Ever been tracking an aircraft when suddenly it shifts off course, heads straight for a different airport and lands, sometimes with a 7xxx code for added urgency? Aircraft diversions may look ‘straight forward’ (or straight backwards for that matter) but there is a lot more going on in the aircraft than the icon on a tracking screen can show.

Here’s a look at what a diversion involves, what it means for the crew, and some ‘notable’ diversions you may have seen on Flightradar24.

What is a diversion (and what causes them)?A diversion is, of course, when an aircraft doesn’t land where originally planned but instead heads somewhere else. In the USA in 2022 (the most recent statistics available), of the approximately 6.7 million operations flown by major US carriers, there were 15,826 diversions (around 0.2%). But don’t be fooled by the smallish size of that statistic – while a dire reason for a diversion of a flight you’re on is unlikely, if you’re a frequent flyer, you may well experience a diversion at some point—particularly if you fly into airports that experience significant weather.

Severe thunderstorms in Denver led to the diversion of this Delta A321 to Grand Junction.Now, there are several types of diversion. For the crew handling it, they are generally going fall into one of four main categories – technical, medical, weather or emergency.

• Technical: something has broken onboard that will prevent the aircraft from continuing with the mission as planned
• Medical: someone is sick (and they need attention urgently)
• Weather: a large storm/high winds/severe winter conditions/too much rain/ too thick fog/etc is preventing the aircraft from getting where it wants to go
• Emergency: Arghhh!!! You can lump ‘disruptive passenger’ into technical or medical depending on how you feel about badly behaved humans.

Diversions can of course occur at any point in the flight, but for decision making purposes (which we will look at in a moment) they can be best thought of as:

• Right after take-off: lots of fuel onboard so might be tough landing immediately, but there is an airport nearby
• En-route: who knows where the best and nearest airport is?
• Just before landing: hardly any fuel and some fairly time critical decisions are needed.

This is because a major factor in any diversion ‘decision making’ (aside from the “we’re on fire, land it now!” scenarios) comes down to how much fuel an aircraft has. Right after take-off, the airplane might have too much fuel onboard for immediate landing performance. En-route, how much fuel determines how far an aircraft can fly (and en-route may mean few nearby options). Just before landing and most aircraft have chewed through the majority of their fuel leading to fewer available options.

But which is the most difficult?Any which are considered ‘time critical’ represent an immediate challenge because without time, options quickly become limited. An en-route emergency such as a fire onboard, major fuel leak or significant failure in systems related to controllability may necessitate an immediate (or land as soon as possible) landing, and if an aircraft is over the North Atlantic or polar region then options can be few and far between. The challenge in that case is balancing where the aircraft can get to with where is most optimal.

But a diversion at any stage involves different challenges for the flight deck. Take, for example, a weather diversion at destination. Big storms can close airports down with little notice, resulting in a whole lot of aircraft needing to divert, and generally no aircraft has much fuel onboard when it reaches its destination. If the weather is big and bad enough, it might also take out the closest alternates leading to multiple aircraft diverting on minimum fuel to places they are not familiar with.

Crew are also rarely going to be familiar with a diversion airport, particularly if it is a remote en-route one, so the airport itself can present challenges even when time is not a critical factor.

Wait, why can’t they land anywhere?Aircraft cannot simply go anywhere. Again, aside from the dire emergencies, crew need to head to airports which are approved. This approval means risk assessed by their company for their aircraft type to make sure it will be able to land, stop, taxi, not crush the runway surface and be able to get out again.

Take a look at a map around the North Atlantic – one of the most flown bits of air – and there ain’t much there. Shannon on one end, Gander (or one of the Canadian spots on the other) and only really Keflavik (Reykjavik) in the middle.

What does the process look like?A crew’s first actions after ensuring the aircraft is safe is to run some sort of decision making process. There are an array of acronyms operators use for this, but they all tend to amount to the same thing:

