Potential remedial measures

1. Introduction

In the following discussion, those damage mechanisms which appear to have contributed to the catastrophic structural failure of Flight PA103 are identified and possible ways of reducing their damaging effects are suggested. These suggestions are intended to stimulate thought and discussion by manufacturers, airworthiness authorities, and others having an interest in finding solutions to the problem; they are intended to serve as a catalyst rather than to lay claim to a definitive solution. On the basis of the Flight PA103 investigation, damage is likely to fall into two categories: direct explosive damage, and indirect explosive damage. 

2. Direct explosive damage

The most serious aspect of the direct explosive damage on the structure is the large, jagged aperture in the pressure hull, combined with frame and stringer break-up, which results from the star-burst rupture of the fuselage skin. Because of its uncontrolled size and position, and the naturally radiating cracks which form as part of the petalling process, the skin's critical crack length (under pressurisation loading) is likely to be exceeded, resulting in unstable crack propagation away from the boundary of the aperture. Such cracks can lead to a critical loss of structural integrity at a time when additional loads are likely to be imposed on the structure due to reflected blast pressure and/or aircraft aerodynamic and inertial loading.

A further complicating factor is that the size of this aperture is likely to be sufficiently large to allow complete cargo containers and other debris to be ejected into the airstream, with a high probability of causing catastrophic structural damage to the empennage. 

3. Indirect explosive damage

Indirect explosive damage (channelling or ducting of explosive energy in the form of both shock waves and supersonic gas flows) is likely to occur because of the network of interlinked cavities which exist, in various forms, in all large commercial aircraft, particularly below cabin floor level. This channeling mechanism can produce critical damage at significant distances from the source of the explosion.

In addition to the structural damage, aircraft flight control and other critical systems will potentially be disrupted, both by the explosive forces and as a result of structural break-up and distortions. The discussion which follows focuses on possible means of limiting structural damage of the kind which occurred on Flight PA103. Undoubtedly, such measures will also have beneficial effects in limiting systems damage. However, system vulnerability can further be reduced by applying, wherever possible, those techniques used on military aircraft to reduce vulnerability to battle damage; multiplexed, multiply redundant systems using distributed hardware to minimise risk of a single area of damage producing major system disruption. Fly by wire flight control systems potentially offer considerable scope to achieve these goals, but the same distributed approach would also be required for the electronic and other equipment which, in current aircraft, tends to be concentrated into a small number of 'equipment centres'.

4. Remedial measures to reduce structural damage

Whilst pure containment of the explosive energy is theoretically possible, in an aviation context such a scheme would not be viable. Any unsuccessful attempt to contain the explosive will probably produce greater devastation than the original (uncontained) explosion since all the explosive energy would merely be stored until the containment finally ruptured, when the stored energy would be released together with massive fragmentation of the containment. 

However, a mixed approach involving a combination of containment, venting, and energy absorption should provide useful gains provided that a systematic rather than piecemeal approach is adopted, and that the scheme also addresses blast channelling. The following scheme is put forward for discussion, primarily as means of identifying, by example, how the various elements of the problem might be approached at a conceptual level and to provide a stimulus for debate. No detailed engineering solutions are offered, but it is firmly believed that the requirements of such a scheme could be met from a technical standpoint. The proposed scheme is based on the need to counter a threat similar to that involving Flight PA103, i.e. a high explosive device placed within a baggage container, however, the principles should be applicable to other aircraft types.

Such a scheme might comprise several 'layers' of defence. The first two layers, one within the other, are essentially identical and provide partial containment of the explosive energy and the redirection of blast out from the compartment via pre-determined vent paths. Although the containment is temporary, it must provide an effective barrier to uncontrolled venting, preventing the escape of blast except via the pre-designated paths.

The third layer comprises a pre-determined area of fuselage skin, adjoining the outer end of the vent path, designed to rupture or burst in a controlled manner, providing a large vent aperture which will not tend to crack or rupture beyond the designated boundaries.

A fourth layer of protection has two elements, both intended to limit the propagation of shock waves through the internal cavities in the hull. The first element comprises the closure of any gaps between the vent apertures in the two innermost containment layers and the vent aperture in the outer skin. This effectively provides an exhaust duct connecting the inner and outer vent apertures to minimise leakage into the intervening structure and cavities around the cargo hold. The second element comprises the incorporation of an energy absorbing lining material within all the cavities in the lower hull, to absorb shock energy, limit shock reflection and limit the propagation of pressure waves which might enter the cavities, for example because of containment layer breakthrough. 

5 Possible application to Boeing 747 type aircraft

5.1 Container Modification

The obvious candidates for the inner containment layer are the baggage containers themselves. Existing containers are of crude construction, typically comprising aluminium sheet sides and top attached to an aluminium frame with a fabric reinforced access curtain, or have sides and top of fibreglass laminate attached to a robust aluminium base section.

These containers are stacked in the aircraft in such a manner that on three sides (except for the endmost containers) the baggage within the adjoining containers provides an already highly effective energy absorbing barrier. If the container is modified so that loading access is via the outboard side of the container rather than at the end, i.e. the curtain is put on the faces shown in Figure E-1, then only the top and base are 'unbacked' by other containers, leaving the outboard face as a vent region.

