Patent Description:
Large passenger aircraft are typically struck by lightning once or twice a year, each lightning bolt striking with up to <NUM>,<NUM> amps of electrical current that seeks the path of least electrical resistance. Many modern passenger aircraft have exterior surfaces made from composite materials which have a very high electrical resistance. There is therefore a high probability of lightning attachment at any of the many metallic fasteners in the exterior surface, which have a much lower electrical resistance. In the wing, some of these fasteners pass through the outer wing skin into the fuel tank.

<FIG> is a side view of a fastener <NUM> passing through a panel <NUM>, which may be a composite or metallic panel. The type of fastener shown in <FIG> is commonly known as a blind fastener, as it allows the fastener <NUM> to be fixed in place from only one side of the panel <NUM>. The blind fastener <NUM> comprises a bolt <NUM> comprising an axially extending shaft <NUM>, a head of the shaft <NUM>, and a tubular sleeve <NUM> fitted around the shaft <NUM>. The shaft <NUM> has a threaded portion on its outer circumference at one end which is the opposite end to the head of the shaft <NUM>. The tubular sleeve <NUM> has a corresponding thread on its internal circumference such that the tubular sleeve <NUM> will travel along the shaft <NUM> as it is rotated. The blind fastener <NUM> also has a collar <NUM> provided around a head end <NUM> of the bolt <NUM>. The collar <NUM> abuts the head <NUM>, and has a flange <NUM> at the other end.

During installation, the fastener <NUM> is slid through an aperture in the panel <NUM> until the flange <NUM> has at least passed through the other side of the panel <NUM>. The shaft <NUM> is then rotated to cause the tubular sleeve <NUM> to be drawn towards the collar <NUM>. When the tubular sleeve <NUM> contacts the collar <NUM>, the tubular sleeve <NUM> deforms along the flange <NUM>, and splays outwardly forming an expanded portion on the opposing side of the panel <NUM> to the head of the shaft <NUM>, therefore acting as a fastener, as shown in <FIG>.

In the event of a lightning strike hitting the panel <NUM> and attaching to the fastener <NUM>, sparking or plasma or out-gassing may occur.

With the above arrangements, the panel <NUM> may provide a fuel tank boundary and the fastener <NUM> may therefore be immersed in fuel or fuel vapour rich gas. A lightning strike at the fastener <NUM> may therefore provide sparking and hot gas ignition sources which could cause ignition of the fuel.

It is well known to suppress such sparking by enclosing fasteners within spark containment caps. <FIG> shows the fastener <NUM> of <FIG> enclosed within an example spark containment cap <NUM> as known in the prior art. The spark containment cap <NUM> is placed over the tail end of the fastener <NUM> that protrudes from the panel <NUM>, and can then be fixed into place with adhesive (not shown). The adhesive can be injected into the skirt <NUM> through the tube <NUM>.

Blind fasteners allow for fastening from a single side of the panel, which can greatly simplify the assembly process. However, the assembly still requires access to the rear of the panel once the fastener hole has been drilled or the fasteners have been positioned to allow for the positioning of the spark containment caps.

A number of different examples of existing spark containment caps can be found in the following prior art documents: <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

Although not claimed, there is herein described a spark containment cap for enclosing an end of a fastener protruding from a structure; the enclosed body having a base and a cover portion; the cover portion extending from the base; the base providing an adhering surface for adhering the spark containment cap to a structure; and the base and the cover portion together defining a cavity within the enclosed body inside which the end of a fastener can be enclosed.

With such an arrangement it is possible to fix the spark containment cap in place on a structure prior to a fastener being fastened to the structure, and even before the hole for the fastener is made. As such, it is not necessary for there to be access required to a rear side of the structure after the hole is drilled and the fastener is put in place. This can enable a much quicker, simpler and more efficient assembly process.

The base may comprise a sacrificial portion into which an opening can be formed, such that the end of a fastener can be received into the cavity through said opening. As such, the hole can be formed after the spark containment cap has been fixed in place, and it eliminates any need for highly accurate alignment of the cap and a hole in the structure.

