Diversion: why flights are rerouted to an alternate airport

The arrivals board suddenly says “diverted”, and on the live map a long-haul jet turns away shortly before its destination. For the passengers that is annoying, for the crew a practiced routine – because every scheduled flight takes off with a plan B in the tank. This guide explains why flights are diverted to an alternate airport, how airlines plan alternates and reserve fuel before takeoff, who decides in the air and which German airports typically come into question as an alternate.

Diversion: the short answer

A diversion is landing at an airport other than the planned destination. It happens when a safe landing at the destination is not possible or not advisable – usually because of weather, sometimes because of a closed runway, an emergency on board or expired operating hours. It is almost never a surprise: for flights under instrument flight rules, the airline names at least one alternate airport during flight planning and carries the fuel to get there. The decision in the air is made by the captain.

If an aircraft returns to the departure airport, that is called a turnback. If it lands en route before it has even come near the destination, that is an en-route diversion. And if it only turns away in the destination area, for example after holding patterns or a go-around, it is the classic destination diversion.

Three kinds of alternate airports

The ICAO rules, and the EU regulations for commercial air operations built on them, distinguish three roles. One airport can take on several of them for the same flight.

Type	Purpose	When required	Typical example
Take-off alternate	Landing shortly after takeoff when the departure airport is unusable	when the weather at the departure airport is below landing minima or a return is not possible for other reasons	dense fog at the departure airport: takeoff allowed, return landing not
En-route alternate	Option to land along the way, for example after an engine failure	mainly on long-haul flights with twin-engine aircraft under ETOPS or EDTO	airports at the edge of the Atlantic, polar routes or deserts
Destination alternate	Landing when the destination cannot be approached	generally on flights under instrument flight rules, with narrowly defined exceptions	thunderstorms over the hub, diverting to a neighboring airport

For the take-off alternate there is a distance limit: for twin-engine aircraft it must be reachable within one flight hour with one engine out, for three- and four-engine aircraft within two hours. The en-route alternate becomes important on long-haul flights when the aircraft gets far from suitable landing sites. How this is regulated for twinjets over the Atlantic is explained in the article ETOPS explained.

The destination alternate may only be dispensed with under certain conditions. These include, for example, that at least two independently usable runways with an instrument approach are available at the destination at the expected arrival time and that the weather is well above minima. If the weather forecast at the destination is poor or missing, the rules even require two alternates. A special case is remote destinations with no reachable alternative: there, airlines plan more fuel and a point of no return.

Plan B in the tank: how fuel is planned

For a diversion to be possible, the aircraft must already have the fuel for it on board. Fuel planning is therefore built up in several layers. Each airline sets the exact values in its operations manual, but the principle is similar everywhere:

• Taxi fuel: for engine start and taxiing to the runway.
• Trip fuel: for takeoff, climb, cruise, descent and approach up to landing at the destination.
• Contingency fuel: an allowance for the unforeseen such as headwinds, detours or a different flight altitude, often calculated as a percentage of trip fuel.
• Alternate fuel: for a go-around at the destination, the flight to the alternate and the approach there.
• Final reserve: the iron reserve. For jet aircraft it is sized for about 30 minutes of holding at 1,500 feet (450 meters) above the airport.
• Extra fuel: additional amounts that the captain or the planning takes along because of expected holding patterns, thunderstorm situations or uncertain weather.

An important logic follows from this: everything beyond final reserve and alternate fuel may be used by the crew at the destination for holding patterns or another approach. Once this buffer is used up, they must turn away to the alternate at the latest. How the circles over fixed points work is shown in the article Holding and holding patterns. The planning itself is usually done by flight dispatchers in the airline’s operations center – what exactly this job involves is in the article on Flight dispatchers.

Minimum Fuel and Mayday Fuel

For the case that things get tight, there are two defined calls. With “Minimum Fuel”, the crew tells air traffic control that any further delay could result in landing with less than the final reserve. This is an advance warning, not yet an emergency. “Mayday Fuel” follows when the calculation shows that the aircraft would touch down even at the nearest suitable airport with less than the final reserve. The flight then has priority over all others. What lies behind distress calls and transponder codes is explained in the articles Mayday and Pan-Pan and Squawk 7500, 7600 and 7700.

The most common reasons for a diversion

Diversions have many causes. Some are foreseeable hours ahead, others arise in seconds.

• Weather at the destination: thunderstorm cells over the approach, dense fog below landing minima, strong crosswind, snowfall or an icy runway. How crosswind makes landing harder is explained in the article Crosswind landings. In fog, it often matters whether airport, crew and aircraft are approved for all-weather approaches – details in the article on the ILS approach.
• Closed runway or airport: for example after an incident, because of construction work, security situations or a disabled aircraft. This is particularly critical at airports with only one runway.
• Medical emergency: If a passenger or crew member becomes seriously ill, the crew lands at the nearest suitable airport with medical care.
• Technical problem: for example faults with the engine, hydraulics, electrics or pressurization. Depending on severity, the nearest suitable airport is chosen or a place with maintenance for the airline itself.
• Disruptive passengers or security situations: Unruly behavior on board, bomb threats or similar can force a stopover.
• Fuel: long holding patterns, repeated go-arounds or detours around thunderstorms eat into the buffer.
• Operating hours: Delayed flights may only reach the destination after the start of the night curfew and have to divert to an airport with night operations.

