Tornados are rotating funnels of air generally associated with severe supercell activity. More often than not, they are relatively short lived and localised, although they can last for much longer and cover a significant distance over the ground. Mesocyclones (a rotation of air in the upper atmosphere) are associated with these supercells, and through significant downdrafts, get dragged down to the surface, creating a focused funnel-like rotating air column. Wind speeds can vary from as little as 60km/h to 200km/h, although wind speeds in excess of 500km/h have been recorded in large storms.
The bottom line for pilots regarding tornados is steer clear! The significant thunderstorm activity itself surrounding where a tornado would form should be avoided. Massive up and down drafts, severe turbulence, potential for lightning strikes, significant hail showers and nasty icing conditions are all key considerations. You would have to be very foolish, or horribly mis-informed to venture into a supercell.