Back-up rings have no intended sealing function. Instead, as their name indicates, they are protective and supporting elements made from extrusion-resistant materials which generally have a rectangular cross-section. They are installed in a groove together with an elastomer sealing element, preferably with a corresponding O-ring in static applications.
Mandate sets 2030 date for use of sustainable aviation fuel
I do hope this doesn't cause costs to UK aviation to rise and lose business to those staying on fossil fuels.