(2) The fuel filler ports should be located below the designated fuel expansion space height to assure that the fuel expansion space cannot be inadvertently filled with fuel. For pressure refueling systems, compliance with this section may be shown with the means provided to comply with § 29.979(b).
(3) Each fuel tank expansion space must comply with the venting requirements of § 29.975.
(4) For multiengine rotorcraft using a single expansion tank to satisfy the requirements of this regulation, the effect of blockage or failure of any vent from this common tank must be considered with respect to compliance with the applicable engine isolation requirements.
AC 29.969A. § 29.969 (Amendment 29-26) FUEL TANK EXPANSION SPACE.
a. Explanation. Amendment 29- 26 was issued so that properly interconnected fuel tanks will not be required to have an expansion space for each tank if adequate expansion space is otherwise provided. This amendment eliminates unnecessary design requirements when simpler designs have been proven to be satisfactory.
b. Procedures. Methods of compliance are not changed with this amendment.
AC 29.971. § 29.971 (Amendment 29-12) FUEL TANK SUMP.
a. Explanation.
(1) Each fuel tank should be provided with a drainable sump which is located at the lowest point in the tank with the rotorcraft at ground attitude in order to allow drainage of possibly hazardous accumulations of water from the system.
(2) The minimum required sump capacity, 0.10 percent of the tank capacity or one-sixteenth of a gallon, whichever is greater, should be effective at any normal attitude and located such that the sump contents cannot escape from the tank outlet opening.
(i) Combined interconnected tanks can be treated as a single tank and utilize only one sump if that sump can be located to allow effective trapment and drainage of the potential combined water accumulation.
(ii) The requirement that sump contents not be allowed to escape through the tank outlet opening is intended to ensure that water, or other impurities which may precipitate from the fuel in the tank(s), does not enter the fuel feed system.
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(3) Section 29.971(c) would ensure that the fuel tank design and installation allows drainage of hazardous quantities of water to the sump with the rotorcraft in the ground attitude.
(4) Section 29.971(d) would ensure that not only are possibly hazardous accumulations trapped, but also that they are drainable with the rotorcraft in the ground attitude.
(5) Proposed Amendments (Notice 84-19) to §§ 29.971(c) and 29.999(a) would require that the tank sumps be designed or arranged to collect water and be drainable in any ground attitude to be expected in service. This proposed provision would require consideration of the effectiveness of the sumps and drains at the sloped landing limits as well as at normal ground attitude.
b. Procedures.
(1) Demonstration of compliance with the minimum sump capacity requirements may be shown by analysis, test, or a combination of both depending on the complexity of the fuel system design.
(2) If minimum sump capacity is to be established by tests, the following procedure has been accepted.
(i) Fuel the aircraft tanks to ensure that all sumps are filled, that any transfer pumps are immersed, and that the fuel level is above the fuel feed pickup point in the tank(s).
(ii) Use the normal fuel feed provisions to remove fuel from the system.
The fuel inlet line at the engine/airframe interface may be disconnected and the fuel pumped overboard. If an engine- supplied suction lift pump is the normal feed mechanism, a suction lift pump of approximately the same capability may be substituted to avoid operating the engine.
(iii) Determine the most critical ground attitude to be expected in service from such considerations as uneven terrain, slope landing limits, etc. The critical attitude for each tank will be that for which the maximum amount of fuel can be withdrawn from the tank using the rotorcraft’s fuel supply system.
(iv) Using a rotorcraft with a fuel system which conforms to the final design specification, position the rotorcraft to the critical attitude for the tank to be tested using leveling jacks, actual terrain of a predetermined slope, or other similar means.
(v) Using the rotorcraft’s fuel supply system, pump fuel from the tank being tested until the supply system will no longer withdraw fuel. This can be done without the rotorcraft engine actually running unless an engine driven pump is an essential component of the fuel supply system. Caution should be exercised if an Page E - 115
engine is to be run to fuel exhaustion since engine surge at the pump cavitation point can result in damaging torsional loads in the transmission drive system.
(vi) When no more fuel can be removed from the tank with the rotorcraft fuel supply system, return the rotorcraft to a normal ground attitude. Completely drain the sump of the tank or tanks being tested into a container and measure the volume drained from each sump. The volume measured must satisfy the minimum capacity requirements of paragraph AC 29.971(a)(2).
(3) If, in the above procedure, a known quantity of fuel is added to initially empty tanks and the total fuel removed (pumped overboard and drained) is recorded, the data may also be used to show compliance with §§ 29.971(d) and 29.999(a).
AC 29.971A. § 29.971 (Amendment 29-26) FU EL TANK SUMP.
a. Explanation. Amendment 29- 26 requires that fuel tank sump designs be arranged so that drainage from the sump area will be effective with the rotorcraft parked in any allowable ground attitude in lieu of “normal” attitude as previously required.
b. Procedures. All of the policy material pertaining to this section remains in effect with the extra requirement that each fuel tank should be provided with a drainable sump which is located at the lowest point in the tank with the rotorcraft at “any” ground attitude in order to allow drainage of possibly hazardous accumulations of water from the system. This provision requires consideration of the effectiveness of the sumps and drains at the sloped landing limits as well as at normal ground attitude.
AC 29.973. § 29.973 FUEL TANK FILLER CONNECTION.
a. Explanation.
(1) Fuel tank filler connections must be designed so that no fuel can enter into any part of the rotorcraft other than the fuel tank during fueling operations. Spilled fuel must be considered as well as fuel entering the fuel filler port.
(2) A recessed filler connection that can retain appreciable quantities of fuel should have a drain that discharges clear of the rotorcraft.
(3) Section 29.1557(c)(1) prescribes the marking of the filler.
(4) The filler cap must be fuel-tight under the pressures expected in normal operation.
(5) For Category A rotorcraft, the filler cap or cover must warn if the cap is not fully locked or seated. An improperly locked and seated fuel cap should be evident on the preflight inspections.
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(6) The parallel Part 23 and 25 requirements specify that, except for pressure refueling connection points, the filling point must have a provision for electrically bonding the aircraft to ground fueling equipment. Though not specifically required by Part 29, rotorcraft manufacturers have included this provision in recognition that the same potential hazard exists for possible discharge of sparks between the fuel dispensing nozzle and the aircraft as would exist for airplanes.
(7) A proposed rule (Notice 84-19) would add a fuel system lightning protection requirement for rotorcraft. The potential for fuel vapor ignition near the filler cap would be a primary concern. (NASA publication 1008, Lightning Protection of Aircraft, and the user’s manual to AC 20–53A, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning, provide further information.)
b. Procedures. Compliance with the requirements of this paragraph can normally be demonstrated by analysis and physical inspection of the fuel filler connection design.
Testing is not normally required.
AC 29.973A. § 29.973 (Amendment 29-35) FUEL TANK FILLER CONNECTION.
a. Explanation.
(1) Amendment 29-35 revised the requirements for fuel tank filler connections.
paragraph (a) is revised to require that all fuel tank filler connections be made crash resistant in accordance with the requirements of § 29.952(f) and its associated advisory material (reference paragraph AC 29.952).
(2) Paragraph (a)(3) is revised to require that all filler caps remain fuel tight under fuel pressures induced during a survivable impact.
(3) Paragraph (b) is revised to require that all transport category rotorcraft (not just Category A as currently required) have a filler cap cover or filler cap that warns when the cap is not fully locked or seated on the filler connection. This change ensures that a loose filler cap will not allow spilled fuel and cause a postcrash fire in an otherwise survivable accident.
b. Procedures.
(1) The compliance procedures for general paragraph (a) are those of § 29.952(f) and those described herein for the three subparagraphs to (a).
(2) The compliance procedures for (a)(1) and (a)(2) can normally be demonstrated by analysis and physical inspection of the fuel filler design. Testing is not normally required.
(3) The compliance procedures for (a)(3) are as follows: The fuel tank filler connection must be shown to be leak free under the worst case fuel pressures (due to Page E - 117
combination of static pressure and sloshing induced head) from both normal operations and from a survivable impact. The worst case loads from these two conditions must be determined. In most cases the load resulting from a survivable impact will prevail. For the survivable impact, normally the worst case combined pressure loading occurs at the time of impact at the fuselage that places the filler tube neck (at the vicinity of the filler cap connection) in a vertical or near vertical attitude. Once the critical load case is determined by analysis, test, or a combination; the fuel tank filler connection (or an approved mockup) can be tested for sealing capability by applying a fluid such as water at the critical pressure at the critical attitude of the tube (with the cap inverted) for a period of at least 5 minutes. If no significant leakage occurs, then compliance has been shown. Significant leakage is defined as leakage in excess of 10 drops per minute at any time during or after the 5-minute test.
(4) Compliance procedures for paragraph (b) are as follows: Visual means, such as placards and alignment marks, and mechanical means, such as detents and locking slots, must both be provided. This is necessary to give both a clear visual and mechanical indication that a filler cap or a filler cap cover is properly installed and fuel tight after each removal and replacement. Visual indications such as alignment marks, that show proper installation should be easily read from a distance of at least 5 feet by anyone making a routine inspection or check.
AC 29.975. § 29.975 FUEL TANK VENTS AND CARBURETOR VAPOR VENTS .
a. Explanation. This section sets forth design requirements that address siphoning of fuel, pressure differentials, moisture accumulation, fumes in personnel compartments, and carburetor vapor vents.
b. Procedures. The design of the vent for the fuel system should be adequate to preclude problems associated with this section. Analysis and/or flight testing may be required to demonstrate this adequacy depending upon the fuel system design. If flight testing is required, the following flight test procedure is one method of verifying proper vent system operation.
(1) Using a rotorcraft with a fuel tank and vent system which conforms to production design specifications, install differential pressure instrumentation to measure the difference between the gas pressure inside each fuel tank expansion space and the air pressure in the cavity or area surrounding the outside of the fuel tank.
(2) Conduct ground and flight tests, recording the differential pressures between the inside and the outside of the fuel tanks. The following conditions should be evaluated:
(i) Refueling and defueling (if applicable).
(ii) Level flight to VNE.
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(iii) Maximum rate of ascent and descent.
(3) Compare the measured differential pressure values with the maximum allowable for the fuel tank design being evaluated. For flexible, bladder -type fuel cells, the pressure inside the tank should not be significantly less than the surrounding pressure to avoid the possibility of collapsing the bladder.
AC 29.975A. § 29.975 (Amendment 29-26) FUEL TANK VENTS AND CARBURETOR VAPOR VENTS.
a. Explanation. Amendment 29- 26 adds § 29.975(a)(7) which requires that fuel tank vent systems be designed to minimize fuel spillage and subsequent fire hazards in the event of rollover of the rotorcraft during landing or ground operations.
b. Procedures. All of the policy material pertaining to this section remains in effect with the added requirement that the fuel tank vent system design should minimize spillage of fuel in the vicinity of a potential ignition source in the event of rollover during landing or ground operation.
AC 29.975B. § 29.975 (Amendment 29-35) FUEL TANK VENTS AND CARBURETOR VAPOR VENTS.
a. Explanation. In addition to the current requirements, Amendment 29-35 revises paragraph (a)(7) to add the requirement that the venting system be designed to minimize fuel spillage through the vents to an ignition source in the event of a fully or partially inverted rotorcraft fuselage attitude following a survivable impact. (A survivable impact is defined in paragraph AC 29.952.) Since rotor action on impact and other impact dynamics have been found in numerous cases to cause rollovers or other unusual postcrash attitudes, compliance with this paragraph would significantly mitigate the postcrash fire hazard by minimizing fuel spills through vents to ignition sources when the postcrash attitude of the rotorcraft would allow gravity and/or post impact sloshing induced fuel spills through a normally open fuel vent.
b. Procedures.
(1) In addition to the compliance procedures for the previous amendment;
installation of design features, such as gravity activated shuttle valves in the vent lines (that are normally open but close under certain predictable, postcrash scenarios that are generated by involvement in a survivable impact that results in either an inverted or partially inverted fuselage attitude) must be accomplished.
(2) Once selected, the design feature chosen for compliance should be shown to function effectively without significant leakage by either full-scale and/or bench tests that apply the total pressure forces that correspond to a 100 percent full, 50 percent full, and 5 percent full fuel load applied to the device in a worst case survivable impact. (If a critical fuel level can be clearly identified, then only that fuel level and the corresponding Page E - 119
critical total pressure load need be utilized for certification approval.) The total pressure forces should be determined and applied in a manner that simulates the magnitude and rate of load onset (due to a combination of gravity and sloshing) that would occur in otherwise survivable impacts that would involve rollover attitudes of 45 degrees (or the minimum spillage roll angle), 90 degrees (rotorcraft on its side), and 180 degrees (rotorcraft fully inverted). (In some designs, the 45- degree attitude may not be the correct initial roll angle at which fuel spillage through a given vent would begin to occur due to the placement of the vents on the fuselage. For these cases, the minimum angle should be determined by analysis.)
(3) Once all test conditions are defined, these tests should be conducted with all structural deformation present in the test set up that is necessary to simulate the actual structural deformation either in or applied to the vent line or system in a worst case survivable impact. The structural deformation to be applied can be determined by rational analysis, analysis, test, or a combination. Significant leakage is defined as leakage of 10 drops per minute, or less, after all testing is complete. The criteria of 10 drops per minute, or less, corresponds to the criteria of 5 drops per minute, or less, per breakaway coupling half (i.e., a total of 10 drops per minute, or less, for the entire separated coupling) specified in the advisory material for § 29.952 (reference paragraph AC 29.952).
AC 29.977. § 29.977 (Amendment 29-12) FUEL TANK OUTLET .
a. Explanation.
(1) This section prescribes a fuel strainer for the fuel tank outlet (suction lift system) or for the booster pump (boosted systems) for both reciprocating and turbine engine installations.
(2) This requirement ensures that relatively large, loose objects which may be present in the fuel tank do not interfere with fuel system operation. The provisions of § 29.997 should ensure protection from smaller contaminants which may occur in service.
b. Procedures.
(1) Section 29.977(a) specifies an 8- to 16-mesh-per-inch strainer for reciprocating engine installations, and a strainer which will prevent passage of any object which could restrict fuel flow or damage any fuel system component for turbine installations.
(2) In addition to the requirement of § 29.977(a), the flow area of the strainer should be at least five times the area of the outlet line. Furthermore, the diameter of the strainer must be at least that of the fuel tank outlet line.
(3) Each finger strainer should be accessible for inspection and cleaning.
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(4) Compliance with § 29.977 is usually verified by inspection, and testing is not required. The ice protection provisions of § 29.951(c) are applicable to the strainer at the fuel outlet, and testing to show compliance with that provision may be required.
AC 29.979. § 29.979 (Amendment 29-12) PRESSURE REFUELING AND FUELING PROVISIONS BELOW FUEL LEVEL.
a. Explanation.
(1) Each fueling system that has the fueling connection below the fuel level in the tanks must prevent the loss of fuel if the fuel entry valve malfunctions.
(2) For pressure refueling systems, a back-up limiting device must be provided in addition to the primary means for limiting the amount of fuel in the tank.
(3) Components of the pressure fueling and defueling systems must be able to withstand an ultimate load that is 2.0 times the maximum pressure (positive or negative) most likely to occur during fueling or defueling. This requirement provides a level of structural integrity for the pressure fueling and defueling system components in the event a system malfunction occurs, which would result in an overpressurization of the fuel system. The fuel tanks and vents are not included in this requirement.
b. Procedure.
(1) Designs which have the pressure refueling and fueling provisions below the fuel level in each tank must demonstrate that when there is a malfunction of the fuel entry valve, no hazardous quantity of fuel will be lost. Generally, any amount of fuel loss in excess of 8 ounces is considered to be hazardous. Any amount of fuel that can come in contact with an ignition source is hazardous and unacceptable. Compliance should be demonstrated by test and supported by a failure mode and effects analysis.
(2) For pressure refueling systems, one of the most hazardous failure modes is an undetected overpressurization of the fuel tank which could lead to a number of potential fuel system failures. The pressure refueling system must contain a device which insures that fuel tank capacity cannot be exceeded. This device can operate on a differential pressure principle or can sense fluid level. A back -up limiting device is required in case of failure of the primary limiting device. Compliance must be demonstrated by test. A failure mode and effects analysis should be performed which verifies that the failure of either the primary or back-up limiting device will not result in the failure of the other limiting device.
(3) The rotorcraft pressure fueling and defueling systems must be designed to withstand an ultimate internal pressure load that is twice the maximum pressure that is likely to occur during fueling or defueling. The maximum pressure will include surges that could occur from the fueling source and/or from any single tank valve or Page E - 121
combination of valves being either intentionally or inadvertently closed. System substantiation may be demonstrated by analysis or test. The substantiation should include all components of the pressure fueling and defueling system except the fuel tank and the fuel tank vents. The rotorcraft defueling system must also be substantiated for a negative pressure application. If tests are conducted, the pressure measurements for both tests (positive and negative) will be made at the fueling connection and the test set-up should conform to the installed system.
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SUBPART E - POWERPLANT
FUEL SYSTEM COMPONENTS AC 29.991. § 29.991 FUEL PUMPS .
a. Explanation.
(1) Section 29.991, paragraph (a) provides a definition of the main pump(s) and § 29.991, paragraph (b) requires an “emergency pump(s).” The main pump(s) that is certified as part of the engine does not fall under § 29.991 requirements. The main pump(s) discussed under § 29.991 should therefore be considered “main aircraft pump(s).”
(2) The main aircraft pump(s) consists of whatever pump(s) is required to meet engine or fuel system operation throughout the range of ambient temperature, fuel temperature, fuel pressure, altitude, and fuel types intended for the rotorcraft. If the main aircraft pump(s) is required to meet the above criteria, then an emergency pump(s) is required.
b. Procedures.
(1) Each pump classified as a main aircraft pump, which is also a positive displacement pump, must have provisions for a fuel bypass. An exception is made for fuel injection pumps used on certain reciprocating engines and for the positive displacement, high pressure, fuel pumps routinely used in turbine engines. The bypass may be accomplished via internal spring check valve and fuel passage, or by external plumbing and a check valve. High capacity positive displacement pumps with internal pressure relief and recirculation passages should be checked for overheating if they may be expected to operate continuously at or near 100 percent recirculation.
(2) Section 29.991, paragraph (b) specifies a requirement for “emergency” pumps to provide the necessary fuel after failure of any (one) main aircraft pump.
(Injection pumps and high pressure pumps used on turbine engines are exempt.) As stated in this rule, the “emergency” pump must be operated continuously or started automatically to assure continued normal operation of the engine. For some multiengine rotorcraft, another main aircraft pump may possibly be used as the required “emergency” pump. In this case, the dual role of this pump requires it to have capacity to feed two engines at the critical pressure/flow condition. Availability of fuel flow from this backup pump must be automatic and this function should be verified in the preflight check procedure. For Category A rotorcraft, a comprehensive fault analysis of the fuel system is mandatory to assure compliance with § 29.903, paragraph (b).
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(3) Section 29.991, paragraphs (c)(1)(i) and (ii) address the situation, usually associated with supercharged reciprocating engines, where fuel pressure must be modulated with respect to carburetor deck pressure. This is accomplished with interconnecting air lines from the carburetor intake (after the supercharger) to the pressure relief connection on the fuel pump(s). A similar connection from the carburetor intake to the vented side of the fuel pressure gauge is needed to obtain correct fuel pressure reading. These systems may require orifices and/or surge chambers to operate correctly.
