reference: 5.3(b)(4)
No specific policy is available as of December 31, 2002.
23.909 Turbocharger systems (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.5
What paragraphs of § 23.909 apply to a turbocharger installation that is not certificated as part of the engine under part 33?
Apply § 23.909(a) through (e) to a turbocharger system (or portions of the system) that is not certificated as part of the engine under part 33.
What paragraphs of § 23.909 apply to a turbocharger installation that is certificated as part of the engine under part 33?
Apply § 23.909(b) through (e) to a turbocharger system (or portions of the system) that is certificated as part of the engine under part 33.
Related Material:
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
AC 23.1521-2 with Change 1, Type Certification of Oxygenates and Oxygenated Gasoline Fuels in Part 23 Airplanes with Reciprocating Engines
AC 33-2B, Aircraft Engine Type Certification Handbook
AC 33.47-1, Detonation Testing in Reciprocating Aircraft Engines
What are the definitions of some of the common terms used relative to turbocharger systems?
Turbocharger System
Includes the following:
1. The turbocharger;
2. All components necessary to control the turbocharger and the associated engine/installation connections;
3. All ducting, heat exchangers, and other plumbing between the turbocharger to the engine intake and exhaust interface points;
4. All plumbing and control devices between the turbocharger and the aircraft; and
5. Mounting provisions for these components.
Turbocharger
Any engine exhaust gas-driven compressor that is a component of one or more of the following systems:
1. Turbonormalized Engine: Turbocharger used to maintain sea level manifold pressure to a critical altitude.
2. Turbosupercharged Engine: Turbocharger used to provide and maintain a manifold pressure greater than sea level pressure to a critical altitude.
3. Cabin Pressurization Systems: Turbocharger used to pressurize air for airframe services.
Intercooler
A device installed in the engine air induction system that lowers the intake air charge temperature. The intercooler can be approved either as part of the engine or the engine installation.
Paragraph 23.909(a)
ADC Cross-reference: 5.5(a)
What engine affects should be considered when adding a turbocharger to a normally aspirated engine?
Adding a turbocharger to a normally aspirated engine will have the following affects on the engine.
1. A turbocharged engine will have different endurance and reliability characteristics than the non-turbocharged engine.
2. A turbocharged engine will have different overhaul characteristics and intervals than the non-turbocharged engine.
3. A turbocharged engine will have different detonation characteristics than the nonturbocharged engines. Detonation margin is affected by the following:
a. Induction air temperature and pressure;
b. Exhaust back pressure;
c. Ignition timing;
d. Fuel grade and flow;
e. Carburetor or injector metering characteristics;
f. Compression ratio;
g. Cooling; and
h. Exhaust gas temperature.
4. A turbocharger has high speed rotating components similar to a turbine engine.
Consider the effects of low-cycle fatigue on turbocharger rotor integrity and durability. The installation should be designed to minimize the effects of an uncontained turbocharger rotor burst event.
5. Expect higher pressures and temperatures with a turbocharged engine induction system due to heat generated from adiabatic compression. An intercooler between the turbocharger and the engine will lower the induction air temperature.
6. Expect higher rates of oil consumption, contamination, and deterioration due to the more severe operating environment. These items may shorten oil change intervals.
7. Expect higher flow rates of hot exhaust gas with a turbocharged engine’s exhaust system, which will increase the temperature of the metal and raise internal pressure.
Consequently, the durability of the exhaust system components is important.
Increased or mandatory inspections of the exhaust system components may be necessary.
8. Both the engine and propeller vibration characteristics of the turbocharged engine installation might be altered by changes in horsepower and by changes in horsepower-to-altitude relations. Readdress the propeller vibration compatibility according to § 23.907 or determine if propeller recertification is necessary.
What are the typical means of compliance with § 23.909(a)?
Turbochargers certificated as part of the engine may use the findings of compliance from the engine certification.
