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
4. The ∆H obtained may be subtracted from the minimum carburetor inlet pressure specified by the engine manufacturer to determine the minimum pressure to be substantiated by the fuel flow tests. (See Figure 1 below in this AC section.)
FIGURE 1
Vent Vent Space Fuel Tank Carburetor Venturi Vent Carburetor Float Bowl Fuel Liquid Traps Airspeed Indicator Line from fuel tank vent space to pitot pressure side of airspeed indicator Line from carburetor float bowl air space to static pressure side of air speed indicator 57
5. A U-tube manometer (see figure 2 below in this AC section) containing actual fuel or any other appropriate fluid may be used instead of the airspeed indicator to determine the pressure differential. Use of the manometer allows reading pressure differential directly in inches of fuel or the chosen fluid. If some other fluid is used, correct the differential height for the specific gravity of the test fluid. A U-tube manometer is generally commercially available or can be constructed of clear plastic or glass tubing.
Vent Vent Space Fuel Tank Carburetor Venturi Vent Carburetor Float Bowl Manometer Pressure Differential Fuel or other test fluid Fuel FIGURE 2
6. Installation of a U-tube manometer, containing fuel, in the cabin can be hazardous;
therefore, take special precautions to prevent damage or spillage. Incorporate provisions to collect and drain any fuel spillage overboard.
Paragraph 23.955(c)
ADC Cross-reference: 5.11(c)
How can I demonstrate the fuel flow rate compliance for a pump fed fuel system?
These tests are required to demonstrate that the airplane fuel system can deliver an adequate fuel supply to the engine.
1. Fuel Flow Rate for Pressure Regulated Systems
Conduct these fuel-flow rate tests by placing the airplane in the same attitude as outlined in the procedure for gravity-feed fuel system tests. Operate the fuel pump either driven by the engine with the pump mounted on the regular engine-fuel pump drive pad or driven by a separate external power source.
If conducted with the pump mounted on the engine, use a separate source of fuel, external to the airplane, to operate the engine at takeoff r.p.m.
If an external power source is used to drive the fuel pump, conduct the test with the pump operating at the same speed as the pump operates when the engine is running at takeoff r.p.m. Mount the pump at the same height as it would be if mounted on the fuel pump pad on the engine.
In either case, measure the pump discharge flow rate using a calibrated flowmeter and correct for inlet condition if necessary.
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 approximately the usable fuel supply has been added. When steady flow has been established, add an additional gallon of fuel to the tank. The fuel-flow tests should be made on the main fuel-pump system followed by a test on the emergency fuel-pump system with the main pump inoperative in the critical mode for fuel delivery.
Part 23 airplanes using aviation or automobile gasoline
For airplanes certified under part 23 using aviation or automobile gasoline, the time for one gallon to flow should be less than or equal to the smaller of the following:
a. At the rate of 0.9 pounds for each TOHP:
Seconds per gallon = 24,000/TOHP
b. At the rate of 125 percent of the actual fuel flow to the engine at maximum takeoff power:
Seconds per gallon = 17,280/(SFC)(TOHP)
Airplanes using jet or diesel fuel
For airplanes using jet or diesel fuels, the time for one gallon to flow should be less than or equal to the following at each intended operating condition and maneuver:
At the rate of 100 percent fuel flow:
Seconds per gallon = 24,480 (JP-5)/(SFC)(bhp or lb. thrust) 23,400 (JP-4, Jet B)/(SFC)(bhp or lb. thrust) 23,760 (JP-7)/(SFC)(bhp or lb. thrust) 24120 (JP-8, Jet A, Jet A-1)/(SFC)(bhp or lb. thrust)
NOTE 1. For the purpose of this discussion, engine fuel flow has been defined by the calculated term (SFC)(TOHP) or (SFC)(bhp or lb. thrust). If the engine manufacturer has specified a higher maximum fuel flow rate, then use that fuel flow rate in place of the calculated term.
NOTE 2. Jet fuels vary considerably in density. For example, at 60 °F:
JP-4, Jet B range is 6.27 to 6.69 lbs./U.S. gallon JP-5 range is 6.58 to 7.05 lbs./U.S. gallon JP-7 range is 6.50 to 6.73 lbs./U.S. gallon JP-8, Jet A, Jet A-1 range is 6.47 to 7.01 lbs./U.S. gallon
Since the flow rates for turbine-powered airplanes are 100 percent, account for the effects of cold fuel, blocked filters, altitude, attitude, maneuvers, acceleration, etc.
Part 04a airplanes using aviation or automobile gasoline
For airplanes certified under Part 04a of the CAR using aviation or automobile gasoline, the time for one gallon to flow should be less than or equal to the following:
At the rate of double the normal flow required for takeoff engine power:
Seconds per gallon = 10,800/(SFC)(TOHP)
2. Fuel Flow Rate for Unregulated Fuel Injection Pump Systems
Some fuel injection systems utilize an integral speed-sensing pump, approved as part of the engine type design, that delivers fuel at an unregulated pressure proportional to engine speed. A fuel injection system with an integral speed sensing pressure pump (refer to AC section on § 23.991) is normally capable of suction lift at the inlet to the pump; therefore, an emergency pump may not be required. Use care in selecting and operating emergency pumps with these types of systems as engine malfunction (as defined in the AC section on § 23.959) can also occur due to excess fuel when operating under high fuel pressure.
