questionable, the slosh and vibration tests of § 27.965 may be accepted as substantiation of this requirement.
(3) The fuel tank clearance required by paragraph (c) may be determined by inspection of the design.
(4) The ventilation and interconnect requirements of paragraph (d) may usually be determined by flight tests which explore maximum rates of climb and descent with sensitive pressure measuring equipment installed inside tanks and in the ventilation airspaces provided to comply with this rule.
AC 27.963A. $ 27.963 (Amendment 27-23) FUEL TANKS: GENERAL.
a. Explanation. Amendment 27-23 added new subsections (e) and (f) that require designs and tests to ensure that no exposed surface inside a fuel tank would, under normal or malfunction conditions, constitute an ignition source. They also set forth standards for the design and qualification of fuel tanks located in personnel compartments. These requirements are needed to ensure freedom from the hazards of fuel tank internal explosions and to ensure that fuel tanks, installed in passenger compartments, present no hazards to the personnel or to the rotorcraft.
b. Procedures. Section 27.963(e) requires the temperature of any exposed surface inside a fuel tank to be at least 50° F lower than the lowest auto-ignition temperature of the fuel or fuel vapors in the tank (reference paragraph AC 27.1185b(3), § 27.1185). For compliance with § 27.963(e), the internal component surface temperatures can be determined by flight or laboratory tests. The most critical flight conditions are established with sensitive temperature and pressure measuring equipment. This equipment is installed inside the tanks and in the ventilation air spaces.
AC 27.963B. § 27.963 (Amendment 27-30) FUEL TANKS: GENERAL.
a. Explanation. Amendment 27-30 adds a new requirement to paragraph (g) that (in addition to the current requirements) requires that the fuel tank bladder or liner be puncture resistant by meeting the TSO C-80, paragraph 16.0, screwdriver test requirements, using a new crash resistance based minimum puncture force of 370 Ibs.
A new requirement is also added to paragraph (f). Paragraph (f) now additionally requires that each fuel tank installed in a personnel compartment be crash resistant by meeting the applicable criteria of the new Crash Resistant Fuel System requirements of § 29.952 (Re: paragraph AC 27.952).
b. Procedures.
(1) Paragraph (g). The procedures for old paragraph (g) still apply under new
(g). In addition, to comply with the added puncture resistance requirement under new
(g), the requirements of § 27.952(h) must be met. Paragraph AC 27.952 gives the Page E - 68
detailed compliance procedures for § 27.952(h). The compliance procedures for § 27.952(h) also provide compliance for puncture resistance under § 27.963(g).
(2) Paragraph (1). The procedures for old paragraph (f) still apply under new (f).
Compliance with the added crash resistance requirement of new (f) can be shown by conducting a thorough design review of each fuel tank compartment to ensure that all the regulatory criteria are met. (All fuel drains and vents should also be reviewed to ensure that they meet applicable § 27.952 requirements.) A basic static loads analysis followed by a stress analysis is typically used to determine that the enclosure protects the fuel tank and provides the crash resistance level necessary for occupant survival in an otherwise survivable impact. The applicable emergency load factors are typically used to design the enclosure. (Section 27.952 contains the corresponding load factors for fuel cells and their attachments.) The emergency load factors are typically adequate for all loading conditions encountered by the enclosure in service. The typical design approach is to design the enclosure to crush at a rate approximately the same as the crush rate of the fuel tank and to ensure that all puncture hazards (such as sharp projections either enhanced or created by impact that would penetrate the fuel tank) are minimized in design. (See paragraph AC 27.952 guidance material for details.)
27.965. § 27.965 (Amendment 27-12) FUEL TANK TESTS.
a. Explanation. This regulation defines the tests that must be accomplished to show compliance for rotorcraft fuel tanks.
(1) Four basic types of fuel tanks are: (1) a metal tank installed in the aircraft or at the wing tip; (2) an integral tank; (3) a nonmetallic self-supporting tank (fiberglass);
and (4) nonmetallic flexible bladder-type tanks.
(2) There are two basic tests required by the regulations. One test procedure substantiates the design by tests and analysis by applying applicable pressure to the tank. The other procedure substantiates the design by vibration and slosh tests of the
b. Procedures.
(1) Pressure Test. The 3.5 or 2.0 PSI pressure test listed in the regulations should be conducted unless the pressure with a full tank for maximum limit acceleration or emergency acceleration is greater. Section 27.337 gives the value for the limit acceleration.
(2) Vibration and Slosh Tests.
There is not an absolute value of what constitutes "large" unsupported or unstiffened flat areas. However, it has generally been considered that any fuel tank with less than 10 gallons capacity, constructed with simple, wide, flat geometric shape and using metal (in metal tanks) of 0.05-inch thickness or greater would not require Page E - 69
tests in accordance with § 27.965(d). Using this basis, a 14- by 14-inch properly constructed tank would not require vibration and slosh tests.
(її) If the tank construction is of a metal or integral design which can be shown to be similar to previously approved tanks with acceptable service history, the vibration and slosh tests may not be required. Similarity would entail comparing the construction technique; i.e., similar panel size, similar sealing methods, skin and angle thickness, loads being similar, etc.
For fuel tanks located in the sponson or stub wing, the entire sponson or wing should be rocked and vibrated unless it can be determined that a certain portion of the tanks is critical. In this case a fixture should be developed such that the portion of the tank being tested is rocked about a pivot point which would produce the same amplitudes of motion for the portion of the tank being tested, as if the whole sponson or wing was being tested. Structure loads in conjunction with these tests have not been required.
(iv) The amplitude of vibration specified in the regulation is double amplitude (peak to peak). Vibration amplitudes less than one thirty-second of an inch must be justified by instrumented tests of the tank installed in the aircraft.
(v) The vibration and slosh procedures listed in Military Specification, MIL-T-6396, have been accepted to show compliance with § 27.965(d).
(3) After all tests have been conducted, the tanks should be leak checked using test fluid conforming to Federal Specification TT-S-735 type III or equivalent.
AC 27.967. $ 27.967 FUEL TANK INSTALLATION.
а. § 27.967(а):
(1) Explanation. This paragraph was added by Amendment 27-30 to create parallelism of both regulatory structure and level of safety between Parts 27 and 29 by the Crash Resistant Fuel System final rule on October 3, 1994. This paragraph sets forth a series of detail requirements for fuel tanks intended to ensure that tank leakage or failure is unlikely. These regulatory requirements pertain primarily to proper support of the tank and protection against chafing.
(2) Procedures. For conventional metal tanks, the support devices, commonly called "cradles," should be designed with wide flanges or cap strips at the contact area with the tank to distribute the loads in the tank material. To prevent chafing, install nonmetallic padding, treated to eliminate absorption of fuel between the tank and the support structure. Cork strips sealed with shellac and bonded to the support structure have been found suitable. Fuel cell sealant material should be applied over rivet heads and in corners. Bladder cells must be designed to fit accurately in the cell cavity in Page E - 70
order to avoid fluid loads in the bladder itself. The interior of the cavity should be smooth to avoid damage to the bladder cells.
b. §27.967(b):
(1) Explanation. This paragraph requires the design to provide ventilation and drainage of spaces adjacent to fuel tanks to avoid accumulation of fuel or fumes to be expected from minor leakage of fuel tanks. This is needed to minimize the possibility of fire or explosion in these spaces. An exception to this requirement is allowed for bladder cells installed in a closed compartment. For this configuration, ventilation may be limited to that provided by compartment drains if the ventilation is adequate to maintain proper pressure relationship between the bladder cell and cell compartment air
(2) Procedures. With the assumption that fuel tank leakage will occur, require the tank compartments to be provided with drains at any low point. These drains should conduct fuel clear of the rotorcraft and should be three-eighths of an inch or larger in diameter to minimize clogging. As with any drain intended to function in flight, verification that reverse flow will not occur due to pressure differentials at each end of the drain is appropriate. Ventilation for these tanks should involve openings in the compartment such that in-flight slipstream and/or rotor downwash will rapidly and continuously purge the tank compartment of fuel fumes. Openings should not be located so the fumes or fuel can reenter the rotorcraft. For flexible tank liner configurations (bladder cells), no specific ventilation is required if the cell is located in a compartment which is closed, except for drain holes. Note that a cell leak may be expected to produce fumes in the compartment airspace which are flammable; thus, items installed in bladder tank cavities shall not create a hazard during either normal or malfunction conditions. The vent system for the interior of the cell must be adequate to ensure that the bladder cell interior pressure is always positive or at least neutral with respect to any other airspace in the cell compartment to prevent collapse of the bladder cell. Drainage of the cell compartment should meet the criteria discussed above.
(3) A light mesh or string network hung between the bladder cell and its compartment walls is recommended to provide seepage channels to facilitate fuel leakage to the low-point compartment drains.
c. § 27.967(c):
(1) Explanation. This paragraph requires a measure of protection for fuel tanks from adverse effects of a fire in a fire zone.
(2) Procedures. Verify that a firewall meeting the requirements of § 27.1185 effectively separates any fuel tank from any engine. To minimize hazards of heat transfer to a fuel tank through a fire wall during an engine compartment fire, verify that at least one-half inch of clear airspace exists between the tank and the firewall.
Page E - 71
d. § 27.967 (d):
(1) Explanation. This paragraph is intended to prevent hazards to integral fuel tanks to be expected by impingement of flames or products of combustion from an engine compartment fire.
(2) Procedures. Review the design for relative positions of engine compartments and integral fuel tanks to estimate the flowpath of fire or heat from an engine compartment fire. Consider autorotation for single-engine rotorcraft and, for multiengine rotorcraft, low power descent as power-on flight in this evaluation. If questionable compliance exists, clear indication of the flow impingement patterns may be identified by ejecting dye from engine compartment openings during flight.
AC 27.969. $ 27.969 FUEL TANK EXPANSION SPACE.
a. Explanation.
(1) Space must be provided in each fuel tank system to allow for expansion of the fuel as a result of a fuel temperature increase. The space provided for this purpose must have a minimum volume equal to 2 percent of the tank capacity.
(2) The fuel tank filling provisions must be designed to prevent inadvertent filling of the fuel tank expansion space when fueling the rotorcraft in the normal ground attitude on level ground.
b. Procedures.
(1) Fuel tanks with interconnected vents need not have provisions for fuel expansion in each tank if equivalent expansion provisions are available in another area.
(2) The fuel filler ports should be located below the designated fuel expansion space height to ensure that the fuel expansion space cannot be inadvertently filled with fuel.
(3) Each fuel tank expansion space must comply with the venting requirements of § 27.975.
(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 27.969A. § 27.969 (Amendment 27-23) FUEL TANK EXPANSION SPACE.
a. Explanation. Amendment 27-23 allows some interconnected fuel tanks to have a common expansion space in lieu of individual expansion spaces. This change Page E - 72
relieves complex design requirements where simpler designs have proven to be satisfactory.
b. Procedures. There is no change to the suggested methods of compliance.
AC 27.971. § 27.971 FUEL TANK SUMP.
a. Explanation.
(1) Each fuel tank must be provided with a drainable sump which is located at the lowest point in the tank with the rotorcraft in a normal ground attitude.
(2) The main fuel supply to any engine may not be drawn from the bottom of any fuel sump.
(3) Each fuel sump drain must comply with the requirements of § 27.999.
b. Procedures.
(1) Each fuel sump should have an effective capacity which is not less than
0.25 percent of the tank capacity or 1/16 gallon, whichever is greater, with the rotorcraft in any ground attitude to be expected in service. This sump capacity will provide a level of safety equivalent with other normal category aircraft (reference § 23.971).
(2) 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.
(3) If minimum sump capacity is to be demonstrated by test, the following general test procedures will produce acceptable results:
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.
(•) 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.
(iii) Using the rotorcraft's fuel supply system, pump tuel 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 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.
Page E - 73
(iv) 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 27.971b(1).
AC 27.971A. § 27.971 (Amendment 27-23) FUEL TANK SUMP.
a. Explanation. Amendment 27-23 prescribed minimum values for fuel tank sump capacity, authorized the use of a sediment bowl in lieu of a sump, and required these sumps or sediment bowls to be effective in any ground attitude which can reasonably be expected in service.
b. Procedures. The policy material pertaining to this section remains in effect.
Additionally, if the rotorcraft is equipped with a sediment bowl or chamber, the capacity should be at least one ounce for every 20 gallons of fuel tank capacity. The sediment bowl or chamber should be located so that water will drain from all parts of the tank to the sediment bowl or chamber when the rotorcraft is in any allowable normal ground attitude. Compliance with the minimum sump capacity or the sediment bowl or chamber requirements may be shown by analysis, test, or a combination of both, depending upon the complexity of the fuel system.
AC 27.973. § 27.973 FUEL TANK FILLER CONNECTION.
a. Explanation. 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 entered into the fuel filler port.
b. Procedures.
(1) Each fuel filler opening must be identified with the markings and placards required by § 27.1557.
(2) Each filler cap should provide a fuel-tight seal for the main filler opening unless the fuel tank is vented through a small opening in the filler cap.
(3) Each fuel filling point should have a provision for electrically bonding the rotorcraft to ground fueling equipment.
(4) Compliance with the requirements of this paragraph can normally be demonstrated by analysis and physical inspection of the fuel filler design. Testing is not normally required.
Page E - 74
AC 27.973А. § 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.
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 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