The last point in particular plays a big role in Germany. At Frankfurt Airport, scheduled flights are banned between 11 p.m. and 5 a.m., while delayed landings with a planned arrival before 11 p.m. are possible until midnight. Other airports operate around the clock. The guide Night flight bans at German airports provides the overview.

Who decides – and how?

The decision on a diversion rests with the captain. In practice, though, the crew seldom decides alone. A typical sequence looks like this:

Air traffic control supports this with current weather data, information on runways and approach aids, and direct routings. How the controllers’ work is organized in Germany is described in the article on DFS Deutsche Flugsicherung.

The weight problem

Anyone who has to turn back shortly after takeoff or land after only a few hours of flight is often still heavy. Many aircraft may also touch down above the maximum landing weight when necessary; a technical inspection follows. Large long-haul jets such as the Boeing 777-300ER or the Airbus A380 often have a system for dumping fuel at high altitude. Many short- and medium-haul jets such as the Airbus A320neo or the Boeing 737-800 do not – they circle for a while if necessary to burn off weight, or land heavy.

What makes a good alternate airport

Not every airport can take every aircraft. Several criteria count at once in the selection:

• Runway length and width: A heavy long-haul jet needs considerably more runway than a regional jet.
• Approach procedures: In fog, only an airport whose instrument landing system and operating procedures allow landings in low visibility helps.
• Firefighting category: It must match the size of the aircraft. What lies behind the categories is explained in the article on the Airport fire service.
• Operating hours: At night, only airports without a curfew or with exceptions for diversion landings come into question.
• Apron and handling: Space for parking, stairs, fuel trucks and staff are needed – with several diversions at once this quickly becomes a bottleneck.
• Weather: The forecast at the alternate must be clearly above minima at the time of planning. Widespread fog situations sometimes affect several neighboring airports at once.
• Onward travel: Airlines prefer airports with their own handling, maintenance or a good rail connection to the actual destination.

Typical alternates in Germany

Which airport is chosen in an individual case is decided by the airline and crew according to the current situation – there is no fixed list. Certain airports do, however, bring features that make them especially suitable as an alternate. The following overview is based on the data in our airport profiles.

Airport	Longest runway	Night operations	What makes it interesting as an alternate
Cologne/Bonn	13L/31R at 3,815 m	no general night flight ban	long main runway, long-distance rail station on the high-speed line toward Rhine-Main, round-the-clock operations
Leipzig/Halle	two runways of 3,600 m each	cargo allowed at night	two parallel long runways, night cargo operations, rail station in the terminal
Frankfurt-Hahn	03/21 at 3,800 m	approved around the clock	very long runway, Category III ILS on 21, just under 100 km as the crow flies from Frankfurt
Nuremberg	10/28 at 2,700 m	no night flight ban, noise protection zones	Category III ILS on 28, location between Munich and Frankfurt
Hanover	north runway 09L/27R	no general night curfew	Category IIIb ILS on the north runway, suburban rail under the terminal
Stuttgart	07/25 at 3,345 m	night curfew with exceptions, including for diversion landings	long single runway in the southwest

On top of that come the big hubs themselves. If a runway is out of service at Munich Airport, for example, Frankfurt can be a sensible alternate and vice versa – provided the same weather isn’t present there and capacity is sufficient. Airports in neighboring countries also play a role, such as Salzburg in the south or EuroAirport Basel-Mulhouse in the southwest.

After landing: what happens at the alternate airport

The diversion isn’t over with touchdown. Depending on the reason, there are different scenarios:

For passengers, a diversion can trigger claims under the EU Passenger Rights Regulation, though depending on the cause it may not – weather is generally considered an extraordinary circumstance. If a flight lands at another airport in the same region, the airline generally has to cover the transfer to the booked destination under Article 8(3) of the Regulation. The details are in the article on Passenger rights under EU 261.

What spotters take away from this

For spotters, diversions are often the most exciting moments of the year: suddenly a widebody jet stands at an airport where it is otherwise never seen. If you know the planning logic, you can better judge when waiting pays off.

• Watch thunderstorm and fog situations at the hub: When thunderstorms build over Frankfurt or Munich, a look at the live map is worthwhile. How to read weather reports is explained in the guide Reading METAR and TAF.
• Keep an eye on late evenings: Delayed flights just before the night curfew are candidates for diversion landings at airports with night operations.
• Interpret holding patterns: When aircraft stack up in the hold, the moment when the first ones turn away is getting closer. The article Recognizing holding patterns on the flight tracker is helpful.
• Live tactics: How to recognize a diversion in real time and then photograph it fairly is described in the practical guide Recognizing a diversion live.

Throughout, stay in public areas and keep in mind that behind some diversions there is a real emergency. How to behave as a spotter in that case is in the article Witnessing an emergency landing or incident.

Conclusion

A diversion is not improv theater but a planned way out. Before takeoff, the airline and crew determine where a flight can divert and load the fuel for it, including the iron reserve. In the air, the captain weighs weather, fuel, aircraft size and operating hours and decides together with air traffic control and the operations center. In Germany, airports with long runways, precise approach aids and night operations benefit – and for spotters, an airline’s plan B often turns into the stroke of luck of the day.