Can airplanes fly over tornadoes

Sometimes yes, but often no. Tornadoes form during severe thunderstorm, extending from the ground up to cloud bases and a good deal above that. Even aside from the tornado, thunderstorms can contain dangerous winds, hail, and icing conditions that planes should avoid. While jet airliners can safely fly over some thunderstorms, the storms that spawn tornadoes are often very tall and their heights can exceed the flight ceilings of commercial aircraft.