Turbochargers that have not been certificated as part of the engine require analysis and testing. Endurance tests should be conducted to meet the reliability standards established by the 150-hour endurance test under § 33.49 for the engine and turbocharger combination. In addition, the engine and turbocharger combination should be tested to determine the effects on the service life of the engine and engine components as required 27
by § 33.7. Any component of the engine and turbocharger installation not substantiated by endurance testing as capable of operating in all normally anticipated flight and atmospheric conditions should be subjected to additional tests or investigations to substantiate this capability as required by § 33.53.
Affects of low-cycle fatigue on turbocharger rotor integrity and durability: A rotor without prior service history should be subjected to a 1,000-cycle start-stop test under the most adverse operating conditions to provide a basis for establishing initial service life.
The applicant should provide instructions for the installation, operation, service, maintenance, repair, and overhaul of the turbocharger and related items as required by § 33.4, Instructions for continued airworthiness, and § 33.5, Instruction manual for installing and operating the engine. Particular attention should be given to component life limits for both the engine and turbocharger.
The exhaust system and supports should be capable of withstanding all vibration and inertia loads at the higher temperatures and pressures to which they are subjected by the turbocharger installation as required by § 23.1123, Exhaust system. Tests or investigations, as necessary, should be conducted to substantiate that the exhaust system meets the standards of airworthiness established by the basic engine 150-hour endurance test as required by § 33.49.
Turbocharger mounts and supporting structure should withstand all vibration and inertia loads to which they are subjected in operation as required by § 23.307, Proof of structure.
Conduct calibration tests and investigations to establish the sea level and altitude power characteristics of the engine and turbocharger combination.
Conduct flight tests to demonstrate that the engine operates throughout its anticipated operating range without exceeding detonation limits at limit temperatures and fuel flow, using the minimum-grade fuel. If necessary, establish the minimum fuel flow required for proper operation. Also, consider the limitations specified in the installer's instructions and as required by § 33.47, Detonation test.
Turbocharger and engine mount provisions should have sufficient strength to withstand loads arising from the conditions prescribed in § 33.23, Engine mounting attachments and structure. Because of relative motion and fire concerns in the event of a failure, experience has shown that turbochargers should not be remotely mounted on the firewall.
Paragraph 23.909(b)
ADC Cross-reference: 5.5(b)
What should be considered when certificating the turbocharger control system?
A turbocharged engine requires additional systems necessary to control the turbocharger, such as the wastegate and wastegate controller. The effects of cold oil on the operating characteristics of the turbocharger wastegate and any oil-operated flow control devices should be considered when certificating the turbocharger control system.
The design of the turbocharger control devices should be such that any probable failure does not result in engine or turbocharger limits being exceeded, which should preclude damage to the turbocharger compressor or turbine.
Guidance for electronic wastegate controllers is contained in AC 33.28-2.
Related Material
AC 33.28-2, Guidance Material for 14 CFR § 33.28, Reciprocating Engines, Electrical and Electronic Engine Control Systems
Paragraph 23.909(c)
ADC Cross-reference: 5.5(c)
What guidance is available when determining compliance with § 23.909(c)?
A turbocharger has high speed rotating components similar to a turbine engine. Consider the possibility of damage from turbine or compressor rotor failure.
If the turbocharger case alone has not demonstrated compliance with § 23.909(c), then the installer may provide additional containment shielding to preclude damage due to rotor failure.
No additional containment shielding is required for turbochargers approved with the engine provided the following conditions are both met. Information concerning the testing for part 33 certification should be obtained from the engine manufacturer.
1. Containment has been demonstrated during part 33 certification.
2. The containment testing was performed at operating conditions appropriate to the airplane and its engine installation.
Related Material
AC 20-128A, Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure
Paragraph 23.909(d)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What affect does a turbocharger have on the engine induction air temperature at altitude?
A turbocharged engine has higher induction air temperatures at altitude because of the increased compression required to maintain engine manifold pressure. Turbocharger speed and compressor discharge temperature increase up to critical altitude.
What should be considered relative to the turbocharger system discharge?
