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:
Make a continuous power-off glide at 1.3 VSO. Simulate turbulent air or smooth condition, whichever is most critical.
Condition c.—Transition from power-off glide.
Starting from the most critical condition established in condition b above, verify that there is no interruption of fuel flow when making a simultaneous application of Maximum Continuous Power (MCP) and a transition to best rate-of-climb speed, VY, from a power-off glide at 1.3 VSO.
Condition d.—Sideslip on approach. Perform the following for 30 seconds or until the first indication of interrupted fuel flow is observed, whichever comes first:
Establish a 1.3 VSO descent in a landing configuration. Maintain at least a 1½- ball or full rudder steady heading sideslip in the direction most critical for the fuel feed system. (Use the maximum sideslip anticipated for the airplane if it is less than 1½-ball.) Maintain a constant heading using sufficient aileron.
Condition e.—Sideslip on approach with balked landing. Perform the following test.
Establish a 1.3 VSO descent in a landing configuration. Maintain at least a 1½- ball or full rudder steady heading sideslip in the direction most critical for the fuel feed system for 60 seconds. (Use the maximum sideslip anticipated for the airplane if it is less than 1½-ball) followed by a maximum power straight ahead balked landing climb for 1 minute. Maintain a constant heading using sufficient aileron.
What ground operational maneuvers are typically used to determine the necessity for further gauge markings?
After determining the unusable fuel supply using the tests above, perform the following tests to determine the need for additional fuel gauge markings and restrictions.
Taxi Turns and Turning Takeoffs
Choose the minimum fuel quantity sufficient to perform the maneuvers described below. This quantity should not exceed the flight unusable fuel supply plus 30 minutes of fuel supply at MCP (see NOTE below). Turns should be in the direction most adverse for the tank being used. Load the airplane to the approximate minimum flying weight or its critical weight. Any engine malfunction due to fuel interruption during this test sequence is not acceptable.
Make a maximum lateral gravity (g’s) 90-degree turn rolling takeoff. (The maximum lateral g’s turn is where lateral skidding is evident.) Immediately follow the takeoff with a 3-minute climb at best angle-of-climb speed for single-engine airplanes or speed determined under § 23.51 for multiengine airplanes. Follow normal flight 69
procedures and maintain operating conditions necessary for safe operations (i.e., proper engine cooling).
Make a 360-degree clearing turn at a rate appropriate to the airplane, immediately followed by a takeoff and a 3-minute straight climb at best rate-of-climb speed for single-engine airplanes or the speed determined under § 23.51 for multiengine airplanes. Follow normal flight procedures and maintain operating conditions necessary for safe operations (i.e., proper engine cooling).
Make a 180-degree turn at normal taxi speed, immediately followed by a takeoff and a 3-minute climb at best rate-of-climb speed for single-engine airplanes or a speed determined under § 23.51 for multiengine airplanes. Follow normal flight procedures and maintain operating conditions necessary for safe operations (i.e., proper engine cooling).
NOTE. If the initial fuel quantity used during the above tests exceeds the 30-minute fuel supply at MCP, take the following actions:
1. Mark the fuel quantity gauge with a yellow arc (appropriately placarded) from the unusable fuel (“zero“ level) to a level corresponding to the amount of fuel used in the above tests.
2. Add an AFM limitation restricting takeoff with the fuel quantity less than the top of the yellow arc.
Related Material
AC 20-88A, Guidelines on the Marking of Aircraft Powerplant Instruments (Displays)
Paragraph 23.959(b)
Does compliance with § 23.959(b) require fuel quantity indicator markings?
The preamble to Amendment 23-51 states paragraph (b) requires that, “The effect of any fuel pump failure on the unusable fuel supply be established. This change would not require any change in the fuel quantity indicator marking required by § 23.1553.”
23.961 Fuel system hot weather operation (Amendment 23-43)
CAR 3 Cross-reference: 3.438
ADC Cross-reference: 5.14
What is the primary concern with fuel systems hot weather operation?
The primary concern is vapor formation resulting in vapor lock in the fuel system. Vapor lock occurs most frequently with hot fuel. Evaluate airplanes using aviation gasoline at the fuel’s critical temperature with respect to vapor formation. Evaluate airplanes using jet or diesel fuel at the more critical temperature with respect to vapor formation of 110 °F (-0, +5°) or the airplane’s maximum allowable outside air temperature.
