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.
Investigate the in-flight restart characteristics of the turbocharged engine. Demonstrate engine in-flight restart procedures.
The fuel system should be free from vapor lock throughout the flight envelope as required by § 23.961.
Because additional moisture might be introduced into the engine crankcase, demonstrate that the engine crankcase breather is adequately protected from ice blockage.
If the turbocharger provides airplane cabin pressurization, a failure of the installation should not contaminate the cabin air. Refer to § 23.831 for cabin Carbon Monoxide (CO) contamination levels and § 23.1121(a) for design requirements to prevent engine exhaust leaks that would allow CO to enter into the cabin area.
Evaluate the propeller-engine-airplane vibration characteristics at altitude to provide continued compliance with § 23.907. Additionally, demonstrate that the engine crankshaft torsion characteristics are within the original engine certificated limits or other safe operational limits.
If the turbocharger installation can affect the windshield defogging system, demonstrate continued compliance with § 23.773(b).
What engine cooling conditions should be considered for a turbocharged engine?
The peak temperature for a turbocharged engine normally occurs at maximum rated power at the altitude above which maximum rated power cannot be maintained. This altitude is called the critical altitude. Cooling air is less effective at altitude due to its lower density. As a general rule, design the engine cooling capacity for the sea level power output, then check the flight envelope cooling capacity at various altitudes. The applicant should investigate enough of the operating envelope to ensure that engine cooling is adequate. The simplest means to determine compliance is by performing a cooling climb test.
What should be considered relative to the turbocharger effect on the engine fuel system?
The increased heating of the induction system for turbocharged engines can affect the vapor lock potential for the engine. In addition, diaphragm-type fuel pumps might be inadequate to supply fuel to the turbocharged engine under all operating conditions. It might become necessary to add a fuel boost pump or make other changes to the pumping system to ensure freedom from vapor lock. The critical conditions should be determined by test, and acceptable fuel system vapor lock characteristics should be demonstrated under all desired operating conditions.
What should be considered if the turbocharger is used to supply pressurized cabin air?
The possibility of contamination of the cabin air during normal operation and after a failure in the system should be considered.
Can the turbocharger lubrication system use pressurized engine oil?
Turbocharger lubrication systems may be self-contained or require oil from an external source. If pressurized engine oil is used, the following effects should be considered:
1. The additional oil requirements should not exceed the capacity of the engine oil pump.
2. Heat rejected by the turbocharger to the oil should not exceed the engine oil system cooling provisions.
3. Air entrained in the turbocharger scavenge oil should not exceed the capability of the engine breather or scavenge systems.
What should be considered relative to turbocharger failures?
The operating procedures to minimize the potential hazards relating to fire and/or loss of engine power due to a turbocharger failure should be specified in the AFM. In particular, the in-flight restart characteristics of the engine with a failed turbocharger should be considered.
23.925 Propeller clearance (Amendment 23-51)
CAR 3 Cross-reference: 3.422
ADC Cross-reference: 5.3(c)
Paragraph 23.925(a)
ADC Cross-reference: 5.3(c)
What is the basis for the minimum clearances specified for ground clearance?
The clearances of seven and nine inches have been in the rules since the 1930’s and are probably based on experience. The primary purpose of these minimums is to provide enough clearance to limit the amount of foreign object damage from debris kicked up from the runway surfaces by the propeller wash and no propeller strike probability. The rule sets out the minimums as well as the airplane and propeller configurations to limit the number of potential test conditions required to demonstrate compliance.
What is the purpose of the positive clearance at the end of the rule if the minimums are met?
The minimums are for normal operational conditions whereas the positive clearance portion of the rule applies with the critical tire deflated and deflected struts. This portion of the rule is used to ensure that in such a condition the propeller will not strike the runway surface. Although tire is singular, the intent of the rule is that the airplane be in the most critical condition necessary to prevent a propeller strike. Thus, for airplanes with more than one critical tire, the interpretation is that tire could be plural.
Paragraph 23.925(b)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
If the positive ground clearance from § 23.925(a) is met, why are there additional requirements for aft-mounted propellers?
The positive ground clearance in § 23.925(a) is for a condition with a deflated tire, whereas this rule is used for normal takeoffs and landings. Propellers mounted aft of the center of gravity of the airplane will be naturally closer to the ground with the airplane nose pitched up as for takeoff and landing. Thus, this rule is used to ensure that aftmounted propellers will not contact the runway surface during these conditions.
Compliance with this paragraph is required in addition to § 23.925(a).
Paragraph 23.925(c)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What is the basis for the minimum clearances specified for water clearance?
