EN 4533-003-2017 pdf download.Aerospace series – Fibre optic systems – Handbook – Part 003: Looming and installation practices.
3.2 System design considerations
3.2.1 Introduction
In the design of a fibre optic harness, the link topology and the available routing path on the platform will
dictate the physical length of the harness and any required branching of the assembly.
If the fibre optic installation is to be installed on an existing platform, then possible routing paths may be restricted (due to existing infrastructure and equipment). However if the platform is a new build, then there may be more freedom to design the routing path.
It is noted that fibre optic design software has been developed to assist in the layout of fibre optic harnessing. This can be used to model different paths and also calculate insertion loss of the link. depending on the path route. A small number of commercial packages are believed to be available. These can also predict losses associated with installation bends of the optical fibre and connector misalignment.
It is further noted that modern fibre optical cable designs are utilising bend tolerant optical fibres and a number of aerospace designs are in existence. These exhibit lower losses when bent to a small radius. Whilst this could allow tighter installation bends to be designed, it needs to be emphasised that the strength of the optical fibre still needs to be respected. Generally tighter bends will increase the strain on the fibre and may reduce lifetime (particularly if there are any defects in the glass). It is therefore generally advisable to ensure typical minimum bend radius limits for aerospace optical fibre cables according to the product standard.
3.2.2 Interconnects
Fibre optic connectors (interconnects) will be required to connect the fibre optic harness to end equipment or to other harness sections on the airframe. It is noted that harnesses may require multi-way connectors (with multiple optical contacts or termini) or single-way connectors depending on the specific design of the harness. This will in turn affect the management of the optical cable leading to that connector and any back shell design.
Careful attention should be made to the number and placement of Interconnects In the fibre optic links. The final choice and location of the interconnect needs to take into account the required performance, reliability and maintenance elements of the system. These aspects often conflict with each other in system design and so some trade-off should be performed at the design stage.
Of primary importance is that the fibre optic interconnects and components do not introduce a loss that exceeds the power budget of the system. Provided that sufficient power budget is available, the use of appropriately positioned production breaks can improve the maintainability of the system. For example, in areas of high maintenance activity where there is an increased likelihood of damage, the use of additional interconnects with short fibre optic links will facilitate a quick and simple replacement of a damaged link In turn this capability has to be traded against the possibility that the additional interconnects may increase the insertion loss of the links reducing the reliability of the system. Experience has shown that failures at or near to the interconnect are not an uncommon failure mode, particularly where loads placed on the links are easily transferred to the interconnect assembly and associated accessories. Careful placement of the interconnect and the correct use of appropriate harness tie-down mechanisms will reduce the likelihood of this type of failure.
Some systems (particularly single-mode systems with laser sources) may also be sensitive to reflections propagating back to the source. Additional connectors added to improve maintainability may significantly increase the total amount of reflection depending on the connector design. Where sensitivity to back reflection exists, low reflection connectors such as IJPC or APC types may need to be considered in the design.
In summary the introduction of additional interconnects should be considered if the attendant increase in insertion loss and back reflection can be accommodated by the system, and the additional interconnects improve maintainability without causing impact on the reliability of the system.
It is noted that connectors and tight bends will generally introduce the largest losses into an optical fibre harness. The material attenuation of the glass fibre will be small over the relatively short harness lengths of an aircraft. In addition to the losses of the cable and connectors in the system it is generally advisable to allow a 3 d13 margin in the harness insertion loss to allow for lifetime ageing of the installation. This should also be factored Into any system design calculations.EN 4533-003-2017 pdf download.
EN 4533-003-2017 pdf download
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