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 insta lations.
(2) In addition to the requirement or g 21. (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
(1) The oil tank should be designed and installed to withstand, without failure, any vibration, inertia, fluid, and structural loads expected in operation.
(2) For reciprocating engines, the expansion space should not be less than
0.5 gallons or 10 percent of the tank capacity, whichever is greater.
(3) For turbine engines, the expansion space should not be less than 10 percent of the tank capacity.
(4) It should not be possible to inadvertently fill the expansion space with the rotorcraft in the normal ground attitude.
(5) Adequate venting should be provided.
(6) Oil overflow from the filler opening into the oil tank compartment should be prevented.
b. Procedures.
(1) The structural analysis of the tank, including the attachments, should ensure that the tank will not leak under the vibration, inertia, fluid, and structural loads expected in service.
(2) The expansion space may be determined by calculating the difference between the volume up to the vent opening and the volume to the spillover level of the filler opening. The expansion space volume must not be less than 10 percent of the volume to the filler spillover level (tank capacity) or not less than 0.5 gallons for reciprocating engine installations with oil tank capacities of 5 gallons or less.
(3) To assure adequate venting under all normal flight conditions, the tank should be vented from the top portion. Traps where condensed water vapor might freeze and obstruct the vent line should be avoided. If other components, perhaps an engine speed reduction gearbox, are vented to the engine oil tank, the oil tank vent line should be sized to handle this additional requirement as well as the air normally entrained in the return oilflow from the engine.
(4) A suitable method to prevent oil spillover from the filler opening from entering the compartment containing the oil tank would be a scupper with an attached drain line that discharges clear of the rotorcraft.
AC 27.1015. § 27.1015 (Amendment 27-9) OIL TANK TESTS.
a. Explanation. This regulation specifies the requirements which the oil tank tests should verify. Each oil tank should withstand, without leakage, an internal pressure of 5 PSI. This regulation also specifies that each pressurized oil tank used with a turbine Page E - 87
engine must withstand, without leakage, an internal pressure of 5 PSI plus the maximum operating pressure of the tank.
b. Procedures. Test procedures for demonstrating these requirements are relatively simple and straightforward. Suitable adapters are fabricated to seal the various tank openings and also a fitting to introduce pressurized air into the tank. The air source needs to be regulated, and a suitable pressure gauge with a current calibration is required. Appropriate methods to check for leakage should also be available. This leak check can be a dip tank, soap and water mixture, or any other method which will provide acceptable results. Test fluid conforming to Federal Specification TT-S-735, Type III, or equivalent, is also an acceptable leak check substance.
AC 27.1017. § 27.1017 OIL LINES AND FITTINGS.
a. Explanation. This regulation outlines the certification requirements for oil lines and fittings.
b. Procedures.
(1) The line should be supported to prevent excessive vibration, and flexibility should be provided between points of relative motion. Advisory Circular 43.13-1A, Chapter 14, Section 2, Paragraph 709, may be used as guidance for the system design.
(2) Flexible hose must be approved. Generally, hoses listed in TSO-C53a or those qualified to equivalent military standards are accepted.
(3) The engine inlet and outlet oil lines should not have an inside diameter less than the corresponding inside diameter of the engine connection, and no line splices are permitted between connections; however, larger lines may be needed to ensure adequate oil flow to the engine or the transmission. Oils which exhibit high viscosity, long oil lines, and arrangements with little or no elevation of the tank outlet with respect to the engine inlet, are design characteristics which should be carefully checked.
AC 27.1019. § 27.1019 (Amendment 27-9) OIL STRAINER OR FILTER.
a. Explanation. This regulation defines the requirements for the engine oil system strainer or filter. If a strainer or filter which meets the requirements of this paragraph is incorporated as part of the type certificated engine, an additional airframe filter is not required.
b. Procedures. This paragraph requires an oil strainer or filter through which all of the oil flows for each turbine engine installation. The strainer or filter should be sized to allow oil flow at the flow rates and within the pressure limits as specified in the engine requirements. The effect of oil at the minimum temperature for which certification is sought should be accounted for.
Page E - 88
(1) For each oil strainer or filter required by § 27.1019(a) which has a bypass, the bypass should be sized to allow oil flow at the normal rate through the oil system with the filtration means completely blocked.
(2) For each oil strainer or filter installed per this rule, the capacity must be such that when operating with oil contaminated to a degree greater than established during engine certification, the oil flow and pressure are within the operating limits established for the engine. The mesh requirements are determined by the engine installation documents for the filtration of particle size and density.
(3) Unless the filter is located at the oil tank outlet, § 27.1019(a)(3) requires an indicator that will show when the contaminant level of the filtration system, as specified in § 27.1019(a)(2), has been reached. The indicator should signal a contaminant level which will allow completion of the flight before the filter would enter a bypass condition.
The indicator may be a pop-out button or other maintenance cue that is checked on each preflight.
(4) An evaluation of the construction and location of the bypass associated with the strainer or filter should be accomplished. The appropriate installation of the filter based on this evaluation would preclude the release of the collected contaminants in the bypass oil flow.
(5) If an oil strainer or filter installed in compliance with this regulation does not have a bypass, there must be a means to connect it to the warning system required in § 27.1305(r). This warning should indicate to the pilot the contamination before it reaches the capacity established in § 27.1019(a)(2).
(6) Section 27.1019(b) covers the blocked oil filter requirements associated with reciprocating engine installations. The lubrication system should be such that the normal oil flow will occur with the filter completely blocked.
AC 27.1019A. § 27.1019 (Amendment 27-23) OIL STRAINER OR FILTER.
a. Explanation. Amendment 27-23 relaxed an unduly restrictive requirement for an "indicator" to indicate the contamination level of oil filters. The rule change allows acceptance of a "means to indicate" the contaminate level to allow a wider range of acceptable methods of compliance.
b. Procedures. All of the policy material pertaining to this section remains in effect except that an "indicator" is not required to indicate the contamination level of the oil filters. Unless the filter is located at the oil tank outlet, § 27.1019(a) (3) requires that the oil strainer or filter have the means to indicate when the contaminant level of the filtration system, as specitied in § 27.1019(a)(2), has been reached. It an indicator is installed, it should signal a contaminant level that will allow completion of the flight Page E - 89