Conduct these fuel-flow rate tests by placing the airplane in the same attitude as outlined in the procedure for gravity-feed fuel system tests, with the following exceptions or additions:
a. The fuel-injector system normally has an engine-driven fuel-injector pump that is approved as part of the engine. It is necessary to show that the fuel system, excluding the injector pump, complies with the applicable regulations. Therefore, any fuel pump capable of providing the flow capacity may be used to conduct the fuel-flow tests. Pressure indicators installed in the fuel lines will be necessary to determine that the system will maintain the pressure specified by the engine manufacturer.
b. Some engine fuel injector systems have a bypass or return-flow fuel system.
These systems may be evaluated by determining that the system can conduct fuel back to the tank at the required flow rate without exceeding the back pressure established by the engine manufacturer at the bypass exit port.
c. When conducting the fuel flow test with a separate fuel pump, the fuel flow requirements of the main fuel system should include the quantity of fuel consumed by the engine plus the quantity returned to the fuel tank.
Airplanes equipped with engine-driven integral speed-sensing pressure pumps can experience engine misfire, stall, or other engine malfunction attributable to excess fuel should the optional emergency fuel pump produce too high an output pressure.
In these cases, the optional fuel pumps are not required by the regulation as emergency pumps but are used to supply fuel pressure at the engine-driven injector pump inlet for engine starting, vapor suppression, and, in some cases, maintaining engine power after an engine-driven fuel injector pump failure. These optional pumps may have a sufficiently high fuel output pressure to adversely affect engine operation when the pump is turned on with the engine-driven pump operating normally. To ensure that the optional fuel pump operates within the maximum allowed fuel pressure limit at the inlet to an engine-driven injector fuel pump, an inflight evaluation with both pumps operating or a ground pressure check is recommended as follows:
a. Block the engine-driven fuel pump outlet line and operate the airplane’s electric fuel pump under a no-flow condition, then
b. Take a pressure reading to ascertain that the fuel pressure falls within the pressure limits specified for the engine at the inlet to the fuel injector or compare the pump manufacturer's no-flow performance pressure with the injector pump's inlet limit.
Is it possible to certify component changes without a full fuel system test?
Changes in components, such as an engine-driven fuel pump or a wobble pump, can often be substantiated by comparative tests of the components themselves without testing the entire airplane system. For example, a replacement fuel pump should be capable of delivering at least the same flow as the original pump when operating at the same speed, suction lift, and delivery pressure.
Paragraph 23.955(d)
ADC Cross-reference: 5.11(d)
No specific policy is available as of December 31, 2002.
Related Material
AC 23-10, Auxiliary Fuel Systems for Reciprocating and Turbine Powered Part 23 Airplanes
Paragraph 23.955(e)
ADC Cross-reference: 5.11(e)
No specific policy is available as of December 31, 2002.
Related Material
AC 23.1521-1B, Type Certification of Automobile Gasoline in Part 23 Airplanes with Reciprocating Engines
AC 23.1521-2 with Change 1, Type Certification of Oxygenates and Oxygenated Gasoline Fuels in Part 23 Airplanes with Reciprocating Engines
Paragraph 23.955(f)
ADC Cross-reference: 5.11(f)
No specific policy is available as of December 31, 2002.
23.957 Flow between interconnected tanks (Amendment 23-43)
CAR 3 Cross-reference: 3.439
ADC Cross-reference: 5.12
Paragraph 23.957(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.957(b)
Can fuel tank vent lines be used to transfer fuel between tanks?
It is not advisable to use the fuel tank vent lines to transfer fuel between tanks as a normal procedure because this will prevent the vent lines from venting and expose the tanks and lines to higher pressures.
23.959 Unusable fuel supply (Amendment 23-51)
CAR 3 Cross-reference: 3.437
ADC Cross-reference: 5.13
Paragraph 23.959(a)
What is the definition of unusable fuel supply?
The following definitions are for terms used throughout this section.
Unusable fuel is the largest total quantity of fuel remaining in the fuel tank(s) when the first evidence of engine malfunction occurs due to fuel interruption. Engine malfunction normally occurs under the most adverse fuel feed conditions depending on such things as tank geometry, fuel system geometry and operating characteristics, and operating maneuvers. Each intended ground operation and flight maneuver should be checked for each fuel tank. See figure 3 on the next page in this AC section for common fuel tank configuration definitions relative to unusable fuel.
VFE is the maximum airplane flap extended speed.
VSO is the airplane stalling speed or the minimum steady flight speed in the landing configuration.
VX is the best angle-of-climb speed.
VY is the best rate-of-climb speed.
VYSE is the best rate-of-climb speed with one engine inoperative.