GB2279930A - Aircraft structures - Google Patents

Abstract

To limit the tearing of the outer skin of an aircraft structure, eg the fuselage, as a result of rupture caused for example by an explosion, the skin of the structure has applied to it strips 2 of crack-arresting material which intersect at spaced intervals likely paths of crack or tear propagation. The crack-arresting material may be adheringly bonded tape reinforced with high tensile strength longitudinal fibres. <IMAGE>

Description

THE PROTECTION OF AIRCRAFT STRUCTURES This invention relates to the protection of aircraft structures, in particular aircraft hulls or fuselages, from the effects of a rupture in the skin of the structure caused by an explosion within the aircraft or other phenomena, typically resulting in decompression of the aircraft hull.

Analysis of the wreckage or damage caused to passenger aircraft as a result of internal explosions or other phenomena causing rupture of the fuselage hull indicates that a major contribution to the survivability of the aircraft in such an incident is the extent to which, following initial rupture of the hull, cracks or tears propagate along the hull causing further detachment of the skin and resulting in a greatly increased aperture. These result from the effects of the outrush of air from the pressurised hull, and the slipstream of the aircraft.

Those incidents in which the aircraft have survived rupture of the hull skin are characterised by relatively well defined ruptures, and it has been observed that any cracks which were generated, propagate only a short distance before terminating or diverting through 900 because of the presence of a substantial bulkhead, the edge of a skin panel or a line of rivets or some other feature of the underlying skeleton of the aircraft.

It has previously been proposed to provide protection against the effects of an explosive device on board an aircraft by providing blow-out panels in the area of the hull closest to the region occupied by cargo containers, ie the lost likely location for an explosive device (see for example International Patent Application No. PCT/GB90/01724 - Publication No. WO 91/07337).

This involves the venting, through pre-weakened blow-out panels in the aircraft skin, of local excess pressure produced by the explosive device.

The purpose of this is to reduce the effects of such overpressures on the remainder of the aircraft structure and to ensure that the rupture(s) in the aircraft skin caused by the pre-weakened panels being blown out by the explosion are confined to predetermined locations and avoid uncontrolled tearing and petalling of the aircraft skin as described above.

A disadvantage of this approach is that it does require pre-weakening of the aircraft structure which is potentially undesirable to aircraft designers, legislators and insurers.

It is therefore an object of the present invention to provide an aircraft structure having means for protecting it from the effects of an explosively or non-explosively induced rupture in the skin thereof during flight, but without inherently weakening it.

According to the present invention, an aircraft structure which includes an outer skin, has applied to the surface thereof crack-arresting strips of material which intersect at spaced intervals the likely paths of crack or tear propagation in said outer skin in the event of a rupture in the outer skin during flight.

Preferably the aircraft structure is a fuselage or hull, and the strips of material are applied to the outer skin both longitudinally and transversely with respect to the axes of the fuselage, preferably defining between them generally rectangular areas of the outer skin. The strips of material are preferably spaced apart at intervals of between 500mm and 1600mm inter alia to limit the size of any likely rupture, and correspondingly, the maximum dimensions of any debris which might pass through the rupture and cause further damage to the aircraft structure.

The strip material is preferably adhesively bonded to the surface of the outer skin and conveniently may comprise self-adhesive tape reinforced with high tensile strength longitudinal fibres, such as glass fibres.

The invention thus provides a solution to the problem of limiting the damage caused to the outer skin of an aircraft structure by, for example, an internal explosion, by arresting the propagation of cracks and tears which have been found to otherwise occur following such a rupture during flight.

The solution proposed by the invention may advantageously be applied as a retrofit to existing aircraft, and even to aircraft of new manufacture.

Furthermore, the invention does not in any way affect or reduce the as-built strength of the aircraft structure as would be the case in other proposed solutions involving pre-weakened panels in the aircraft skin.

The invention is preferably used in conjunction with other measures for protecting the aircraft structure from the effects of an on-board explosive blast. These include the use of hardened cargo containers designed to at least partially contain the effects of an explosive blast within the container as described, for example, in copending International Patent Application Nos. PCT/GB93/00893 and PCT/GB92/02379.

Alternatively, for narrow-bodied aircraft, the cargo hold may be lined with blast-absorbing materials such as those described in our copending International Patent Application No. PCT/GB90/01723 (Publication No.

WO91/07275), which materials may also be used in the construction of hardened luggage containers.

Additionally, blast suppressing and absorbing materials and valves may be incorporated into the air channels and manifolds around the aircraft structure to inhibit the propagation of blast wave energy along these channels and manifolds as desribed, for example, in our US Patent No.

5190248.

The invention will now be described by way of example only with reference to the accompanying drawing which is a schematic side elevation of part of an aircraft embodying the invention.

Referring now to Figure 1, a region of the hull of the passenger aircraft 1 which is susceptible to an on-board explosive blast, ie the region 3 containing the cargo hold (shown outlined by broken lines), is provided with a latticework of crack-arresting strip material 2 adhesively bonded to the outer surface of the fuselage skin.

The strip material 2 is in the form of a tape similar to the fibre reinforced "shipping tape" used to bind up large parcels, cardboard, or other types of box or crates of merchandise. It consists of several hundred strands of 1 GPa tensile strength glass fibres laid in parallel in a flexible, adhesive epoxy resin base. The epoxy resin should be capable of retaining its strength at temperatures between -600 and +600 centigrade, immune from effects of ultra violet radiation and resists creep and delamination. It also be compatible with the paints used on aircraft and remain unaffected by de-icing fluids.