The eighteen-inch clearance has been in the rules since the 1930’s and is probably based on experience. The primary purpose of these minimums is to provide enough clearance to limit the spray kicked up from the water by the propeller wash. The rule sets out the minimum to limit the number of potential test conditions required to demonstrate compliance with § 23.925(c) and § 23.239.
Paragraph 23.925(d)
ADC Cross-reference: 5.3(c)(2)
What is the basis for the minimum clearances specified for structural clearance?
These minimum clearances have been in the rules since the 1930’s and are probably based on experience. The purpose of these minimums is to provide enough clearance to prevent the propeller from striking any other part of the airplane during operations and to limit propeller induced fuselage fatigue. These are considered the minimums, but care should be taken to identify any propeller or structural vibration modes that occur during operations that may lead to propeller strikes.
23.929 Engine installation ice protection (Amendment 23-51)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.4
What components of the engine installation does this apply to?
This section applies to propellers, and cooling inlets for oil, generators and other components, when the applicant has requested certification with the ice protection provisions of § 23.1419. This section does not apply to engine induction system icing protection, which is covered under § 23.1093.
Who is responsible for engine installation ice protection certification?
Ice protection is the responsibility of the type certificate applicant. Propeller type certification requirements do not include any type of propeller ice protection requirements, unless accomplished with special conditions or special tests. In most instances, the propeller ice protection components are not included in the propeller type design.
Since the ice protection equipment is normally mounted on the propeller, include the propeller manufacturer in the process of assuring that the installation or failure of ice protection equipment will not harm the propeller. For example, an electrical short on a deicing boot could burn a hole in a composite blade or locally change the heat treatment of an aluminum blade. The resulting damage could make the propeller un-airworthy.
Can a deicing system that is certificated as part of the propeller be used to show compliance with § 23.929?
Deicing systems that are certificated according to the requirements of 14 CFR part 35 do not fully meet the requirements of § 23.929. Part 35 requirements address only propeller structural and vibration characteristics and require that the system functions. System failure modes are not addressed with part 35 certification of deicing systems.
The typical analysis report from the deicing boot manufacturer is not sufficient by itself to show compliance to § 23.929. The typical report calculates intercycle ice thicknesses for various flight and icing conditions, but it does not calculate the effect on propeller efficiency, which must be done to show no appreciable loss of thrust. It would be acceptable to show that intercycle ice is equal to or less than the accretions obtained on the same propeller on an airplane that was flight-tested in icing conditions and shown to have no appreciable loss in thrust.
The typical deicing boot manufacturer report also contains a caveat that it does not address propeller runback ice. Similarity to another propeller that was flight tested in icing conditions is usually done to address runback. Similarity would include propeller and deicing boot aerodynamic and thermal similarity, deicing timer, propeller RPM, and flight conditions. Note that metal and composite propellers have different thermal masses.
As a final qualitative check for both intercycle and runback ice on new airplane programs, airplane performance is checked during flight tests in icing conditions.
Related Material
AC 20-73, Aircraft Ice Protection
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
AC 23-1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions
23.933 Reversing systems (Amendment 23-51)
CAR 3 Cross-reference: None
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What are the origins of this rule?
This rule reads exactly the same as § 25.933, which was developed in response to uncommanded, in-flight deployments of thrust reversing systems in turbine powered transport airplanes.
Paragraph 23.933(a)
If the reversing system can be restored to forward thrust, why must the airplane be capable of continued safe flight with the reverser in any possible position?
During uncommanded reversing system deployment, there is a possibility of damage to the system due to the dynamic nature of the deployment that might prevent the system from being restored to forward thrust. The nature of this damage is considered unpredictable, and it is necessary that the airplane be capable of continued safe flight in such an event.
Paragraph 23.933(b)
Is propeller reversing system certification covered as part of the propeller certification?
The requirement of 14 CFR part 35, § 35.21, applies to components of the propeller reversing system. If the reversing system components are included in the propeller type design (this can be verified with the propeller manufacturer or the FAA office controlling the propeller type design), the components have demonstrated compliance with § 35.21.
The compliance data used to certify the propeller may be used to support the demonstration of compliance with § 23.933(b).
Components that are not included in the propeller type design may or may not have demonstrated compliance with § 35.21. Those components that have not been certificated as part of the propeller are covered under and must show compliance with § 23.933(b).
Are there other rules in 14 CFR part 23 that are related to § 23.933?
Requirements related to the operation of propeller reversing systems are contained in 14 CFR part 23, § 23.1155.
Related Material
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes