AC 27.973A. § 27.973 (Amendment 27-30) FUEL TANK FILLER CONNECTION.
a. Explanation. The original, single unlettered paragraph of old § 27.973 is redesignated as paragraph (a) by Amendment 27-30. The new (a) has three subparagraphs. These changes have been made to both make § 27.973 parallel to § 29.973 and to incorporate the new crash resistant fuel system requirements of § 27.952 (re: paragraph AC 27.952).
(1) New paragraph (a) is revised to require that all fuel tank filler connections be made fuel tight under both normal operations and during a survivable impact in accordance with the requirements of § 27.952(f) and its associated advisory material.
(2) New paragraph (a)(1) is added to require that each filler be marked as prescribed in § 27.1557(c)(1).
(3) New paragraph (a)(2) is added to require that each recessed filler connection that can retain an appreciable amount of fuel have a drain that discharges clear of the rotorcraft.
(4) New paragraph (a)(4) is added to require that each filler cap provide a fuel tight seal under the fluid pressures expected in service and in a survivable impact.
(5) New paragraph (b) is added to require that each filler cap or cap cover warn when the cap is not fully locked or seated to a fuel tight condition on the filler connection.
b. Procedures.
(1) The compliance procedures for general paragraph (a) are those of § 27.952(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 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 Page E - 75
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 27.975. § 27.975 FUEL TANK VENTS.
a. Explanation.
(1) Each fuel tank for which an expansion space is required per § 27.969 must be vented from the top part of the expansion space.
(2) Fuel tank vents must be designed to minimize the probability of the vent being restricted or completely clogged by dirt or ice.
(3) Vents of fuel tanks having interconnected outlets must be interconnected as required per § 27.963.
b. Procedures.
(1) There should be no point in any vent line where moisture can accumulate with the rotorcraft in the ground attitude or level flight attitude unless drainage is provided.
(2) Each vent should be constructed to prevent siphoning of fuel during any normal operation.
(3) No vent line or drainage provision should be terminated at a point where the discharge of fuel from the outlet would constitute a fire hazard or from which fumes could enter any personnel compartment.
(4) The vent system capacity and installed configuration should maintain acceptable differences of pressure between the interior and exterior of tank. Analysis and/or flight testing may be required to demonstrate this capability depending on the fuel system design. If flight testing is required, the following flight test procedure is one method of verifying proper vent system operation.
(i) Using a rotorcraft with a fuel tank and vent system which conforms to production design specifications, install differential pressure instrumentation which will Page E - 76
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.
(ii) 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:
(A) Refueling and defueling (if applicable).
(B) Level flight to VNE.
(C) Maximum rate of ascent and descent.
(iii) 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 27.975A. § 27.975 (Amendment 27-23) FUEL TANK VENTS .
a. Explanation. Amendment 27-23 added a new paragraph § 27.975(b) that requires 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 operation.
b. Procedures. The policy material pertaining to this section remains in effect.
Additionally, 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 27.975B. § 27.975 (Amendment 27-30) FUEL TANK VENTS .
a. Explanation. In addition to the current requirements, Amendment 27-30 revises paragraph (b) 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 27.952.) 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 Page E - 77
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 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 § 27.952 (re:
paragraph AC 27.952).
AC 27.977. § 27.977 (Amendment 27-11) FUEL TANK OUTLET.
a. Explanation.
(1) This provision 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 is intended to ensure that relatively large, loose objects which may be present in the fuel tank do not interfere with fuel system operation. The provision of § 27.997 should ensure protection from smaller contaminants which may occur in service.
b. Procedures.
(1) Section 27.977(a) specifies an 8- to 16-mesh-per-inch strainer for reciprocating engine installations, and a strainer which will prevent passage of any Page E - 78
object which could restrict fuel flow or damage any fuel system component for turbine installations.
(2) In addition to the requirement of § 27.977(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.
(4) Compliance with § 27.977 is usually verified by inspection, and testing is not required. The ice protection provisions of § 27.951(c) are applicable to the strainer at the fuel outlet, and testing to show compliance with that provision may be required.
Page E - 79
SUBPART E - POWERPLANT
FUEL SYSTEM COMPONENTS AC 27.991. § 27.991 FUEL PUMPS.
a. Explanation.
(1) Section 27.991(a) provides a definition of the main pump(s) and § 27.991(b) requires an “emergency pump(s).” The main pump(s) that is certified as part of the engine does not fall under § 27.991 requirements. The main pump(s) discussed under § 27.991 should therefore be considered the “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. Airframe supplied pumps intended for use during engine starting only are not considered to be main aircraft pumps and do not require emergency backup pumps.
