SAE AIR 4069C-2016 Sealing of Integral Fuel Tanks.pdf

1、_ 6$(7HFKQLFDO6WDQGDUGV%RDUG5XOHVSURYLGHWKDW7KLVUHSRUWLVSXEOLVKH GE6$(WRDGYDQFHWKHVWDWHRIWHFKQLFDODQGHQJL neering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising tKHUHIURPLVWKHVROHUHVSRQVL

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4、0 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/standards.sae.org/AIR4069C AEROSPACE INFORMATION REPORT AIR4069 REV. C Issued 1990-10 Revised 2016-11 Superseding AIR4069B Sealing of Integral Fu

5、el Tanks RATIONALE This SAE AIR is being revised, 5 year review, to update applicable sealant specifications and review/update engineering designs and sealing practices for producing reliable aircraft integral fuel tank sealing systems. TABLE OF CONTENTS 1. SCOPE 4 2. REFERENCES 5 3. SEALING PHILOSO

6、PHY 5 3.1 Sealing Objectives and Basic Categories . 5 3.1.1 Designs Providing Interior Access 5 3.1.2 Designs with Limited Access . 6 3.2 Redundancy in Sealing to Mitigate Technical Risk . 8 3.2.1 Faying-Surface Sealing . 8 3.2.2 Wet Installation and Overcoating of Fasteners . 8 3.2.3 Fillet Sealing

7、 - Extrudable Sealant (Class B) Over Brushable (Class A or Class C) Fuel Tank Sealant 9 3.2.4 Topcoats (AMS-S-4383 Buna-N) Over Fuel Tank Sealant . 9 3.2.5 Adhesion Promoters for Polysulfide/Polythioether Sealants . 9 3.2.6 Redundancy in Sealing Groove Designs and Adhesively Sealed Designs . 9 3.3 C

8、omposite Construction - Sealing Philosophy 9 4. SUBSTRATES 10 4.1 Corrosion-Preventive Coatings . 10 4.2 Titanium 10 4.3 Composite Surfaces 10 5. SURFACE PREPARATION (REFERENCE AS5127) . 11 5.1 Cleaning 11 5.1.1 Important Points to Remember Concerning Surface Cleaning . 11 5.2 Special Considerations

9、 Regarding Surface Preparation . 12 5.3 Adhesion Promoters 12 5.3.1 Polysulfide/Polythioether Sealant Adhesion Promoters 12 5.3.2 Polyurethane Sealant Adhesion Promoter 13 SAE INTERNATIONAL AIR4069 and discusses the most practical and conservative methods for producing a reliable, sealed system. Alt

10、hough this AIR presents practices for sealing of integral fuel tanks, the practices presented within this report are practices that are carried throughout sealing that include both pressure and environmental aircraft sealing. Design preferences for optimum sealing are not within the scope of this do

11、cument. Such discussions can be found in the United States Air Force (USAF) sponsored report, entitled Aircraft Integral Fuel Tank Design Handbook, AFWAL-TR-87-3078. Key objectives of the fuel tank sealing process are to produce a sealing plane that is leak-free and corrosion resistant, especially a

12、t fastener locations, at environmental and operational conditions expected for the life of each aircraft. Factors that can influence the outcome of this process are: a. How well the basic design lends itself to good sealing (key design factors to consider include accessibility and minimal movement,

13、among others). b. The choice of sealant; where it is applied; how it is applied. c. How good substrate surfaces are prepared. d. Whether sealant fillet dimensions are optimum for aircraft configuration and flight dynamics. e. The degree of resistance sealant has to fluid and thermal environments. f.

14、 The degree of engineering insurance - i.e., application of adhesion promoters to sealant bond surfaces, application of organic topcoats over sealants, proper drainage of the fuel tank, etc. - employed for technical risk reduction. Effective and efficient sealing of aircraft fuel tanks are prime con

15、siderations in both commercial and military aircraft designs. Note, fillet seals inside fuel tanks are considered to be the primary fuel barrier seals. Of nearly equal importance is corrosion control. It is generally accepted, for example, that a major objective of faying-surface sealing is corrosio

16、n control. The faying-surface seal is not considered to be a primary fuel barrier seal, except in adhesive-bonded systems; the faying surface seal, however, does play an extremely important role as a secondary fuel barrier seal. It limits the length of a leak path and is a permanent, stable, protect