The enclosed body may be shaped substantially as a hollow conical frustum. This is a space efficient shape which is able to closely encapsulate a blind fastener, and reduces wasted space inside the structure taken up by the spark containment cap.

The base may comprise a plurality of projections on the adhering surface. As a result, the plurality of projections serve to maintain a separation distance between the base of the cap and a surface against which it is positioned, and can improve the strength of an adhesive bond between the two.

The enclosed body may be manufactured as a single piece and may be manufactured using a blow moulding process. As such, the spark containment cap can be manufactured very simply, and at very low cost compared to a more traditional spark containment cap which tends to be formed as a complex injection moulded part.

The enclosed body may comprise one or more alignment features. Each of the one or more alignment features may be a rib formed in the cover portion of the enclosed body, or may be a surface marking on the outside surface of the cover portion of the enclosed body. This may aid the accurate positioning of the spark containment cap on the structure, particularly if the cap is positioned prior to a hole being drilled in the structure for the fastener.

The diameter of the cap may be greater at the base than at the top of the cover.

The base may comprise an adhesive pad on the adhering surface. The adhesive pad may allow the spark containment cap to be held temporarily in place on structure whilst an adhesive is applied and cured, and may prevent accidental movement of the cap prior to the adhesive being cured.

The adhesive pad may be positioned over less than half of the diameter of the base of the cap. As a result, there is still enough area of the base outside of the adhesive pad to act as an adhering surface to allow for a strong enough adhesive bond between the spark containment cap and the structure.

An edge formed between the base and the cover portion may be rounded, and the rounded edge may have a first radius.

When the spark containment cap is positioned on a structure with an edge having a second radius, the first radius and the second radius may be substantially the same. As a result, the spark containment cap can be positioned close to features or components on the structure such as ribs and rib feet, which may be beneficial for the optimal positioning of the fastener, allowing for smaller, lighter fasteners, and/or fewer fasteners, which can help to save cost and weight of the assembled product.

Although not claimed, there is also described herein a fastening system comprising a spark containment cap as described in the preceding statements, and a fastener, the spark containment cap being configured to receive part of the fastener.

The fastening system may further comprise an uncured adhesive, the adhesive being curable to fix the body to the structure.

The uncured adhesive may comprise non-deformable beads. As a result, the non-deformable beads serve to maintain a separation distance between the base of the cap and a surface against which it is positioned, and can improve the strength of an adhesive bond between the two.

There is also described herein a fastener joint comprising: a structure; a fastener having a fastener end protruding from a surface of the structure; and a spark containment cap including: an enclosed body for enclosing the fastener end protruding from the surface of the structure; the enclosed body having a base and a cover portion; the cover portion extending from the base; the base providing an adhering surface for adhering the spark containment cap to the structure; and the base and the cover portion together forming a cavity within the enclosed body inside which the end of a fastener is enclosed; the base comprising a drilled hole through which the end of a fastener is received into the cavity; a cured sealing material provided between the base and the surface of the structure which secures the spark containment cap to the structure and seal a volume of gas within the cavity.

According to the invention, there is provided a method of securing a fastener and a spark containment cap to a structure according to claim <NUM>.

With such a method, true one-sided drilling and assembly can be achieved, with the spark containment cap being fitted to the structure prior to the drilling and assembly process. This method also reduces or even eliminates the need for access to the rear side of the structure after drilling has taken place, which can greatly simplify the assembly process, and improve the time taken to assemble.

The step of fixing the spark containment cap onto the surface of the structure may comprise applying adhesive between the base of the spark containment cap and the structure, and curing the adhesive.

The step of fixing the spark containment cap onto the surface of the structure may further comprise using an adhesive pad positioned between the base of the cap and the surface of the structure to hold the spark containment cap in place while the adhesive is cured. As a result, this may prevent accidental movement of the cap prior to the adhesive being fully cured.

The step of forming a hole through both the structure and the base of the spark containment cap may comprise drilling a common hole through the structure and the base of the affixed spark containment cap. With such a method, there is no need for accurate positioning of the cap with an existing hole in the structure, greatly simplifying the assembly process and reducing the time taken.

