Sometimes, a pilot may decide to change the predetermined flight path while airborne, and this can be down to a number of factors from bad weather, even to low fuel. In addition to that, airline dispatches often work closely with the pilot to not only help analyze the weather and wind conditions between the start and endpoints of the flight but to also help determine the most economic route. Though this is usually done while creating the flight plan, these sorts of situations can occur mid-flight, as external factors (like the weather) are never set in stone and always changing.
So, in other words, if you find yourself on a flight and hear over the speakers that your pilot has had to detour or change the flight plan in some way, there will almost always be no cause for panic, as they will have made a calculated and well-informed decision to do so. This is because, as we mentioned above, when a pilot chooses to stop following a flight plan, they will have carefully made the decision to do while in communication with Air Traffic Control and other personnel, such as the flight dispatcher.
In addition to this, pilots may also need to change the flight plan due to something known as ‘Holding’, which is when bad weather or congestion occurs at an airport. Instead of being able to land the plane as per the flight plan, the pilot has to re-route slightly and circle in a designated pattern near the airport’s control tower. Usually, if a flight dispatcher or pilot thinks that there is a chance that this may occur, the estimated holding time and fuel requirements will be specified in the flight plan before departure. However, unforeseen circumstances can sometimes mean that the pilot has to enter the holding period without it being accounted for in the flight plan.