Why flights are delayed: delay codes, aircraft rotations and flow control

The arrivals board flips to “delayed,” the long-haul jet you came to the fence early for arrives an hour late – and nobody tells you why. Yet flight delays follow a fairly clear logic. Behind almost every delay is one of a few cause groups that airlines document with delay codes, and a large part of it is simply inherited from the previous flight. This guide explains how rotations, codes and flow control fit together – and how to turn that into a real planning advantage at the spot.

Why flights are delayed: the short answer

A flight is delayed because somewhere in the chain of aircraft, crew, passengers, handling, airspace and airport, something takes longer than planned – and because this delay is carried over to later flights through the aircraft’s rotation. The industry therefore distinguishes between the primary delay, which newly arises at an airport, and the reactionary delay, which a flight merely passes on.

How large the second share is, the figures from the European air navigation organization Eurocontrol show: in 2024, the average delay per flight in Europe was 17.5 minutes. Of that, 8.0 minutes were due to reactionary delays – 46 percent of all delay minutes. In second place were causes within the airlines themselves at 4.3 minutes per flight, followed by flow control in en-route airspace at 2.3 minutes and airport restrictions at 1.7 minutes.

The source of the figures is the annual report of Eurocontrol’s delay service CODA for 2024. The values fluctuate considerably from year to year and month to month; in July 2024, for example, delay from flow control in en-route airspace reached a record 5.7 minutes per flight.

The rotation: why a delay stays with you all day

To understand reactionary delays, you have to think in rotations. A short-haul jet such as the Boeing 737-800 or the Airbus A321neo often flies four, six or more sectors in a row in one day. Between two flights is the planned ground time, in which the aircraft is deplaned, unloaded, cleaned, refueled, loaded and boarded again – the turnaround.

For every aircraft type and airport, each airline plans a minimum ground time, the shortest time in which a turnaround reliably succeeds. If the planned ground time is above that, there is buffer in the rotation. This buffer is the only thing that can shrink an incoming delay. Once it is used up, the aircraft departs just as late as it arrived – plus anything else that goes wrong on the ground.

In addition, it is not only the aircraft that moves. Crews also switch between aircraft, and connecting passengers or transferred baggage link flights together. If a flight waits for a late crew from another aircraft, that is also a reactionary delay, just via a different path.

You can estimate how much of a delay carries over with simple rotation logic yourself:

For you at the fence, an important rule follows from this: in the morning the schedule is most reliable, in the evening least. The first waves start with aircraft that stood at the departure airport overnight and with rested crews. Over the course of the day, disruptions accumulate, and in the afternoon and evening some aircraft carry the delay of three or four previous flights.

The IATA delay codes at a glance

So that airlines, handlers and airports can analyze where things stick, every departure delay is assigned a cause. A standardized system of two-digit IATA delay codes exists for this. The codes are transmitted in the movement messages exchanged between stations, together with the number of minutes. If a delay has several causes, several codes with their respective minutes can be given. Many airlines add their own subcategories to the codes internally; these are not uniform, though.

The first digit tells you the cause group:

Code group	Cause area	Typical examples
0x	internal airline use	free for individual airlines’ own purposes
1x	Passengers and baggage	late acceptance of passengers (11), boarding problems such as missing passengers at the gate (15)
2x	Cargo and mail	delays with cargo documents or loading
3x	Aircraft handling	late or faulty load documents such as the loadsheet (31), loading, refueling, catering
4x	Technical	technical defect on the aircraft (41), maintenance, replacement aircraft
5x	Damage and IT failures	damaged aircraft, system failures
6x	Flight operations and crew	late arrival or boarding of the crew (63), flight planning, crew shortage
7x	Weather	weather at the departure point (71), at the destination, en route; de-icing
8x	Air traffic control, airport, authorities	flow control in en-route airspace (81), restrictions at the destination airport (83), restrictions at the departure airport (89)
9x	Reactionary delays and miscellaneous	aircraft arrived late from its previous flight (93), waiting for crew from another flight (95), miscellaneous (99)

Important for putting it in context: a code always describes only the immediate cause at that departure. If an aircraft arrives later because of a thunderstorm in the morning and therefore also continues late in the afternoon, the afternoon flight does not show the weather code but 93. The actual cause only appears on the first affected flight. That is exactly why the share of reactionary delays in the statistics is so large.

Primary causes: what slows things down on the ground and in the air

If you subtract the reactionary delays, the primary causes remain. They fall into a few large areas.

Airline and handling

According to Eurocontrol, the largest block of primary delays lies with the airlines themselves. This includes technical problems that have to be fixed or cleared before departure, but also bottlenecks in handling: the baggage belt is late, the fuel truck is still at the neighboring aircraft, the load documents are not ready, or a passenger is missing whose suitcase has to be offloaded again. How tightly these processes are timed is shown by the guides on baggage handling and on the work of ramp agents on the apron. The crew belongs here too: if someone drops out at short notice or a crew would exceed its permitted duty time because of the delay, a replacement is needed.

