Fatal plane crashes are not common but often receive substantial media attention when they do happen. Hijackings, sabotage and military action have all been reasons for fatal plane crashes in the past, but far and away the most common causes are human error and mechanical failure. Recorded statistics for fatal plane crashes usually only refer to those carrying more than ten passengers. Those with fewer passengers, such as private aircraft and military planes are not normally included. The worst disaster in aviation history (in terms of passenger fatalities) was the Tenerife airport disaster of 1977, when two Boeing 747 airliners collided on the runway, resulting in 583 fatalities.