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Sector Over-deliveries and Overloads

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Over-deliveries and overloads - a recurrent issue

Background - Delivering the optimised flight plan

Flight plans and associated update messages are submitted for all IFR/GAT flights, including the IFR portions of mixed IFR/VFR flights, entering, overflying or departing the IFPS Zone (IFPZ).

The filed flight plan is the result of the preparation by Aircraft Operators (AOs) to ensure, as far as possible, the most efficient and economic profile for the intended flight.

  • The route and levels are determined by many factors such as aircraft type, payload, weather conditions, route charges, etc.
  • The flight plan data, departure and arrival times, waypoints, route and levels are key elements in the preparation for airports, ACCs and CFMU to manage the flight in a safe and optimum manner.

The forecast traffic demand and available ATC capacity is identified by Air Navigation Service Providers (ANSPs) at ATC sector level and thus appropriate scenarios and sector configurations can be put in place for the day of operations. This assessment of traffic counts and sector loads, when and where appropriate, may result in a request to the Network Manager's Operations Centre (NMOC) to put protective ATFCM regulation in place.

  • ATFCM is implemented for airspace where the traffic demand exceeds the defined ATC capacity.
  • Flow and/or capacity management measures such as departure slots, level-capping or rerouting scenarios may then be needed to prevent sector overloads

Why is there a problem?

Daily across Europe, regulations are put in place to protect ATC from receiving more traffic than the controller can handle safely. However, it regularly happens that more aircraft than planned enter these protected sectors, exceeding their capacities by more than 10%, which is regarded as an ATFCM "over-delivery". To protect ATC from those consequences is one of the core operational tasks of the Network Manager, i.e. enhance safety by preventing congestion of air traffic.

When investigating those occurrences in most cases it is found that the additional flights entered the concerned sector as a result of:

  • not flying at the initial requested flight level (RFL); or,
  • departing at times different from the original estimated off block time (EOBT) or calculated take off time (CTOT); or,
  • deviating from their original planned route; often direct routeing (DCT).

Possible reasons for non-adherence are multiple and vary from inefficient turnover process, weather problems, lack of trust in load figures, under estimation of the network effects, pressure to make up lost time, etc.

In one third of all ATFCM reported over-deliveries during 2009, the actual FL was different to the RFL in the Flight Plan. The network impact of this over-delivery can result in:

  • wasted capacity in some sectors,
  • an overall lack of confidence in the accuracy of forecast traffic counts,
  • protective capacity reduction,
  • increased workload, stress or working conditions such that the ability of ATC controller may be significantly impaired and thus a safety issue

Why is there a lack of adherence to FPL & ATFCM measures at airports?

NMOC is systematically monitoring the performance on ATFCM slot adherence. At some holiday destinations, a few flights can be hours not minutes away from their CTOT without notification to ATM. These changes to the planned timing, route or flight level, if not communicated, may cause disruption to a handling ACC but also to the destination airport. Who has not arrived early on a flight at an airport to find no stand available?

So where it is experienced, what are the principal causes for the lack of FPL adherence at airports? It can be caused by:

  • AOs not submitting DLA/CHG messages.
  • Poor management of delays.
  • Failure to adhere to CTOT and to update EOBT for non-regulated flights.

Accurate flight plan data is essential. If change is not communicated then this immediately introduces an element of inaccuracy into traffic projections for both en route and destination airports.

NMOC can allocate a calculated take-off time (CTOT) to protect a congested ATC sector. In such circumstances:

  • AOs should plan the departure of their flight so that the aircraft will be ready for start in sufficient time to comply with the CTOT.
  • Slot tolerance (-5min +10min) is available to ATC to organise the departure sequencing.
  • Airports and controllers should have effective practices to monitor EOBT & CTOT.
  • ATC has a joint responsibility with AOs in CTOT adherence (ATFCM Users Manual).
  • ATC may deny start up clearance to a flight unable to meet its CTOT until coordination with the ATFCM unit concerned.

Why is there a lack of adherence to FPL & ATFCM measures in the en-route phase of flight?

There are many professional reasons for pilots to request different flight levels (conserve fuel, headwinds/tailwinds, CAT, lighter payloads, etc.) and consequently deviate from the flight level originally requested in the FPL. Carrying and burning the minimum amount of fuel is also commercially important, and so all pilots are trained to fly an optimised fuel trajectory wherever and whenever possible.

Most ANSPs provide guidelines to controllers which typically state that changes to the requested FL should only be initiated in exceptional circumstances such as for weather or for ATC reasons, however, it is often the case that controllers ask the crew for their requested level and if circumstances permit it is readily granted.

If for example the flight has been level-capped to avoid a regulated sector, the “requested level” from ATC is often interpreted as being the original higher flight level rather than the flight level envisaged in the flight plan. Thus the flight may end up back in the sector which regulation had sought to avoid.

In some cases “low filers” have sought to avoid regulated airspace and then request a higher level in flight. If the higher level is granted, they will achieve a more efficient cruising level than that for which they filed and avoid the delays incurred by other airlines, who had genuinely filed at the higher level.

