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
b. Add the difference to the pressure that the engine TCDS indicates the pump is required to deliver to the carburetor or injector.
How can the minimum inlet fuel pressure for reciprocating engines be determined if it is not available from the engine manufacturer?
Should the minimum carburetor or injector-inlet fuel-pressure figure not be available from the engine manufacturer, the following process may be used to determine the acceptable inlet pressure:
1. Connect a gravity flow fuel tank to the engine inlet that can be varied in height in relation to the carburetor inlet. Large tubing may be used to minimize the effects of fluid friction;
2. With the airplane in the ground attitude position, run the engine at full takeoff r.p.m. and manifold pressure. Consider using a propeller that will allow full takeoff r.p.m. and satisfactorily cool the engine;
3. Start with the fuel tank high enough to permit proper operation of the engine. Vary the fuel level by lowering the tank until the first evidence of engine malfunction occurs, then measure and record the fuel head available to the engine. This measurement is to be made from the carburetor fuel inlet to the top of the fuel level in the tank;
4. Engine malfunction may be detected by use of an exhaust gas temperature indicator, which will indicate change in the fuel-air mixture. Any change in fuel-air mixture will result in an exhaust gas temperature change and impending engine malfunction. Also, any noticeable change in engine r.p.m. or manifold pressure will indicate a change in engine horsepower; and
5. Establish the minimum fuel pressure 10 percent above the measured fuel pressure (at the carburetor inlet) required for acceptable engine operation.
Are there any special considerations?
If a fuel flowmeter is installed, block the flowmeter during the flow test and measure the fuel flow through the bypass according to § 23.955(a)(2).
If a fuel filter with bypass provisions is installed, block the fuel filter during the flow test and measure the fuel flow through the bypass.
For engines operating on kerosene (jet) or diesel fuels, evaluate the effects of fuel density, low temperature, altitude, attitude, and water saturation.
Paragraph 23.955(b)
ADC Cross-reference: 5.11(b)
How can I demonstrate the fuel flow rate compliance for a gravity fed fuel system?
This method is applicable only to airplanes with reciprocating engines. Use this method only if the airplane is being certificated without the use of airplane supplied fuel pumps for normal operation. The airplane may be equipped with optional auxiliary or emergency pumps provided they are installed but not used during this test.
This test demonstrates the adequacy of the engine fuel supply under the most adverse operating condition. Measure the flow directly at the carburetor inlet (for example, if a reducing nipple is used at the carburetor inlet, measure the fuel flow through the nipple).
Position the airplane on the ground with the thrust or fuselage level line at the most critical attitude for fuel flow. A bench test using the pertinent fuel system components located at relative elevations to represent actual airplane critical attitudes may be used. If the airplane is tested in the level flight attitude, determine the appropriate fuel head for the critical attitude by analysis and test, and add this fuel head to the fuel system in the level flight attitude.
Reduce the fuel pressure to the minimum operating pressure for an acceptable takeoff power mixture as recommended by the carburetor and engine manufacturer. An acceptable method of simulating the minimum pressure required at the carburetor inlet is to disconnect the fuel-feed line at the carburetor and raise the end of the line above the carburetor a distance equal to that of the required minimum operating pressure in inches of fuel.
Compensate for the differential pressure (calculated as shown below under Normally Aspirated, Carbureted, Reciprocating Engine Fuel System Pressure Differential), by subtracting it from the required minimum carburetor inlet pressure. For example, if the minimum permissible fuel inlet pressure is 19 inches of fuel and a system pressure differential is 7 inches (produced between the fuel tank vent space and the carburetor float bowl airspace), then run the test with the fuel line assembly raised 12 inches above the fuel inlet fitting of the carburetor. (The pressure, in psi, can be estimated at approximately one psi for each 40 inches head of fuel.)
Alternatively, compute the sum of the head pressure associated with best rate-of-climb and carburetor inlet requirements, and demonstrate (with the fuel restricted by a valve installed at the engine-end of the system to simulate this pressure) that fuel flow is at least equal to the regulatory minimum.
Completely drain the fuel system before beginning the test. Set the system to feed from one tank only. Slowly add fuel to the tank until a steady flow is established at the inlet to the carburetor or fuel injector unit. Steady flow should be established when slightly more 54
than the unusable fuel supply has been added. When steady flow has been established, add an additional gallon (or the fuel quantity necessary to complete the flow test) to the tank.
Record the time in seconds for at least one gallon of fuel to flow from the feed line. The time for one gallon of fuel to flow should not be more than the figure computed from the following equations:
1. For reciprocating engine powered airplanes, certificated under Part 3 of the CAR or 14 CFR part 23, at the rate of 150 percent of the actual fuel flow to the engine at maximum takeoff power:
Seconds per gallon = 14,400/(SFC)(TOHP)
Where SFC = Engine Specific Fuel Consumption (SFC) (at Takeoff Power (TOPH)) expressed in (lb./bhp/hr.)
TOHP = Engine brake horsepower (bhp) at takeoff
2. For reciprocating engine powered airplanes certificated under Part 04a of the CAR, at the rate of double the normal flow required for takeoff engine power:
Seconds per gallon = 10,800/(SFC)(TOHP)
NOTE 1. For the purpose of this discussion, engine fuel flow has been defined by the calculated term (SFC)(TOHP). If the engine manufacturer has specified a higher maximum fuel flow rate, then use that fuel flow rate in place of the calculated term.
These calculations are based upon a fuel density of 6 lbs. per U.S. gallon.
NOTE 2. The fuel flow for airplanes powered by a turbine engine(s) under part 23 should meet the requirements discussed under Fuel Flow Rate for Pressure Regulated Systems below.
How do I account for the Normally Aspirated, Carbureted, Reciprocating Engine Fuel System Pressure Differential?
A pressure differential normally exists between the fuel tank airspace and the carburetor float bowl airspace at various climb attitudes and climb speeds. Account for this pressure differential when calculating the minimum fuel head for conducting fuel-flow tests. One way to determine the fuel system pressure differential is as follows:
1. Connect a calibrated airspeed indicator between the fuel tank vent and the carburetor float bowl airspace. Connect the pitot pressure connection to the fuel tank vent airspace and the static connection to the carburetor float bowl airspace. Use a liquid trap on each side of the airspeed indicator. The carburetor float bowl should be vented to the atmosphere at the normal carburetor air passage during the test.
2. With the airspeed indicator installed as described above, fly the airplane at critical weight (using the regular airspeed indicator) at the best angle-of-climb airspeed (VY) with the carburetor inlet air heat off and a full rich mixture setting (if mixture is controllable). Record the airspeed indicated on the instrument connected between the fuel tank and the carburetor.
3. Calculate the pressure differential (∆H) between the fuel tank vent airspace and the carburetor float bowl using this equation:
∆H = 6.81 (V/100)2 Where ∆H = Pressure differential in inches of fuel V = Airspeed indicator reading in miles per hour 56