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 27.991(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.) To ensure adequate pressure, the “emergency” pump should produce 100 percent of the engine flow requirement. In addition, to allow for pump or fuel system deterioration or possible filter impediments, 125 percent of takeoff flow at minimum pressure should be provided by the “emergency” pump. As stated in this rule, the “emergency” pump must be operated continuously or started automatically to ensure 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 all 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. The flight or ground crew should be Page E - 80
provided with a means to determine that a main pump failure has occurred so that it can be replaced in a timely manner.
AC 27.991A. § 27.991 (Amendment 27-23) FUEL PUMPS.
a. Explanation. Amendment 27-23 revised § 27.991 to clarify fuel pump redundancy requirements. Redundancy for fuel pump failure includes consideration of both the pump and the pump motivating device.
b. Procedures. All of the policy material pertaining to this section remains in effect with the following clarification: Airframe supplied fuel pumps that are intended for use only during engine starting are not considered as “main” airframe pumps and do not require “emergency” backup pumps.
AC 27.993. § 27.993 (Amendment 27-2) 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. The service life of TSO C53a approved high pressure fuel hoses is not established by regulation. Service life is determined by the aircraft manufacturers and included in their quality control system which is monitored by the FAA/AUTHORITY.
(2) Verify that 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 ensure 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) Fuel system lines and fittings located in any area subject to engine fire conditions must comply with the requirements of § 27.1183.
(5) Compliance with § 27.999 requires that fuel system lines contain no low points from sagging or looped routing unless drains are provided which will completely drain the system with the rotorcraft in its normal attitude on level ground.
Page E - 81
(6) 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 27.995. § 27.995 FUEL VALVES.
a. Explanation. Valves must be provided in the fuel supply system to each primary and auxiliary powerplant which will permit positive fuel flow feeding and shutoff from each fuel supply source. Although the engine throttle control system will provide one positive fuel shutoff means at the engine fuel control, additional fuel shutoff valves will normally be required in each fuel supply system to satisfy the requirements of paragraph (d) of this rule and § 27.1189(c).
b. Procedures.
(1) The fuel valve control must be located within easy reach of the appropriate crewmember and must satisfy the requirements of §§ 27.1141(c) and 27.1189(b).
(2) If independent fuel supply sources are provided, the fuel valve or valves must allow independent feeding and shutoff of fuel from each supply source.
(3) Multiengine rotorcraft fuel systems must have fuel valves which comply with the requirements of § 27.953(b)(1).
(4) No fuel valve may be located on the engine side of any firewall. Each valve should be supported so that loads resulting from its operation or from accelerated flight conditions are not transmitted to the lines connected to the valve.
(5) If check valves are included in the fuel supply system, each check valve should be constructed, or otherwise incorporate provisions, to preclude incorrect installation of the valve.
AC 27.997. § 27.997 (Amendment 27-20) 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 contaminates, both solid and liquid, from the fuel. This section is not intended to require installation of the filter between the fuel tank outlet and the first fuel system component which is susceptible to restricted fuel flow because of contaminates (such as a fuel heater or ice trap equipment).
b. Procedures.
Page E - 82
(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 27.997(d) sets forth a requirement for filter capacity. 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 § 27.1305(a)), does not impair the ability of the fuel system to deliver fuel at pressure and flow values established as minimum limitations for the engine. The filter mesh must be sized to prevent passage of particulate matter which cannot be tolerated by the engine. Part 33 requires that the degree and type of filtration be established for the engine. This information, available in the FAA/AUTHORITY-approved Engine Installation Manual, should be the basis for selection of the airframe filter mesh.
Although a test may be devised and conducted, data from the filter manufacturer usually are acceptable to verify compliance. Note that when the filter capacity is reached, continued flow of contaminated fuel may result in engine failure. A flight manual note regarding precautionary procedures is appropriate.
(3) Part 33 (through Amendment 33-6) has an identical requirement for a fuel filter for engine fuel systems; however, it is not intended that two filters should be required.
AC 27.997A. § 27.997 (Amendment 27-23) FUEL STRAINER OR FILTER.
a. Explanation. Amendment 27-26 requires that a fuel strainer or filter should be installed between the fuel tank outlet and the first fuel system component that is susceptible to fuel contamination. Components that will be protected from contamination include but are not limited to fuel metering devices which control flow rate, fuel heaters, and positive displacement pumps. The amendment also requires a sediment bowl and drain (unless the bowl is readily removable for drain purposes) to facilitate separation of solid and liquid contaminants from the fuel.
b. Procedures.
(1) The fuel strainer or filter should be accessible for draining and cleaning. It should incorporate a screen or other element that is easily removable. It should be mounted so that its weight is not supported by the inlet or outlet connections of the strainer itself, unless it can be shown that adequate strength margins exist in the lines and connections.
Page E - 83
(2) The fuel strainer or filter should have a sediment trap and drain (unless the trap is readily removable for drain purposes). The volume capacity of the sediment trap is specified in § 27.971(a) (0.10 percent of the tank capacity or 1/16 of a gallon).