For controllers there is a problem of knowing whether a flight has been subject to regulation in the first place; so some major ANSPs are beginning to address this need to respect the requested flight level in a more systematic way rather than a verbal alert.

  • Skyguide have a tool on the basis of NMOC data in development to monitor specific regulated sectors and to detect flights deviating from their original flight plans. If required, on a tactical basis, a correction of the flight level as filed in the FPL will be requested before it enters the regulated airspace.
  • Maastricht UAC new FDPS will be able to show active flow regulation on flights through its airspace.

ATC may also clear a flight to take a more direct route (DCT) than that originally envisaged and thus the flight profile may deviate substantially from the FPL route and its planned times. Provided the controller meets the sector exit conditions it is not often regarded as an issue, and further, it is a rare occasion when a pilot refuses a shorter route!

  • There are many positive reasons why the use of DCT is an appropriate course of action for a controller e.g. low traffic volumes, a flight wishing to make up time, military airspace has been made available but principally because use of DCT is seen as providing a more economic flight profile.
  • However, DCT may also be a cause for flights entering sectors earlier, or indeed entering a sector which had not anticipated this traffic.
  • Approach timing and sequencing is increasingly an issue and an accurate “time over” on approach will become more important for ground handling; thus an early arrival can be just as much an issue for an airport and AO as arriving late.

“After all, the more imprecise the projected traffic loads for airspace sectors prove to be, the bigger the safety margins will have to be which are built into their declared capacity limits. So controllers are really doing the system no favours by letting individual crews climb to higher flight levels over and over again”. (Dr. Klaus Affholderbach, Head ATFCM Skyguide).

Towards Business Trajectory

In the future with SESAR there will be a progressive move from airspace to trajectory based operations where the Shared Business / Mission Trajectory (SBT) is made widely available for ATM planning purposes to authorized users. The Reference Business/Mission Trajectories (RBT) represents an agreed 4D profile between AO and the ANSP which takes account of route, charges, weather, approach sequencing and predictable arrival times for ground handling etc. This trajectory is the choice of the AO. It will be the role of ATC and the cockpit to match that agreed profile to the extent possible and the flight plan will increasingly be regarded as a contract between parties. The use of the business trajectory is a fundamental element of SESAR, their use and definition is sought by airlines and ATM has to prepare itself to meet this requirement. The filed route, flight levels and timing will need to be respected through training, system support and practice.

The Regulatory Context

The texts below are extracts only from the original publications. The complete documents are accessible on the respective websites.

The Principle of Adherence to Flight Plans stems from ICAO Doc Annex 2 to the Convention on International Civil/Aviation/Rules of the Air.

In accordance with Chapter 3, article 3.6.2 “an aircraft shall adhere to the current flight plan or the applicable portion of a current flight plan submitted for a controlled flight unless a request for a change has been made and clearance obtained from the appropriate air traffic control unit, or unless an emergency situation arises which necessitates immediate action by the aircraft, in which event as soon as circumstances permit, after such emergency authority is exercised, the appropriate air traffic services unit shall be notified of the action taken and that this action has been taken under the emergency authority.

ICAO Doc 7030/4 (EUR)

“ATC is responsible for departure slot monitoring at departure aerodromes…” and “…Aircraft operators shall inform themselves of and adhere to… current ATFM measures (e.g. specific measures applicable on the day in question such as ATFM slot or flight suspension)”.

“Any changes to the EOBT of more than 15 minutes for any IFR flight within the IFPZ shall be communicated to the IFPS”.

The Network Operations Handbook

“ATC may deny start up clearance to a flight unable to meet its CTOT until coordination with the ATFCM units concerned has been effected and a revised CTOT issued.”

According to the Network Operations Handbook the ATC Flight Plan Proposal Message (AFP) message shall be submitted to the IFPS by an ATS unit where that unit has new or revised information concerning an aircraft filed as IFR/GAT within the IFPZ that is already in flight.”

Commission Regulation (EC) No 1033/2006 (Procedures for Flight Plans in the Pre-Flight Phase)

Commission Regulation (EC) No 1033/2006, laying down the requirements on procedures for flight plans in the pre-flight phase for the single European sky, applies as from 1st January 2009:

“The operator shall ensure that the conditions of acceptance of a flight plan and any necessary changes thereto as notified by IFPS to the originator are incorporated into the planned flight operation and communicated to the pilot.” and “The operator shall ensure prior to operation of the flight that the content of the initial flight plan correctly reflects the operational intentions.”

Commission Regulation (EC) No 255/2010 (ATFM Implementing Rule)

Commission Regulation (EU) No 255/2010 of 25 March 2010 lays down common rules on air traffic flow management. Subject matter is defined as follows: “This regulation lays down the requirements for air traffic flow management (hereinafter ATFM) in order to optimize the available capacity of the European air traffic management network (hereinafter EATMN) and enhance ATFM processes”.

It includes:

  • the general obligations of Member States,
  • the obligations of Member States concerning the central unit for ATFM
  • the general obligations of ATS Units
  • the general obligations of operators
  • the general obligations of airport managing bodies

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