Also described herein but not claimed is an aircraft comprising at least one of a spark containment cap, and a fastening system as set out in the statements above.

<FIG> shows a cross sectional side-view of a spark containment cap <NUM>. The spark containment cap <NUM>, which may be referred to herein more simply as cap <NUM>, is shown positioned on a structure <NUM>. The structure <NUM> may be, for example, an aircraft skin panel 31a joined to another component, such as another panel 31b. The structure <NUM> in this embodiment is a composite aircraft structural component, but may be a hybrid composite-metallic component.

The cap <NUM> comprises an enclosed body <NUM>. The body <NUM> has a base <NUM> and a cover portion <NUM> which extends from the base <NUM>. The base <NUM> provides an adhering surface that can be used for adhering the cap <NUM> to the structure <NUM>, more details of which will be provided below. The base <NUM> and cover portion <NUM> together define a cavity <NUM> within the enclosed body <NUM>. It is into this cavity <NUM> that the end of a fastener can be received, which will be described in more detail below.

The cap <NUM> is manufactured as a single piece, and due to its simplistic form can be manufactured by blow moulding. Often, due to their complex shape, spark containment caps such as the prior art example shown in <FIG> are typically manufactured by injection moulding. However, blow moulding has lower tooling and production costs compared to injection moulding, and offers a far quicker, easier and cheaper manufacturing alternative, and as such the cap <NUM> may be considerably cheaper than existing spark containment caps.

The cap <NUM> is required to be formed of a material which can be blow moulded, but which is also resistant to long term exposure to fuel. Examples of such a material could be Nylon PA6, PA66 and PA12. Another example may be ULTEM™ <NUM>, although this material would need to be injection moulded, not blow moulded. These materials may require additional treatment or primer paint application to enhance adhesive properties depending on the requirements of the cap <NUM>.

The blow moulding process requires there to be a small hole in the moulding to enable the injection / inflation of the plastic into the mould. This hole will preferably be provided in the base <NUM> of the cap <NUM>. The reason for this will become more apparent later, but to summarise briefly this is because a portion of the base <NUM> is intended to be sacrificial, and a hole will be drilled through it anyway. As such, the blow moulding process hole would be smaller in diameter than the subsequently drilled hole and would not have any impact on the installation or effectiveness of the spark containment cap <NUM>.

The diameter of the cap <NUM> at the base <NUM>, as indicated by double arrow A is larger than the diameter of the cap <NUM> at the top of the cover <NUM> as indicated by double arrow B. The body <NUM> of the cap <NUM> therefore substantially takes the shape of a hollow conical frustum. This frusto-conical shape allows for the insertion of the tail end of a blind fastener into the cavity <NUM>, and also allows additional space in the cavity <NUM> near the base <NUM> for deformation of a blind fastener sleeve. The shape of the body <NUM> therefore is efficient with respect to the space that it takes up. As these spark containment caps <NUM> are often installed inside an aircraft wing, and within a fuel tank, it is important that space is not taken up unnecessarily which would reduce the effective fuel tank volume and clash with other components within the wing such as pipes. In an alternative embodiment, however, the spark containment cap could be generally cylindrical or domed in shape.

One of the biggest advantages of the cap <NUM> is that, as it has an enclosed body <NUM>, it is not necessary for the cap to be accurately aligned with an existing hole in the structure <NUM> that is intended for a fastener. Instead, the enclosed cap <NUM> can be fitted and fixed to the structure <NUM> at a position that coincides with an intended fastening point. Steps in this method will be described with reference to <FIG>.

The cap <NUM> is slightly oversized compared to the size actually required for it to properly enclose the tail end of a fastener. This slight oversizing compensates for any error in positional accuracy of a subsequent drilling step, axial misalignment of a fastener, and/or an error in the positioning of the cap <NUM> itself. For example, manufacturing and assembly techniques typically give rise to errors of up to +/- <NUM> in drilling and cap placement, and axial misalignment of a fastener tolerance is around +/- <NUM> degrees.