The proposal is therefore that a modified container is developed in which the access is changed from the end to the outside face only, and which is modified to improve the resistance to internal pressures and thus encourage venting via the new access curtain only. How the container is actually modified to achieve the containment requirement is a matter of detail design, but two approaches suggest themselves, both involving the use of composite type materials. The first approach is to adopt a scheme for a rigid container which relies on a combination of energy absorption and burst strength to prevent uncontrolled breakout of explosive energy. The second approach is to use a 'flexible' container, i.e. rigid enough for normal use, but sufficiently flexible to allow gross deformation of shape without rupture. This, particularly if used with a backing blanket made from high performance material to resist fragmentation, could deform sufficiently to allow the container to bear against, and partially crush, adjoining containers. In this way, the shock energy transmission should be significantly reduced and the inherent energy absorption capability and mass of the baggage in adjoining containers could be utilised, whilst still retaining the high pressure gas for long enough to allow venting via the side face. Clearly, care would need to be taken to ensure that the container vent aperture remained as undistorted as possible, to ensure minimal leakage at the interface.

5.2 Cargo bay liner

The existing cargo bay liner is a thin fibreglass laminate which lines the roof and sidewalls of the cargo hold. There is no floor as such; instead, the containers are supported on rails running fore and aft on the tops of the fuselage frame lower segments. In a number of areas, there are zipped fabric panels let into the liner to provide access to equipment located behind. The liner 'ceiling' is suspended on plastic pillars approximately 2 centimeters below the bottom of the main cabin floor beams. The purpose of the liner is solely to act as a general barrier to protect wiring looms and systems components.

The proposal is to produce a new liner designed to provide the second level of containment, essentially at 'floor' and 'roof' level only [Figure E-1]. The dimensional constraints are such that potentially quite thick material could be incorporated (leaving aside the weight problem), permitting not only a rigid liner design, but semi-rigid or flexible linings backed by energy absorbing blanket materials.

The liner would be designed to provide an additional barrier at the base and roof of the containers, which unlike the sides, are not protected by adjoining containers. The outside ends of these barrier elements must effectively seal against the vent apertures in the containers, to minimise leakage into the fuselage cavities. 

5.3 Structural blow-out regions.

The final element in the containment/venting part of the scheme is a line of blow-out regions in the fuselage skins, coinciding exactly with the positions of the vent apertures in the cargo containers and cargo bay liner. These should extend along the length of the cargo hold, zoned in such a way that rupture due to rapid overpressure will occur in a controlled manner. The primary function of the blow-out regions would be to provide immediate pressure relief by allowing the inevitable skin rupture to take place only within pre-determined zones, limiting the extent of the skin tearing by means of careful stiffness control at the boundary of the blow-out regions.

The structural requirements of such panels are perhaps the most difficult challenge to meet, particularly for existing designs. However, it is believed that by giving appropriate consideration to the directionality of fastening strengths, and the use of external tear straps, it should be possible to design the structure to carry the normal service loads whilst creating a pre-disposition to rupturing in a controlled manner in response to gross pressure impulse loading.

The implementation of such features will need carefully balanced design in order to provide local stiffening, sufficient to control and direct the tear processes, without creating stiffness discontinuities which could lead to fatigue problems during extended service. However, the degree of reinforcement needed at the blow-out aperture need only be sufficient to limit tearing and to sustain the aircraft long enough to complete the flight unpressurised.

All aircraft have pre-existing strength discontinuities, despite the efforts of the designers to eliminate them. By choosing the positions of butt joints, lap joints, anti-tear straps and similar structural features in future designs, so as to incorporate them into the boundary of the blow-out panel region, the natural "tear here" tendencies of such features could possibly be turned to advantage. In the case of current generation aircraft, the positions of existing lines of weakness at such features will determine the optimum position for structural blow-out areas, and hence the positions of the container and cargo bay liner blow-out panels. A limited amount of local structural reinforcement (e.g. in the form of external anti-tear straps), carried out as part of a modification program, could perhaps fine tune the tearing properties of existing lines of weakness, potentially producing significant improvements.

5.4 Closure of cavities

There are four main classes of cavity which will need to be addressed on the Boeing 747, and most other modern aircraft. These are: 

 

(i)	The channels formed between fuselage frames
(ii)	The cross-ship cavities between cabin floor beams
(iii)	Longitudinal 'manifold' cavities on each side of the cargo deck, running fore and aft in the space behind the upper sidewall areas of the cargo bay liner.
(iv)	Air conditioning vents along the bottom of the cabin side-liner panels, which connect the side cavities below cabin floor level with the main passenger cabin.

 

 

If the containment barriers (i.e. modified cargo containers and cargo hold liner) can be made to prevent blast breakthrough into these cavities directly, then the only area where transfer can occur is at the interface between the container/cargo hold liner vent apertures and the fuselage skins at the blow-out region. This short distance will need to be sealed in order to form a short 'exhaust duct' between the container vent aperture and the fuselage skin. Since the shock and general explosive pressure will act mainly along the vent-duct axis, the pressure loading on the vent duct walls should not be excessive.

5.5 Attenuation of shock waves in structural cavities

To prevent the 'ducting' of any blast which does enter the fuselage cavities, either because of partial penetration of the containment barriers or leakage at the vent duct interfaces, the scheme requires the provision of lightweight energy absorbing material within the cavities to limit reflection and propagation of pressure waves within the cavities, and radiation of shock waves into the cabin from the conditioning air vents. Materials such as vermiculite, which are of low density yet have excellent explosive energy absorption properties, may have application in this area, perhaps in lieu of the existing insulation material.

Since the existing cavities often serve as part of the air conditioning outflow circuit, some consideration will need to be given to finding an alternative route. However, the flow rates are small compared with the total cross-sectional flow potential of the cavities and this function could be served by separate air conditioning ducts, or perhaps by restricting access to one or two cavities only (thus limiting the risk), or by using some form of blast valve to close off the air conditioning vents. Similarly, the requirement to vent pressure from the cabin in the event of a cargo bay decompression would also need to be addressed.