The strip material 2 is attached to the cleaned metal of the aircraft skin prior to painting of the aircraft. The effect of painting over the top of the strip material would be to fair it in.

The choice of where the strip material 2 is to be affixed to the aircraft skin depends upon the local structure of the aircraft. The effect of such an arrangement is to create a series of areas of hull in the region of the cargo bays, which, while not weaker than the rest of the hull, will cause any explosive-induced cracking and hull loss initiated within those areas to be confined to those areas. The generation of ragged-edged tears in the hull, which invite further damage in the airflows from both the slipstream and the outflow of cabin air, will thus be suppressed and further damage to the hull will be minimised.

The optimum location of these areas are along the region adjacent the cargo bays and at other locations, dependent upon the internal structure of the aircraft, where there is a possibility of the aircraft skin being burst by local overpressures resulting from an explosion within the aircraft. These latter locations cannot be stated very precisely in general but can be determined for individual aircraft structures. They depend critically on the internal structure of the aircraft and the distances travelled by, and degree of channelling of the gases from the explosion and may be several metres away from the location of the explosive device.

The criteria to be followed, in addition to those pertaining to the preferential lines of adhesion of the strip material, are that the areas to which the cracking and skin separation is to be confined should be as small as possible in order to minimise the amount of debris discharged into the slipstream, usually as a result of decompression of the aircraft cabin.

The preferred locations for the application of strip material in accordance with the invention are at lap or butt joints in the skin of the aircraft hull, especially when such joints are associated with the presence of ribs, along strong longerons (spacers or stretchers), along window belt or passenger or cargo door reinforcement, and at strong bulkheads.

The exact positioning of the tape relative to rivet lines is important. Rivet or screw heads should remain exposed as far as possible, to facilitate checking during maintenance. However, if the choice is between covering rivet or screw heads and not having the adhesive tape placed in the optimum position, the latter condition should be paramount. The locations of the strip materials should be arranged so as to optimise the cracksuppressing features of the skin, reinforcing them where they exist already, and introducing them where they do not.

Crack arresting strip material 2 is applied in both longitudinally and circumferentially with respect to the axis of the aircraft hull. This is because it has been found that when a crack propagating along the aircraft skin meets an intersecting crack-arresting feature, eg the strip material 2, whilst propogation of the crack in its original direction is arrested, the crack is forced through 900 and 'rung-off' by the strip material. Thus providing strip material in both longitudinal and circumferential directions serves to arrest propagation of a crack in both its original and 'rung-off' directions thereby confining it to the area defined by the strip material The location of the longitudinal tapes should be relatively easy to determine, as they will be largely dictated by the regions of the cargo holds and the window belts.However, especially on wide bodied aircraft, attention needs to be paid to protecting regions of the upper part of the fuselage. Where there are horizontal hull panel joints, the location of the longitudinal tapes suggest themselves. However, if there is a step in a join, then there must also be a circumferential tape to "ring off" a regularly shaped area, rather than allow the possibility of ragged holes developing.

The positioning of the circumferential or transverse tapes should be such that they are no more than about three bays apart. This corresponds to about 1525mm. In some circumstances it might be less, say 2 bays, or 1020mm, but much depends upon the disposition of the joints in the different sections of the hull skin. The general shape of each of the areas to be protected is preferably rectangular, and it makes no difference if the long axis is vertical or horizontal. However, it is preferred that neither axes be more than about 1600mm in length in the region of a cargo container hold. The reason for this is that it restricts the size of the aperture and prevents the egress of whole cargo containers and other large items of debris. Although it is not possible to prevent the egress of smaller items of debris, it should be possible to prevent the loss of any individual item which could so badly damage the aircraft's aerodynamic surfaces that the aircraft would be rendered uncontrollable.

It will be appreciated that whilst the specific embodiment of the invention described employs self-adhesive epoxy tape reinforced with glass fibres, other forms of strip material may be used, the only requirement being that it serves to arrest or limit the propogation of cracks or tears in the skin of the aircraft structure.

Claims (9)

1. An aircraft structure which includes an outer skin has applied to the surface thereof strips of crack-arresting material which intersect at spaced intervals likely paths of crack or tear propagation in said outer skin in the event of a rupture in the outer skin during flight.

2. An aircraft structure as claimed in Claim 1, wherein the aircraft structure is a fuselage or hull and the strips of material are applied to the outer skin both longitudinally and transversely with respect to the axis of the fuselage or hull.

3. An aircraft structure as claimed in Claim 1 or Claim 2, wherein the strips of material define between them generally rectangular areas of the outer skin.

4. An aircraft structure as claim in any one of Claims 1 to 3, wherein the strips of material are spaced apart at intervals of between 500mm and 1600mm.

5. An aircraft structure as claimed in any preceding Claim, wherein the strips of material are adhesively bonded to the surface of the outer skin.

6. An aircraft structure as claimed in Claim 5, wherein the strip material is self-adhesive.

7. An aircraft structure as claimed in Claim 5 or Claim 6, wherein the strip material is reinforced with high tensile strength longitudinal fibres.

8. An aircraft structure as claimed in Claim 6, wherein the strip material comprising an epoxy resin tape reinforced with high tensile strength longitudinal glass fibres.

9. An aircraft structure as claimed in Claim 1 and substantially as described with reference to the accompanying drawing.