Safety Nets

Sometimes, the safest systems fail. That is why safety nets are needed - to provide additional System Safety Defences.

Safety nets are ground-based or airborne system functions that alert controllers or pilots to an increased risk to flight safety.

These pages focus on ground-based safety nets which comprise alerting logic that works primarily on the basis of surveillance data. They function in the short term, using prediction and warning times of up to 2 minutes.


  • Ensuring Effectiveness Guide
    A call to action; a tool for getting started; a roadmap for effective safety nets; a total lifecycle approach.
  • Awareness Package
    Interactive training on safety nets.
  • Safety nets and SESAR
    Safety nets are also addressed within several SESAR projects. SESAR is the technological and operational dimension of the Single European Sky (SES) initiative to meet future capacity and air safety needs.

Ground-based and airborne safety nets operate independently, often within the same airspace. The ground-based safety nets Short Term Conflict Alert (STCA) and Area Proximity Warning (APW) work alongside their airborne equivalent Airborne Collision Avoidance System (ACAS), also known as Traffic alert & Collision Avoidance System (TCAS).

STCA directly assists in preventing collision between aircraft whilst APW does so indirectly by predicting or detecting violations of traffic segregation rules.

Both Minimum Safe Altitude Warning (MSAW) and Approach Path Monitor (APM) assist in preventing controlled flight into terrain accidents, and should work in concert with their airborne equivalent (Enhanced) Ground Proximity Warning System ((E)GPWS), also known as Terrain Awareness and Warning System (TAWS).

Specifications & guidance material

The Safety Nets: Planning Implementation & eNhancements (SPIN) Task Force was created in 2005 to draft specifications and guidance material for STCA, MSAW, APM and APW, propose ECIP Objectives for ECAC-wide implementation of those specifications.

It was a true community effort. The people involved included operational, technical and safety experts in ANSPs, working together with Industry and EUROCONTROL. These people have all been represented on the multinational working groups that developed the specifications and guidance materials for safety nets, as well as the many resources available to assist with implementation and tuning.

EUROCONTROL Guidelines material has been reviewed and updated in 2017 and provide guidance for evolutionary improvement of ground-based safety nets.

The role of SPIN today

In May 2008, the Task Force became a Sub Group and changed its name to Safety nets Performance Improvement Network - to enable it to continue to develop and support enhancement of safety nets as well as a coordinated overall concept for airborne and ground-based safety nets.

SPIN is the main stakeholder group for EUROCONTROL's safety nets work. The group meets twice yearly and welcomes new members from ANSPS, regulators and industry.