Procedures for conducting hot weather fuel tests on airplanes using automobile gasoline (autogas) are contained in AC 23.1521-1B, Type Certification of Automobile Gasoline in Part 23 Airplanes with Reciprocating Engines.
Test results referred to in this section were made at the William J. Hughes Technical Center in Atlantic City, New Jersey.
What considerations are applicable to evaluating hot weather performance for airplane fuel systems?
Several factors are involved in the evaluation of an airplane fuel system for hot weather.
The applicant should consider at least the following:
1. Fuel System Design
Fuel system designs should be in compliance with 14 CFR part 23. In particular, do not route fuel lines close to hot exhaust systems as that may cause increased fuel temperature in the fuel lines.
Any fuel system (including gravity feed systems) using aviation or automobile fuels is conducive to vapor formation. Systems using a fuel pump with suction lift are highly susceptible to vapor formation.
2. Determination of Critical Fuel Temperature
The likelihood of vapor formation and subsequent interruption of engine power increases with elevated fuel temperature up to 110 °F. The temperature value of 110 °F was derived from the distillation data pertinent to aviation and turbine fuels.
The tendency of various fuels to form vapor is different. Fuels such as automobile gasoline (autogas) have an initial boiling point of approximately 85 °F, whereas some turbine fuels have an initial boiling point of approximately 120 °F.
The distillation data for any other fuel should be analyzed and the appropriate test temperature chosen. If, for example, an applicant wanted to test an airplane with liquid hydrogen fuel, it would not be reasonable to require the fuel to be heated to 110 °F since the boiling point is -423 °F at one atmosphere pressure. The same logic should apply, regardless of what alternate fuel is being tested.
Distillation data, provided from the National Institute for Petroleum and Energy Research (NIPER), using the ASTM D-86 distillation method for ASTM D-910 aviation gasoline and winter-grade automotive gasoline, revealed autogas heated to 110 °F in a vented vessel (i.e., an airplane fuel tank) will boil off approximately 18 percent of its mass in the form of non-recoverable vapors. These lost constituents are primarily the highly volatile fractions, mainly pentane and butane, the fuel constituents that can cause vapor lock problems. In contrast, aviation gasoline will boil off approximately 1 percent of its mass when heated to 110 °F.
Since the initial boiling point is a vapor temperature, the actual fuel temperature for the initial boiling point is greater than the vapor temperature. The tendency to form vapor may be related to Reid Vapor Pressure (RVP) with the higher values being more conducive to vapor formation.
3. Test Requirements for Demonstrating Compliance
A flight test is required for the initial certification of an airplane to complete the hot weather operation tests at the critical operating conditions, including the following:
maximum fuel flow, highest angle-of-attack (for tractor propeller aircraft), maximum fuel temperature, etc. Supplemental ground testing should closely simulate flight conditions. Derivative models of an airplane may use ground testing and analysis using the original flight test data to demonstrate compliance.
In the case of small airplanes with shallow depth fuel tanks, the actual fuel level in the tanks has little affect on hot weather test results.
4. Other
Fuel temperatures will be significantly higher than the ambient air temperature if the airplane has been parked in direct sunlight.
What are the definitions of common terms used when talking about fuel?
Initial boiling point
The vapor temperature at which the first drop of liquid fuel is observed after passing vapor through a distillation test apparatus.
Fuel volatility
The fuel’s tendency to evaporate (change to a vapor from a fluid).
Vapor lock
Vapor lock is the tendency of a liquid to form vapor in fuel lines, pumps, etc., that would restrict liquid fuel flow to the engine and interrupt combustion.
True vapor pressure
True vapor pressure is the pressure exerted by its vapor in equilibrium with the liquid at a specific temperature with the absence of air over the fuel.
Weathering
Weathering is the reduction of fuel volatility when given sufficient time, agitation, temperature cycles, and/or pressure changes.
What are the requirements for running hot weather compliance tests?
Perform hot weather operation tests only with outside air temperatures at 85 °F or higher measured at 4 feet to 6 feet above the runway surface.
Conduct the tests with the fuel system operating normally according to the Airplane Flight Manual (AFM). Do not operate the emergency fuel pumps if they are being considered for use as backup pumps. This test may be used to establish the maximum pressure altitude for operation with the pumps off.
