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
23.969 Fuel tank expansion space (Original)
CAR 3 Cross-reference: 3.443
ADC Cross-reference: 5.18
Does this apply to a pressure refueling system?
Yes, these requirements are applicable, and are related to compliance with § 23.979(b).
NOTE. Fuel tank expansion space is illustrated in Figure 3 of this AC under section 23.959.
23.971 Fuel tank sump (Amendment 23-43)
CAR 3 Cross-reference: 3.441
ADC Cross-reference: 5.19
Paragraph 23.971(a)
ADC Cross-reference: 5.19(a)
What is the purpose of the fuel tank sump?
The fuel tank sump functions as an operational trap for contaminants. The fuel tank sump may be located either within the fuel tank, or outside of the fuel tank, provided the tank is drainable to the sump.
A single fuel tank sump may serve multiple fuel tanks provided:
1. The fuel tanks are interconnected to function as a single tank, and
2. The fuel tanks are completely drainable to one tank.
While part 23 does not require fuel cross-feed capabilities, airplanes with fuel cross-feed systems should also incorporate fuel tank sumps to prevent contaminated fuel from entering the engine.
Paragraph 23.971(b)
ADC Cross-reference: 5.19(a)(2)
Is there guidance as to the time it should take for water to migrate from any part of the fuel tank to its sump?
Design each fuel tank to allow drainage of any “hazardous” quantity of water from any part of the fuel tank to its sump, with the airplane in the “normal ground attitude” within a reasonable period of time. The amount of time it takes for water in a fuel tank to migrate to the tank sump is dependent on many different factors, including temperature and size and design of the tank. Because of this, a time limit cannot be assigned. The fuel tanks should be designed to provide the most direct path from the different areas of the tank to the sump.
What is a hazardous quantity of water?
A hazardous quantity of water is generally taken to be the amount necessary to affect engine operation if it were taken up into the fuel system and made its way to the engine.
Coordination with the engine manufacturer may be required to determine the quantity of water. The term “hazardous” was incorporated by Amendment 23-43 (55 FR 40598, October 3, 1990) based on service experience that reciprocating engine airplane fuel systems are susceptible to water collecting in the fuel tanks. Prior to Amendment 23-43, tank sumps or a sediment bowl/chamber arrangement could be approved that does not always prevent water from reaching the engine, especially in tank configurations with flexible liners.
Related Material
AC 20-43C, Aircraft Fuel Control
AC 20-116, Marking Aircraft Fuel Filler Openings with Color Coded Decals
Paragraph 23.971(c)
ADC Cross-reference: 5.19(a)(2)
No specific policy is available as of December 31, 2002.
Paragraph 23.971(d)
ADC Cross-reference: 5.19(d)
No specific policy is available as of December 31, 2002.
23.973 Fuel tank filler connection (Amendment 23-51)
CAR 3 Cross-reference: 3.445
ADC Cross-reference: 5.20
Paragraph 23.973(a)
ADC Cross-reference: 5.20(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.973(b)
ADC Cross-reference: 5.20(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.973(c)
ADC Cross-reference: 5.20(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.973(d)
ADC Cross-reference: 5.20(d)
No specific policy is available as of December 31, 2002.
Paragraph 23.973(e)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Why are there fuel-filler opening dimensional restrictions?
The fuel-filler opening dimensional restrictions specified in § 23.973(e) and (f) were incorporated by Amendment 23-43 (55 FR 40598, October 3, 1990) to prevent turbine fuel from being put into tanks of airplanes that operate only on gasoline. The dimensions were established through discussions with airplane manufacturers, fuel suppliers, and pertinent industry associations. In addition, airplanes in production at that time were found to comply with the proposed dimensions.
Related Material
AC 20-122A, Anti-misfueling Devices: Their Availability and Use
Paragraph 23.973(f)
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
See paragraph (e) above.
23.975 Fuel tank vents and carburetor vapor vents (Amendment 23-51)
CAR 3 Cross-reference: 3.446
ADC Cross-reference: 5.21
What other sections of this document are pertinent to § 23.975?
Some general information on this subject is found in this AC under sections 23.951,
23.955, and 23.959 of this document.
Paragraph 23.975(a)
ADC Cross-reference: 5.21(a)
What is considered excessive pressure as mentioned in § 23.975(a)(3)?
Section 23.975(a)(3) refers to excessive pressure differences between the interior and exterior of the fuel tank. The characteristics and consequences of excessive difference in pressure will depend on the type and material of the tank. For instance, excessive differences in pressure for a bladder-type tank would result in deformation in the bladder resulting in an erroneous fuel quantity indication. Adequate positive pressure should remain within the bladder tank. For an integral tank system, excessive difference in pressure may result in collapse or structural failure of the tank and supporting structure.
What are acceptable means of compliance to § 23.975(a)?
Compliance with the requirements of § 23.975(a) is typically accomplished through a combination of design review, analysis, and testing (both ground and flight testing).
What are acceptable means of compliance to § 23.975(a)(1) for icing if the airplane is not being certificated for flight into icing conditions?
Compliance to § 23.975(a)(1) for icing if the airplane is not being certificated for flight into icing conditions can be shown using an impingement analysis at critical airspeeds and angles of attack throughout the flight envelope. The intent is to show that the location and design of the vents is such that they are in areas of zero collection efficiency. It is possible for ice to accrete on flush inlets and this should be analyzed.
Similarity to a design that has been either analyzed or tested, or both, may be also be used as the method of compliance. If the collection efficiency is not zero, it should be shown by either analysis or test, or both, that the critical ice accretion would not prevent the vent from functioning.
Paragraph 23.975(b)
ADC Cross-reference: 5.21(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.975(c)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.977 Fuel tank outlet (Amendment 23-43)
CAR 3 Cross-reference: 3.448
ADC Cross-reference: 5.22
Paragraph 23.977(a)
ADC Cross-reference: 5.22(a)
Why are the requirements different for reciprocating and turbine engine powered airplanes?
The requirement in § 23.977(a)(1) is that the strainer has 8 to 16 meshes per inch. This requirement was introduced with Civil Air Regulations (CAR) 3 in 1945, and the mesh size was found desirable by field service experience. Section 25.977 has the same requirement. The § 23.977(a)(2) requirement for turbine engines is different because field experience showed that this mesh size produced strainer icing problems with turbine engine fuel. Turbine engine fuels have a greater affinity for water than aviation gasolines, and there is a greater chance for entrained water to enter turbine engine fuel systems. The strainer mesh size for turbine engines and turbine fuel is typically larger (4 meshes per inch) than for aviation gasoline. Sections 23.951(c), 23.955, 23.997,
23.1305(b)(4), and 23.1305(c)(8) contain related applicable requirements. For example, the requirements of § 23.997, Fuel Strainer or Filter, are in addition to § 23.977 and must be considered.
Paragraph 23.977(b)
ADC Cross-reference: 5.22(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.977(c)
ADC Cross-reference: 5.22(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.977(d)
ADC Cross-reference: 5.22(d)
No specific policy is available as of December 31, 2002.