623/23 AC 29-2C, Chg 9 AC 29.955B. § 29.955 (AMENDMENT 29-59) FUEL FLOW.
a. Explanation. Amendment 29-59 revised § 29.955(a)(7) to correct a reference from § 29.1305(a)(17) to § 29.1305(a)(18). When § 29.1305 was updated at Amendment 29-40, the numbering sequence was changed. The fuel filter contamination warning, § 29.1305(a)(17), was referenced in § 29.955(a)(7) but was not updated at Amendment 29-40 to account for the change in numbering sequence.
b. Procedures. The policy pertaining to the procedures in this section remains unchanged, however the reference to the § 29.1305 fuel filter contamination indicator has been corrected.
Page E - 102.1
AC 29.957. § 29.957 FLOW BETWEEN INTERCONNECTED TANKS.
a. Section 29.957(a):
(1) Explanation. This paragraph sets forth a design requirement that prohibits approval of a fuel tank interconnect arrangement wherein gravity or acceleration-induced flow between tanks will result in overflow through a tank vent.
(2) Procedures. The design of the vent for the receiving tank should be sufficiently elevated to preclude gravity or flight accelerations from causing overflow through the vent. A flight test may be needed to determine the effectiveness of the arrangement. Check valves in the vent system to prevent overflow should be discouraged because of reliability aspects.
b. Section 29.957(b):
(1) Explanation. For fuel system arrangements which permit fuel to be pumped from one tank to another, design precautions to prevent structural damage to the receiving tank in the event of overfilling are required as well as a design means to warn the crew before overflow through the vents occurs.
(2) Procedures. The design of the receiving tank should have large vent lines or a recirculation line back to the original tank to prevent overfilling of the receiving tank.
Alternatively, a float switch may be used to de-energize the transfer pump, providing that faults in the system do not adversely affect safety. A float switch may be used to warn the crew that overfilling of the receiving tank is impending. If a float switch is used, review the system reliability requirements of § 29.901(c).
AC 29.959. § 29.959 UNUSABLE FUEL SUPPLY.
a. Explanation. This rule requires the applicant to establish a value for unusable fuel for each tank. This value for unusable fuel may be selected by the applicant to facilitate compliance with § 29.1337(b)(1) provided the amount is equal to or greater than the actual unusable fuel. The actual unusable fuel is the amount of fuel in the tank when, in the critical flight attitude, evidence of system or engine malfunction occurs, or in the case of transfer tanks, when flow to the receiving tank is interrupted.
b. Procedures.
(1) The unusable fuel for each tank can be determined by flight tests which involve flight in the critical attitude or maneuver until indication of a malfunction. For boosted systems, the “first evidence of malfunction” may be a pressure fluctuation to below the fuel pressure minimum redline, engine power fluctuation, or boost pump failure warning indication. For suction lift systems, the indication may be a low fuel pressure warning light. In some instances, particularly for suction- lift systems, special test instrumentation for fuel pressure is required, and, since an accurate measurement Page E - 103
of the remaining fuel in the tank should be obtained, a method to close off flow from that tank would be needed. For transfer tanks, or tanks which are limited to use only during cruise flight, the flight regimes usually can be limited to level flight or hover at the c.g.
condition which, by inspection, would create the maximum unusable fuel. For tanks for general use, the flight regimes should also include takeoff and landing using steady pitch attitudes to be expected, as well as hover and level flight conditions. The possible adverse effects of extreme lateral c.g. should be considered.
(2) Normally, these tests are conducted with all equipment (pumps, ejectors, etc.) operating as prescribed by the design. However, values for unusable fuel with pump failures, if significantly different, should also be determined and listed in the flight manual. These values for unusable fuel need not be considered in the empty weight of the aircraft.
c. While the procedures of paragraph b(1) are acceptable, fuel exhaustion during critical flight test conditions must be expected. To minimize this possible flight test hazard, the applicant may in many cases, utilize analysis and/or ground tests involving normally available flight test data on aircraft attitudes, tank configuration studies, and critical flight condition studies to determine unusable fuel. Any questionable results, however, should be resolved by actual flight test or introduction of conservatism into the finding.
