Using the future in a clause of time, instead of the present tense
I'll see you when I shall come back.
I'll see you when I come back.
If the verb in the main clause is in the future, the verb in the time clause must be in the present tense
Spectacular Hiking Trails Around The World
Tour du Mont Blanc
This three country, 105 mile hike circles the 15,770 foot Mont Blanc Massif, the highest peak in Western Europe. One of the most popular long distance walking trails, the Tour du Mont Blanc takes hikers through mountain passes, snowfields, lush forests, glacial valleys and secluded Alpine villages over a span of about 10 days.