In case you are wondering what happens if a plane is struck by lightening, Paul of Ask a CFI.com has written a detailed post about lightening and the potential damage it can do to airplanes. As Paul explains, aircraft designers work under the assumption that aircraft will be struck by lightening and they have tried to 
Nevertheless, lightening strikes still happen and happen with surprising frequency. In fact, it is estimated that most commercial aircraft are struck by it at least twice a year and usually the only evidence left behind is a small lightning entry and exit point on the body of the aircraft. However, Paul also mentions that lightening strikes can do more serious damage such as pop circuit breakers, cause some glass cockpit displays to flicker or even fail and punch large holes through aluminum surfaces. He even includes some pictures of lightening damage to aircraft in his post but he is quick to point out that crashes where lightening is a contributing factor are extremely rare.
However, since lightening strikes can still do damage to the aircraft, it makes sense to avoid the risk of getting hit by not flying in thunderstorms and to avoid any thunderstorm by at least 30 kilometers or 20 miles that is identified as severe or one that gives off an intense radar echo. After all, its better to be safe than sorry!