"Routes aren't known until one or two hours before a flight when the dispatcher makes a decision as to which route to operate, and then files it with air traffic control," said former airline dispatcher Phil Derner, founder of aviation-enthusiast site NYCAviation.com. "And once a route is filed, it can change multiple times—it doesn't become 'Bible' once it's filed."
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Once a flight plan is filed with the FAA, however, the itinerary information then becomes available to third-party sites and apps, such as FlightStats and PlaneFinder, which do an admirable job of showing a plane's current location. Others, including FlightAware and WindowSeat, show complete route information but are limited to North America and, of course, rely on flight plans that are subject to subsequent changes.
Determining an international flight's exact path is even harder. "The timeline for filing routes is about the same," Derner said. "But there are a lot of areas in the world that don't have as amazing resources to track weather, volcanic activity or war zones."
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Complicating matters further is the fact that different aviation authorities approach problem situations differently. While the FAA had previously prohibited U.S. carriers from flying over parts of Ukraine back in April, European authorities only did so after Malaysia Airlines Flight 17 was shot down.