commercial airliners don't crash into mountains any more, but small civil aviation craft do with some regularity, particularly when sightseeing and entering into or flying "up" canyons, ravines, etc. The optics can be deceptive, and the sides close in, and, running out of room, they can't turn tight enough or gain altitude quick enough. Also, at a higher altitudes and in hot weather, the less dense air provides much less lift, and the propulsion is less effective, and higher altitudes and nice weather is generally the meeting place for flatlander sightseers and mountains.
I've been watching a lot of plane crash youtubes.