In the middle of an air, when you are in a plane, you would never want your phone, or anything in that matter to catch fire. There would a huge panic if that happens and you fear that any moment, the plane will be on fire.
Something like this happened in Australia. During a flight between Lismore and Sydney, an Australian passenger’s iPhone suddenly started emitting smoke. Moments later, it started glowing red.
The iPhone caught fire and the fire was successfully extinguished and the flight landed safely, with all passengers unharmed. However, it is still not known how and why did the fire start. Battery issues are being seen as a theory as the burnt phone was either an iPhone 4 or an iPhone 4S.
Source: iDownload Blog