Once the cap <NUM> has been placed in a desired position on the surface of the structure <NUM>, as shown in <FIG>, the cap <NUM> is then fixed to the structure using either an epoxy adhesive, shim material or polysulphide sealant. For example, the epoxy adhesive may be a two-part epoxy adhesive such as <NUM>™ Scotch-Weld ™ Structural Epoxy Adhesive <NUM> or <NUM>™ Scotch-Weld ™ Structural Epoxy Adhesive EC-<NUM> B/A.

In one embodiment, an adhesive may be used that contains non-deformable beads. The non-deformable beads act to maintain a separation distance between the base <NUM> of the cap <NUM> and the surface of the structure <NUM>. This ensures a minimum bond thickness equal to the diameter of the beads, thus strengthening the bond between the cap <NUM> and the structure <NUM>. An example of such an adhesive is <NUM>™ Scotch-Weld ™ EC-<NUM> B/A Epoxy Adhesive *IPS <NUM>-<NUM>-<NUM>-<NUM> which contains glass beads that ensure a minimum bond line thickness of <NUM>.

As described earlier, the base <NUM> provides an adhering surface that can be used for adhering the cap <NUM> to the structure <NUM>. Adhesive is applied to the base <NUM> and the cap is positioned on the surface of the structure <NUM> and allowed to cure. Excess adhesive may form a bead <NUM> around the periphery of the cap as shown in <FIG>. This bead <NUM> helps to strengthen the bond between the cap <NUM> and the structure <NUM>, and also later acts to seal the cavity <NUM> inside the body <NUM> once a fastener is fitted in place in order to prevent escape of outgassing products. The seal between the cap <NUM> and the structure <NUM> also prevents the ingress of fuel, water or other contaminants into the cavity <NUM>, and also prevents plasma or other out-gassing products from exiting the cavity in the event of a lightning strike.

As shown in <FIG>, once the cap <NUM> is fixed to the structure <NUM>, a hole <NUM> is drilled through the structure <NUM> through which a fastener can be inserted. A drill bit <NUM> is used to drill through the structure <NUM> at the intended fastening point position, extending through the structure and then back out again, as represented by the arrow C. The drill bit <NUM> is shaped such that a countersunk hole <NUM> is produced to correspond with the shape of the blind fastener to be used. As the cap <NUM> is now fixed to the rear of the structure <NUM> at the intended fastening point position, the drill bit also creates a hole <NUM> through a sacrificial portion of the base <NUM> of the cap <NUM>. As such, a common hole is created through the structure and the base of the affixed spark containment cap.

In the next step, shown in <FIG>, a blind fastener <NUM> is inserted through the hole <NUM>. The blind fastener <NUM> is of the same type described in relation to <FIG>, and comprises a bolt <NUM> comprising an axially extending shaft <NUM>, a head of the shaft <NUM>, and a tubular sleeve <NUM> fitted around the shaft <NUM>. The shaft <NUM> has a threaded portion on its outer circumference at one end which is the opposite end to the head of the shaft <NUM>. The tubular sleeve <NUM> has a corresponding thread on its internal circumference such that the tubular sleeve <NUM> will travel along the shaft <NUM> as it is rotated. The blind fastener <NUM> also has a collar <NUM> provided around a head end of the bolt <NUM>. The collar <NUM> abuts the head, and has a flange <NUM> at the other end.

During installation, the blind fastener <NUM> is slid through the drilled hole in the structure <NUM> until the flange <NUM> has at least passed through the other side of the structure <NUM> and the base <NUM> of the spark containment cap <NUM>. The shaft <NUM> is then rotated as shown by arrow D to cause the tubular sleeve <NUM> to be drawn towards the collar <NUM>, indicated by arrows E. When the tubular sleeve <NUM> contacts the collar <NUM>, the tubular sleeve <NUM> deforms along the flange <NUM>, and splays outwardly forming an expanded portion (sometimes referred to as a bulb) on the inside of the base <NUM> of the cap <NUM>.

