23.934 Turbojet and turbofan engine thrust reverser systems tests (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Are there other rules in 14 CFR part 23 that are related to § 23.934?
Requirements related to the operation of thrust reverser systems are contained in 14 CFR part 23, § 23.1155.
Related Material
AC 20-18A, Qualification Testing of Turbojet Engine Thrust Reversers
23.937 Turbopropeller-drag limiting systems (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.6
Paragraph 23.937(a)
Is a failure probability analysis an acceptable means of showing compliance to § 23.937?
A failure probability analysis may be used to show compliance with this paragraph.
Show that the probability of an accident occurring because of any failure of a propeller drag limiting system (either automatic or manual) is extremely improbable.
Related Material
AC 23.1309-1C, Equipment, Systems, and Installations in Part 23 Airplanes
Paragraph 23.937(b)
Is there any policy available for compliance with § 23.937(b)?
This paragraph was added at Amendment 23-43 in 1993 to define a propeller drag limiting system as manual or automatic devices that, when actuated after engine power loss, can move the propeller blades toward the feather position to reduce windmilling drag to a safe level. This does not include negative torque sensing systems. Prior to this, there was confusion as to whether these systems included negative torque sensing systems or only propeller feathering systems.
23.939 Powerplant operating characteristics (Amendment 23-42)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.7
Related Material
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
AC 25.939-1, Evaluating Turbine Engine Operating Characteristics
Paragraph 23.939(a)
ADC Cross-reference: 5.7(a)
Is it possible to extrapolate engine operability compliance for airfields at higher elevations than those tested when showing compliance with § 23.939(a)?
There currently are no extrapolation procedures for showing compliance with the requirements of § 23.939(a) for airplanes certificated to part 23. Traditionally, testing up to the field elevation for which the airplane is to be approved has been used to show compliance. This is the most desirable method since it does not require parameter variation analysis to account for the higher airfield elevations. Extrapolations have been used for part 25 airplanes, and the same considerations may be used for part 23 airplanes.
When considering extrapolation for compliance with § 23.939(a), data must be presented that substantiates the proposed extrapolation procedures. This data should include supportive flight test and engine operations data. The applicant should determine, understand, and substantiate the critical operating modes of the engine including a determination and quantification of the effects the installation and variations in ambient conditions have on these modes. Advisory Circular 25.939-1 may be used for turbine engines to determine and evaluate the engine critical operating modes. If the proposal is appropriately substantiated, the extrapolation of the engine data for compliance with the operability requirements of § 23.939(a) may be performed up to a maximum of 3,000 feet above the airfield elevation tested.
Paragraph 23.939(b)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
Paragraph 23.939(c)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.943 Negative acceleration (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What methods of compliance are acceptable?
This requirement can be satisfied by flight test demonstrations that consider critical airplane, engine, and APU configurations (for an APU approved for use in flight).
Guidance concerning these tests is included in AC 23-8B. The durations of the tests specified in Chapter 196 of AC 23-8B are intended to represent anticipated abnormal operational events such as atmospheric upsets and collision avoidance maneuvers. Fuel, oil lubrication, and electrical systems are typically monitored during these tests. Fuel tanks should not be more than half full to allow room for fuel to lift and uncover fuel line inlets and boost pump pickups during the negative accelerations.
Related Material
AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes
FUEL SYSTEM
23.951 Fuel System General (Amendment 23-43)
CAR 3 Cross-reference: 3.429, 3.430
ADC Cross-reference: 5.8
Related Material
AC 23-10, Auxiliary Fuel Systems for Reciprocating and Turbine Powered 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
Paragraph 23.951(a)
ADC Cross-reference: 5.8(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.951(b)
ADC Cross-reference: 5.8(b)
Under what conditions can fuel be drawn from more than one tank at a time?
Section 23.951(b) allows fuel (for the purpose of direct engine feed) to be drawn from more than one tank at a time provided:
1. There are means to prevent introducing air into the fuel system; and
2. The pilot has a means to draw fuel from a single tank.
What are some of the considerations for such systems?
