Were does it say that flex lines must be replaced every 7 years on part 91 aircraft?
The "RECOMMENDATION" to change flammable fluid hoses comes from the manufactuerer not from the FAA regulations. Each manufactureer has a different recommendation from 5 years to on condition.
Under part 91 no one has to do "MANUFACTUERER RECOMMENDED" maintenance.
in 135 and such, I have hardly ever seen anyone change hoses every 7 years in GA Aircraft.
For what its worth, from an EX 135 maint guy.
As I recall.
Part 135 operators are supposed to follow the recommendations of the manufacturer, unless they get a waiver.
That would happen if you could prove that the recommendations of the manufacturer were more stringent than necessary or even just arbitrary, and that the alternate method of compliance was satisfactory for continued airworthiness.
Be advised, each Air Taxi operation has to have their own waiver, they are not transferable.