Wingtip Tanks, Winglets And Tail Boom Fairings For C336 Or C337.

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Actually, not.
If you read many of the RTW flight reports, it is a simple way of adding extra fuel, in a non-permanent manner. In some places, it makes adding fuel easier, as well, because the drum can be taken to the fueling point, then pumped into the tanks.

Certainly, the installation of the plumbing to the tanks must have a field approval.

Oh, the tank is "Owner Supplied Parts"