As such it can be said that there is a fastening system provided in <FIG>, where the fastening system comprises the spark containment cap <NUM> together with the blind fastener <NUM>. The adhesive <NUM> that is used to fix the spark containment cap <NUM> to the structure also forms part of the fastening system.

<FIG> shows a fastener joint <NUM> that is the end result once the blind fastener <NUM> has been fastened to the structure <NUM> as described above. In the completed fastener joint <NUM> of <FIG>, the spark containment cap <NUM> is fixed to the structure <NUM> by a cured sealing material in the form of adhesive <NUM> provided between the base <NUM> and the surface of the structure <NUM>. The blind fastener <NUM> protrudes through the hole <NUM> formed in the structure <NUM> and the base <NUM> of the cap <NUM>, such that the end of the fastener is enclosed within the cavity <NUM> in the enclosed body <NUM> defined by the base <NUM> and the cover portion <NUM>.

<FIG> shows an alternative embodiment of a spark containment cap <NUM>. As with the previously described cap above, the cap <NUM> comprises an enclosed body <NUM>. The body <NUM> has a base <NUM> and a cover portion <NUM> which extends from the base <NUM>. The base <NUM> provides an adhering surface that can be used for adhering the cap <NUM> to a structure, and comprises a sacrificial portion that can be drilled to create a hole in the base <NUM>. The base <NUM> and cover portion <NUM> together define a cavity <NUM> within the enclosed body <NUM>.

The cap <NUM> also comprises a number of projections <NUM> provided on the outside surface of the base <NUM>. The projections <NUM> act to maintain a defined separation distance between the base <NUM> of the cap <NUM> and a surface of a structure to which the cap is to be affixed. This provides an alternative to requiring the adhesive that contains non-deformable beads described above. Instead, the projections <NUM> ensure a minimum bond thickness equal to the height of the projections, thus ensuring a strong bond between the cap <NUM> and a structure. To provide a stable base, there are preferably at least three projections <NUM> provided in a spaced apart configuration on the base <NUM>.

<FIG> also shows an adhesive pad <NUM> which can be fixed (signified by arrow F) to the adhering surface on the base <NUM> of the cap <NUM>. The adhesive pad <NUM> is able to temporarily hold the cap <NUM> to a structure whilst the adhesive or other sealing material is curing. This reduces any risk of the cap moving or becoming dislodged before curing is complete. The size of the adhesive pad <NUM> allows for a satisfactory temporary hold, but without significantly affecting the effectiveness of the adhesive/sealant bond. For instance, the adhesive pad would be less than half of the diameter of the base <NUM> of the cap <NUM>. Of course, it will be understood that an adhesive pad <NUM> may not be required if the thixotropic properties of the sealant or adhesive are sufficient to hold the cap <NUM> in position during curing.

Also shown in the embodiment of <FIG> are alignment features in the form of ribs <NUM> formed in the cover portion <NUM> of the enclosed body <NUM>. The ribs <NUM> allow for sufficiently accurate positioning of the cap when positioning it onto a structure to which it is to be fixed. Some assembly methods may use robots to position and fix the cap <NUM> in place, and in these methods, the robot would likely have built-in positioning systems to ensure accurate placement of the cap <NUM> onto a structure. However, if the cap <NUM> is to be fitted manually, then alignment features, such as the ribs <NUM> can aid the person carrying out the task.

Another embodiment is shown in <FIG> which shows a top view of a spark containment cap <NUM> positioned on a structure <NUM>. The structure <NUM> is provided with markings shown as dotted lines 82a and 82b. The markings 82a and 82b form a target, indicating where an intended fastening point is to be where the lines 82a and 82b cross. The point at which they cross is obscured by the cap <NUM>, but the lines extend far enough to be visible outside the outer diameter of the cap <NUM>. The cap <NUM> is provided with alignment features in the form of surface markings 83a, 83b, 83c, 83d. The surface markings 83a, 83b, 83c, 83d can be aligned with the markings 82a and 82b on the structure <NUM> in order to position the spark containment cap <NUM> correctly.

