In the world, lightning strikes about 3 billion times a year and kills an average of 6000 people. Lightning strikes on commercial planes are daily occurrences. In, fact, aircraft often trigger lightning when flying through a heavily charged region of a cloud. In these instances, the lightning flash originates at the airplane and extends away in opposite directions.
For safety reasons, planes hit by lightning mid-flight undergo inspection after landing but in most cases, the aircraft is either unharmed or only minor damaged but it costs airline more than $2 billion per year. Prior to the next flight, the aircraft is grounded and thoroughly inspected for damage, affecting its availability.
The last major accident caused by lightning was in 1967, when lightning cased a catastrophic fuel tank explosion. since then, more techniques have been determined to reduce the threat of lightning.
Although passengers and crew can see a flash and hear a loud noise, if lightning strikes their plane, nothing serious should happen because of the lightning protection in the aircraft. Initially, the lightning will strike to a nose or wing tip. the airplanes then flies through the lightning flash. The current will travel through the conductive exterior skin and structure of the aircraft and exits through the tail.
Most aircraft skins consist primarily of aluminium. which conducts electricity very well. Some modern aircraft like Boeing 787 and the Airbus A350 are made of advanced composite material like carbon fibers, resulting in a reduced electrical conductivity of the fuselage and wings.
Bottom line, lightning strikes are not a severe problem from a safety issue. If the plane diverts its route, it is mainly because of hail, rather than lightning.