ANSP performance is gathered an published online on https://ansperformance.eu. They monitor complexity, delays, efficiency and other operational data.

I also noticed they have two ways of describing delays with a formal definition:

  1. En-route delays

The en-route ATFM delay provides an indication of ATFM delays on the ground due to constraints en-route.

  1. Airport delays:

The Airport Arrival ATFM Delay provides an indication of ATFM delays on the ground due to constraints at airports.

My question is: What does this exactly mean? Are these different ways of describing the same delays? Or do they describe two different sources of delays.

One reason of my doubt is that both categories use the same classification, from A (accident/incident) - W (weather). Could someone shine some light on this?

  • $\begingroup$ If an aircraft has to divert around a major storm, that can delay the arrival time. The weather may be clear at the destination, but between "here and there" is a major storm. $\endgroup$
    – Ron Beyer
    Nov 22, 2019 at 13:20

1 Answer 1


The delays you mentioned are imposed by Eurocontrol to manage traffic flow all over Europe:

Even when an aircraft is ready to depart, it may still be delayed by ‘flow management’ (ATFM). You may well hear that your flight is ‘waiting for its Air Traffic Control slot’ and this is explained in another article. In short, if there is a problem that will delay an aircraft either en-route or on the approach to the destination airport, then it’s safer and cheaper to hold the aircraft on the ground, not burning fuel, and give it a delayed take-off time.

EUROCONTROL as Network Manager looks at all the flight plans and identifies where and when there are too many flights/too little capacity. This might be as a result of a peak in demand (aviation has rush hours too!), weather problems, limited capacity at the destination airport or many other possibilities. In total, flow management delays account for about 16% of total delays (in 2017), mostly relating to issues at the destination airport.


ATFM departure slots are allocated centrally by the European Network Manager upon the request of the local Flow Management Position (FMP), when an imbalance between demand and capacity is foreseen at airports and/or en-route. The purpose of such regulation is to hold aircraft on the ground.

(Eurocontrol ATFM Slot Adherence)

To the pilots, this comes as a so called CTOT (Calculated Take-Off Time):

  1. A time calculated and issued by the Central Flow Management unit, as a result of tactical slot allocation, at which a flight is expected to become airborne.

  2. An Air Traffic Flow & Capacity Management (ATFCM) departure slot, forming part of an Air Traffic Control (ATC) clearance, which is issued to a flight affected by Network Management regulations. It is defined by a time and tolerance (-5 to +10 minutes) during which period the flight is expected to take-off.

(Eurocontrol ATM Lexicon)

For each of these delays, there is an associated Reference Location where the reason for the delay is located. This defines, which delay category applies:

  • En-route delay:

    1. Delay caused by regulation based on traffic volume which has a reference location classified as AS (Airspace) or SP (Special Point).

    2. ATFM delay caused by regulations applied by the CFMU at the request of the FMP to protect en-route ATC sectors from overload.

    (Eurocontrol ATM Lexicon)

  • Airport Delay:

    Delay caused by regulation based on traffic volume which has a reference location classified as AZ (Aerodrome Zone) or AD (Aerodrome)

    (Eurocontrol ATM Lexicon)

Examples for delaying a departure could be:

  • enroute weather does not allow to fly the filed route at the moment (En-route delay)
  • weather at destination is not expected to be good enough to land at the estimated arrival time (Airport delay)
  • there will be too many aircraft in one air traffic control sector (En-route delay)
  • there will be too many aircraft arriving at the destination airport at the same time (Airport delay)
  • $\begingroup$ Wow thanks. That is very clear, and quick. It is thus just a matter of source/location. So if I would like to investigate how ATC (e.g. staffing, performance) affects delay, I would need to look at both en-route and airport, because ATC can impact either, right? $\endgroup$ Nov 22, 2019 at 13:47
  • 1
    $\begingroup$ @RobinKramer In principle yes, but I don't think ATC staffing is the limiting factor for an airport. That is usually coming from available runways or weather. En-route however, the number of aircraft one controller is allowed to handle at any time is limited: "The capacity of the current ATC system is primarily limited by the maximum number of aircraft that a controller can handle in a sector." (eurocontrol.int/archive_download/all/node/9730) $\endgroup$
    – Bianfable
    Nov 22, 2019 at 13:51

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