Planes divert to airports other than their destination for reasons ranging from medical emergencies and mechanical problems to severe weather and runway closures. Most diversions are precautionary rather than dramatic, and pilots plan for them before every flight by identifying alternate airports along the route and carrying extra fuel to reach them.
Medical Emergencies
In-flight medical events are one of the most common triggers. A large study published in JAMA Network Open found that about 1.7% of in-flight medical incidents result in a diversion, which sounds small until you consider the tens of thousands of flights operating daily worldwide. The conditions most likely to force a landing are neurological issues (about 41% of medical diversions) and cardiovascular emergencies (about 27%).
Not every medical event warrants a diversion. Pilots and crew typically consult ground-based physicians through a radio or satellite link to assess the situation. The conditions with the highest odds of triggering a diversion are suspected stroke (roughly 20 times more likely to cause a diversion than a routine medical call), acute cardiac emergencies (about 8 times more likely), and altered mental status (about 7 times more likely). These all share the common thread of being time-sensitive: the faster the patient reaches a hospital, the better the outcome.
Weather at the Destination
Severe weather is probably the most familiar reason for diversions. Thunderstorms, heavy fog, ice storms, and high crosswinds can all make a runway unsafe or impossible to use. When weather deteriorates at the destination, air traffic control may hold arriving flights in a circling pattern, hoping conditions improve. If they don’t improve before the plane’s fuel reserves start running low, the crew diverts to a nearby airport with clearer skies.
Sometimes weather closes an airport entirely. A tornado warning near a major hub, for example, can shut down all arrivals and departures for an extended period, forcing dozens of inbound flights to scatter to surrounding airports. Winter storms that coat runways with ice faster than crews can clear them have the same effect.
Mechanical and Technical Problems
Aircraft systems are built with layers of redundancy, so a single failure rarely means the plane is in danger. But redundancy has limits, and regulations require pilots to land within a certain timeframe once key backups are lost. Common mechanical triggers include engine trouble, pressurization failures (which force a descent to lower altitude where breathable air is thicker), hydraulic leaks, and electrical system faults.
This is especially relevant over oceans. Twin-engine planes flying long routes over water operate under rules called ETOPS (Extended Operations), which specify the maximum time a plane can be from a suitable diversion airport. Ratings range from 120 minutes to well beyond 180 minutes, depending on how reliable the specific airplane-engine combination has proven to be. An engine that shuts down in flight over the Atlantic, for instance, means the crew heads for the nearest airport within that approved time window, which could be in Iceland, the Azores, or Newfoundland rather than the original destination.
The FAA tracks in-flight engine shutdown rates closely. To qualify for 120-minute ETOPS approval, an engine model must demonstrate no more than 0.05 shutdowns per 1,000 flight hours across the global fleet. For routes requiring 180 minutes or more from an alternate, the threshold drops to 0.02 or lower. These strict reliability standards are what make long overwater flights possible in the first place.
Fuel Considerations
Every commercial flight departs with more fuel than it needs to reach its destination. FAA regulations require enough fuel to fly to the destination, then onward to the most distant alternate airport if needed, and then 45 more minutes of cruising on top of that. This built-in cushion is what makes diversions logistically possible without creating a fuel emergency.
But fuel margins can shrink in flight. Extended holding patterns, stronger-than-forecast headwinds, or detours around weather all burn through reserves. If the crew calculates they no longer have enough fuel to reach the destination safely with the required reserves intact, they’ll divert to a closer airport while they still have comfortable margins. Pilots don’t wait until the tanks are nearly empty. The decision typically happens well before the situation becomes urgent.
Runway and Airport Closures
An airport can become temporarily unusable for reasons that have nothing to do with weather. A plane with a blown tire sitting on the only open runway, a security incident that triggers an evacuation, or a drone sighting in the approach path can all shut things down. When closure is expected to last longer than a plane can circle overhead, diversions follow.
Some airports also have noise curfews that restrict operations during certain hours. Hollywood Burbank Airport in California, for example, enforces a nighttime curfew. Minneapolis-St. Paul has implemented nighttime restrictions as well. A flight that’s running significantly late might not be allowed to land if it arrives after the cutoff, forcing a diversion to a nearby airport that operates around the clock.
Disruptive Passengers
Unruly behavior on board, from intoxicated passengers who become violent to bomb threats, can prompt the captain to land at the nearest suitable airport. These diversions prioritize getting the disruptive individual off the plane so law enforcement can handle the situation. The crew considers factors like how far along the route they are, which direction offers the shortest path to a suitable airport, and whether the behavior poses an immediate safety risk to other passengers or the crew.
How Pilots Choose the Diversion Airport
The alternate airport isn’t chosen at random. Pilots weigh several factors quickly, and in many cases they’ve already identified options during pre-flight planning. The key considerations include distance (how quickly the plane can get on the ground), runway length (a fully loaded widebody jet needs a long runway), available emergency services like firefighting equipment rated for the aircraft’s size, current weather conditions at the alternate, and whether the airport has customs and immigration facilities if the flight is international.
A plane diverting with a medical emergency might pick the closest airport with a major hospital nearby, even if it’s a smaller regional field. A plane with an engine issue on a transatlantic route might aim for a military airfield in the Azores that exists largely to serve as an ETOPS alternate. A domestic flight dealing with a late-night delay might simply land at the next major hub down the road.
Ground infrastructure matters more than people realize. An airport’s rescue and firefighting capability is rated by index categories that correspond to the size of aircraft it can safely handle. A small regional airport might not have the equipment to support an emergency landing by a large widebody jet, which means the crew might bypass a closer but less-equipped field for one slightly farther away that can handle their aircraft.
What Happens After a Diversion
Once on the ground at an unplanned airport, the airline scrambles to figure out next steps. For mechanical issues, a maintenance team may need to travel to the diversion airport to make repairs, which can take hours or longer. For weather diversions, passengers often wait on the plane or in the terminal until conditions clear, then resume the flight. For medical diversions, the affected passenger is taken to a local hospital while the plane typically continues to its destination after a short delay.
Your rights as a passenger depend on where you’re flying. In the European Union, regulations require airlines to provide meals, refreshments, and hotel accommodations during long delays caused by diversions, regardless of the reason. Compensation for the delay itself depends on whether the cause was within the airline’s control (like a mechanical failure) or an extraordinary circumstance (like weather). In the United States, rules are less prescriptive, and airlines set their own rebooking and accommodation policies.
Most diversions add a few hours to your travel day. Occasionally, when the diversion happens late at night or the mechanical issue is complex, passengers end up spending an unplanned night in a city they never intended to visit. Airlines will typically rebook you on the next available flight and, depending on the carrier and circumstances, cover hotel and meal costs.