17、ed, and essentially non-dislodgeable seal that is sandwiched between two mating surfaces. Extensive use of faying-surface sealing is highly recommended. Sealing philosophies differ within industry and government. However, as stated in 3.1, there is much greater agreement than dispute. If a particula

18、r fuel tank sealing approach appears to be clearly more reliable, keeping production cycle time and costs in mind, it will be identified as a preferred method. This report is based on technical opinions from a broad cross-section of engineering experts who specialize in aircraft sealing. The user sh

19、ould consider the engineering requirements and options provided by this report; then develop an individual course (or plan) of action from a somewhat more informed position. SAE INTERNATIONAL AIR4069 or (2) retightening or replacing certain types of fasteners from the exterior; or (3) gaining access

20、 to the interior of the tank through disassembly of structure, as necessary, to reach the leak source (see Section 12). Except for channel sealing, all sealing is performed inside the fuel tank. Factors to be considered in sealing integral fuel tanks for the two basic categories are given in the fol

21、lowing paragraphs. 3.1.1 Designs Providing Interior Access Leak paths found in interior access designs originate from skin splice joints, spar-to-web joints, dry-to-wet bay interfaces, and access doors. Faying surfaces, lap joints, butt joints, fastener holes, and any other possible path between wet

22、-to-dry sides represent opportunities for fuel leakage and for moisture intrusion, which may produce corrosion. (Moisture from water that has combined with fuel inside the tank also is a potential source of corrosion.) It is generally accepted by the industry that the primary seal plane is formed by

23、 sealant applied inside the fuel tanks at butt joints, lap edges, and fasteners. Fay-surface sealing is also considered to be an important type of sealing because it limits the length of leak paths, contains fuel, prevents corrosion, and protects against fretting. Fay-surface sealing is an essential

24、 adjunct to fillet and fastener sealing to produce a reliable, sealed fuel tank that does not leak during the operational life of the aircraft. 3.1.1.1 Fasteners and Joints All fasteners and rivets (even interference fit fasteners) at fuel tank boundaries should be wet installed and overcoated to en

25、sure that the tank does not leak. All fasteners that penetrate dissimilar metals must be wet installed with fuel tank sealant; for example, see Figure 2 of ARP4118. If pre-cleaned fasteners are not available, the fasteners to be wet installed shall be cleaned, using AMS3167, prior to installation. I

26、t is mandatory that cadmium-plated fasteners be coated with a fuel resistant epoxy primer before applying adhesion promoter. Solid-film lubricants that meet the engineering material property requirements of MIL-PRF-46010 need not be removed provided adhesion promoter is used. MIL-L-23398 solid-film

27、lubricant coating, on the other hand, must be removed and adhesion promoter should be applied before sealant application. Corners and joggles shall be injected or pre-packed with fuel tank sealant, except at a junction where three planes meet. Such a location shall be fitted with suitable reinforcem

28、ent such as corner fittings or screens. Surge boxes and vent boxes should be sealed on both sides. Both sides of common fuel and pressure bulkheads should be sealed. All butt joints, lap joints, and seams should be sealed with full fillets. 3.1.1.2 Faying Surfaces Fay-surface sealing should be emplo

29、yed on all lap joint surfaces in fuel tank boundaries and under all attachments to the fuel tank. Major examples of fay-surface sealing applications include: a. Skin-to-stringer interfaces. b. Web-to-spar chord. c. Spar chord-to-wing skin. SAE INTERNATIONAL AIR4069 the aircraft manufacturing industr

30、y refers to this method as SUHVVXULHGJURRYH-LQMHFWLRQVHDOLQJ 3.1.1.3 Considerations in the Adhesive Sealing of Tanks Adhesive sealing of faying surfaces of an integral fuel tank with a structural film adhesive requires particular design characteristics and special processes and equipment. Mating sur

31、faces must not mismatch by more than 0.015 inch (0.38 mm). Tooling must be inspected and verified periodically. After application of film adhesive, full clamp-up of structural fasteners must be accomplished to produce the necessary mechanical force (or pressure) to ensure satisfactory adhesive bondi

32、ng. Lockbolts, threaded fasteners, and crimp pins installed with non-metallic seal washers must be located in wet-to-wet areas of the fuel tank. The entire assembly must be placed in an oven to cure the adhesive per manufacturer instructions. Usually, structural voids at fuel tank boundaries are fil

33、led with polysulfide or polythioether sealant after oven cure is completed. Sometimes a high-temperature sealant (conforming to AMS3276, AMS3277, AMS3281) can be used to prepack voids. 3.1.2 Designs with Limited Access Designs with limited (or no) access should use pressurized groove sealing or chan

34、nel sealing to facilitate fuel tank sealing rework or repair. SAE INTERNATIONAL AIR4069 while others have the groove in-line with the fasteners, so that sealant will surround each fastener. The groove design offers distinct advantages in thin-winged aircraft, such as military fighters, where interna

35、l access is extremely limited or impossible. This design offers a major advantage to this type of aircraft, since repairs can be made quickly under battle conditions using a high-pressure sealant injection gun and an adapter. Fuel tanks need not be emptied to repair a leak. This design is selected f

36、or other aircraft due to the simplicity of maintenance and repair. Grooves are machined into aluminum or titanium alloy component parts of the fuel tank faying surfaces; they are molded into carbon-reinforced epoxy, carbon-reinforced bismaleimide, or other composite parts. In the channel for pressur

37、ized, non-curing groove-injection sealant, fastener spacing should be a maximum of six times the diameter of the fastener. The minimum diameter of fastener used in channel seal grooves is 0.250 inch (6.35 mm). Ribs that do not assemble to skin splices, but which form the internal structure, do not n

38、ormally require channel sealing. Self-sealing fasteners are used (Figure 1). Fasteners not located in the groove path are of self-sealing and can be tightened or replaced from the outside. Figure 1 - Rib/Skin assembly with self sealing fastener The size of the groove, the distance between the inject

39、ion ports, and design features that can inhibit the flow of sealant through the groove are factors related to design that greatly affect the ease of sealing and, eventually, seal integrity. In many groove designs, joint intersections produce voids or open spaces in the groove line. Restrictions, cal

40、led dams, made with polysulfide or polythioether sealant (AMS-S-8802, AMS3276, AMS3277, or AMS3281) or epoxy adhesive are formed-in-place to preserve the continuity of the groove. Dam configuration includes chamfered lands at the ends (or terminations) of each dam. Metal clips (or retainers) may als

41、o be used to hold the dams in place. It is best to fay surface seal around the grooves to provide the best seal. Shims should be avoided as much as possible. However, if a shim be required, it should not go across the groove and should be fay surface sealed. Liquid shims made of epoxy, or adhesively

42、 bonded solid or laminated shims, are used to improve the basic structural fit-up. Except for all fuel tank boundary locations, liquid shims may be used in integral fuel tanks, unless specifically prohibited by the engineering drawing. Note: Flexible liquid shims may be used in fuel tank boundaries

43、provided testing is performed to determine thickness requirements and have been approved for use by appropriate engineering. The use of shims should be controlled by engineering drawings or applicable specifications for allowable thickness and number of shims in accordance with the aircrafWPDQXIDFWX

44、UHUVGRFXPHQWV SAE INTERNATIONAL AIR4069 but these bead-filled sealants can be more difficult to extrude because of their relatively-high viscosity. One technique that may be used to alleviate this problem is to warm the bead-filled channel sealant to approximately 95 F (35 qC) maximum, to improve th

45、e extrudability (or flow) of this sealant. Sealant is injected at pressures of several thousand pounds per square inch by sealant guns designed to augment the incoming plant air pressure of 80 to100 psi (552 to 690 kPa) by approximately 40 to 50 times. Under these high injection pressures, it is pos

46、sible to damage structure or distort aircraft wing skins, thus injecting (losing) sealant into the tank. Design thicknesses for tank components in the area of channel sealing shall be able to withstand injection load pressures without causing damage to the surrounding structure. Pressure regulators

47、on high-pressure channel sealant injection guns are essential. Sealers must be made aware of injection pressure restrictions for the particular design they are servicing. Groove sealant can be extruded with as little as 40 psi (276 kPa) input pressure from plant air, using a 70:1 ratio. Common failu

48、re mechanisms causing leaks in channel-sealed aircraft include: a. Excessive mismatch of parts i.e., greater than 0.005 inch (0.13 mm). b. Large joint deflections. Particular attention must be given to selection of a channel sealant that ensures good performance, consistent with the aircrafts design configuration and expected operational environment. 3.2 Redundancy in Sealing to Mitigate Technical Risk Fuel leaks have been one of the mos