You asked "Some time ago there was talk about a software upgrade to enable the GNS XLS to become P NAV approved. Does any one know whether this ever happened"?
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Honeywell says "Honeywell's GNS-XLS Enhanced Flight Management System offers new software upgrades which extend the life of this reliable system and meet new operating standards in both the United States and Europe.
Precise Area Navigation (PRNAV) compliance to allow certification under the Eurocontrol's Technical Guidance leaflet (TGL) - 10."
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Question answered.