If you are going for 100% completion don't bother wasting time flying around extra to unlock this achievement. With searching for the letter scraps, nuclear waste, UFO parts, submarine pieces and doing all the other missions, you should cover enough ground to unlock this achievement without doing any extra flying.
*NOTE* You may still have to fly to a few spots after getting 100% completion. This solution was meant as an alternative for people who did not want to fly around the whole map just to unlock this achievement. By waiting until you get 100% completion you will waste much less time flying around to unlock this achievement.