Flow control in the airspace

European airspace is divided into sectors that can each safely handle only a certain number of aircraft per hour. If a sector expects more traffic than it can manage – because of air traffic control staff shortages, thunderstorms, military use or simply peak traffic – flow control (Air Traffic Flow Management, ATFM) kicks in. How national air traffic control plays its part is described in the guide to DFS German Air Traffic Control.

Airport and weather

Airports also have a limited capacity per hour, depending on runways, runway direction in use and visibility. In fog, the spacing between landing aircraft has to increase, in strong crosswinds a runway drops out at some airports, and in winter de-icing costs extra minutes on every aircraft. What determines runway capacity is explained in the article on airport capacity. If a destination airport is full and this has not already been managed on the ground, aircraft end up in a holding pattern.

ATFM slot and CTOT: waiting on the ground instead of in the air

Flow control in Europe has a clear principle: it is better to let an aircraft wait on the ground than to let it circle in the air. On the ground it uses no fuel for holding patterns and does not burden an already full airspace. That is why Eurocontrol’s Network Manager Operations Centre assigns a calculated takeoff time, the Calculated Take-Off Time (CTOT) – colloquially the “slot” – when there are bottlenecks.

Here is how that works:

For you as a spotter, this has a surprising consequence: a delay you see at the Munich fence can have its cause in an airspace sector over France that the aircraft will only fly through hours later. And a regulation at the destination airport holds aircraft at dozens of departure points at the same time.

Do not confuse the ATFM slot with the airport slot. The latter is an airline’s long-term allocated right to take off or land at a coordinated airport at a particular time. How this works is explained in the guide Slots at airports. At many large airports such as Frankfurt or Munich, operations are additionally coordinated through Airport Collaborative Decision Making (A-CDM): airline, handler, airport and air traffic control share their time forecasts so that pushback and takeoff match the slot time as closely as possible.

On time or delayed: where the line is

In aviation statistics, a flight generally counts as on time if it arrives or departs less than 15 minutes after the schedule. The 72.4 percent arrival punctuality that Eurocontrol determined for Europe in 2024 also refers to this threshold. The usual comparison is with block times, that is, leaving the parking position and reaching the parking position – not with liftoff or touchdown.

That also explains why airlines build buffer into their schedules: the planned block time of a route is often somewhat longer than the pure flight time under ideal conditions. A flight that leaves the stand late can still arrive on time this way. For passenger rights under EU Regulation 261/2004, by contrast, the 15-minute mark plays no role; separate, longer thresholds apply there.

What this means for your planning at the spot

With this knowledge, the arrivals board becomes a tool. Instead of just looking at the displayed time, you check the chain behind it.

The most important step: follow the aircraft, not the flight number. On flight tracking sites you can use the registration to see where the aircraft is right now and when it finished its previous flight. How to do this in a targeted way is shown in the guides on flight tracking with ADS-B and on the accuracy of arrival forecasts on flight tracking sites. For rarities, notifications for registrations and types are also worthwhile.

Also plan by time of day. If you want to photograph a particular short-haul aircraft, its morning rotation is the safer choice. Long-haul flights, by contrast, are less affected by long rotation chains because they usually only make one or two flights a day – but they depend more on weather, tailwinds and the situation at the destination airport. A pleasant place like the visitors’ hill at Munich Airport or the Zeppelinheim viewing platform in Frankfurt fortunately makes waiting time bearable.

What to watch out for

• The board only knows the forecast. Displayed arrival times are recalculated continuously. Shortly before landing they are accurate, hours earlier often only a rough estimate.
• Aircraft swaps are common. In case of larger disruptions, airlines swap aircraft between rotations to limit delays. Your target registration then flies a completely different route. So always check the registration.
• Night flight rules set limits. If it gets very late in the evening, a flight at airports with night restrictions can be canceled or diverted. Which rules apply where is covered in the overview of the night flight ban at German airports.
• Delay can also be an opportunity. If an evening flight shifts into twilight or an aircraft arrives from the other side because of a changed runway direction in use, unplanned shots arise.

Conclusion

Flight delays seem random but follow a clear pattern. A primary cause – technical, handling, crew, weather, air traffic control or airport – sets the chain in motion, and the aircraft’s rotation carries it on until sufficient buffer absorbs it. In Europe, this passed-on share, at almost half of all delay minutes, is the largest single item. Flow control via CTOT shifts the waiting time from the air to the ground, often far away from the actual bottleneck. If you know this mechanism, you read the arrivals board with different eyes and stand at the fence in vain less often.