Fuel systems that supply engine fuel from more than one tank at a time do not consume fuel at the same rate from both tanks. This is attributable to differences in vents, filter condition, line losses, pumps, fuel return flow, fuel consumption rates (multiengine airplanes), etc. It is probable that one tank will run out of usable fuel while there is still usable fuel on board. To prevent air being drawn into the engine fuel supply line, use a method to seal the fuel tank outlets whenever the tank is out of usable fuel.
Substantiation tests are required for systems that draw fuel from more than one tank demonstrating that air is not drawn into the fuel system.
A series of fuel tanks can be interconnected to function as a single tank. For example, two left-handed wing tanks using gravity to feed from one to the other with interconnected vent lines function as a single tank provided that fuel is drawn from only one location. For the purposes of this discussion, these tanks may be treated as a single tank.
Interconnecting the vent lines on two independent tanks (i.e., left-hand and right-hand wing tanks feeding to a common fuel selector valve) does not cause the tanks to function as a single tank. As above, this configuration may allow air into the fuel system when either independent tank has consumed its usable fuel unless there is a means to prevent the introduction of air.
(Many in-service airplanes were certified under the Civil Air Regulations (CAR). For background, CAR § 3.430, which preceded § 23.951(b), required the fuel system arrangement to “permit any one fuel pump to draw fuel from only one tank at a time.” In the past, this has sometimes been interpreted (incorrectly) as “a fuel pump may only draw fuel from one tank at a time.” The CAR regulation is actually more permissive than § 23.951(b) as it did not address the possibility of introducing air into the fuel system.)
Paragraph 23.951(c)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Is the use of fuel anti-icing additives an acceptable means of compliance for § 23.951(c)?
Yes, the fuel icing requirements of § 23.951(c) may be satisfied by using the anti-icing additives as specified in AC 20-29B, Use of Aircraft Fuel Anti-icing Additives.
Related Material
AC 20-29B, Use of Aircraft Fuel Anti-icing Additives
Paragraph 23.951(d)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Is this requirement covered under § 23.903(a)(1)?
The portion of the fuel system that is certificated with the engine is covered under § 23.903(a)(1). The remainder of the airplane fuel system, including the tanks, pumps, fuel lines, and firewall shutoff valves, is covered under § 23.951(b).
23.953 Fuel system independence (Amendment 23-43)
CAR 3 Cross-reference: 3.431
ADC Cross-reference: 5.9
What parts of the fuel supply system should be considered when assessing its independence for each engine?
The independence of a fuel supply system for each engine should be assessed starting at the fuel supply pickup point within the tank and continue to the fuel inlet at the engine.
Related Material
AC 23-10, Auxiliary Fuel Systems for Reciprocating and Turbine Powered Part 23 Airplanes
Paragraph 23.953(a)
ADC Cross-reference: 5.9(a)
What is the intent of requiring no immediate action by the pilot to prevent the loss of thrust to more than one engine?
In the event a multiengine airplane experiences a loss of thrust on one engine as a result of a fuel system failure, the pilot will be occupied with flying the airplane in the new thrust configuration, as well as trouble shooting the problem engine. The intent of § 23.953(a) is that the system should be designed such that, in this event, the remaining engine(s) will not be affected by the failure so the pilot will not be required to deal with another engine thrust loss during this time of increased workload.
Paragraph 23.953(b)
ADC Cross-reference: 5.9(b)
No specific policy is available as of December 31, 2002.
23.954 Fuel system lightning protection (Amendment 23-7)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.10
Is it possible to comply with § 23.954 with operational limitations only?
Operational limitations only are not acceptable means of compliance for standard category airplanes. However, this has been allowed for special purpose, restricted category, single-engine agricultural airplanes using the normal category requirements of part 23 as a certification basis. A limitation in the Airplane Flight Manual, such as “Operation of the airplane in and around thunderstorms is prohibited,” has been determined to be sufficient to address this regulation for these non-standard category airplanes.