(4) Section 29.991, paragraphs (c)(2) and (3) requires seal drains which drain safely. A drain impingement test is normally required to verify safe drainage. Use of a colored dye to simulate fuel discharge at the drain line exit or a fluid sensitive coating (Bon Ami) on the aircraft skins will facilitate evaluation of the safety aspects of drain impingement. Pump seal drain requirements would not be applicable for tank immersed pumps.
AC 29.991A. § 29.991 (Amendment 29-26) FUEL PUMPS .
a. Explanation. Amendment 29- 26 revises § 29.991 to clarify fuel pump redundancy requirements. Redundancy for fuel pump failure includes consideration of both the pump and the pump motivating device.
(1) Section 29.991(a)(1) now stipulates that a single fuel pump failure should not jeopardize the capability of the fuel system from delivering the fuel necessary to satisfy the requirements of § 29.955. This stipulation excludes any fuel pumps that are approved as a part of the type certificated engine.
(2) Section 29.991(a)(2) expands the stipulation of § 29.991(a)(1) by including any component(s) required to drive the fuel pump (such as electric motors or generators for electric pumps). This section also stipulates that if the pump is engine driven, failure should not affect more than the engine served by the pump.
b. Procedures. The method of compliance for this section is unchanged.
AC 29.993. § 29.993 FUEL SYSTEM LINES AND FITTINGS .
a. Explanation. This rule outlines design requirements for fuel system lines.
b. Procedures.
(1) Compliance is usually obtained by employing routing and clamping as described in paragraph 709, Chapter 14, Section 2, of AC 43.13-1A and by monitoring the arrangement throughout the developmental and certification test period.
Requirements for approved flexible lines may be resolved by utilizing lines listed as TSO C53a approved for installation in either normal or high temperature areas as appropriate.
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(2) Verify adequate clearance exists between lines and elements of the rotorcraft control system at extremes of control travel, including control deflections and, for flexible lines (hoses), possible variations in routing.
(3) Flexible lines inside fuel or oil tanks require special evaluation to assure that the external surfaces of these lines are compatible with the fluids involved and that fluid sloshing will not cause line failure. Lines inside tanks should be routed to avoid impingement by fuel or oil filler nozzles.
(4) Good design practice suggests that all flammable fluid lines should be routed to minimize the possibility of rupture in the event of a crash or from engine rotor disc failure.
AC 29.995. § 29.995 (Amendment 29-13) FUEL VALVES .
a. Explanation. This regulation requires that fuel valves be supported so that no loads resulting from their operation or from accelerated flight conditions are transmitted to the lines attached to the valve.
b. Procedures. Compliance with this rule is usually accomplished by designing the installation of the fuel valve so that the valve is supported by either primary or secondary airframe structure.
AC 29.997. § 29.997 (Amendment 29-10) FUEL STRAINER OR FILTER.
a. Explanation. This rule provides for a main in- line fuel filter designed to collect all fuel impurities which could adversely affect fuel system and engine components downstream of the filter. The rule also requires a sediment bowl and drain (or that the bowl be removable for drain purposes) to facilitate separation of contaminations, both solid and liquid, from the fuel.
b. Procedures.
(1) The filter should be mounted in a horizontal segment of the fuel line to facilitate proper action of the sediment bowl. If the filter is located above the fuel tank, it becomes necessary to activate a fuel boost pump to achieve positive drainage of the filter bowl. Without pump pressure, air may enter the fuel system during the filter draining operation and, for turbine engines, result in transient power surges or engine failure during subsequent engine operation. A flight manual note to require pump(s) to be “on” during filter draining would be appropriate.
(2) Section 29.997(d) sets forth a requirement for filter capacity and for filter mesh. The capacity requirement may be substantiated by showing that the filter, when partially blocked by fuel contaminates (to a degree corresponding to the indicator marking or setting required by § 29.1305(a)(17)), does not impair the ability of the fuel Page E - 125