b. Procedures. Section 29.963(e) requires the temperature of any exposed surface inside a fuel tank to be at least 50° Flower than the lowest auto-ignition temperature of the fuel or fuel vapors in the tank (reference AC Paragraph 586b(3), § 29.1185). For compliance with § 29.963(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.
454B. § 29.963 (Amendment 29-35) FUEL TANKS: GENERAL.
a. Explanation. Amendment 29-35 adds a new Paragraph (b) that includes the requirements previously contained in Paragraph (c) that each flexible fuel tank bladder or liner be either FAA/AUTHORITY approved or be suitable for each particular installation. In addition, the new Paragraph (b) adds the requirement that the fuel tank bladder or liner be puncture resistant by meeting the TSO C80, Paragraph 16.0, screwdriver test requirements, using a new crash resistance based minimum puncture force of 370 Ibs. The requirements previously contained in Paragraph (b) are replaced by the crash resistant fuel system requirements of § 29.952 (including load factors). A new Paragraph (e) is also added. Paragraph (e) requires that each fuel tank installed in a personnel compartment be isolated by fume-proof and fuel-proof enclosures that are drained and vented to the exterior of the rotorcraft. Further, the design and construction of the enclosures must provide the necessary protection for the tank, must be crash resistant by meeting the applicable criteria of the new Crash Resistant Fuel System requirements of § 29.952, and must be adequate to withstand the loads and abrasions to be expected in personnel compartments.
b. Procedures.
(1) Paragraph (b). The procedures for Paragraph (c) prior to Amendment 29-35 still apply to new Paragraph (b). In addition, to comply with the added puncture resistance requirement under new Paragraph (b), the requirements of § 29.952(g) must be met. Paragraph 447 of this document gives the detailed compliance procedures for § 29.952(g). The compliance procedures for § 29.952(g) also provide compliance for puncture resistance under § 29.963(b).
(2) Paragraph (e). Compliance with Paragraph (e) can be shown by conducting a thorough design review of each fuel tank and its enclosure that is installed in a personnel compartment to ensure the regulatory criteria are met. (All fuel drains and vents should also be reviewed to ensure that they meet applicable § 29.952 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.
740 Par 454A
(Section 29.952 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 447 guidance material for details.) The design of the enclosure should also be reviewed for overal durability and resistance to all reasonable occupant abuses that could cause a hazard to the integrity of the enclosure, the tel tank, its vents and its drains.
455. § 29,965 (Amendment 29-13) FUEL TANK TESTS.
a. Explanation.
(1) This section prescribes the fuel tank structural tests to be accomplished without failure or leakage.
(2) Section 29.965(b) prescribes pressure testing for conventional metal tanks, integral tanks, and for nonmetallic tanks with walls that are not supported by the rotorcraft structure.
(3) Section 29.965(c) prescribes pressure testing for nonmetallic tanks with walls supported by the rotorcraft structure.
(4) Section 29.965(d) prescribes slosh and vibration testing for tanks with large unsupported or unstiffened flat areas.
b. Pressure Tests.
(1) Each conventional metal tank, integral tank, and each nonmetallic tank without supporting rotorcraft structure should be subjected to pressures of at least
3.5 PSI gage.
(i) If the pressures developed during maximum limit acceleration or emergency deceleration with a full tank exceeds the 3.5 PSI value, a hydrostatic pressure test (or equivalent) should be used to duplicate these acceleration loads as far as possible.
(її) Pressures need not exceed 3.5 PSI on surfaces not exposed to the acceleration loading.
(iii) Section 29.337 gives the value for the maximum limit acceleration.
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(2) Section 29.965(c) applies to nonmetallic tanks with walls supported by the rotorcraft structure. Section 29.965(c)(1) does not require that the tank alone be capable of withstanding 2.0 PSI. Rather, the tank may be mounted in the supporting structure and subjected to the testing of § 29.965(c)(2).
(3) Pressure tests may be conducted by slowly applying a controlled, gaged air source to the tank with sealed vents and fluid entrances and exits. The air pressure source should then be positively sealed and the tank should retain the prescribed pressure.
(4) Tank and surrounding structure should be carefully examined during and after pressure testing to ensure that there is no damage.
(5) If the prescribed 3.5 PSI or 2.0 PSI, depending on the type of tank, will be exceeded on some surfaces during maximum limit acceleration loading, hydrostatic testing may be preferred. High density fluids have been used to apply the acceleration loads to lower surfaces with supplemental air pressure used above the liquid surface to provide the appropriate pressure on upper surfaces.
(6) For fuel tanks in those areas designated by § 29.967(1), the pressure tests may be designed such that compliance with that paragraph also demonstrates compliance with § 29.965 pressure test requirements.
(c) Slosh and Vibration Tests
(1) The test requirements of § 29.965(d) are very specific and require little explanation.
(2) 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 a 10-gallon capacity, constructed with a simple, wide, flat geometric shape and using metal (in metal tanks) of 0.05-inch thickness or greater would not require tests in accordance with § 29.965(d). Using this basis, a 14- by 14- inch properly constructed tank would not require vibration and slosh tests.
(3) 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, similar loads, etc.
(4) For fuel tanks located in a 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 742 Par 455
7/30/97 АС 29-2B amplitudes of motion for the portion of the tank being tested, as if the whole sponson or wing was being tested. Structural loads in conjunction with these tests have not been required.
(5) The amplitude of vibration specified in the regulation is double amplitude (peak-to-peak). Vibration amplitudes less than one thirty-second of an inch should be justified by instrumented tests of the tank installed in the aircraft.
(6) The vibration and slosh procedures listed in Military Specification MIL-T-6396 have been accepted to show compliance with § 29.965(d).
(7) 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.
456. § 29.967 FUEL TANK INSTALLATION.
a. § 29,96Z(a):
(1) Explanation. This paragraph sets forth a series of detail requirements for fuel tanks intended to ensure that tank leakage or failure is unlikely. These 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 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. § 29,967(0):
(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 spaces.
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(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 walls 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. § 29.967 (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 § 29.1191(e) effectively separates any fuel tank from any engine. To minimize hazards of heat transfer to a fuel tank through a firewall during an engine compartment fire, verify that at least one-half inch of clear airspace exists between the tank and the firewall.
a. § 29.967 (a):
(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 a dye from engine compartment openings during flight.
744 Par 456
e. § 29,967 (e):
(1) Explanation. This paragraph is primarily intended to provide a standard for installing fuel tanks in personnel compartments. The primary safety concern is to isolate fuel or fumes from personnel in event of a leak in the tank.
(2) Procedures. Assume a leak in the tank and determine that, through the use of additional walls, bulkheads, enclosures, etc., that fuel and fumes will be safely drained and/or purged to the exterior of the rotorcraft. Note that, in order to perform their intended function, the enclosure material and structure should withstand the mechanical stresses and abrasions to be expected from crew and passenger activities within the compartment.
f. § 29.967(1):
(1) Explanation. This paragraph is intended to require the design to prevent fuel tank or tank support failure when exposed to the minor crash loads of § 29.561 if such failure could result in fuel entering personnel compartments or fire hazard areas.
(2) Procedures. If a review of the design indicates that tanks are in or adjacent to passenger compartments, or are adjacent to combustion heaters or engines (including APU's), further evaluation of the structural integrity of the tank and its support teatures must be accomplished. Normally, this involves a quantitative analysis of the tank support structure to confirm that it can sustain the minor crash loads plus one or more pressure tests to simulate the fluid loads on the tank interior to be expected when the minor crash loads are applied. This latter requirement may be, in many cases, satisfied by the qualification requirements of §§ 29.963 and 29.965. Pressure tests tend to overstress upper surfaces of a tank in order to achieve the required stress in the lower surfaces. To minimize this, some applicants have filled the tank to be tested with high density fluids and applied only supplemental pressure to the airspace at the top of the tank. High density fluids are available from the petroleum industry.
456A. § 29.967 (Amendment 29-35) FUEL TANK INSTALLATION.
a. Explanation. Amendment 29-35 removes Paragraph (e) from § 29.967 and places the identical criteria in a new Paragraph (e) to § 29.963. This was done to make §§ 29.963 and 29.967 parallel with §§ 27.963 and 27.967.
b. Procedures. The procedures specified in Paragraph 456, subsection (e) now apply under Paragraph 454B. Thus there is no change in the certification requirements or the compliance methodology, only a change in their location in the FARs and Advisory Material, respectively.
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457. § 29,969 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 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.
457A. § 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.
458. § 29.971 (Amendment 29-12) FUEL TANK SUMP.
a. Explanation.
746 Par 457
(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.
(il) 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.
(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.
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).
Par 458 747
(її) 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.
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 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 458(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).
458A. § 29.971 (Amendment 29-26) FUEL 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.
748 Par 458
459. § 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.
(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.
459A. § 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 Par 459 749
resistant in accordance with the requirements of § 29.952(f) and its associated advisory material (reference Paragraph 447).
(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 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 five 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.
750 Par 459A
460. § 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:
Refueling and defueling (if applicable).
(i) Level flight to VNE•
(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.
460A. § 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.
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