Flexible use of airspace

The EUROCONTROL Concept of the Flexible Use of Airspace (FUA) is that:

  • Airspace is no longer designated as purely "civil" or "military" airspace, but considered as one continuum and allocated according to user requirements. 
  • Any necessary airspace segregation is temporary, based on real-time usage within a specific time period.
  • Contiguous volumes of airspace are not constrained by national boundaries.


The FUA Concept provides the Air Traffic Management (ATM) system with the potential to increase capacity.

The FUA Concept allows the maximum shared use of airspace through enhanced civil/military co-ordination. The application of the FUA Concept ensures that any airspace segregation is temporary and based on real use for a specified time period. Flexible airspace structures have been established that are suited to temporary use.

The Flexible Use of Airspace (FUA) Concept has been developed at the three Levels of Airspace Management that correspond to Civil/Military co-ordination tasks. Each Airspace Management (ASM) level has an impact on the others:

  • Strategic Level 1 – definition of the national airspace policy and establishment of pre-determined airspace structures;
  • Pre-tactical Level 2 – day-to-day allocation of airspace according to user requirements;
  • Tactical Level 3 – real-time use of airspace allowing safe Operational Air Traffic & General Air Traffic (OAT & GAT) operations.

Effective application of the FUA Concept requires the establishment in each of the European Civil Aviation Conference (ECAC) States of a national High-Level Airspace Policy Body. This body is tasked with the re-assessment of national airspace, the progressive establishment of new flexible airspace structures and the introduction of procedures for the allocation of these airspace structures on a day-by-day basis.

The practical application of the FUA Concept relies on national Airspace Management Cells (AMCs) for the daily allocation and promulgation of flexible airspace structures in the Airspace Use Plan (AUP) and Updated Use Plan (UUP), and on the Centralised Airspace Data Function (CADF) for the dissemination of information to aircraft operators through daily European AUP and UUPs (EAUP/EUUP) via NOP Portal or eAMI message for those using B2B service.

The States should also establish real-time civil/military co-ordination facilities and procedures to fully exploit the FUA Concept.


The introduction of the Flexible Use of Airspace Concept is based on the fundamental principle that airspace is one continuum to be allocated for use on a day-to-day basis to accommodate user requirements.

The FUA Concept will allow the maximum joint use of airspace by appropriate civil/military co-ordination to achieve the required separation between civil and military flights, hence reducing airspace segregation needs.



The Implementation of the Flexible Use of Airspace Concept has already benefited both civil and military aviation with:

  • Increased flight economy offered through a reduction in distance, time and fuel;
  • the establishment of an enhanced Air Traffic Services (ATS) route network and associated sectorisation providing :
    - an increase in Air Traffic Control (ATC ) capacity;
    - a reduction in delays to General Air Traffic;
  • more efficient ways to separate Operational and General Air Traffic;
  • enhanced real-time civil/military co-ordination;
  • a reduction in airspace segregation needs;
  • the definition and use of temporary airspace reservation that are more closely in line with military operational requirements and that better respond to specific military requirements.


Flexible Airspace Structures

The Flexible Use of Airspace Concept is based on the potential offered by adaptable airspace structures and procedures that are especially suited to temporary allocation and utilisation:

Conditional Route (CDR)

A non-permanent Air Traffic Services (ATS) route or portion thereof which can be planned and used under specified conditions.

According to their foreseen availability, flight planning possibilities and the expected level of activity of the possible associated Temporary Segregated Areas (TSA), Conditional Route (CDRs) can be divided into the following categories:

  • Category One : Permanently Plannable CDR,
  • Category Two : Non-Permanently Plannable CDR,
  • Category Three : Not Plannable CDR.

Temporary Reserved Area (TRA)

Airspace temporarily reserved and allocated for the specific use of a particular user during a determined period of time and through which other traffic may be allowed to transit under Air Traffic Control (ATC) clearance.

Temporary Segregated Area (TSA)

Airspace temporarily segregated and allocated for the exclusive use of a particular user during a determined period of time and through which other traffic will not be allowed to transit.

Cross-Border Areas (CBA)

Temporary Reseverd Airspace (TRA) or Temporary Segregeted Airspace (TSA) established over international boundaries.

Reduced Co-ordination Airspace (RCA)

A specified portion of airspace implemented when Operational Air Traffic (OAT) is light or has ceased and within which General Air Traffic (GAT) is permitted to operate outside the ATS route structure without requiring General Air Traffic (GAT) controllers to initiate co-ordination with Operational Air Traffic (OAT) controllers.

Prior Co-ordination Airspace (PCA)

A given block of controlled airspace within which military activities can take place on an ad-hoc basis with individual General Air Traffic (GAT) transit allowed under rules specified in Letter of Agreements between civil and military Air Traffic Services (ATS) units concerned.

Establishment of Airspace Reservation/Segregation From the joint/shared use of airspace to the temporary reservation/segregation of airspace, an airspace segregation scale can be defined as described below:

flexible airspace use

Establishment of Conditional Routes (CDRs)

The Conditional Route (CDR) concept encompasses, by definition, all non-permanent Air Traffic Services (ATS) routes. The Conditional Routes are non-permanent parts of the published ATS route network that are usually established:

  • through areas of potential temporary reservation (e.g. TRA or TSA), with CDR opening/closure resulting from associated military activities, and/or
  • to address specific Air Traffic Control (ATC) conditions (e.g. traffic restrictions or ATC sectorisation compatibility) with CDR opening/closure resulting from purely civil needs.

Conditional Routes will be established by the Level 1, allocated at Level 2 by the Airspace Managment Cell (AMC) and utilised at Level 3 by Area Control Centres. CDRs will usually be established and utilised as pre-planned routing scenarios. CDRs will permit the definition of more direct and alternative routes by complementing and linking to the existing ATS route network.

A Conditional Route (CDR) can be established in one or more of the three following categories:

CDR categories