When used in environments such as on aircraft, it can be beneficial to install spark containment caps in close proximity to structural features such as rib fillets. By locating fasteners close to such structural features, a stronger and more reliable fastening can be achieved. As a result, smaller or fewer bolts may be necessary, which in turn can provide an optimised design that is cheaper and lighter. However, it can be challenging to fit spark containment caps close to said structural features. <FIG> shows how a spark containment cap <NUM> can be positioned on a structure <NUM> next to a rib fillet <NUM>.

The edge <NUM> formed between the fillet <NUM> and the structure <NUM> has a natural radius. The cap <NUM> is manufactured such that the edge <NUM> formed between the base <NUM> and the cover portion <NUM> has a radius the substantially matches the radius of the edge on the structure. By matching the radius on the cap <NUM> with the radius on the structure <NUM>, it is possible to fit the cap <NUM> as close to the fillet <NUM> as possible whilst not reducing the amount of adhesive area and bonding strength. For example, the cap may be manufactured such that the edge <NUM> has a radius of around <NUM>, and this will enable fitment into close proximity of a fillet on a structure having an edge of similar radius of around <NUM>.

The spark containment caps, fastening systems, fastener joints and corresponding methods of assembly may be used in any application, but most preferably the are used in an aircraft such as the aircraft <NUM> shown in <FIG>. The aircraft <NUM> includes a fuselage <NUM>. Two wings <NUM> extend from the fuselage <NUM>. It will be appreciated that the fuselage <NUM> and wings <NUM> may take a variety of different planned formed shapes and profiles depending on the particular application. Fuel tanks <NUM> are formed in the fuselage <NUM> and wings <NUM>. One such fuel tank <NUM> is schematically shown in <FIG>. The upper and lower boundaries of the tank <NUM> are provided by upper and lower skins of the wing <NUM>, and the fore and aft boundaries are provided by forward and rear spars of the wing. In the embodiments described above, the spark containment cap and the tail of the blind fastener may be located within the interior of the fuel tank <NUM>.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

For example, the adhesives described above which can be used to fix a spark containment cap to a structure are all two-part epoxy adhesives. However, in an alternative embodiment, an adhesive could be used which is cured using UV light. In this embodiment, it may be beneficial to form the spark containment cap from a transparent material which can allow UV curing of the adhesive through the cap.

Claim 1:
A method of securing a fastener (<NUM>) and a spark containment cap (<NUM>, <NUM>, <NUM>, <NUM>) to a structure (<NUM>, <NUM>), the method comprising:
fixing a spark containment cap (<NUM>, <NUM>, <NUM>, <NUM>) onto a surface of the structure (<NUM>, <NUM>), the spark containment cap (<NUM>, <NUM>, <NUM>, <NUM>) comprising:
an enclosed body (<NUM>, <NUM>);
the enclosed body (<NUM>, <NUM>) having a base (<NUM>, <NUM>, <NUM>) and a cover portion (<NUM>, <NUM>, <NUM>); the cover portion (<NUM>, <NUM>, <NUM>) extending from the base (<NUM>, <NUM>, <NUM>);
the base (<NUM>, <NUM>, <NUM>) providing an adhering surface for adhering the spark containment cap (<NUM>, <NUM>, <NUM>, <NUM>) to the structure (<NUM>, <NUM>); and
the base (<NUM>, <NUM>, <NUM>) and the cover portion (<NUM>, <NUM>, <NUM>) together defining a cavity (<NUM>, <NUM>) within the enclosed body (<NUM>, <NUM>);
once the cap (<NUM>) is fixed to the structure (<NUM>), forming a hole (<NUM>) through both the structure (<NUM>, <NUM>) and the base (<NUM>, <NUM>, <NUM>) of the spark containment cap (<NUM>, <NUM>, <NUM>, <NUM>);
inserting a blind fastener (<NUM>) through the hole (<NUM>) and tightening the fastener (<NUM>) to secure it to the structure (<NUM>, <NUM>).