Do not allow the test fuel to weather, which includes exposure to the atmosphere overnight, as this may have a significant effect on the RVP. This ensures that the fuel has the maximum RVP. Take fuel samples just prior to the test to establish that the fuel’s vapor pressure is correct.
Set the airplane weight with critical fuel level in the critical fuel tank, minimum crew necessary for safe operation, and the ballast necessary to maintain the center of gravity within allowable limits. If the critical fuel tank cannot be positively identified, then the hot weather operation tests should be repeated for the other tank(s). The critical fuel level in most cases would be low fuel; however, full fuel can be critical.
If required, heat the fuel in the shortest time period possible without causing excessive local temperature conditions at the heat exchanger, optimally less than 90 minutes but not to exceed 180 minutes. The temperature of the fuel during this time period should not exceed the critical values below. Exceeding this temperature will “weather” the fuel by driving off the volatile elements. This will make the fuel less susceptible to vapor lock and defeat the purpose of this test. It is not necessary to provide additional heat to the fuel system after this initial heating is completed. Raise the temperature of the fuel to the critical value with respect to vapor formation as follows:
1. For aviation gasoline, 110 °F (-0 to +5 °F), and
2. For turbine fuel, 110 °F (-0 to +5 °F).
There are several methods of heating the fuel such as the following:
1. Circulating hot water or steam through a heat exchanger placed in the fuel tank to increase the fuel temperature, or
2. Placing black plastic or other material on the fuel tanks in bright sunlight, or
3. Blowing hot air over the fuel tank.
NOTE. Do not agitate or excessively handle the fuel during the heating process.
CAUTION: Heating and handling fuel at elevated temperatures is hazardous to ground and flight personnel; therefore, every safety precaution should be taken.
If the outside ambient conditions interfere with the ability to conduct a valid test, it may be necessary to insulate fuel tank surfaces, fuel lines, and other fuel system components.
Perform the following as soon as possible after the fuel in the tank reaches the required test temperature:
1. Perform a maximum power takeoff and climb. (This ensures maximum fuel flow rates.)
2. Use the same climb airspeed used in demonstrating the requirements specified in § 23.65.
3. Continue the climb to the maximum operating altitude approved for the airplane.
NOTE. It is not necessary for the engine oil temperature to be at maximum, although it should be at least the minimum recommended for takeoff.
Record the following data, at minimum, throughout the test:
1. Fuel temperature in the tank;
2. Fuel pressure at the start of the test and continuously during climb, noting any loss of pressure, fluctuation, or variations;
3. Main and emergency fuel pump operation, as applicable;
4. Pressure altitude;
5. Ambient air temperature, total or static as applicable;
6. Airspeed;
7. Engine power (i.e., engine pressure ratio, gas generator speed, torque, r.p.m.), turbine inlet temperature, exhaust gas temperature, manifold pressure, and fuel flow, as appropriate;
8. Comments on engine operation;
9. Fuel quantity in the fuel tank(s) during takeoff;
10. Fuel vapor pressure for autogas only, determined prior to test; and
11. Fuel grade or designation, determined prior to test.
The test is passed if:
1. The fuel pressure is at or above the minimum prescribed by the engine manufacturer;
2. The fuel pressure does not fluctuate excessively; and
3. There is no engine malfunction due to fuel interruption throughout the test.
If fuel pressure fluctuations occur, but the test is otherwise passed, consider additional testing requirements to determine that pressure failure may not occur during any expected operating mode. Also, evaluate the fuel system for vapor formation during cruise flight at maximum approved altitude in smooth air at low to moderate power setting, and at low fuel flow and idling approach to landing.
Document instructions for proper fuel pump use during hot weather operation in the AFM. Also, document any limitations on the outside air temperature in the AFM.
23.963 Fuel tanks: General (Amendment 23-51)
CAR 3 Cross-reference: 3.440
ADC Cross-reference: 5.15
Paragraph 23.963(a)
ADC Cross-reference: 5.15(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.963(b)
ADC Cross-reference: 5.15(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.963(c)
ADC Cross-reference: 5.15(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.963(d)
ADC Cross-reference: 5.15(d)
No specific policy is available as of December 31, 2002.
Paragraph 23.963(e)
ADC Cross-reference: 5.15(e)
No specific policy is available as of December 31, 2002.