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.
23.965 Fuel tank tests (Amendment 23-51)
CAR 3 Cross-reference: 3.441
ADC Cross-reference: 5.16
Paragraph 23.965(a)
ADC Cross-reference: 5.16(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.965(b)
ADC Cross-reference: 5.16(b)
When is the vibration testing in § 23.965(b) required?
The vibration tests are necessary for any tank with a capacity of more than 10 gallons constructed in a wide flat shape with no baffles. It is acceptable to test the portion of the wing encompassing the tank(s) with a suitable amount of structure extending on all sides provided the support is representative. If there are several tanks spanwise, it may be desirable to test the entire wing, thus substantiating all tanks at the same time. If the entire wing span is wet, then test the entire structure.
The tests may not be required if:
1. The tanks include internal baffles (not exceeding 15 inches apart in length and width); or
2. The tank is constructed of a .050 inch or more thick aluminum alloy and the tank uses “suitable” joints. Joining methods can be determined “suitable” based upon design characteristics and relevant service experience.
The tests are necessary, even if the above are met, if any permanent set is observed in the baffles or their attachments during the pressure test.
Refer to AC 23-15, Small Airplane Certification Compliance Program, for fuel tanks with less than a 10-gallon capacity.
What test setup and procedures are typically used for the slosh test in §§ 23.965(b)(5) and 23.965(d)?
The test cell is a 24x30x30 inch cell containing fittings representative of those used in tank construction for airframe installation. The test cell, complete with filler cap, vents and typical outlet fittings, is installed in a suitable mounting structure, which is mounted on the support jig and rocker assembly. The cell is oriented such that the 24-inch dimension is vertical. Flexible hoses connect the vent and outlet fittings on the cell with fittings rigidly attached to the support jig and are installed and supported in a manner representative of an actual installation in an aircraft.
The tank mounting structure is representative of an actual aircraft fuel cell compartment.
Recommendations of the cell manufacturer for supporting or mounting the fuel cell in the aircraft fuel cell compartment are to be incorporated. The interior of the support jig should be completely lined with brown paper held in place by a suitable adhesive to assist in identifying leakage.
The test specimen is filled two-thirds full with the applicable test fluid containing a suitable dye. For fuel cells, one-half gallon of water is added. At the conclusion of this test, the test cell is completely filled with the applicable test fluid and thoroughly inspected for leakage or other evidence of failure.
This test setup and procedure is also used to comply with § 23.1015(b)
Related Material
AC 23-15, Small Airplane Certification Compliance Program
TSO-C80, Flexible Fuel and Oil Cell Material
Paragraph 23.965(c)
ADC Cross-reference: 5.16(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.965(d)
ADC Cross-reference: 5.16(d)
See § 23.965(b).
23.967 Fuel tank installation (Amendment 23-43)
CAR 3 Cross-reference: 3.442
ADC Cross-reference: 5.17
Paragraph 23.967(a)
ADC Cross-reference: 5.17(a)
What should be considered when determining if the tanks are sufficiently supported?
Consideration should be given to installed fuel system components such as fuel quantity capacitance probes, fuel level floats, fuel lines, and any other components that could induce concentrated loads on primary structure or tank liner attach points during the slosh and vibration test in § 23.965.
What is the definition of minor spillage?
Minor spillage applies to ground operation only. The definition of minor is “upon removal of the filler cap, no fuel makes it from the filler opening to the surface upon which the airplane is resting.”
What operations does this rule apply to?
Current policy on § 23.967(a)(6) applies the prohibition against fuel siphoning only to bladder (flexible) fuel tanks. A legal review of the regulation has concluded that the intent of this paragraph is to prevent the fuel loss and associated fuel quantity measuring inaccuracies in collapsed bladder cells due to filler openings being left uncovered. This policy applies to ground and flight operations.
Paragraph 23.967(b)
ADC Cross-reference: 5.17(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.967(c)
ADC Cross-reference: 5.17(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.967(d)
ADC Cross-reference: 5.17(d)
No specific policy is available as of December 31, 2002.
Paragraph 23.967(e)
ADC Cross-reference: 5.17(e)
What are acceptable means of compliance with § 23.967(e)(1)?
Compliance with § 23.967(e)(1) may be demonstrated by analysis alone, provided sufficient margin can be shown to ensure that fuel leakage is such that no hazard to occupants results.
Related Material
AC 20-107A, Composite Aircraft Structure