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.
A B C E D Usable Fuel Unusable Fuel Drainable Sump Undrainable Vent Expansion Space Tank Capacity Sump Drain VentFuel Filler Pump Inlet Fuel to Engine
Fuel Level Representations
A Completely empty tank.
B Remaining fuel after draining tank using sump drain. (Normal ground attitude) C Remaining fuel after fuel pump has run dry. (At most adverse fuel condition in flight) D Fuel quantity after tank has been “topped off.” (Normal ground attitude) E Fuel quantity where fuel first enters vent system. (Normal ground attitude)
FIGURE 3
What are acceptable means of compliance for § 23.959?
The applicant must conduct tests to determine the most adverse fuel feed flight condition and the in-flight maximum unusable fuel amount. This will define the Red Line identifying ZERO on the fuel quantity indicator according to section 23.1337(b)(1).
Further tests must be conducted to determine if any additional indicator marking in the form of a Yellow Arc is needed to assure satisfactory fuel flow during taxi turns and turning takeoffs.
The fuel system and tank geometry should be analyzed to determine what flight maneuvers, appropriate to the airplane flight envelope, are critical for the specific tanks being tested. Radical or extreme maneuvers are only investigated if they are part of the airplane’s flight envelope. Ground tests using equipment that accurately simulates the 65
airplane fuel systems and in-flight effects may be used. Alternatively, the applicant may establish a conservative unusable fuel quantity and conduct necessary tests to show that engine malfunction will not occur.
A tank that is not used in all flight regimes need only be tested in the flight regime where it is used (e.g., cruise). Either the Airplane Flight Manual (AFM) or placards, or both, may be used to provide limitations for the use of this tank.
The guidance presented in this AC is based on service experience. It is the applicant’s responsibility to conduct sufficient tests to determine the most adverse fuel feed condition that is appropriate to the airplane.
What are the general guidelines for testing?
When testing an airplane with a single fuel tank, an auxiliary tank must be installed.
Many general aviation airplanes have symmetrical fuel systems. Test planning should determine, on a symmetrical system, that the unusable fuel results from one side are applicable to the other or whether one side could be more critical and if the results are applicable and conservative for the other side.
The test configuration must be able isolate the test tank after each test condition to not alter the resulting quantity of unusable fuel. Consider isolating vapor return flow to prevent crossover flow through vent interconnects and loss of fuel due to attitude changes into header tanks from the tank being tested.
The airplane best angle of climb (Vx) airspeed adjusted to light weight, should be determined and used for flight tests where high positive deck angles are desired. Also, the airplane configuration and airspeed that results in the most negative deck angle should be determined.
What criteria should be used to determine engine malfunction due to fuel interruption during the testing?
For the purpose of these tests, only malfunctions attributable to fuel flow interruption are considered. Some examples are the following:
1. Engine roughness;
2. Partial or total loss of power;
3. Fuel pressure reduced below minimum; or
4. Fuel flow fluctuations.
To ensure the most conservative unusable fuel supply value for each tank, another fuel tank should be selected at the first indication of engine malfunction due to fuel 66
interruption. During the test, the system should be configured to direct the fuel return lines from the engine to the non-test tank. The fuel remaining in the test tank at the time of malfunction is the unusable fuel. If header tanks (small tanks that accumulate fuel from one or more fuel tanks and supply the engine directly) are utilized, the fuel remaining in the header tank should be added to the unusable fuel, but it is not shown on the fuel gauge marking.
What flight maneuvers are typically used to determine the unusable fuel quantity, according to § 23.959, for establishing the gauge marking, according to § 23.1337(b)(1), on the fuel quantity indicator?
The following tests should be performed as applicable to determine the unusable fuel supply at the most critical condition during flight. The unusable fuel quantity is the largest quantity of fuel necessary to accomplish the critical maneuvers. This quantity should be used to set the fuel gauge zero mark.
1. Level Flight: Maximum Recommended Cruise Power
NOTE. If the airplane manufacturer does not specify Maximum Recommended Cruise Power, then operate engine at the maximum allowable power for continuous operation.
Condition a.—Straight coordinated flight. Perform the following until the first indication of interrupted fuel flow is observed:
Maintain straight coordinated flight (bank angles not exceeding 5 degrees).
Condition b.—Turbulent Air. Perform the following until the first indication of interrupted fuel flow is observed:
Simulate turbulent air with +/- half-ball width oscillations at approximately the natural yawing frequency of the airplane in pitch, roll, and yaw.
Condition c.—Skidding turn in the direction most critical with respect to fuel feed.
Repeat the following until the first indication of interrupted fuel flow is observed:
Maintain a skidding turn with 1-ball width skid for 30 seconds and return to coordinated flight for 1 minute.
2. Climb: Maximum Climb Power at Best Angle-of-Climb Speed, VX
The climbs at Maximum Climb Power and Vx should be performed uninterrupted to obtain the most conservative results.
NOTE. If the airplane manufacturer does not specify Maximum Climb Power, then operate engine at either the maximum allowable power for takeoff operation 67
(maximum airplane weight over 6,000 lbs.) or maximum allowable power for continuous operation (maximum airplane weight 6,000 lbs. or less).
Condition a.—Straight coordinated flight. Perform the following until the first indication of interrupted fuel flow is observed:
Maintain straight coordinated flight (bank angles not exceeding 5 degrees).
Condition b.—Turbulent air. Perform the following until the first indication of interrupted fuel flow is observed:
Simulate turbulent air with +/- half-ball width oscillations at approximately the natural frequency of the airplane.
Condition c.—Skidding turn in the direction most critical with respect to fuel feed.
Repeat the following until the first indication of interrupted fuel flow is observed:
Maintain a skidding turn with 1-ball width skid (or full rudder if 1-ball width cannot be obtained) for 30 seconds and return to coordinated flight for 1 minute.
3. One-Engine-Inoperative Climb: Maximum Climb Power at One-Engine- Inoperative Best Rate-of-Climb Speed, VYSE
NOTE. If the AFM does not specify Maximum Climb Power, then operate engine at either the maximum allowable power for takeoff operation (maximum airplane weight over 6,000 lbs.) or maximum allowable power for continuous operation (maximum airplane weight 6,000 lbs. or less).
Set the critical engine to zero thrust to simulate a critical engine-out condition (do not shut down the critical engine). Conduct a straight climb at the best rate-of-climb speed for one engine inoperative (VYSE). Utilize the bank angle and ball displacement used in determining single-engine performance. Continue climb until a malfunction occurs.
4. Descent and Approach
Condition a.—Emergency descent or rapid descent. Perform the following until the first indication of interrupted fuel flow is observed:
Make a continuous power-off straight descent at VFE with gear and flaps down or follow emergency descent procedures contained in the AFM.
Condition b.—Perform the following until the first indication of interrupted fuel flow is observed: