"In a very strange oversight for one of the pivotal missions in the game, President Kimball's helicopter in You'll Know When It Happens is not despawned as soon as it crosses out of view. In fact, it takes well over two minutes before it finally despawns (either that or it could be a proximity thing).

Once the quest completes, killing the vertibird (and by extension the President) can no longer cause you to fail that quest, and since Bear Force One does not disappear after flying over the ridge, you can destroy it without consequence. You do this by crossing over towards Boulder City, essentially doing a wide U-turn to get around the hills, where you'll see it floating stationary above the horizon.

This is where you take out your gun.


Of note, in my game, Kimball refused to leave the podium after his speech, which would have ruined the save if not for the Legion engineer watching in the crowd. Killing him doesn't count as disrupting or failing the mission, but it does cause the President to run off."