AC 29.961. § 29.961 FUEL SYSTEM HOT WEATHER OPERATION.
a. Explanation.
(1) Section 29.961 specifies that a hot fuel test be conducted on suction lift systems, and on other fuel systems conducive to vapor formation, to ensure that the system is free from vapor lock at a fuel temperature of 110° F under critical operating conditions.
(2) Pressure boosted systems would not ordinarily require hot fuel tests unless- -
(i) There are high points in the fuel system which would allow accumulation of vapor; or
(ii) The engine fuel inlet pressure is negative relative to tank pressure because of low boost pump pressure or high fuel system pressure losses (but still within fuel pressure limits).
(iii) The airframe boost pump is not actually submerged such that a portion of the system is suction lift.
(3) Boosted system vapor lock difficulties, at relatively low system flows compared to pump capacity, have occurred in at least two instances.
Page E - 104
(i) If the fuel pump is a positive displacement type with an internal bypass and the pump capacity significantly exceeds system demand, excessive recirculation within the pump may significantly raise the local fuel temperature resulting in pump cavitation.
(ii) Parallel pump systems, where one supplies the majority of the fuel while the other “deadhead” pump supplies only a negligible amount of fuel, may experience vapor lock and cavitation of the deadhead pump due to excessive recirculation of fuel as described in a(3)(i).
(4) The requirement to use 110° F fuel is a carryover from the recodification of CAR Part 6, although the use of hotter fuel at the same Reid Vapor Pressure would tend more toward vapor formation.
(5) The term “vapor lock” means a change in normal engine operation as a result of the formation of fuel vapor-air mixtures in the fuel feed system.
(6) Section 29.961(b) and (c) inappropriately specify a particular flight condition, weight, and power spectrum which may not be critical. Hence, a demonstration of compliance to the specifics of § 29.961(c) will probably be inadequate for compliance with § 29.961(a)(2). NPRM No. 84–19 proposes to revise § 29.961 to delete these unnecessary, detailed regulations with a simple requirement to show satisfactory operation under critical operating conditions with hot fuel. The guidance which follows should be sufficient to establish compliance with § 29.961, in total, without regard to the misleading specifics of § 29.961(b) and (c).
b. Procedures.
(1) The fuel type to be used should be that with the highest true vapor pressure (TVP) at the 110° F condition.
(2) The fuel should be heated as rapidly as possible since the longer fuel is heated the more vaporization occurs resulting in unconservative test results. Likewise, heating the fuel above the target temperature, then allowing it to cool will “weather” the fuel excessively resulting in a reduction in Reid Vapor Pressure and unconservative testing.
(3) If the test is performed at cool ambients, the fuel lines, tanks, etc., may have to be insulated to ensure that the fuel inlet temperature is approximately the same as would be experienced on a hot day. This should be verified by instrumenting the fuel temperature at the engine inlet.
(4) The fuel level should be the lowest consistent with test safety. The reference to full fuel tanks in § 29.961(c)(2) is misleading because:
Page E - 105
(i) Section 29.955(a)(2) would require adequate fuel flow under low fuel level conditions.
(ii) The provision of § 29.961(a)(2) to verify satisfactory hot fuel operation “under critical operating conditions” would mean verification at maximum rate of climb and maximum fuel suction head. The maximum fuel suction head would occur with lowest fuel level.
(5) The flight tests to the service ceiling should include maximum power climbs to selected intermediate altitudes where various maneuvers including the following are performed:
(i) Low power descent with rapid transition to takeoff power.
(ii) Turns and cyclic pull-ups with load factors comparable to the flight strain survey.
(iii) For multiengine rotorcraft with 30-minute and/or 2.5-minute OEI power ratings, conduct a rapid single-engine acceleration from low power to engine topping power followed by cruise at 30 minute OEI power.
(6) The flight test maneuvers should be repeated at the service ceiling.
(7) Except for transients and descents, the power available used should correspond to a 100° F sea level day lapsed 3.6° F/1,000-foot pressure altitude.
(8) Engine operation throughout the test should be normal; i.e., no surge, stall, flameout, etc., and the engine fuel inlet requirements should not be violated.
(9) Alternative tests on appropriate test rigs may be conducted ensuring proper simulation of altitude, ambient temperature, fuel temperature, fuel flow, and load factors.
AC 29.961A. § 29.961 (Amendment 29-26) FUEL SYSTEM HOT WEATHER OPERATION.
a. Explanation. Amendment 29- 26 simplifies and restates the fuel system hot weather certification requirements. This eliminates detail requirements in the existing rule which were, to some extent, redundant, or not necessarily critical for some rotorcraft. The phrase, “including, if applicable, the engine operating conditions defined by §§ 29.927(b)(1) and (b)(2),” was added to ensure that certain critical certification aspects are properly considered.
b. Procedures. This paragraph specifies that all suction lift systems and any other fuel system that may be conducive to vapor formation, show satisfactory engine fuel inlet conditions (within criteria established by the engine manufacturer) when using the fuel with the highest true vapor pressure (TVP) at 110° F fuel temperature. Engine Page E - 106
operating conditions should include those defined by §§ 29.927(b)(1) and 29.927(b)(2).
Compliance can be shown by analysis, testing, or a combination of both.
AC 29.963. § 29.963 FUEL TANKS: GENERAL.
a. Explanation. Section 29.963(a) sets forth general requirements for fuel tank structural aspects. Paragraph (b) requires design features to react forces defined by § 29.561 without leaking fuel. Paragraph (c) requires that whenever flexible fuel tank liners are used, they must be FAA/AUTHORITY approved. Paragraph (d) requires that integral fuel tank interiors be inspectable and repairable.
b. Procedures.
(1) For compliance with § 29.963(a), the tests of § 29.965 are normally adequate if performed in conjunction with the reliability test of § 21.35 or other simulation tests.
(2) For compliance with § 29.963 (b), a structural analysis is usually required to show adequate strength under the loads of § 29.561. Testing, if proposed, may also be an acceptable method of compliance.
(3) For compliance with § 29.963(c), prior FAA/AUTHORITY approvals should be reviewed to ensure compatibility with current project requirements. Also, if a new approval is required as part of the project, then analysis and/or tests should be conducted as appropriate to ensure compliance.
(4) For compliance with § 29.963(d), a review of the design data and/or a visual inspection of any prototype available for inspectability and repairability considerations is usually sufficient to determine compliance. Features such as inspection ports and access panels are typical methods of compliance.
AC 29.963A. § 29.963 (Amendment 29-26) FUEL TANKS: GENERAL .
a. Explanation. Amendment 29- 26 adds § 29.963(e) that requires designs and tests to ensure that no exposed surface inside a fuel tank would, under normal or malfunction conditions, constitute an ignition source. It also sets 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 29.963(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 29.1185b(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 Page E - 107
conditions are established with sensitive temperature and pressure measuring equipment. This equipment is installed inside the tanks and in the ventilation air spaces.
AC 29.963B. § 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 lbs. 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 AC 29.952 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.
(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 AC 29.952 guidance material for details.) The design of the Page E - 108
enclosure should also be reviewed for overall durability and resistance to all reasonable occupant abuses that could cause a hazard to the integrity of the enclosure, the fuel tank, its vents and its drains.
AC 29.965. § 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.
(ii) 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.
(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, gauged 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.
Page E - 109
(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(f), 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 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).
Page E - 110
(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.
AC 29.967. § 29.967 FUEL TANK INSTALLATION.
a. Section 29.967(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. Section 29.967(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 spaces.
(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, Page E - 111
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. Section 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.
d. Section 29.967(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 a dye from engine compartment openings during flight.
e. Section 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.
Page E - 112
f. Section 29.967(f):
(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 features 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.
AC 29.967A. § 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 AC 29.967, subsection (e) now apply under paragraph AC 29.963B. 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.
AC 29.969. § 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.
Page E - 113