Can a C-5 cargo plane cross the Atlantic ocean without refuelling?

No, a C-5 cargo plane cannot cross the Atlantic Ocean without refuelling. The maximum range of a C-5 Galaxy with a full payload is approximately 2,000 nautical miles (2,300 miles or 3,700 kilometres). The distance between Newfoundland, Canada, and Lisbon, Portugal (the approximate crossing points for a transatlantic flight) is over 2,900 nautical miles (3,300 miles or 5,300 kilometres).