(3) The fuel strainer or filter mesh should provide the filtration stipulated in the FAA/AUTHORITY-approved engine installation manual that is prepared for the type certificated engine (FAR Part 33).
(4) The fuel strainer or filter should have the capability to remove any contaminant that would jeopardize the flow of fuel that is necessary to meet the requirements of § 27.955. In addition, the strainer or filter should have a bypass system with an impending bypass indicator (Refer to § 27.1305(a)(17)). When the strainer or filter is partially blocked with contaminants, to the degree that the fuel flow requirements of § 27.955 can no longer be achieved, the impending bypass indicator should be activated. At this point, the strainer or filter should not yet be bypassing unfiltered fuel.
Although a test may be devised and conducted, data from the filter manufacturer usually are acceptable to verify compliance. Note that when the filter capacity is reached, continued flow of contaminated fuel may result in engine failure. A flight manual note regarding precautionary procedures is appropriate.
(5) Section 33.67(b) has an identical requirement for a fuel filter for engine fuel systems; however, it is not intended that two filters should be required.
AC 27.999. § 27.999 (Amendment 27-11) FUEL SYSTEM DRAINS.
a. Explanation. This regulation provides for fuel system drains and defines the requirements which the system must meet.
b. Procedures.
(1) The location and function of the fuel system drains are an integral part of any fuel system. There may be several drains required dependent upon the fuel system design. Each fuel tank sump and certain types of fuel strainers or filters require a means to drain (reference §§ 27.971 and 27.997).
(2) Selection of the location and orientation of the drain discharge in the design phase is important to assure that there is no impingement on any part of the rotorcraft.
To show compliance with the requirement may require tests dependent upon whether the applicant has a previously approved design which is similar or if the system is a new design for which no previous experience is available.
(3) The location of the drain valve should be selected so that the requirements for accessibility, ease of operation, and protection are met.
(4) Spring-loaded fuel drain valves conforming to MIL-V-25023B, TSO-C76, or equivalent, may be approved as “positive locking” valves for those installations where Page E - 84
the person operating the valve can visually confirm that the valve is closed, provided the applicant has shown that the valve will not open inadvertently under any foreseeable operating condition.
AC 27.999A. § 27.999 (Amendment 27-23) FUEL SYSTEM DRAINS.
a. Explanation. Amendment 27-23 adds the requirement that fuel system drains be effective with the rotorcraft in any allowable ground attitude including uneven terrain.
In addition, the change amended § 27.999(b)(2) to require fuel drains have a means to ensure positive closure, as contrasted to positive locking, when in the “off” position.
This will accommodate designs featuring spring-loaded drain closures that have been found to be satisfactory.
b. Procedures. All of the policy material pertaining to this section remains in effect. Additionally, selection of the location and orientation of the fuel drain discharge in the design phase is important to assure that there is no impingement upon any part of the rotorcraft. The location and orientation should also ensure effective fuel drainage when the rotorcraft is parked on uneven terrain. To show compliance with the requirement, tests may be required, dependent upon whether the applicant has a previously approved design that is similar, or the system is a new design for which no previous experience is available.
Page E - 85
AC 27-1B, Chg 4 3/31/2014
SUBPART E - POWERPLANT OIL SYSTEM.
AC 27.1011. § 27.1011 (Amendment 27- 23) ENGINES: GENERAL.
a. Explanation.
(1) This regulation defines the general oil system requirements for the engine.
(2) Each engine oil system should be independent of the system for the other engine(s).
(3) The minimum acceptable usable oil capacity, in terms of rotorcraft endurance and engine maximum oil consumption, is specified.
(4) The oil cooling provisions should be capable of maintaining the oil inlet temperature at or below the maximum allowable value.
b. Procedures.
(1) The requirement for an independent oil system for each engine should ensure continued adequate lubrication of each engine in the event of failure of the opposite engine(s) or of that opposite engine’s oil system. The provision does not require that the engine oil system be independent of other components; e.g., the use of the engine’s oil system for rotor drive system component lubrication is not precluded by this regulation.
(2) The usable oil capacity for each engine’s oil system should not be less than the product of the maximum endurance of the rotorcraft times the engine’s maximum oil consumption, plus some margin to ensure adequate circulation and cooling.
(3) Instead of a rational analysis of rotorcraft endurance and engine oil consumption rate, a usable oil capacity of 1 gallon for each 40 gallons of usable fuel may be used. (This concept should apply only to reciprocating engines.)
(4) Flight tests should be required to show adequate oil cooling provisions (reference § 27.1041).
AC 27.1013. § 27.1013 (Amendment 27-9) OIL TANKS.
a. Explanation. This regulation, along with § 27.1015, defines the oil tank design and installation requirements.
Page E - 86