• How much time do we have? Time available is the most critical question to answer. If you’re on fire, there is not point heading off over the horizon looking for a lovely company destination airport. If you have loads of time (and probably fuel onboard) then flying a visual circuit onto the 1000m runway directly underneath you probably isn’t so wise either.
• What is the problem? This doesn’t mean pointing out what has just broken/made a loud bang/generally stopped working. It means diagnosing the impact on the planned flight, which is not always easy. An engine failure means one less engine, sure. But it might also mean the aircraft has to descend (terrain clearance concerns), fly at a lower level (fuel burn concerns), has lost hydraulic or electrical systems (capability concerns).
• Where can we go? This is based on the above time and diagnosis parts, and looks at what airports are available to which the aircraft can go. The options are also impacted by weather, NOTAMs, runway length, engineering support etc. So at this stage there is a lot of information gathering and ‘weighing up’ to be done in order to determine the most optimal option to opt for.
• What do we need to do to get there? This comes after the decision has been made on where to go, and it considers the management side of the diversion – who do the crew need to talk to, what information do they need, what calculations should they run?
• Has anything changed? The review stage is a continuous process during a diversion, because it is the crew’s chance to keep asking questions like this. What have we missed? Oh, that ILS is actually not working and the weather isn’t good enough for a visual. Oh, we thought it was a fuel leak out of the left engine but actually it’s the entire tank, we ain’t gonna make plan A… Any change, any missed bit of information, any improvement in the situation might require a change to the decision to avoid flying into a corner, so to speak. Managing the diversionFor the flight crew, once the initial problem has been dealt with, a decision made and the aircraft is pointing where it needs to go, then a diversion becomes a mission in management and coordination of resources.

Crew need to advise the relevant people of what is going on, and by relevant this means:

• The Cabin Crew: Yup, they don’t much like it when an airplane touches down at an unexpected airport and no-one has told them. But more importantly, they need to be aware in order to secure the cabin. Pilots will give a NITS briefing to the crew which includes the Nature (of the problem), the Intentions, Time available (to sort the cabin) and any Special Instructions (from expect an emergency landing and “brace brace” call to bring me a coffee please, we have hours left to go).
• The Passengers: For much the same reason as above. The crew will try to give enough information to keep them aware without scaring them.
• ATC: Re-routes to the intended diversion airport will need to be coordinated.
• The Company: A diversion doesn’t finish once the aircraft is on the ground, there are a zillion other things that come next.

Actually, let’s dig in deeper on that point for a moment.

Getting to the diversion airport is rarely the issue, ATC coordinate this and off the airplane goes. But the challenges start once there because unfamiliar airports may have taxi restrictions, lack of engineering support, or lack of ground support for passengers, crew and aircraft if the airplane is stuck there.

A medical diversion shouldn’t result in an aircraft remaining on the ground – the plan is generally to drop said sick passenger off then resume the flight, but this requires some planning around crew duty times, uploading more fuel and potential engineering support to turn the aircraft around.

So, communications with Company are critical, and often happen earlier in the decision-making process.

Notable diversionsWe could fill this page with a thousand examples, but here are some of the more interesting ones.

In 2015 Jet Airways flight 9W 555, a Boeing 737, was heading from Doha to Cochin, but when it reached Cochin, the crew found the weather to be significantly worse than expected with a low cloud base that prevented them from being visual with the runway on their first approach attempt.

As is fairly common, the crew made the decision to hold and see if the weather improved, and after around 20 minutes they attempted a second approach but were unsuccessful. Not to be dissuaded, they held again for around 30 minutes before attempting approach number three. Also ending in a go-around.

At this point, the crew made the (possibly sensible) decision to divert, and headed for Bangalore (a little less sensible) which was the primary alternate for the flight. Partway there though they realized their fuel was insufficient and changed course, routing instead to Thiruvananthapurum (Trivandrum for those who can’t pronounce the full name). The weather here was not much better and their first attempt (fourth overall) resulted in a go-around due lack of visual contact.

The crew were, at this point, rather lacking in fuel, and with no options to divert again, continued to make approach attempts numbers five and six, both of which were unsuccessful. Despite still not being visual, the aircraft landed on the seventh approach (they kind of had to at this point) with only 349kg of fuel left onboard. Even for a 737 that’s little more than fumes.

A summary, with pictures, and link to the final report from the subsequent safety investigation is available here.

In September 2023, Ural Airlines flight 1383 from Sochi to Omsk, operated by an Airbus A320 suffered a hydraulic failure which impacted their braking systems. With their planned destination runway too short to allow for reasonable margins on landing, the crew elected to head for Novosibirsk instead.

However, the hydraulic issue had also led to the gear doors remaining open. Big bits of airplane flapping in the wind equal a lot of drag, increasing the fuel burn significantly. Enough in fact to lead to the crew having to ‘divert’ into a field in Russia where they carried out an emergency landing. As of May 2024, the aircraft remains stuck in the field.

So next time you’re tracking an aircraft on Flightradar24 and watch it suddenly change course, consider what the crew are going through up there and what that diversion might actually involve.

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