How can a pilot fly a distance of 594 NM connecting two VOR's? I was researching worldwide airway designations when I stumbled upon this airway in Brazil: G678 with an MEA of 15,000 feet and a distance of 594 NM (nautical miles) between navaids.

It's drawn between the BSI and ATF VOR's. Mid-airway around S12°50.12' W52°4.38'.

Flying at 15,000' with BSI at elev ~3500', the line-of-sight is 166 NM. And as far as I know a high altitude VOR reaches out to 130 NM, maybe more, but certainly not 300 NM, right?

The "G" designation is for a non-area navigation route per ICAO annex 11. So it's a non-RNAV route. It's even colored black (vs. blue) on skyvector.


  A, B, G, R for routes which form part of the regional networks of ATS routes and are not area navigation routes.


So, how can it be flown?
You are correct that VOR's rarely have a range of more than 200 NM. However, Alta Floresta VOR (ATF) is colocated with an NDB (and DME). In spite of less accuracy, NDB's have a much greater range (partly because their signals can travel as ground waves). This makes it possible to have a very long segment on the G678 airway without other navigation aids. When leaving the coverage of Brasília VOR/DME (BSI), you should be able to pick up ATF NDB.