Turbochargers on engines with carburetors produce combustible mixtures (at elevated pressure and temperature) in portions of the induction system. Induction system leaks should be minimized. Consider the relation of the turbocharger to the point of fuel introduction into the induction system. For turbocharged engines with carburetors, a combustible mixture at elevated pressure and temperature exists in portions of the induction system and should require special precautions. Consider that engine backfire combined with combustible mixtures in the induction system might produce fire in the system.
What methods of compliance are typical for § 23.909(d)?
Demonstrate the strength of the intercooler and mounting provisions as required by § 33.23.
Evaluate the intercooler installation for adequacy of:
1. The intercooler and mounting provisions;
2. Vibration response;
3. Engine cooling;
4. Additional instrumentation required for the intercooler to ensure that the engine ratings are not exceeded; and
5. Any cowling or airframe protrusions located in front of the firewall for fire resistance.
Demonstrate that the induction system can withstand pressure differentials (positive and negative) and temperature levels resulting from the turbocharger under the most critical operating conditions expected.
Determine the effect of turbocharger installation on ice accretion characteristics of the induction system according to § 33.35(b). The induction system should meet the requirements of § 23.1091 and incorporate means to prevent and eliminate ice accumulation within the engine and turbocharger induction system, as required by § 23.1093. Associated heat rise requirements should also be evaluated.
Paragraph 23.909(e)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What methods of compliance are typical for § 23.909(e)?
It is difficult to thermodynamically match a turbocharger to an engine by analysis alone.
A combination of test data and analysis is required for performance predictions and correction factors.
Power (Performance)
For airplanes required to comply with the performance requirements of § 23.45, Amendment 23-21, establish the actual calibrated power available at all altitudes. If performance information is required by § 23.45, Amendment 23-50, determine the actual engine power at altitude.
Investigate each engine installation at all operating points to ensure engine power output at the specified manifold pressure.
On intercooled installations, evaluate the effects the intercooler has on the engineapproved limits. These installations may require redefining the engine ratings. For example, increased engine horsepower output at the same manifold pressure and engine speed would require a method of limiting engine horsepower.
The Maintenance Manual should identify any turbocharger control system calibration checks required to ensure that the engine is maintaining the approved power output. This is especially important on installations with automatic turbocharger control systems.
Cooling
If turbocharger lubrication is provided by the engine oil system, the capacity of that system should be adequate to serve the engine and turbocharger under the most adverse combination of oil temperature and bearing clearances.
External lines and fittings containing flammable fluids as part of the turbocharger installation should comply with §§ 23.1017 and 23.863.
If the turbocharger oil system is self-contained, the capacity should be adequate to provide lubrication and cooling under the most adverse conditions of turbocharger oil temperature and bearing clearance.
Powerplant cooling provisions should be capable of maintaining the temperature of all powerplant components, parts, and fluids at or below the maximum established safe values under all ground and flight operating conditions as required by § 23.1047.
All portions of the engine exhaust system and turbocharger should be isolated from lines or components carrying flammable fluids, the accessory section, and primary structure by firewalls and shrouds, or by adequate separation as required by § 23.1121. Where required, adequate cooling air should be provided so that there is no adverse change in service life or failure rate of adjacent components.
Any ducting that bypasses the firewall and/or cowling should be fireproof.
Operating Limits
Define all operating limitations or instructions in the AFM and revise placards as required.
Demonstrate, using an engine conforming to type design data, that the approved power is not exceeded at any altitude.
Demonstrate through test that engine and turbocharger limits are not exceeded under any intended operating condition. In particular, establish that the turbocharger is operating within its speed and turbine inlet temperature limits.
Establish turbocharger oil temperature and pressure limits according to § 33.39, Lubrication system.
Specify temperature limits for any components requiring special cooling as required by § 33.21, Engine cooling.
Determine whether propeller blade, crankshaft, and prop shaft stresses are within acceptable limits. The applicant should readdress the propeller vibration compatibility according to § 23.907. Additionally, propeller recertification may be required.
Demonstrate compliance with § 23.955 if the maximum fuel flow has increased or the operating condition where maximum fuel flow is required has changed.
Operating Procedures
Demonstrate compliance with § 23.939(b), which includes a demonstration that the engine installation is free from compressor surge at all operating points.