MoD line on dual wall pipework is that it is more difficult to replace seals and more difficult to find the source of leaks.
SFAR 88
Airworthiness Notice 55 issued by the CAA and EASA
Bottom line (one of them)is that on modified fuel systems a lot of work / rework needs to be carried out with regards to reducing risk of fire / explosion. This is not only on old Aircraft, 4 year old BBJ are being modified IAW the directive. Operators that do not comply will have to have the Aux fuel system made inoperable.
Part of this mod is to install "dual wall" fuel pipes. At present, it is
allowable to shroud a standard rigid pipe with a larger flexible hose
type assembly, with a gland drain that can indentify the source of a
leak to within a certain distance. The hose will go.
Lord knows what the CAA would do with AAR capable airliners.
Beags do you have any gen on the AAR pipework on FSTA?