Related Material
AC 20-53A, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning
AC 23-15, Small Airplane Certification Compliance Program
AC 23.1521-2 with Change 1, Type Certification of Oxygenates and Oxygenated Gasoline Fuels in Part 23 Airplanes with Reciprocating Engines
Paragraph 23.954(a)
ADC Cross-reference: 5.10(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.954(b)
ADC Cross-reference: 5.10(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.954(c)
ADC Cross-reference: 5.10(c)
No specific policy is available as of December 31, 2002.
23.955 Fuel flow (Amendment 23-51)
CAR 3 Cross-reference: 3.433, 3.434, 3.435, 3.436, 3.437(e)
ADC Cross-reference: 5.11
Paragraph 23.955(a)
ADC Cross-reference: 5.11(a)
What are the acceptable means for showing compliance with § 23.955(a)?
Demonstrate fuel-flow characteristics with the entire fuel system installed in the airplane during the airplane type certification program. A suitable mock-up of the fuel system can also be used to demonstrate fuel-flow capacity. Analysis may be used to reduce the tests required, provided the analysis has been validated through appropriate tests.
Some applicants may modify their airplanes in such a way as to affect the original fuel system. Changes in the type of fuel used are also considered a change to the fuel system.
In these cases, additional fuel-flow testing may be required to substantiate continued compliance of the fuel system with applicable regulations and engine manufacturer's requirements.
In-service fuel that has the correct physical properties and octane ratings should flow at the rate specified by regulation and at the pressure established in compliance with 14 CFR part 33, § 33.7, Engine ratings and operating limitations. This information is provided in the engine installation manual.
As stated in § 23.955(d), the test conditions and tests provided in this section are also applicable to auxiliary fuel systems and their associated fuel transfer systems.
What test conditions should be considered?
1. Airplane Attitude
Flow tests should be conducted at the critical attitude for the airplane. The critical attitude may be determined with the use of an inclinometer by flying the airplane at critical weight.
Conventional tractor airplanes normally have fuel tanks aft of the carburetor or engine injector inlet. Conditions grow less favorable for fuel flow as the airplane rotates in the “nose-up” attitude. Likewise, pusher airplanes normally have the fuel tanks forward of the carburetor or engine injector inlet making conditions less favorable for fuel flow as the airplane rotates in the “nose-down” attitude.
2. Minimum Inlet Pressure
The engine manufacturer typically establishes the minimum and maximum fuel pressure limits necessary to obtain acceptable engine operation during certification to 14 CFR part 33, §§ 33.7(b)(5)(i) and 33.7(c)(6)(i). These limits are on the applicable FAA Engine Type Certificate Data Sheet (TCDS).
The fuel pressure limits are shown in a number of different ways:
a. Minus or plus “x” pounds per square inch (psi);
b. “y” psi above true vapor pressure; and
c. “z” inches fuel head differential between carburetor fuel inlet fitting and float bowl chamber.
Most of these inlet pressure limits require no conversions. Some turbine engine manufacturers specify either the minimum inlet fuel pressure as “x” psi above the true vapor pressure of the fuel used with a Vapor/Liquid (V/L) ratio of zero, or as the maximum V/L ratio for emergency use that should not exceed a specific value. These fuel pressure limits will require an analysis of the fuel to determine true vapor pressure of the fuel with the corresponding V/L. Alternatively, a laboratory test may be conducted to determine the engine minimum fuel inlet pressure.
3. Supercharged Engine Fuel System Inlet Pressure
This applies to engines using either a mechanical or an exhaust-driven (turbocharger) device to pressurize the intake air charge.
The required fuel pressure at the carburetor or injector inlet is stated on the engine TCDS and is normally referenced to the engine inlet air (manifold) pressure. The manifold pressure is an absolute pressure and is typically stated in inches of mercury (Hg). Be sure to use compatible units when calculating the (carburetor or injector) fuel pressure minimum limit.
The inlet air (or upper deck) pressure is maintained up to the critical altitude, while the fuel tank pressure decreases with increasing altitude. This usually results in the greatest difference between the fuel tank pressure and the inlet air pressure at the critical altitude. Therefore, conduct pressure and flow tests at conditions that account for the difference between the tank pressure at the actual test altitude and the critical altitude. The pressure and flow tests may be done in two ways:
a. Reduce the fuel tank pressure, or