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.
23.979 Pressure fueling systems (Amendment 23-51)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.23
Paragraph 23.979(a)
ADC Cross-reference: 5.23(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.979(b)
ADC Cross-reference: 5.23(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.979(c)
ADC Cross-reference: 5.23(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.979(d)
ADC Cross-reference: 5.23(d)
No specific policy is available as of December 31, 2002.
FUEL SYSTEM COMPONENTS
23.991 Fuel pumps (Amendment 23-43)
CAR 3 Cross-reference: None
ADC Cross-reference: 5.24
Paragraph 23.991(a)
ADC Cross-reference: 5.24(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.991(b)
ADC Cross-reference: 5.24(b)
How are § 23.991(b) and § 23.991(c) related?
Paragraphs (b) and (c) are interrelated, and the interpretation of § 23.991(c) should consider § 23.991(b).
Both the main and emergency fuel pumps may operate continuously (simultaneously) provided there is a means to immediately indicate to the crew (in-flight) a malfunction of either pump.
A main fuel pump(s) supplies sufficient fuel to sustain the engine(s) during normal operations. An emergency fuel pump can sustain engine operation at full power in the event the engine-driven fuel-injector pump fails. An auxiliary fuel pump can provide some fuel flow during emergency operations but not enough to sustain engine operation at full power. Its function is to aid in priming the engine and suppressing fuel vapors.
Some fuel injection system designs utilize an integral speed-sensing pump, approved as part of the engine type design that delivers fuel at a (unregulated) pressure proportional to engine speed. Such systems have fuel lift capability that enables the system to function with a negative inlet pump pressure within specific limits as indicated by the engine type certificate data sheet (TCDS). For these types of fuel systems, an emergency fuel pump is not required when the injection pump is approved as part of the engine under § 23.991(b).
Other fuel injection system designs require an essentially constant (regulated) fuel pressure to the fuel-injector inlet. The acceptable pressure limits are specified on the engine TCDS. In these systems, the engine-driven fuel pump is functionally independent of the engine and not tied to the type design (regardless of the fuel pump supplier).
Consequently, these systems do require an emergency pump (according to 14 CFR part 23, § 23.991(b)) to ensure engine operation is sustained should the engine-driven pump fail.
Paragraph 23.991(c)
ADC Cross-reference: 5.24(c)
See paragraph (b) above.
Paragraph 23.991(d)
ADC Cross-reference: 5.24(d)
No specific policy is available as of December 31, 2002.
23.993 Fuel system lines and fittings (Amendment 23-43)
CAR 3 Cross-reference: 3.550
ADC Cross-reference: 5.25
Related Material:
AC 23-10, Auxiliary Fuel Systems for Reciprocating and Turbine Powered Part 23 Airplanes
Paragraph 23.993(a)
ADC Cross-reference: 5.25(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.993(b)
ADC Cross-reference: 5.25(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.993(c)
ADC Cross-reference: 5.25(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.993(d)
ADC Cross-reference: 5.25(d)
What does the term suitable for a particular application mean?
The term “suitable for a particular application” was incorporated by Amendment 23-43 (55 FR 40598, October 3, 1990) to remove the words “must be approved or.” This wording made § 23.993(d) more restrictive. All components are “approved” if they are installed on a certificated airplane; however, this in itself does not guarantee suitability.
For example, fuel hoses could be “approved” through the Technical Standard Order (TSO) system but still not be “suitable” for the environment of a particular installation.