Hello gentlemen, this has nothing to do with Captain Sim still I hope someone can info me out if they know.

I invite you to enter (not fly) a flight plan into your FSX Flight Planner, from Reunion Island's Guillot (FMEE) to Toamasina, Madagascar (FMMT) with a high-altitude airway routing. It seems that the area around Reunion is where some airways end, so the only high-altitude airway route is quite mmm uneconomical. The direct distance is 383.9 nm and the H-Alt route is 810.7 nm. Air Austral operates this route with the ATR 72, that does not operate FL260+ which is the minimum altitude for those airways, and their flights --apparently-- last 50 minutes making a direct route of more or less 380 nm.

If they (Air Austral) some day for some reason have to fly this route with their B772 for example, would they follow the 500+ nm route instead of flying direct? or airlines are given flexibility of cruising off airways in some cases?

If I'm wrong or unexact on something please let me know, thanks!  :)

Say you were flying from Oxnard to Watsonville, in California. You would take the V-25 airway part way, entering in Santa Barbara, exiting Salinas; the route doesn't, by any means, have to follow the airway for its entirety, only small portions. Now, an aircraft needs not exit an airway at an interesection, most especially when there is no exit near the airport. Not all routes need to be on an airway anyways, flightplans plan out any number of waypoints that the aircraft flies to on its route, whether it be on an airway or not. The airway is meant as a means of organizing flights so that the airspace at said altitudes aren't crowded; they aren't necessary at all times. Remember, each day a flightplan may be different, depending on teh wind, and other weather conditions, airways may take one over airspace that permission to fly over isn't there one day, and means are meant to move around that.